United States
Environmental Protection
Agency
Research and Development
Office of Energy. Minerals, and
Industry
Washington DC 20460
EPA 600 7 79 103
April 1979
Evaluation of
Present Chemical
Standards in
Relationship to
In Situ Marine
Water Quality
Measurements
Interagency
Energy/Environment
R&D Program
Report
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
energy supplies in an environmentally-compatible manner by providing the nec-
essary environmental data and control technology, Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
systems; and integrated assessments of a wide range of energy-related environ-
mental issues.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
-------
EPA-600/7-79-103
April 1979
EVALUATION OF PRESENT CHEMICAL STANDARDS
IN RELATIONSHIP TO IN SITU MARINE WATER
QUALITY MEASUREMENTS
by
D. G. Deliman
D. G. Harden
L. L. Launer
M. D. Sands
H. G. Stanley
Interagency Agreement No. D5-E693
Program No. EPA-78-BEA
Program Element No. 1 NE 625C
Project Officer
Gregory D'Allessio
Office of Energy, Minerals and Industry
U.S. Environmental Protection Agency
Washington, DC 20460
This study was conducted as part of the
Federal Interagency Energy/Environment
Research and Development Program
by Interstate Electronics Corporation
Ocean Engineering Division
Anaheim, California
Prepared for
National Oceanic and
Atmospheric Administration
Rockville, MD 20852
Office of Energy, Minerals and Industry
Office of Research and Development
U.S. Environmental Protection Agency
Washington, DC 20460
-------
DISCLAIMER
This report was assembled by the Test and Evaluation Laboratory, National
Ocean Survey, National Oceanic and Atmospheric Administration, from informa-
tion received under contract with the Ocean Engineering Division, Interstate
Electronics Corporation, Anaheim, California. Approval does not signify that
the contents necessarily reflect the views and policy of the National Oceanic
and Atmospheric Administration or the U.S. Environmental Protection Agency,
nor does mention of trade names or commercial products constitute endorsement
or recommendation for use.
-------
FOREWORD
The Department of Commerce, National Oceanic and Atmospheric Administra-
tion (NOAA), has been given the responsibility to monitor seawater pollution
related to energy exploration and development. In situ measurements of chemi-
cal parameters are becoming more important because of the large volumes of
data which are required for monitoring seawater pollution, but which cannot
be achieved with conventional discrete sampling and subsequent laboratory
analysis. The more detailed information that is needed to understand the
complex chemical processes involved requires that sensors be used for contin-
uous in situ monitoring and/or profiling with depth. Inevitably, however,
some confusion exists about the accuracy and precision of data collected from
different in situ instrument arrays. Present standards, calibration proce-
dures, and uncertainties in the data collected with in situ water quality
systems, as well as how these standards, calibration procedures, and systems
contribute to this confusion, will be evaluated in this report. Parameters
most commonly measured and those which show promise for future development
are discussed.
"James P. Sullivan
Contracting Officer's Technical
Representative
Test and Evaluation Laboratory/National
Oceanic and Atmospheric Administration
m
-------
ABSTRACT
This report represents the findings determined by Interstate Electronics
Corporation Oceanic Engineering Division during a 9-month study entitled
"Evaluation of Present Chemical Standards in Relationship to In Situ Marine
Quality Measurements" (Contract 6-35234), performed for the U.S. Department of
Commerce, National Oceanic and Atmospheric Administration (NOAA). The purpose
of the program was to determine what standards are currently available and
their suitability for validating present in situ water quality measurements,
and to assess the accuracy of measurement traceability from the in situ
instrument to a recognized standard. The assessment included the determina-
tion of measurement uncertainty and resulted in recommendations for the
development of necessary standards.
Mhile development and use of in situ measurement systems in the marine
environment is in its infancy, the ease with which in situ systems gather
large volumes of data for environmental studies, pollution monitoring, and
other applications is such that their continued use and growth is assured.
Because their use is recent, inattention to establishing standards has made
suspect the absolute accuracy of the data and, thus, the comparability between
data sets. Most users have devised their own techniques of assuring the rela-
tive accuracy of their in situ data, but no recognized standard techniques
exist.
It was found that deficiencies exist in two areas: (1) lack of an ade-
quate (multiple point) primary standard for some parameters (e.g., salinity
and dissolved oxygen); and (2) failure by in situ system users to follow
standard methods (where they exist) for calibration and standardization. The
study recommends: (1) the establishment of standard reference materials and
methods for calibration and standardization to assure accuracy of parameters
measured in situ; and (2) the development of specific means to achieve trace-
ability to these standards.
IV
-------
CONTENTS
Foreword iii
Abstract iv
Figures vi
Tables vi
Abbreviations vii
Symbols viii
Acknowledgments ix
1. Introduction 1
2. Conclusions 3
3. Recommendations 4
Calibration and standardization materials 4
Calibration procedures 4
Documentation procedure 5
Support services 5
4. Traceability of Calibration to Common Standards 7
Standard references 7
Calibration and standardization techniques 10
Accuracy and reliability 14
Alternate calibration techniques 17
Measurement degradation 18
5. Reference Standard Requirements for Chemical In Situ
Instrumentation 19
Reference standards for currently measured parameters .... 19
Extention of in situ instruments to parameters not
now measured 21
Survey of national accuracy requirements 23
Instrumentation 25
References 35
Appendices
A. Description of two variations of in situ instrumentation
systems 37
B. Experimental cruise for comparison of calibration
accuracies 40
Glossary 49
-------
FIGURES
Number
1 Intercalibration cruise sampling station locations,
TABLES
Number Page
1 Representative Instrumental Capabilities 15
2 Representative Analytic Capabilities 16
3 Common Standards for Calibration 17
4 Ranges of Common Oceanic Parameters 19
5 Parameters Measured In Situ—Present and Future
Applications 22
6 Estimates of Best Oceanic In Situ Accuracies 25
7 Partitioned Standard Deviations from Intercomparison
Experiment 30
8 95% Confidence Intervals with Paired T-Tests for Significant
Difference Between Sample and Calibration Measures 31
B-l In Situ Systems Used for Experimental Cruise 42
B-2 Laboratory Methods Used on Discrete Samples for
Experimental Cruise 42
B-3 Summary of System Accuracies 43
B-4 Horiba: U-7 Water Quality Checker 44
B-5 Hydrolab: 6D Surveyor 45
B-6 Martek: Mark V Digital Water Quality Analyzer 46
B-7 Plessey Environmental Systems: Model 9400
Telemetering Sensor System 47
B-8 Yellow Springs Instruments: Model 57 Dissolved
Oxygen Meter 47
B-9 Beckman Select-Mate 48
B-10 Orion pH Meter 48
B-ll Beckman RS 7-C Induction Salinometer 48
VI
-------
LIST OF ABBREVIATIONS
ABBREVIATIONS
ASTM
CalCOFI
CI
cm
CTD
CUE
DMRP
DO
GEOSECS
Hz
I.A.P.S.O. --
KHz
kg
mA
mg/1
MIL/STD
ml
mm
mS/cm
mv
NA
NBS
NORPAX
OCS
ppm
ppt
QC
SI
SRM
STD
TDWG
American Society for Testing Materials
California Cooperative Fisheries Investigation
confidence interval
centimeter
in situ conductivity, temperature, and depth measuring system
Coastal Upwelling Experiment
Dredged Material Research Program
dissolved oxygen
Geochemical Ocean Sections
Hertz
International Association of the Physical Sciences in the Ocean
kiloHertz
kilogram
milliampere
milligram per liter
military standard
milliliter
millimeter
mi Hi Siemens per centimeter
millivolt
not available
United States National Brueau of Standards
North Pacific Experiment
Outer Continental Shelf
parts per million
parts per thousand
quality control
The International System of Units
standard reference material
in situ salinity, temperature, and depth measuring system
Test Documentation Working Group
Vll
-------
LIST OF SYMBOLS
SYMBOLS
°C -- temperature in degrees Celsius1
Eh — oxidation reduction potential in millivolts
KC1 -- potassium chloride
n — number
Na2S — sodium sulfide
Q£ -- oxygen
% -- percent
%o -- parts per thousand
pH -- negative logarithm of hydrogen ion concentration
Vac — alternating current voltage
Vdc — direct current voltage
-------
ACKNOWLEDGMENTS
Because of the amount of interest and the number of personal contribu-
tions to this study, it is impossible to acknowledge individually all of the
contributors to the program. In addition to the information generously pro-
vided by instrument manufacturers throughout the world, there were over 300
personal communications from correspondents in the United States and 24
foreign countries. This evidence of worldwide interest in accurate oceanic
measurements should provide the necessary incentive to develop uniformly
accepted calibration and standardization techniques.
The staff of the Oceanic Engineering Division would like to express
special thanks to the volunteers from Saddleback College and Fullerton College,
the teams of marine technicians for the experimental cruise, Moss Landing
Marine Research Center, LFE Environmental, Inc., and the following instrument
manufacturers who provided instruments:
Hydrolab Corporation
Yellow Springs Instrument Company
Martek Instruments, Inc.
Horiba Instruments, Inc.
Beckman Instruments, Inc.
Orion Research, Inc.
Great Lakes Instrument, Inc.
Pi Instruments, Inc.
Appreciation is extended to the Contract Officer's Technical Representa-
tive, Mr. J. Sullivan of the Test and Evaluation Laboratory of the National
Oceanic and Atmospheric Administration, for his patience, technical support,
and guidance, and to Mr. R. Farland of the NOAA Office of Ocean Engineering
for his assistance in program development and technical review. A special
thanks is given to Ms. C. Gariepy of the Test and Evaluation Laboratory for
typing the manuscript.
IX
-------
SECTION 1
INTRODUCTION
Under this study, Interstate has considered the chemical parameters
(dissolved oxygen and pH) and two supporting physical parameters (temperature
and salinity) which are routinely measured in situ in the marine environment.
The study evaluated present standards in relationship to these water quality
measurements, and has explored the traceability of those standards. While
this study emphasizes investigation of dissolved oxygen, pH, temperature, and
salinity, it is noted that currently 19 parameters are measured in situ, and
59 other parameters are under consideration. Estimates of the range and
accuracy required of in situ measurement, present and future, were determined
by surveying major groups using in situ sensors and by reviewing manufactur-
er's recommended procedures and published specifications.
In situ marine monitoring systems remotely sense the oceanic parameters,
or transferable property from which the parameter can be derived, while the
sensor package is submerged. Frequently considered as in situ monitoring are
those pumped systems that provide a continuous flow from the location of
interest. Sub-classes of in situ monitoring systems can be characterized by
the nature of the platforms (unmanned moored buoys, drifting buoys, or manned
ships) or by the method of measurement (automated, continuously profiling
systems, or towed systems). User contacts reveal a significant increase in
the use of continuous profiling systems for coastal zone water quality mea-
surement and for pollution control and monitoring. While they are a rela-
tively recent innovation, they deserve special consideration because of their
potential.
Today's emphasis on conservation of the marine environment results in
the greatest number of measurements being made in the relatively shallow
waters of the coastal zone, where depths typically are less than 50 meters.
The vertical gradients of water properties in shallow bodies of water are
frequently great and can vary widely over short time spans. Therefore, there
is considerable interest in maintaining accuracy and in sampling at short
intervals. Since rapid sampling frequency cannot be met using conventional
discrete sampling techniques such as Nansen casts, there has been an increase
in the use of continuous profiling systems. A brief description of two
classes of in situ instrumentation is provided in Appendix A.
The design of profiling systems integrates the sampling process with on-
board data processing and analysis. This is highly desirable in order to
cost effectively meet the mission objectives of some users, such as waste-
water treatment districts that perform nearly continuous monitoring at out-
falls. The cost of post-survey data analysis by hand methods is prohibitive
1
-------
and introduces unacceptable errors. The automatic data reduction and analy-
sis capabilities of the continuous profiling systems results in great quanti-
ties of data quickly and neatly produced and ready for user interpretation.
Of significant importance in saving labor costs is the system's ability to
print the data on the various Federal, State, and local government report
forms with a minimum of hand labor, such as typing and drafting of charts.
A discussion of the components of error in and between in situ systems
and a demonstration during an experimental survey of the errors induced by
different participants using different in situ instruments under nonstandard
procedures are offered as a compelling argument for the procedural recommend-
ations that follow. Because of difference in usage of a number of terms such
as "Precision Standard" and "Calibration," definition of selected terms as
they are used in this report appear in the glossary. The pertinent cruise
data which form the basis for comments in the text are included in Appendix B.
-------
SECTION 2
CONCLUSIONS
1. An addendum to the contract document states, "What the Government
needs to know is that if a measurement is made in situ, would one get the
same answer if a sample were taken at the same place and the same time and
analyzed in a laboratory with laboratory methodologies?" The results of this
study answers this question with an emphatic "NO!" While this situation must
be remedied, a successful solution will require a well-planned, long-term
commitment from the Federal Government that includes recognizing the impor-
tance of data quality in the evaluation of proposals, grants, and Government
conducted oceanographic measurement programs.
2. In situ measurement is not at a state of routine data comparability
between different instruments measuring the same parameter or for a single
instrument operating at different time intervals. Accuracies attained are
less than desirable for absolute measurements associated with research activi-
ties. The relative accuracy of in situ systems, coupled with their ability
for continuous sampling and rapid data collection, is often justification for
their use in monitoring programs or for coastal research programs.
3. Inter-user calibrations are not presently part of routine survey
operations. The GEOSECS program does come close to the practice by central-
izing the responsibility for the calibration and maintenance of instrumenta-
tion used by all investigators within the program. However, there is no
interface with investigators outside the program who may utilize the data.
4. Standard reference materials and/or calibration methods are not
utilized by institutions or programs. Variations in standardization methods
may exist. This situation is very widespread. Manufacturer's specifications
and Recommendations vary; users' application of manufacturers' recommendations
varies as well. Data derived from in situ measurements of a parameter by
different instruments or by different programs do not seem to be comparable.
5. For current monitoring needs, adequate standards are currently avail-
able for temperature and pH. There are inadequate, multiple-point standard
reference materials for salinity and dissolved oxygen.
-------
SECTION 3
RECOMMENDATIONS
A comprehensive program plan for quality assurance must be developed for
in situ marine water quality measurements, their calibration, and standardi-
zation. This program should be under the guidance of a single agency oriented
to the marine environment to insure the goals will be attained. Observations
of past programs of similar nature conducted by committees of agencies reveal
that the dilution of authority results in failure to achieve the objectives.
Program elements would include:
CALIBRATION AND STANDARDIZATION MATERIALS
The program must include a standard reference material with well-defined
accuracy for all parameters. Specifically:
1. Develop a series of standards for salinity that encompass the range
from 0 to 40 ppt in 5-ppt increments with an accuracy to 0.002 ppt.
2. Develop a series of dissolved oxygen standards for the range from 0
to 10 ppm in increments of 2 ppm with an accuracy of 0.05 ppm.
CALIBRATION PROCEDURES
Instrument calibration procedures utilized on Government-funded programs
must be required to satisfy traceability to accepted standards and have formal
documentation. A major source of error in any system is the calibration and
standardization routine. The military and space programs have used this type
of procedure for many years. These programs require manufacturers to develop
a recommended calibration procedure which is verified, with the calibration
traced to a recognized standard. The verification process is commonly con-
ducted by an independent calibration laboratory or at the manufacturer's
facility under the supervision of a Government inspector. The verification
process also recommends calibration frequency. Inputs from the manufacturer
for calibrating marine in situ monitoring instruments would be considered by
the agency responsible for procedure development. Current user practices for
calibration would also be considered for the procedural development.
It is expected that responses from users and manufacturers will be
diverse and dependent on instrument capabilities and on program accuracy
requirements. But these must be considered for the development of a uniform
procedure. For example, the frequency of calibration may vary with class of
required measurement and type of instrumentation. A temperature sensor may
-------
require recalibration and certification once a year to ensure 0.5°C accuracy,
or as frequently as bimonthly for an accuracy of 0.02°C.
The verification of instrument calibration could be indicated by a certi-
fied seal or sticker. The sticker provides information on the date of cali-
bration and the required date for recalibration. The official calibration
procedures should be distributed by a central issuing agency that is respon-
sible for receiving and updating as required.
Questions that require resolution of differences between methods would
be resolved by a Test Documentation Working Group (TDWG) that includes repre-
sentatives from the sponsoring agency, instrument designers, standards organi-
zation, instrument test operators, and data users. In addition to the devel-
opment of calibration procedures, the TDWG would provide information on future
requirements for parametric standards and sponsor the development of standards.
DOCUMENTATION PROCEDURE
A standard documentation procedure with the information basic to
assessing data quality includes:
1. The standard reference material used including the date of receipt
and handling since receipt.
2. A uniformly practiced and accepted Calibration and Intercalibration
Procedure which would include method accuracy and precision, the frequency
required for calibration, and the time lapsed since the previous calibration.
The calibration procedure must also include specific instructions for field
standardization procedures. The field standardization procedure should
include the limits of acceptable variation.
3. A standard data reporting format including the number of replicates,
mean and standard deviation of the calibration, and standardization procedure.
SUPPORT SERVICES
The sponsoring agency should consider undertaking the following program
support:
1. Preparation of guidelines for evaluation of proposals for marine
water quality measurement to insure that a proportion of any project is
dedicated to calibration of instruments and to the supporting tasks for data
quality assurance. It is strongly recommended that data quality assurance
requirements be listed in the contract specifications.
2. Compilation and maintenance of a comprehensive directory of manu-
facturers and users of in situ chemical oceanographic instrumentation and the
data types and quality resulting from these programs.
3. Preparation of test, calibration, and standard traceability proce-
dures for in situ instruments.
-------
4. Preparation and maintenance of a comprehensive handbook on marine
instrumentation and procedures.
5. The organization and scheduling of seminars to transfer this tech-
nology to the user community.
-------
SECTION 4
TRACEABILITY OF CALIBRATION TO COMMON STANDARDS
STANDARD REFERENCES
In situ measurements in the marine environment are subject to most of the
problems that degrade laboratory measurements, with additional problems
attributable to field conditions. Investigation of the error sources and
their effect on the accuracy of the measurement can be classified as qualita-
tive or quantitative, following the Shewhart (1) concept. The number assigned
to a distance or a chemical concentration is quantitative. Qualitative
sources of error arise from the apparatus, reagents, operator, sequence of
operations, and ambient conditions involved in quantifying the measurement.
A statement on the accuracy of the quantitative number is an indication
of the information content of that number within the framework of the intended
application. The application will usually dictate the degree of accuracy
sought and the effort required to achieve it [Eisenhart (2)]. The accuracy
achievable when calibrating a system of measurement to a primary reference
material with an assigned value (systematic error) depends on the precision
of the measurement technique and operation involved, as well as on the accu-
racy of the assigned value of the primary reference material.
In making a measurement, the individual involved must take care to assure
the measurement is of a unique property and not a combination of properties,
such as interference from other sources (specificity). Second, no systematic
errors should be permitted to cause a bias in the distribution of repeated
measurements of the property. Third, the degree of precision, which is the
ability to reproduce the same number on repeated measurements of the same
material, must be at least as good as the desired accuracy of measurement.
When these three aspects of the measurement process are assured, the accuracy
of the measurement can approach its state-of-the-art level.
Accuracy is measured by referencing a measurement to a standard using a
methodology that has been shown to achieve acceptable results. The practice
of establishing reference standards (the primary reference units and the
methodology of measurement) has been set out in some detail at the National
Bureau of Standards (NBS) by Cali et al (3). The development of a measurement
system has five major components as stated by NBS:
"Component 1 : A Rational. Self-Consistent System of Units of Measure-
ment. International agreement suggests this system should be the
System Internationale de Unites (SI) [Cali et al (3)].
°l
'Component 2: The Materials Necessary to Realize in Practice the SI
Units and Their Derivative's"! In much of the world, these well-charac-
terized materials are called Standard Reference Materials (SRM's) and
7
-------
are prepared, measured, and certified, in most instances, by national
standards laboratories. The key characteristic of an SRM is that the
properties of interest be measured and certified on the basis of accu-
racy. At NBS there are three routes that are used to accomplish this
goal :
(a) Measurement of the property using a previously validated reference
method. By definition, a reference method is a method demonstra-
ted to be accurate and reproducible.
(b) Where previously established reference methods do not exist, two
or more independent, reliable measurement methods are used. A
reliable method is one of high precision, but one whose system-
atic biases have not been fully discovered and evaluated.
(c) The third route is a variation of the second. Where a previous
issue of an SRM is available to be used to assure intralaboratory
quality control, many laboratories can be formed in an ad hoc net-
work to perform the work. Each laboratory uses the method felt to
be most reliable (and accurate) under that laboratory's operating
conditions, but must run the known SRM in parallel with the
unknown as a check. The results will be used only when the certi-
fied value of the prior SRM is obtained.
"Component 3: Reference Methods of Measurement Used With or Based on
SRM's. A reference method is defined as "a method of proven and
demonstrated accuracy." These have been called umpire methods,
referee methods, standard methods, and so forth. Absolute accuracy,
implying methods with no systematic biases, is an unattainable goal,
not achievable by mortals. It is important to realize that the cost
of obtaining greater accuracy increases exponentially. Therefore,
only that degree of accuracy required should be sought, making allow-
ance for advances in the state-of-the-art. A good guideline is to
strive for a reference method whose accuracy is three times better
than that currently required by the end user. The definition and
development of reference methods is a time-consuming, expensive, and
complex process, involving the following steps:
(a) A group of experts surveys the literature to choose a candidate
method—one expected to have small systematic biases. They also
decide what the accuracy goal should be for the reference method,
considering the required end use.
(b) A central laboratory is chosen to coordinate the work; develop the
statistical design; prepare and distribute samples that have been
previously measured by the central laboratory using an independent
method of known accuracy, but one not usually available to the
field in question; and distribute the SRM (a necessary precondi-
tion being the availability of the appropriate SRM).
(c) The group of experts, in conjunction with the central laboratory,
writes the first version of a detailed procedure (protocol) and
helps select a group of measurement laboratories (usually 6 to 10
8
-------
laboratories) willing to cooperate in performing the work called
for in the protocol.
(d) The central laboratory distributes the protocol, sample, SRM, and
instructions to the cooperating laboratories. The cooperating
laboratories perform the work according to a schedule. The ana-
lytical data plus other pertinent information are returned to the
central laboratory.
(e) The group of experts and qualified personnel from the central
laboratory, including statisticians, analyze the data, identify
sources of error, then revise the protocol to eliminate the
errors.
(f) Steps di and e^ are repeated as often as necessary until the accu-
racy goal is achieved.
(g) The protocol is written in final form and published in a journal,
a collection of reference methods, or another appropriate publica-
tion.
"Component 4: Establishment of Compatibility into a Wider Area of
Technology via the SRM and Reference Method. Components 1, 2, and 3
are sufficient in themselves to bring about accurate measurements in a
few well-qualified laboratories. The real problem is, however, to
improve the quality of and make compatible the measurement in the
average laboratory on a routine basis. There are two aspects to this
problem, one involving the field (routine) methods per se, the other
concerned with commercially produced (in-house) working standards.
As reference methods and SRM's become available, responsible groups
should begin the assessment of the various field methods currently in
use. Mhen the test materials to be used in the assessment process are
characterized on an absolute (accuracy) basis via the reference method
and SRM, the inaccuracies of the tested field methods will become
readily apparent.
"Component 5: Assuring the Long-Term Integrity of the Measurement
Process. Measurement systems are notorious in one respect—unless
carefully monitored, they tend to get out of control. Loss of preci-
sion is usually the first indication that the measurement process is
not in a state of quality control. In most measurement laboratories,
this question is one of almost daily concern and one that has been
extensively studied and addressed. Although each individual labora-
tory must ultimately be responsible for assuring its own quality con-
trol, professional societies and governmental agencies can, and often
do, provide a mechanism that helps to assure, to a degree, long-term
control.
If SRM's and reference methods are available, the mechanism for assur-
ing the long-term integrity of the measurement process in a large
number of measurement laboratories is quite straightforward:
-------
(a) The sponsoring or testing agency prepares a series of test samples
incorporated in a suitable matrix that cover the range of values
likely to be encountered in real life.
(b) The properties are determined by the sponsor's laboratory (or
laboratories) using the reference method to obtain values of known
accuracy.
(c) The test samples, as unknowns, are distributed.with suitable
instructions and reporting forms to the laboratories under test
who perform the work as instructed. In true blind studies, these
samples will not be differentiate from daily, routine samples.
(d) Results are returned to the sponsoring agency and statistically
analyzed. In a well-designed and controlled program, each labora-
tory should receive back the following information for each prop-
erty tested: its day-to-day precision within the laboratory; the
accuracy of the method used; its rank compared to other labora-
tories using the same methodology; the accuracy of its method
compared to alternative methods; a statement of acceptability of
the results (if norms for that technology have been established).
Through a survey of the more visible users and manufacturers of in situ
instruments in the marine environment, an overview of current needs and prac-
tices for calibration and standardization evolved. Then through a field
experiment the amount of error arising from these practices was demonstrated.
In the following sections the current needs and practices are described in the
context of the NBS recommendations.
CALIBRATION AND STANDARDIZATION TECHNIQUES
Definition of Terms
The terms "calibration" and "standardization" are widely used by a number
of individuals often implying the same meaning. The differences however are
significant. Calibration is the process of examining instrumental response
to a series of prepared standards that bracket the expected concentration to
be measured in the environment. The standards must be traceable, that is, of
a certified concentration. Calibration must be performed at multiple points
over the analyses range to adequately determine the instrumental response to
the concentration change. Primary standards are directly traceable to a
certified source such as the National Bureau of Standards. Secondary chemical
standards may be prepared by dilution of a primary standard or prepared from
reagent grade chemicals in the laboratory by the analyst. They are regarded
as secondary standards because of the potential errors involved during prepa-
ration. The errors include: analytical balance accuracy and time since last
inspection, purity and quality of the reagents, and the quality of the water
used to prepare the solutions. Other variables include the cleanliness of the
preparation area, type and condition of the mixing and storage container,
stability of the solution, and human expertise.
Standardization is a procedure performed in the field that brings the
instrumental response into the previously prepared calibration curve.
10
-------
Generally, at least two points in the field should be taken with every profile
or with 8-10 discrete measurements. This can only be accomplished when
sampling from a sampler that houses the in situ probe and a series of water
bottles. When an in situ measurement is taken, a water bottle must be sig-
naled to collect a sample. This procedure will limit the variability
resulting from spatial and temporal differences in the water column. Cer-
tainly, as R. A. Home (4) points out in "Marine Chemistry," the sample will
go through some changes as it is brought to the surface and the various gases
in solution rearrange in proportions. The bottle composition and cleanliness
must be controlled so as to not contaminate the sample. However, there is
presently no other acceptable way of evaluating how effectively the in situ
probe is operating in the environment. The differences tend to increase with
the distance of the water column sample from the surface. For example, carbon
dioxide is nearly twice as soluble at 1,000 meters as in surface waters. The
concentration of carbon dioxide, of course, plays a significant role in deter-
mining the pH of the water.
The sensitivity of an instrumental measurement is defined as the change
in signal or instrument output per unit change of concentration. The range
of measurement is regarded as the span of concentrations in which the probe
may be used to provide acceptable data within the stated performance specifi-
cations. The limit of detection is the lowest detectable signal which is
normally equal to twice the ambient or normal background noise.
One-Point Calibration
A one-point calibration implies that the instrumental response is linear,
can be internally normalized, or the slope of the response curve is linear
over the measurement range. This assumption cannot be made unequivocally for
all instruments under all conditions. In addition, care must be taken to
insure that the response curve has not rotated around the calibration point.
To justify one-point calibration, adequate field and laboratory data must be
collected to verify that the response for the measurement type does not rotate
or otherwise change under ambient conditions where problems are identified.
A variety of corrective procedures may be carried out. As an example, con-
sider the galvanic dissolved oxygen probe which must be calibrated using
salinities similar to the salinity of the environment being measured. Cali-
bration remains a particularly difficult problem where significant salt con-
centration gradients occur, such as between surface and deeper waters or in
an estuarine stream where the salinity may range from less than 5 parts per
thousand to 35 parts per thousand. While commercial probes have recently
become available to compensate for this effect, it is not clear how well they
actually operate. Additionally, the speed of water across the membrane may
affect the metered response. Flow must be below the minimum level which dis-
torts the membrane, yet above the level of stagnation. Because this effect
(flow versus measured oxygen) plateaus, some instrument manufacturers have
installed agitators near the probe surface to reach the effective speed where
small changes in flow do not significantly alter response. Other physical
variables which affect one point calibration for in situ instruments include
the length of the cable between the probe and electronics packages, pressure,
temperature, and trace inorganic constituents (i.e., sulfide).
The selection and use of a one-point calibration is not recommended
11
-------
unless adequate field and laboratory data have been collected to support the
linear curve assumption with full consideration of various field conditions,
such as the effects of pressure, temperature, salinity, flow across the mem-
brane, cable length, and concentrations of chemical constituents.
Recommended Means of Instrument Calibration and Standardization
All in situ marine measurements must be related directly to an accurate
standard. The integrity or exact value of the standard must meet or exceed
the level of accuracy achievable by the instrument and satisfy the program
objectives. The National Bureau of Standards recommends a standards accuracy
three times better than the user's required accuracy [Cali et al (3)]. There
is limited value to water column profiling with an instrument which can only
reflect relative rather than absolute values, particularly when most moni-
toring programs require several years worth of data to assess potential sub-
lethal trend impacts which necessitate data inter-comparability. Data integ-
rity derives from adequate documentation which traces instrument calibration
with verified field standardization. The calibration and standardization
procedures must be formalized and documented to spell out the approach, scope,
and limits of in situ monitoring.
The documentation required includes the data of instrument calibration,
type and integrity of standards, standardization values, and succeeding cali-
bration results. Instrument repair and maintenance should also be documented.
Ideally, instruments could be categorized in a manner that would indicate the
type preferred for each application (e.g., deep sea) and for achievable levels
of accuracies.
Documented calibration procedures should be required in every oceano-
graphic program. Often when commercial firms are responding to a Government
or industrial request for proposals that involve monitoring, price is a major
award factor. As such, the low bidder may short cut data integrity in order
to come up with the lowest price. The presence of a universally accepted and
endorsed calibration and standardization procedure would close this obvious
gap. Additionally, the procedures should be under yearly review to incorpo-
rate the most recent technology.
Training is a key element in performing the calibration. The procedures
must be clear, concise, and supported with performance data and literature
references. A variety of in situ instruments are available at different
prices. Each has the manufacturer's suggested calibration procedures. This
divergence and a lack of adequate documentation cause difficulty in training,
with a consequent source of error in data comparability.
Standards used for calibration must serve a large community for a variety
of needs ranging from compliance monitoring to the pure research scientist.
They should be accessible to the analyst, relatively inexpensive, convenient
to use, made of constituents which are stable over time, and of a certifiable
concentration.
For each of the parameters under study, at least two levels of data
quality assurance should be incorporated into marine in situ monitoring pro-
grams. First, instruments should be calibrated at several points over the
12
-------
expected range of measurement, under conditions approximating critical envi-
ronmental conditions, and with universally accepted standards. Second, in
the field, samples should be collected in parallel with the in situ monitoring
to verify instrument operation. Comparable techniques must be used for cali-
bration and standardization. In the following sections, methods are presented.
Salinity--
Induction-type conductivity or salinity in situ probes, because of their
size, are often very difficult for the user to calibrate. Usually manufac-
turers calibrate the units at their facilities prior to delivery and recommend
return annually. The survey of users indicated that very rarely are the
instruments returned unless a major repair is required.
A typical calibration procedure using the available standards is
described. Briefly, a series of 200-liter tanks are prepared to contain a
specific salinity water. A bench or laboratory salinometer, calibrated to
Standard Seawater, is used to verify the salinity tank values. The in situ
probe is then placed in each tank, and the values are recorded. A control
sample of the water used to prepare the salinity tanks must also be analyzed.
The accuracy of the method can be determined quite simply and is traceable
back to a primary standard. The method is not without limitations. Ideally
the laboratory salinometer should be calibrated with a primary standard at
several points. However, at the present time, only one salinity standard is
available (Standard Seawater). Also typical variations in environmental
conditions, beyond pressure and temperature, should be incorporated into the
calibration program.
Field standardization is required for data quality assurance. In the
field, the analyst must collect discrete water samples at the probe during the
time of measurement and analyze them by a reference method such as a labora-
tory salinometer. At least two samples should be collected with each profile
or each daily use in the case of a fixed monitoring device. The analyses of
these samples should be performed in real time while on station to verify
calibration, otherwise whole sets of data might later be found unacceptable.
Standard Seawater is the only primary salinity or conductivity standard
available to the analyst; it has a nominal salinity of 35 ppt. This standard
has a reported accuracy of ±0.001 ppt, however systematic variations exceeding
this level have been observed. The name "Eau de Her Normale" was changed in
1960 to "Standard Seawater" and the name of the agency which distributes this
standard was changed from "Depot d'Eau Normale" to "Standard Seawater Service."
The "Standard Seawater Service" is an agency under the "International
Association of the Physical Sciences of the Ocean" (I.A.P.S.O.). "Standard
Seawater" is certified in chlorinity only, but has been widely used as a
standard for salinity determinations from conductivity measurements. Addi-
tional certified standards should be available at 10 ppt, 20 ppt, 30 ppt, and
40 ppt. These standards would limit the extrapolation errors in calibration.
Pressure effects should be considered but they cannot be easily simulated at
a low cost in the laboratory.
13
-------
Temperature--
Temperature sensors should be routinely verified against an NBS cali-
brated platinum thermometer in the laboratory with a water bath accurate to
0.01°C. Currently both quartz and platinum reference thermometers are in use
as standards, with the platinum predominating. An accuracy of .004°C is
readily achievable according to NBS (5). Response should be checked at
several points over the intended range of use.
In the field, standardization procedures can be performed with the aid
of reversing thermometers mounted on the water bottles.
Dissolved Oxygen--
The conventional method of in situ dissolved oxygen probe calibration is
the modified Winkler titration. Air-saturated seawater at room temperature is
typically measured with the probe, and then by the Winkler and by adjustments
made to the instruments. Some analysts check the probe response to zero by
immersing the unit in a solution to which a small amount of sodium sulfite
crystals has been added, or in seawater purged with nitrogen gas. A micro
burette procedure using dilute reagents delivers exceptional accuracy. Typi-
cally in situ dissolved oxygen instruments are calibrated at one point in the
field. No chemically certified standard is produced or available in the
United States for dissolved oxygen calibration. Recommendations have been
made for a physical standard preparation. This procedure would involve
purging a seawater solution with various proportions of atmospheric air and
nitrogen gas to provide incremental solutions to saturation. The actual con-
centration of the standards may be determined by the modified Winkler method
or by other conventional methods such as gas chromatography (J. Sullivan,
NOM/NOS pers. comm.) or colorimetry. However, these methods are time con-
suming and require more expensive instrumentation.
In the field, standardization must be determined by simultaneous collec-
tion of discrete water samples at the probe using a rosette or other multiple
sampling device.
pH-
pH buffers are available from a variety of commercial manufacturers or
may be prepared by the analyst according to NBS specifications. Whatever the
case, pH meters must be laboratory calibrated at several values over the
range of use, and standardized at a minimum of two points. The narrow range
of pH values in ocean waters enhances the ability to calibrate instruments
properly.
For field standardization, a discrete water sample from a water bottle
located adjacent to the probe must be collected.
ACCURACY AND RELIABILITY
Replication Error
Replication errors are observed when different values are measured for a
known homogenous environment. However, replication problems may occur if the
water sample collected in the environment is from a different point in time
or different water mass. In situ probes must be carefully calibrated to
14
-------
achieve the stated accuracy of the manufacturer. Typically, manufacturers
quote precision-accuracy figures for their instruments which often differ from
the analytic figures. Also, the operator tends to read the meter value as an
absolute value. If a manufacturer's specifications for a dissolved oxygen
unit are ±0.1 ppm, the user should only record the display value to this
significant figure. However, the analyst may ascribe an accuracy to the mea-
surement which cannot be verified by the analytic technique.
TABLE 1. REPRESENTATIVE INSTRUMENTAL CAPABILITIES
DATA FROM MANUFACTURERS SPECIFICATION SHEET
Parameter
Manufacturer
(Model No.)
Dissolved Oxygen
Plessey (9050)
YSI (Model 57)
Montedoro-Whi tney
(Dor-IB)
Ocean Data
Salinity
Plessey (9050)
YSI (33)
Temperature
Plessey (9050)
Plessey (4005)
YSI (Model 33)
Montedoro-Whi tney
(MK III A/B)
Ocean Data
PH
Plessey (4005)
Montedoro
(DPH-1B)
Ocean Data
Accuracy
±0.15 ppm
NA
±0.10 ppm
±0.20 ppm
NA
+0.02 ppt
±0.9 ppt at 40 ppt;
±0.7 ppt at 20 ppt
±0.02°C
NA
NA
NA
NA
NA
±0.005 pH units
NA
Precision
±0.10 ppm
±0.10 ppm
NA
NA
±0.20 ppm
±0.003 ppt
NA
±0.004°C
±0.50°C
±0.1°C at -2°C;
±0.6°C at 45°C
±0.03°C
±0.1°C
±0.02 pH units
NA
±0.1 pH units
Range
0 to 15 ppm
0 to 25 ppm
0 to 10 ppm
0 to 20 ppm
0 to 20 ppm
30 to 40 ppt
0 to 40 ppt
-2 to 35°C
-2 to 35°C
-2 to 50°C
-5 to 45°C
-5 to 40°C
2 to 14 pH units
0 to 14 pH units
0 to 14 pH units
NA - Not Available
15
-------
The accuracy and precision typically reported by manufacturers, based on
instrumental capabilities for dissolved oxygen, range in accuracy from 0.1 to
0.2 ppm and from ±0.1 to ±0.2 ppm in precision. The applicable measurement
range is commonly from 0 to 20 ppm, Salinity probes typically have a reported
accuracy ranging from 0.02 to 0.9 ppt, with a precision of ±0.003 ppt. The
greater accuracy range is available with an instrument of 30 to 40 ppt range.
Temperature units have an accuracy of 0.02 to 0.1°C, with a precision ranging
from ±0.004 to ±0.1°C over a usual range of -2 to 35°C. pH probes generally
have a precision of ±0.02 to ±0.1 pH units over the range of 2 to 14 pH units.
The instrumental capabilities of representative in situ instrumentation are
summarized in Table 1.
Analytical Error in Calibration
The accuracy of standards can only be verified by the best analytical
techniques. The technique must be relatively simple, and should be low cost
and relatively quick to perform. The modified Winkler as it appears in
Strickland and Parsons (6) has a reported precision of ±0.079 ppm over the
range of 0.081 ppm to super saturation. pH standards are available to four
significant figures through NBS, while the Environmental Protection Agency (7)
lists a precision of ±0.1 pH units. Salinity as performed by the wet chemical
method in Strickland and Parsons (6) offers the analyst two ranges from 4 to
40 ppt, with a precision of ±0.06 ppt at 30 ppt or 30 to 40 ppt with a
precision of ±0.023 ppt at 33 ppt. The end point may best be determined by
the conductivity titration method. Temperature may be measured with NBS cali-
brated quartz thermometers to an accuracy of 0.02°C over a very wide range.
The representative analytic techniques for calibration are listed by parameter
with precision and range data in Table 2; Table 3 lists the common standards
used for calibration.
TABLE 2. REPRESENTATIVE ANALYTIC CAPABILITIES
Parameter/Source
Precision
Range
Dissolved Oxygen
Strickland &
Parsons (1972)
Environmental
Protection Agency
(1974)
PH
EPA
Salinity
Strickland &
Parsons (1972)
±0.079 ppm
±0.20 at 7.5 ppm
±0.1 at 3.5 pH units
±0.12 at 7.7 pH units
±0.12 at 8.0 pH units
±0.06 ppt at 30 ppt
±0.023 ppt at 33 ppt
0.081 - 12.96 ppm
NA
0 - 14 pH units
4 to 40 ppt
30 to 40 ppt
NA - Not Available
16
-------
TABLE 3. COMMON STANDARDS FOR CALIBRATION
Parameter
Sal inity
(through
conductivity)
Dissolved
Oxygen
pH
Temperature
Accepted
Standard
Standard Seawater
Winkler - Mod. Azide
NBS Buffers
NBS Buffers
NBS Buffers
NBS Buffers
NBS Buffers
NBS Buffers
NBS Buffers
National Bureau of
Standards Quartz
Calibration
Thermometer
Cone./
Accuracy
35 ppt
(nominal )*
Variable
3.557**
4.004**
6.863**
7.415**
7.669**
9.183**
10.014**
-80 to
250°C/
±0.002°C
Appl i cable
to
In Situ
Calibration
No
No
No
No
No
No
No
No
No
Yes with
water bath
Several
Point
Cal.
No
Variable
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
*at 15°C and 1 atmos. with batch-to-batch variations as high as 0.01 ppt;
within batch variation of 0.001 ppt.
**uncertainty of ±0.005 at 25°C.
ALTERNATE CALIBRATION TECHNIQUES
Temperature
The options open to the analyst for temperature sensor calibration are
few. All that can be determined by calibration is that the sensor is
responding accurately. The type of water bath used is not critical but should
be calibrated against a recently checked NBS standard thermometer which is
accurate to at least the level of probe accuracy required.
Salinity
Salinity standards may be prepared in the analytical chemical laboratory
from a synthetic seawater mixture or with the proper proportions of at least
reagent-grade chemicals. However, because of the number of variables involved
(i.e., micro-balance accuracy, age and conditions purity of the reagents,
analytical techniques, and the quality of the water used to prepare the solu-
tions), the accuracy of the final solutions cannot be assured beyond two to
three significant figures.
17
-------
Dissolved Oxygen
A technique some manufacturers recommend for dissolved oxygen probe cali-
bration is to wave the probe in the air. The meter value should correspond to
a chart value taking into account the barometric pressure and temperature.
Frequently these data are not sufficient for statistical treatment but ade-
quately demonstrate that the readings did not correspond to the Winkler values
when the probe was calibrated by waving it in air. This method is not a
preferred technique for one-point calibration or standardization. Discussions
with NOAA personnel indicate that it is not possible to obtain reproducible
results.
The analyst may prepare his own standards from the analytical chemical
laboratory, reagent-grade chemicals. This option is also not recommended for
reasons outlined in the salinity discussion.
MEASUREMENT DEGRADATION
Measurement degradation cannot be easily determined without standardiza-
tion. The accuracy and precision of a unit can only be evaluated by compari-
son with a standard reference technique. For this reason calibration with
standardization is essential to collect meaningful data.
Most in situ instruments automatically check for proper electronic opera-
tion but have no means for determining the condition of the sensor. This
obvious disadvantage must be confronted in future design efforts.
The variance as a result of instrument degradation is presently undefin-
able, but could be resolved by repetitive calibration. Sensors may slowly
degrade until the malfunction is obvious. The period between new part instal-
lation and degradation is a wide band where several errors may occur.
Cleaning, replacement, and other activities affect probe calibration, sensi-
tivity, and accuracy. While these activities are required, frequent calibra-
tion and standardization are essential to detect when measurements begin to
diverge systematically from the calibrated accuracy.
18
-------
SECTION 5
REFERENCE STANDARD REQUIREMENTS FOR CHEMICAL IN SITU INSTRUMENTATION
REFERENCE STANDARDS FOR CURRENTLY MEASURED PARAMETERS
The evaluation of standards for in situ measurements must be related to
the present instrument accuracy and precision capabilities. Ideally, the
standard reference material should be more accurate than the probe, which
would allow for instrument improvement with time. Allowances for variations
in instrument capabilities and the user's requirements should also be made.
Sensitivity
The sensitivity of in situ probes to changes in concentration of dis-
solved oxygen and salinity is reportedly less than the manufacturer's stated
overall accuracy. Only pH sensors are more sensitive than required by both
research and survey groups. Clearly, care should be used in reporting data
to the correct degree of precision. As an example, a conductivity probe with
a sensitivity of only 0.25 mS/cm used with an instrument having a stated
accuracy of OJO mS/cm is not providing data to the stated accuracy of the
instrument.
Probes are not totally selective for one parameter. Dissolved oxygen
probes respond to such energy sources as light and water movement. Various
ions present in seawater may produce a response in the probe. (O.H. Carpenter,
University of Miami, pers. comm.).
Limit Of Detection For Parameters Now Measured
The normal oceanic range of values of temperature, salinity, dissolved
oxygen, and pH are found in Table 4.
TABLE 4. RANGES OF COMMON OCEANIC PARAMETERS
Minimum
Maximum
Temperature
(°C)
-2
+30
Salinity
(ppt)
20
40
Conductivity
(mS/cm)
17
60
Dissolved
Oxygen
ppm
0
15
PH
(Units)
7.7
8.5
19
-------
Interferences Associated With Parameters Now Measured
Temperature--
Temperature probes are generally stable under environmental conditions
and are considered reliably accurate for up to 6 months of in situ use (8).
Conductivity—
Conductivity may be measured by seawater-coupled transformers or elec-
trode devices; however, electrode devices are easily fouled, polarized, and
corroded in seawater and are seldom used. Conductivity cells are still sub-
ject to hysteresis effects, and techniques using STD and CTD devices should
be rigidly controlled to minimize this problem.
Salinity--
Salinity measurements in situ are conversions by electronics circuits of
conductivity and temperature sensors. Lack of linearity and inability to
perform the necessary complex calculations reduce accuracy. Digital elec-
tronics may reduce the analog errors now present.
Dissolved Oxygen—
Membrane electrodes used to measure oxygen have a very slow response time.
Nonmembrane (naked) electrodes are sensitive to hydrodynamic properties of the
environment and are easily fouled. Compensation for temperature and pressure
effects has been successfully applied to depths of 3,000 meters (9).
pH-
pH system nonlinearity is commonly caused by reference electrode mal-
functions and drift (10). Although this is not large in laboratory tests
(0.003 pH in 24 hours), it is commonly severe in field instruments due to
fouling of the glass membrane after about 24 hours of use (11). Another prob-
lem is with cracks in the reference cells which frequently are not detected
with a one-point calibration.
Accuracy For Parameters Now Measured
Accuracy, as used here, refers to how closely a set of measurements
cluster about a chosen reference standard value, and as such includes random
and systematic errors. The accuracy of an in situ measurement system depends
on the integrated functioning of its components, including the operator, under
field conditions. Although often reported, laboratory accuracies of equip-
ment designed to operate in situ are not adequate descriptions of instrument
accuracy. In tracing a measurement from the field environment through the
measurement device to the final recorded value, a number of possible errors
are encountered.
First is the error due to inexact correspondence between the probe's
response and the ambient condition; this is called sensitivity. The response
in the probe must then be translated into an electric potential which is
transmitted to a measuring device. The error in this step is called probe
accuracy.
The electrical and electronic instability of the device results in two
additional errors. The first error is more or less random over a long period
20
-------
of time and is referred to as repeatability. The second is vectored over
time and is usually measured as 24-hour stability, or drift. The display of
the measured value results in the error called readability. Manufacturers
often lump these errors in a total instrument accuracy value, which may or may
not reflect the greater portion of the actual system accuracy.
In addition to the errors accumulated when making a measurement under
controlled conditions, in situ instruments are variously affected by pressure,
temperature, and other physical and chemical conditions present in the envi-
ronment. Compensation for environmental conditions may be built into the
instrument or, as in the GEOSECS program, it may be desirable to correct raw
measures by means of shipboard or laboratory computer systems. The error due
to inexact compensation for environmental conditions should be considered when
reporting total accuracy.
Platinum temperature transducers have a representative overall accuracy
of ±(K01 percent. Reversing thermometers have an overall accuracy of about
±0.02°C. Inductively coupled transformers for salinity measurements have
a transfer function accuracy of about ±0.02 ppt. Dissolved oxygen electrodes
have a reported accuracy of ±1 percent (±0.1 ppm) as stated in manufacturers'
literature.
The International Biophysics Corporation reported the problem of inter-
preting manufacturers' stated accuracies for dissolved oxygen. "It is our
opinion that, with the present state-of-the-art of dissolved oxygen measure-
ment, it is impossible for most manufacturers to meet their stated accuracy
specifications across the entire dissolved oxygen and temperature ranges."(12)
Temperature transducers now used for oceanographic purposes introduce
errors due to nonlinearity, stability, response time, usable life, and main-
tenance. Platinum thermometers can be obtained with a drift of only 0.011°C
over 6 months (8). Copper and platinum thermometers have a linearity of
±0.01°C from -2°C to +20°C using the Callender-Van Dusen equation (5).
Temperature thermistors of the oxide type exhibit an aging effect and an
increase in resistance with time. About 0.5 to 1.5% of the resistance varia-
tion occurs within the first week of use. Changes of 1% may still occur after
several months of use. Exposure of the thermistor to a temperature slightly
higher than that to which it will be subjected (in a process also called
aging) may reduce drift to as little as 0.04% per year (8). Since thermistors
are nonlinear, best least-square fits with third- or fourth-degree polynomials
yield a temperature compensation of no better than ±0.01°C (Q). A three therm-
istor compensation network has shown an actual linearity of only ±0.04°C.
EXTENSION OF IN SITU INSTRUMENTS TO PARAMETERS NOT NOW MEASURED
Reference standard requirements should be established for parameters not
now measured. Manufacturers and researchers are currently developing a wide
range of instruments designed for in situ use. Reference standard development
should proceed in parallel with in situ instrument development to assure data
accuracy and reliability.
Currently, 19 parameters are measured in situ and other parameters (Table
21
-------
5) are under consideration for application to seawater. Studies of stability
and accuracy have begun for some of the specific ion electrodes. It is diffi-
cult to predict the future acceptance for routine application of any system.
However, the rate of growth of human impact on the environment will encourage
more continuous monitoring of parameters. Parameters of more imminent concern
might be identified from review of the environmental problems under study and
the parameters of concern to the studies. Heavy metals should be a leading
contender for in situ measurement. Cadmium, mercury, and lead are among the
more important. Vanadium has been found to be associated with the heavy
fractions of petroleum.
TABLE 5. PARAMETERS MEASURED IN SITU
Presently Measured
Calcium
Chloride
Chlorophyll
Conductivity, electrical
Dissolved oxygen
Fluoride
Fluorescein (flushing studies)
Hydrocarbons
Light attenuation
Possible Future Application
Aluminum
Ammonia
Antimony
Arsenic
Barium
Bromine
Cadmium
Calcium
Carbon dioxide
Cesium
Chlorine
Magnesium
Oxidation-Reduction Potential
PH
Potassium
Rhodamine-B (flushing studies)
Salinity
Sodium
Temperature
Transmittance, optical
Turbidity
Chromium
Cobalt
Copper
Dysprosium
Europium
Fish "species" density
Fluorine
Gallium
Germanium
Gold
Indium
continued
22
-------
TABLE 5. (continued)
Possible Future Application
Iodine
Iron
Lithium
Lawrencium
Lead
Magnesi urn
Mercury
Molybdenum
Neodymium
Nickel
Niobium
Nitrate-Nitrogen
Nitrite-Nitrogen
Reactive phosphate
Osmi urn
Particulate matter
Plankton "species" density
Platinum
Potassium
Selenium
Silicates
Silver
Sodium
Strontium
Sulfide
Sulfur
Tin
Titanium
Tungsten
Total Kjeldahl nitrogen
Total Phosphorus
Uranium 238
Vanadium
Yttrium
Zinc
Zirconium
SURVEY OF NATIONAL ACCURACY REQUIREMENTS
For the purposes of this study, manufacturers and instrument users were
contacted in the United States. Also, users from Japan, West Germany, the
Soviet Union, France, Poland, Australia, the United Kingdom, and Kenya were
contacted. A limited field survey of operational in situ equipment calibra-
tion and standardization techniques, as well as present and desired levels of
accuracy, was conducted. Individuals contacted were involved in major marine
sampling programs, including the Coastal Upwelling Experiment (CUE), the
California Cooperative Fisheries Investigation (CalCOFI), the North Pacific
Experiment (NORPAX), Geology-Chemical Sections (GEOSECS), Outer Continental
Shelf (OCS), and the Dredge Material Research Program (DMRP). These programs
represent sponsorship by the Bureau of Land Management, the U.S. Army Corps
of Engineers, the Environmental Protection Agency, the National Science Foun-
dation, and the National Oceanic and Atmospheric Administration, which includes
the National Marine Fisheries Service. While it is not an exhaustive review
of the practitioners, it accomplishes the intended purpose of indicating the
current practices and establishing the scale of accuracies sought by major
users. The survey included 60 individuals representing 10 institutions opera-
ting within 8 major programs under 6 sponsors. Additional information was
gathered from the industrial and scientific literature.
23
-------
U.S. Government research and funding agencies such as the Department of
Commerce's National Oceanic and Atmospheric Administration; the Environmental
Protection Agency; the Department of the Interior's Fish and Wildlife Service,
Bureau of Land Management, and Geological Survey; the U.S. Army Corps of
Engineers; and the National Science Foundation routinely support water quality
measurements with in situ sensors.
State and local environmental monitoring agencies often acquiesce to
specifications of U.S. Government agencies; some establish their own accuracy
specifications for water quality. Some of these local government programs
have grown in scope and have eclipsed the more widely known national program.
A wide range of international programs are under way with still another
specification of accuracy. The most common in situ measurements among all
users are temperature, electrical conductivity, pH, dissolved oxygen, and
pressure (or depth). A summary of the accuracy requirements for these param-
eters (except depth) among research and environmental survey groups is given
in Table 6. As part of the teaching program, schools and other groups often
perform cursory surveys which include measurements on the parameters under
study. These measurements will sometimes find their way into public archives,
although such accessory measurements are often considerably less accurate than
those required by research and survey groups. Survey requirements correspond
closely with manufacturers' stated best accuracies. Frequently, researchers
require more accuracy (except for salinity) than possible with commerical
equipment. For both groups, the observed accuracy reported for dissolved
oxygen is considerably less than the desired accuracy. In summary, the survey
revealed:
°For the most part, instruments in present use have been used for several
years or more. Reliability is generally conceded as good for all but
one CTD unit at the University of Washington which was a relatively
early model.
°No one expressed much enthusiasm over future prospects to develop sen-
sors for additional parameters or for refining those sensors presently-
available. The need for more accuracy and finer resolution was recog-
nized by two or three individuals but not to the extent that they would
promote such an undertaking. The problem arises from the very low con-
centrations found for most environmental parameters and the high noise
band likely to surround the measurement itself.
°Approximately the same attitude, that little benefit would be gained,
persisted in answer to the question of improvements in calibration and
standardization. The parties recognizing the need for standards were
survey or monitoring groups, whereas research groups were concerned with
instrument capabilities.
°While it cannot be quantified, it seems as if those individuals using
the in situ instrumentation in long-term monitoring are able to operate
with less exacting equipment and are more concerned with standardization
between installations. By contrast those in the experimental group seek
24
-------
exact data and are satisfied to assure their values within their own
experimental "structure." This "structure" may include one investigator,
a department, or a program.
TABLE 6. ESTIMATES OF BEST OCEANIC IN SITU ACCURACIES
Error Sources
Probe Sensitivity
Probe Accuracy
Repeatability
Readability
In Situ Errors
Drift (24 hour)
Temp. Compensation
Pressure Compensation
(10,000 m)
Water Agitation Error
Manufacturer Stated
Accuracy
Research Observed
Required Accuracies
Research'
Survey
Accessory
Temperature
(°C)
0.01
0.015
0.01
0.01
0.5
0.01
0.81
-
0.02
0.01
0.01
0.05
1.0
Dissolved
Oxygen
(ppm)
0.25
0.1
0.1
0.05
-
0.008
-
0.05
0.01
0.5
0.01
0.1
1.0
Salinity
(ppt)
0.25
0.30
0.003
0.002
-
0.33
-
-
0.02
0.30
0.03
0.03
0.5
PH
(units)
0.0025
0.004
0.001
0.01
0.003
0.02
-
-
0.01
0.10
0.02
0.5
0.5
INSTRUMENTATION
Frequency of Standardization Required
The majority of users were found to use Standard Seawater only when cali-
brating by an automated laboratory salinometer. The standardization procedure
is only carried out a few times during an experiment, or once in several
months. Errors from instrument drift, degradation, fouling, and reliability
indicate standardization should be carried out frequently.
Cross checking between two instruments provides a partial quality check
on resulting values (J.H. Carpenter, University of Miami and K. Grasshoff,
25
-------
University of Kiel, pers. comm.). Cross checking of discrete samples against
a continuous profiler generally cannot be used to verify more than a few per-
cent of continuous monitoring data, but should be used in conjunction with in
situ instruments no less frequently than once in every profile, or every few
days for non-mobile instrument packages (A. Bainbridge, GEOSEC, pers. comm.).
Present Uncertainty In Instrument Measurements
An attempt to quantify an estimate of uncertainty in instrumental measure-
ments was terminated when the review of user practices disclosed a lack of
traceability to a common reference material or reference method. Quantitative
values of percent uncertainty should be estimated as average percent deviation
from the known mean. Percent error of the measurement is determined by
attempting to repeat a single measurement many times on a parameter of known
value. The spread of the data points estimates this error. In lieu of quan-
tification, a discussion of the sources of uncertainty and the potential quan-
tifiable uncertainty as a function of the standards available is provided.
Manufacturers' Standardization Procedures--
Nonlinearity of an instrument results in a curve when data points derived
from that instrument are plotted against the actual values. This in itself
does not constitute an error. However, when converting the instrument output
to standard units of measure some error is produced. This error is measured
by the magnitude of difference between the resulting converted measure value
and the actual value. It should be noted that simplistic analog conversions
within the instruments are often considerably nonlinear with a resulting large
conversion error. For this reason GEOSEC has removed analog conversions from
their oxygen meters and are using ship-board computer conversions. The GEOSEC
technique is a considerable improvement over commonly used techniques,
although some researchers do not consider the shipboard routine conversion to
fully meet their standards.
Temperature—Manufacturers of CTD's and STD's recommend the system be
returned to their lab approximately once per year for calibration. The CTD
or STD is placed in a constant temperature-salinity water bath (verified by
NBS methods), and the thermistor and conductivity cell are then adjusted. NBS
has a standard adequate for calibrating temperature sensors.
Salinity--!.A.P.S.O. Standard Seawater is the standard reference for cal-
ibration of both in situ and laboratory salinometers. It is in routine use in
the oceanographic community and is the salinity reference. It is considered
difficult to duplicate the solution in the laboratory. Currently, Standard
Seawater is being distributed by the Institute of Oceanographic Sciences in
Surrey, England. This standard solution has a certified uncertainty of
±0.001 ppt chlorinity. Calibration to the standard using potassium chloride
techniques has a reported uncertainty maximum of ±0.01 ppt salinity. The best
American refractometers have a reported accuracy of ±0.05 ppt salinity; how-
ever, a Russian interferometer system has a reported accuracy of ±0.01 ppt
salinity.
Dissolved oxygen-- Sagami Standard Seawater Reference Solution Source
prepares and distributes worldwide CSK standard seawater reference solutions,
including one for dissolved oxygen. The solution is used without dilution,
26
-------
and the colorimeter technique is based on a standard color development to
calibrate absorbance in a spectrophotometer. In general, the Sagami samples
are designed to remain stable for at least 1 year after preparation. The most
widely used dissolved oxygen reference standard worldwide are modifications of
the Winkler procedure.
Calibration of dissolved oxygen probes often uses air saturated water as
a standard. The maximum accuracy of this technique is 0.1 percent, due to
problems with variations in air quality and supersaturation at low tempera-
tures encountered in the field (13).
£H--Standard pH buffer solutions are prepared by many chemical and manu-
facturing companies to the exact formulas prescribed by the National Bureau
of Standards. The certified accuracy of these standards is ±0.01 pH units.
There is good reason to conduct seawater sensor calibration at something
greater than a pH value of 7.0. Testing at a pH of 8.0 rather than 7.0 has an
immediate advantage of being closer to the seawater average of pH 8.0 to 8.3.
Buffer solutions are less stable than powder buffers. Solutions in
storage are stable up to a year, whereas buffer powder or buffer tablets can
be stored indefinitely. However, with buffer powders, the mixing methods in
the field must be as accurate as normal analytical methods followed in an
equipped chemistry laboratory.
Instrument standardization to a pH buffer standard has a maximum reported
accuracy of ±0.02 pH units at 25°C as measured with a precision pH meter.
Problems of comparing standard pH buffer solutions to measurements in seawater
result in an additional error due to interferences that should be solved by
setting up rigid standards for seawater buffer solutions and standardization
methodology.
Users' calibration and standardization—Users often choose different
standards since a common standard is not recognized for most of the parameters.
The procedure for calibration standardization is produced from manufacturers'
instructions, laboratory manuals, research journals, and text books, or are
invented to meet the user's specific needs. Considering these circumstances,
comparison of data between independent researchers is questionable. The need
for comparable water quality measurements necessitates the development of
reproducible standard analytical techniques.
Users introduce four additional errors into the measurement process.
Without proper control of these errors, data cannot be considered standardized.
"Procedures may be modified by a laboratory to meet their needs. Such
modifications change the procedural error and are referred to as pro-
cedural modification errors.
°Human errors are procedural errors resulting from the measurement being
taken by different ope""-1-™"
Training error result:
make the measurement.
II Ml IIUII t I IV/IO U I C (JlUUCUUlUl C
taken by different operators.
°Training error results when operators of different levels of learning
ma Uo + ha mnaciiv»om««•*-
27
-------
°Technology errors result when new instruments are developed. The spread
of data points due to different instruments making the same measurement
estimates this error. It is necessary to subtract from this value
instrument error, standardization error, and, if necessary, operator
error, as well as sampling error to obtain the actual value.
The necessity of using rigid procedures during calibration, standardiza-
tion, and measurement is stressed. The inaccuracies resulting from inade-
quately trained personnel following many manufacturers' techniques are docu-
mented by the cruise results (Appendix B). Improved procedures, such as are
documented in the CalCOFI/GEOSECS procedures manuals, should be used at all
times. (Note: Errors may still arise from other facets of sampling design.)
The GEOSECS program determined that calibration of equipment at exces-
sively short intervals actually resulted in a degradation in overall accuracy.
If the instrument can be determined to have a known bias, the data can be
readily corrected by computer programming or manual data correction.
"Temperature sensors usually must be calibrated by the manufacturer,
while most users are only able to make a few crude comparisons (±0.1°C)
to a laboratory thermometer.
°CTD and STD systems are usually field standardized by simultaneously
profiling and collecting discrete water samples, then comparing the CTD
or STD temperature and salinity data with measurements of the water
samples taken by reversing thermometers and laboratory salinometer.
°For salinity and pH, the user will make dilutions of the standard solu-
tion to obtain a set of working standards to calibrate the sensor or
probe. Ideally, the sample value will fall within the range of the
working standards. The user often prepares his own standards by dis-
solving reagent grade (99% or greater purity) salts into distilled
de-ionized water. To eliminate the matrix effect, an ionic strength
buffer should be added to the standard solution. The standard method
of calibrating in situ salinity instrumentation is by the laboratory
salinometer method. The standard instrument for salinity measurements
is considered to be the high accuracy-precision laboratory salinometer.
This type of salinometer uses the inductive coupling technique to mea-
sure conductivity and is automatically temperature- and pressure-compen-
sated.
°Dissolved oxygen sensors can be coarsely standardized with air-saturated
seawater. If the temperature of the water is known, solubility tables
will provide an "accurate" value of the percent saturation.
In situ dissolved oxygen measurements are usually calibrated against the
standard Winkler method performed on water bottle samples. Calibration is
performed using distilled water for freshwater work. Even though bottle
samples are not exactly comparable to in situ measurements, in situ measure-
ments are often terminated when discrepancies are found between reversing
thermometer or water bottle sample values and in situ values (A. Bainbridge,
GEOSECS, pers. comm.). Some users cross-calibrate between two in situ systems
without close calibration to any outside standard. The University of Miami
recalibrates both in situ systems if a difference of greater than 0.1 ppm Of
28
-------
oxygen is found (J. Carpenter, University of Miami, pers. comm.).
Cruise Reports
The at-sea operation was carried out April 13, 1977, to further study the
problem of standardization and calibration technqiues, and to determine the
comparability of measurements obtained using the calibration and standardiza-
tion techniques recommended by equipment manufacturers. The at-sea operation
utilized electronic in situ instrumentation systems and standard sampling
techniques for dissolved oxygen, pH, temperature, and salinity. The survey
was conducted at five stations in Monterey Bay, California. Temperature,
salinity, dissolved oxygen, and pH were measured at the surface, and at depths
of 5, 10, 20, 30, 40, and 50 meters (Appendix B).
Calibration of the equipment prior to the cruise was performed according
to manufacturers' specified procedures: dissolved oxygen was calibrated
against air-saturated seawater; conductivity with Standard Seawater; pH by
standard NBS buffer solutions; and temperature probes by the constant tempera-
ture bath method in comparison with a platinum thermometer. The instrument
systems were under manufacturers' calibration. A representative of LFE par-
ticipated in the cruise procedures along with Interstate Electronics Corpora-
tion personnel. Four in situ systems were tested: Horiba, Hydrolab, Martek,
and Plessey Environmental Systems. All four parameters were measured by the
Horiba, Hydrolab, and Martek systems; the Plessey system did not measure pH
and dissolved oxygen.
In addition to the in situ instruments, a Beckman (RS7C) Laboratory
Salinometer was used to measure conductivity, Winkler titrations were used to
measure dissolved oxygen, and a reversing thermometer was used to measure
temperature. These methods were used as checks on the in situ instrument
performance. Conductivity measurements were performed by inductive coupling
and multiple electrode sensors (from which conductivity ratios were determined
and subsequently converted by use of tables). Dissolved oxygen was measured
by membrane polarographic electrodes; pH was measured by glass electrodes; and
the temperature sensors were resistance transducers. All instruments were
capable of measuring the oceanic range of each parameter. All water samples
were collected with standard Nansen bottles. Depth was calculated by triangu-
lation when necessary after measuring the wire angle.
Statistical methods--
The 95% confidence interval was calculated on each system for each param-
eter as a measure of the accuracy of the method employed. The confidence
intervals were calculated using the method given by J. Freund (14) for small
samples. The sample variance was estimated for each system as the sum of the
squared deviations from the calibration procedure. The following procedures
were assumed to be calibration procedures:
Winkler method: dissolved oxygen
Calibrated reversing thermometer: temperature
Beckman laboratory salinometer: salinity
Statistical mean pH value: pH
29
-------
The mean square differences were calculated between the in situ method
and the calibration method by the paired value method to eliminate distribu-
tion dependency and the effects of depth and location (15). Tests for signif-
icant differences between techniques were performed by means of the small
sample paired t-test for comparing a sample mean to a theoretical mean (14).
This total interval may be partitioned into the uncertainty caused by the
analytical method (precision) and the uncertainty attributable to systematic
error (calibration). Partitioning of the interval into two t-statistic inter-
vals (precision and systematic) is shown in Table 7.
TABLE 7. PARTITIONED STANDARD DEVIATIONS FROM INTERCOMPARISON EXPERIMENT
Parameter
Temperature
Salinity
Dissolved oxygen
PH
Methodology
0.010
0.102
0.175
0.113
Systematic
0.18
0.470
1.007
0.365
Total
0.28
0.572
1.182
0.478
Cruise observation results--
Considerable differences were found between the values reported by the
various in situ instruments. Only one temperature system, two dissolved oxy-
gen systems, and one pH system demonstrated a lack of significant difference
from the chosen calibration system (Table 8).
Sampling was inadequate to determine behavior of the measurements due to
pressure differences. Statistical interpretation is also limited by the lack
of replication. Even under the best of circumstances, it is essentially
impossible to obtain a true replicate measure in such a heterogenous environ-
ment. Since all measurements were made during a 10-hour cruise, no discussion
of drift, marine biofouling, durability, or reliability is warranted.
All parameters showed a greater standard deviation due to systematic
error than to methodology (Table 7). This suggests considerable calibration
error, and points out the problem of independent calibration of systems by
manufacturers and the nonequivalency of calibration techniques used for
different instruments.
It is suspected that part of the error between methods could be reduced
by a uniform calibration procedure. No single procedure was devised that
could accommodate the differences in equipment structure. Ship-environment
interaction, such as bilge effluent, propwash, and engine cooling effluent,
was a potential problem. Precautions were taken to avoid such problems, and
the data did not reflect these influences (increased standard deviation values
of surface water quality measurement).
30
-------
Depth compensation requires an accurate measurement of pressure. Two
systems incorporated pressure transducers in their in situ systems. The other
systems used cable length markings and triangulation (when wire angle was
greater than 5°) to estimate depth. One system uses the bonded strain gage,
which is considered a very accurate method of determining depth (8). This
transducer is automatically temperature-compensated; accurate compensation is
important since gage impedance varies significantly with temperature (8). No
comparison between cable lengths, pressure transducers, or triangulation tech-
niques was attempted. Fortunately, since wire angles were slight on this
particular cruise, the inaccuracy of depth measurements was not considered a
major problem. In rougher seas, the cable length and triangulation method of
depth estimation is much less accurate than a pressure transducer. Placement
of more weight in the sonde would improve some of the systems environmental
applicability. The sonde unit is very light and easily responds to ship move-
ment and currents. This is a very undesirable characteristic as water depth
uncertainty becomes a problem.
TABLE 8. 95% CONFIDENCE INTERVALS WITH PAIRED T-TESTS FOR
SIGNIFICANT DIFFERENCES BETWEEN SAMPLE AND CALIBRATION MEASURES
Parameter
Temperature
Salinity
Dissolved Oxygen
pH
System
A
B
H
C
D
A
B
H
D
C
B
A
G
A
B
H
n
13
24
18
24
20
11
25
19
22
25
24
11
24
13
23
19
95% C.I.
0.171
0.103
0.115
0.195
0.111
9.02
0.127
3.28
0.016
0.374
0.525
1.69
0.07
0.03
0.04
0.05
Std. Error
0.097
0.050
0.055
0.095
0.053
4.049
0.061
1.489
0.008
0.183
0.508
0.816
0.008
0.015
0.018
0.023
Probability
of Difference
from Standard
3.38*
4.22*
3.27*
4.29*
2.67
3.28*
4.82*
4.35*
3.56*
3.91*
2.12
0.64
16.0*
2.72
3.21*
3.43*
* = 99% probability of a difference
(* = significance level of better than 0.01)
Temperature—The 95% confidence interval covers 2.8% of the average
sample mean, or ±0.28°C. One was consistently high, recording values outside
31
-------
the confidence interval 69% of the time. All the values recorded by another
system were lower than the sample mean (by an average of 3%) and outside the
confidence interval in 48% of all measurements. This consistently large range
between systems suggests calibration (systematic) error.
The reversing thermometer and only one thermistor system showed close
agreement at all stations and depths. The response time for all systems
varied between 30 and 60 seconds. During the cruise, all the temperature
measuring systems operated properly, and no repairs or replacements were
necessary. All systems were reliable, relatively sensitive, and had a short
response time. Previous studies yielded similar results (8).
The reading uncertainty standard deviation for one system was ±0.16°C,
which is 0.10°C greater than the uncertainty reported by NOIC Instrument Fact
Sheet #75004. The uncertainty for another system was ±.09°C, which is 0.08°C
greater than the uncertainty reported by NOIC Instrument Fact Sheet #76012.
Sa1inity/conductivity--Previous CalCOFI studies have determined that the
expected salinity range of Monterey Bay is 33.0 to 34.0 ppt for the summer
months. Since values from two systems were far outside this range, these
values were not included in the sample mean and confidence interval calcula-
tions. One system reported values of about 18.8 ppt, and the other system
about 27.3 ppt. The 95% confidence interval calculated from readings of the
remaining three systems was ±0.57 ppt, or approximately +1.5% of the sample
mean.
Two systems showed significant agreement at all stations and depths.
Both systems recorded salinities lower than a third system; however, reported
values were six and five significant figures, while the third gave three
significant figures. This difference in data reporting with the resulting
recording errors could partially account for the range between salinity values.
Conductivity is a function of salinity, temperature, and pressure.
Therefore to achieve accurate conductivity values, precise measurement of
these three parameters is essential. Compensation for temperature and pres-
sure variations is included in all salinity data presentations. All salinity
recording systems compared in this study measured conductivity. Two systems
used inductive coupling techniques, and the others used multiple electrodes.
Polarization could have been a problem since all instruments used less
than the recommended 1 KHz power source (8). Response time varied between 1
and 3 minutes for all systems. Mechanically the five systems were reliable,
excepting the gross errors in two of the systems.
Dissolved oxygen--The 95% confidence interval for dissolved oxygen is
±1.18 ppm, or approximately ±15% of the sample mean. The wide range of values
reported for the same sample yields a relatively large confidence interval,
often as large as 15-20% of the sample mean. No two systems agreed more
closely than 0.2 ppm.
During the measurement of dissolved oxygen, the expected sample collec-
tion problems were accompanied by a significant number of system malfunctions.
Approximately one half of the membranes had to be replaced. After the membrane
32
-------
was replaced, the system's response was tested but additional performance data
were not collected. One of the six electrode systems developed a white
clogging precipitate. After 22 readings, two others did not function at all.
One system probe was accidently set down in the bottom sediment, which
resulted in the failure of all its sensors (temperature, dissolved oxygen, pH
and conductivity).
Improper membrane installation could account for some of the inconsist-
encies. Manufacturers' instructions for the replacement of membranes are
sometimes vague and confusing. Other possible problems include uncertainty of
sensor's depth, electronic drift, and other previously stated factors.
The response times for the in situ dissolved oxygen systems ranged from
20 seconds to 4 minutes; in almost all cases they were greater than the manu-
facturer's specification. The response time of all sensors of this type
(polarographic membrane) will increase with decreasing temperature, and opera-
tion at 0°C would thus require longer equilibrium times according to Sectarian
Instrument communication to NOIC Fact Sheet #76011, 1976.
In this study, the data accuracy (compared to Winkler titration) and
precision values were much poorer than the values stated in the manufacturer's
specifications.
£H--For field measurements of pH, the 95% confidence interval was ±0.47
pH units. Two systems were very close to the mean of the four systems.
Comparable deviations from replicate readings (precision errors) were observed
between the shipboard lab and each in situ system. Three responded similarly
with changes in station and depth. These systems agreed closely at approxi-
mately one half the stations. Another system's values were inconsistent with
the sample mean and the pH profile. All systems appeared to function properly
and no sensor repair or replacement was necessary. Response time was gener-
ally between 1 and 3 mintues. The large uncertainty under field conditions is
in agreement with previous studies (8).
Environmental Perturbations
In situ measurements eliminate errors incurred through sampling and
sample handling. However, in situ measurements are considered more difficult
to make. Standardization and calibration are a serious problem, but when a
discrete sample is obtained by water bottle from the ocean "...its temperature
and pressure changes. These changes displace rapid equilibria so that by the
time our sample has reached the surface its chemical composition has already
changed." (4 ) Contamination from containers, addition or losses of dis-
solved gasses, settling, precipitation, and decomposition of life forms
further contribute to non-in situ measurement error (4 ). In situ measure-
ments should reflect ambient conditions with greater accuracy than non-in situ
techniques. However, in situ measurements also have a set of problems.
Instruments which function accurately in the laboratory under closely con-
trolled environmental conditions must still function accurately under a
rigorous range of pressure, temperature, water movement, turbidity, light, and
chemical and biochemical conditions. Ionic strength and complex formation may
shock dissolved oxygen electrodes (F. Millero, University of Miami, pers.
comm.).
33
-------
Temperature compensation methods are somewhat better understood than
pressure compensation methods. Plessey and Hydrolab in situ systems include
temperature-compensated, bonded strain-gage transducers to measure depth. The
YSI 5514 instrument uses a depth-compensated system. However, the combined
temperature and pressure effects on dissolved oxygen and pH are only partly
understood. Temperature measurements should be pressure-compensated if a
high degree of accuracy (±0.01°C) is required, as there is a change in
indicated temperature caused by pressure effect on the sensor. An effect as
high as 0.81°C at 10,000 meters has been reported (16). Accurate compensation
can only be carried out by means of a shipboard computer system; commercial
conversion procedures are often inadequate to meet the needs of researchers
(A. Bainbridge, GEOSEC, pers. comm.).
The problem of measuring perturbations of measures due to pressure
effects, pollutant interference, temperature compensation, and buffering
effects is partly due to the lack of standards of reference. The lack of
known values of comparison contributes to the problem of defining the meaning
of an in situ value. Additional errors may arise due to the unexpected
behavior of parameters in situ, and the variability of the quality of
standards.
34
-------
REFERENCES
1. Shewhart, W. A. Statistical Method from the Viewpoint of Quality Control.
U.S. Department of Agriculture, Washington, DC, 1939.
2. Eisenhart, C. Realistic Evaluation of the Precision and Accuracy of
Instrument Calibration Systems. Journal of Research, U.S. National
Bureau of Standards, 67C (2): pp. 161-187, 1963.
3. Call, J. P., T. V. Meters, R. E. Michaelis, W. P. Reeo, R. W. Senaro,
C. L. Stanley, H. T. Yolken and H. H. Ku. The Role of Standard Reference
Material in Measurement Systems. NBS Monograph 148, U.S. National Bureau
of Standards, Washington, DC, 1975.
4. Home, R. A. Marine Chemistry. John Wiley and Sons, New York, NY, 1969.
568 pp.
5. National Bureau of Standards, U.S. Platinum Resistance Thermometry. U.S.
National Bureau of Standards, Washington, DC. Monograph 126, 1973.
129 pp.
6. Strickland, J. D. H. and T. R. Parsons. A Practical Handbook of Seawater
Analysis. Fisheries Research Board of Canada, Ottawa, Canada. Bulletin
167, 2nd edition, 1967. 310 pp.
7. Environmental Protection Agency, U.S. Methods of Chemical Analysis of
Waters and Wastes. U.S. Environmental Protection Agency, Washington, DC,
1974. 239 pp.
8. Texas Instruments, Inc. The State of the Art of Oceanographic and
Meterological Sensors. Texas Instruments, Inc., Dallas, Texas. Volume 1,
1970. 150 pp.
9. Green, M. W., R. D. Gafford, and D. G. Rohsbaugh. A Continuous Profiling,
Deep-Submersible Dissolved Oxygen Monitor. Marine Technology Journal,
Volume 2, 1970. pp. 1485-1502.
10. Westcott, C. C. Selection of a Reference Electrode. Industrial Proce-
dures IP-EC-1 , Beckman Corporation, Scientific Instruments Division,
Irvine, California, 1976. 4 pp.
11. Phelps, K. D., and A. D. Beck. Sensing and Simulating the Marine
Environment. U.S. Environmental Protection Agency, Narragansett, Rhode
Island, 1973. pp. 361-369.
35
-------
12. National Oceanographic Instrumentation Center, U.S. Discrete Oxygen
Analyzer. International Biophysics Corporation, Irvine, California.
U.S. Department of Commerce, National Oceanic and Atmospheric Administra-
tion, National Ocean Survey, National Oceanographic Instrumentation
Center. Instrument Fact Sheet (IFS-76009), 1976.
13. Carpenter, James H. New Measurements of Oxygen Solubility in Pure and
Natural Water. Limnology and Oceanography, (11): pp. 264-277, 1966.
14. Freund, J. Mathematical Statistics. 2nd edition, Prentice Hall, Engle-
wood, NJ, 1971. 274 pp.
15. Sokal, R. and J. Rohlf. The Principles and Practice of Statistics in
Biological Research. Biomedics, W. H. Freeman and Company, San Francisco
California, 1969. 776 pp.
16. Zobell, C. E. Thermal Changes Accompanying the Compression of Aqueous
Solubility to Deep Sea Conditions. Limnology and Oceanography (4): pp.
463-471 , 1959.
17. Hulse, G. L. The Plunkett In Situ Monitoring System. New York State
Marine Science Research Center, State University of New York, Stony Brook,
NY, 1975. 201 pp.
18. Bradshaw, A., and K. E. Schleicher. The Effect of Pressure on the Elec-
trical Conductance of Seawater. Deep Sea Research (12): pp. 151-162,
1965.
36
-------
APPENDIX A
DESCRIPTION OF TWO VARIATIONS OF IN SITU INSTRUMENTATION SYSTEMS
THE PLUNKETT SYSTEM
The Plunkett is a semi automated system that permits continuous sampling
in vertical and horizontal planes. The system was developed in 1969 at the
State University of New York, using proven, commercially available components.
The Plunkett system has three major components: the sampling unit, the wet
lab, and the electronics module. The sampling unit incorporates a submersible
centrifugal pump at the lower end of a flexible hose. Vertical sampling can
be accomplished from the surface to 80 meters. For horizontal sampling, a
second centrifugal pump operates through a hull intake. Parameters measured
directly at the pump intake include salinity, temperature, and depth. Sea-
water is pumped to the wet lab unit on deck where pH, dissolved oxygen, in
vivo chlorophyll, and turbidity are routinely measured. Taps are provided
for unfiltered seawater for additional analyses..
The electronics module includes signal processing and analytical instru-
ments for determining the height of the sampling unit above the seabed, the
depth of the sampling unit below the surface, temperature, pH, dissolved
oxygen, salinity from the direct and backup salinometer (thermosalinograph),
and underwater photometry. Graphic recording equipment in the form of a
multipoint analog stripchart recorder provides a continuous indication of
system performance and real-time sensor information, and acts as a backup
recording medium in the event of failure of the digital data acquisition
system. The data acquisition system can accommodate up to 40 channels.
System Calibration
Calibration of the system was conducted in two separate programs: labor-
atory and field. The salinometer is calibrated against Standard Seawater.
The thermistor bridge system for direct temperature measurement uses the
manufacturer's calibration method and curves. System calibration consists of
substituting precision resistors for the thermistor in the bridge. A 12-point
calibration is used, and this may be expanded over a narrow temperature range.
The resistors are compared to a National Bureau of Standards 1 ohm primary
standard resistor. Total accuracy is rated at no greater than the greatest
error present, the 0.1°C accuracy of the thermistor. There is no regular
calibration of the thermistor against a primary standard; therefore, changes
in thermistor characteristics due to physical aging, pressure, damage, etc.,
would not be apparent to the operator.
The pressure transducer for intake depth measurements is calibrated only
37
-------
in the field by lowering it on a handline marked in 10-meter increments to 50
meters. The calibration potentiometer is adjusted for a voltage input of 50
mV to the data acquisition system. The transducer is then raised in 10-meter
steps, and the voltage on the digital voltmeter is checked. The accuracy of
the depth reading is expressed as ±0.5 digit values on the meter.
On-Deck Measurements
The dissolved oxygen probe is calibrated against the Winkler titration
using the instrument company's field calibration procedure. This procedure
consists of placing the probe in a plexiglass sampling chamber, pumping sea-
water through the system, and comparing the values obtained with the modified
Winkler titration.
pH calibration is conducted by comparison of the probe value against a
laboratory pH meter value, submerging the sensor in a standard buffer solution.
The probe is equipped with an automatic temperature compensator. The accuracy
of the calibration method is reported as ±0.002 at the buffer points and
±0.005 with the buffer and sample in the same range between points.
The in vivo chlorophyll system utilizes a fluorometric determination.
Calibration consists of blanking the fluorometer by the Turner and Lorenzen
methods, followed by calibration against acetone extractions with a minimum of
15 samples and a standard chlorophyll reference.
The turbidity system is calibrated by blanking the fluorometer with
particle-free distilled water. Discrete samples of seawater of known volume
are collected from the discharge line. The samples are then filtered through
a preweighed millipore filter. The filter is re-weighed, and the weight of
particulate matter is plotted as suspended solids per volume of seawater
sample by which fluorometer values can be checked (17).
THE "FISH"
The "fish," which was developed and used by the Institute of Marine
Resources of the University of California, Berkeley, is towable at speeds up
to 5 knots, and can be programmed for sampling from the surface to 30 meters,
permitting both vertical and horizontal profiling. The system monitors the
following parameters to the indicated accuracies:
"Depth: 0-30 m ±0.1 m
°Temperature, full range: ±0.02°C
°Conductivity, full range: 0.3%
"Dissolved oxygen, full range: 1% of full range
°pH, sulphide ion, and Eh: within 1 mV of electrode voltage
°Ambient light: detects changes within 1% of full scale
Similar design concepts have been available from commercial manufacturers
uch as the Hydrospace Corp., Towed Automatic Profiler, and Hermes Ltd., which
ffers the "Batfish."
38
-------
Calibration Techniques for the "Fish"
This type system has unusual calibration requirements. The parameter
values are recorded with a precision of ±1 digit on either a 16-mm single
frame movie camera or alternately on a digital cassette recorder. In turn,
the basic accuracy of the overall system is dependent upon the accuracy of
the internal reference voltage. Therefore, the aim of the calibration proce-
dure was to ensure the internal reference accuracy rather than traceability
to a defined standard. For example, traceability of the temperature sensor
to the manufacturer's test and calibration of the thermistor.
Dissolved oxygen calibration is performed in a seawater bath containing
a sample of fully oxygenated water of known salinity. The sample is agitated
to read oxygen saturation equilibrium. Calibration of the dissolved oxygen
probe is performed every time the membrane is replaced.
pH calibration is accomplished with standard buffer solutions and is
corrected for alkaline error due to the presence of cations and temperature
effects.
The sulfide sensor is calibrated in the laboratory with a solution of
variable S activity. The solution is made by bubbling gaseous H2S in a 0.25
molar Na2S and 0.5 molar KCL solution and by changing the pH by adding 1
molar acetic acid.
The Eh sensor is checked occasionally by measuring the Eh potential of a
standard oxidizing solution that has an Eh value of 430 mV. Experience indi-
cates that the Eh value obtained is within 10 mV of the standard value with-
out any special electrode cleaning.
The conductivity sensor is calibrated against a temperature-compensated
laboratory-grade salinometer. Pressure effects are neglected during calibra-
tion but corrections for pressure are incorporated during the data analysis
phase using the Bradshaw and Schleicher tables (18).
39
-------
APPENDIX B
EXPERIMENTAL CRUISE FOR COMPARISON OF CALIBRATION ACCURACIES
An intercomparison cruise was conducted on April 13, 1977, in Monterey
Bay aboard the Ship R/V Oconostota for the purpose of directly comparing the
in situ measurement accuracy of instruments measuring dissolved oxygen, con-
ductivity (salinity), temperature and pH to laboratory results. Four in situ
systems, each with an individual operator, were simultaneously operated at a
number of discrete depths to a maximum of 50 meters at five widely spaced
stations to offer a range of environmental variability. The station locations
sampled appear in Figure 1. All analytical systems were calibrated before the
cruise according to the manufacturers' recommended procedures: dissolved
oxygen using air-saturated water; conductivity using Standard Seawater; and pH
using buffer solutions. Temperature and pressure sensors were calibrated by
the manufacturer using constant temperature baths and high pressure chambers
respectively. LFE Environmental Labs supported Interstate's staff by cali-
brating instruments, performing laboratory analyses, and by participating in
the supervision of cruise operations.
Laboratory analysis for the traceability of standards for dissolved
oxygen and conductivity were performed at LFE's certified laboratory located
in Richmond, California. To ensure proper handling of samples from point of
origin to laboratory, senior laboratory personnel participated in the opera-
tion, collected the samples, and were responsible for transport.
In Situ Instruments
Specifications for parameters measured by each of the in situ systems
and laboratory systems (shipboard and shore based) used appear at the end of
this section in Tables B-l to B-ll. In situ salinity was measured with in
situ and laboratory systems using either inductive coupling or multiple elec-
trode techniques. Dissolved oxygen membrane polarographic electrodes and
glass pH electrodes were used in both laboratory and in situ measurements.
Temperature-compensated bonded strain gage transducers were used by the
Plessey and Hydrolab system to measure in situ depth. Temperature sensors
were all resistance transducers with one platinum transducer and four types of
thermistors.
Most of the systems were capable of measuring the full pH range, 0 to 14.
All systems were capable of measuring temperature from 0 to 35°C, and three
systems could accurately measure in the 5 to 45°C range. All instruments
were capable of measuring dissolved oxygen concentration levels from 0 to 20
ppm. Many scales were available for the measurement of electrolytic conduc-
tivity in mS/cm. The most common range of measurement was 0 to 60 mS/cm.
The systems predominantly employed an automatic temperature compensation
arrangement.
40
-------
if.
SOQUEI
MONTEREY BAY
SCALE 1:210 668
WATSONVIUE
. y» J f tt I™'*'*! LOnoi
^ / /[/Moil Londi
Figure 1. Intercalibration cruise sampling station locations.
41
-------
TABLE B-l . IN SITU SYSTEMS USED FOR EXPERIMENTAL CRUISE
Manufacturer
Parameters Measured
Horiba:
Hydrolab:
Martek:
Plessey:
Dissolved Oxygen, pH,
Conductivity, Temperature.
Dissolved Oxygen, pH,
Conductivity, Temperature,
Depth (Pressure).
Dissolved Oxygen, Tempera-
ture, Conductivity, pH.
Conductivity, Temperature,
Depth (Pressure).
TABLE B-2. LABORATORY METHODS USED ON
DISCRETE SAMPLES FOR EXPERIMENTAL CRUISE
Method or
Instrument
Parameters Measured
Where Measured
Reversing
Thermometer
Orion pH meter
Beckman pH meter
YSI D.O. meter
Winkler titration
Beckman Salinometer
Temperature
pH
pH
Dissolved Oxygen
Dissolved Oxygen
Salinity
Shipboard Laboratory
Shipboard Laboratory
Shipboard Laboratory
Shipboard Laboratory
Shore Based Laboratory
Shore Based Laboratory
42
-------
TABLE B-3. SUMMARY OF SYSTEM ACCURACIES
MANUFACTURER
Horiba
Hydrolab
Martek
Plessey
Environmental
Systems
SYSTEM
MODEL
U-7
Surveyor-
GDI 2
MK V
CTD9400-5
ESTIMATED
COST
2,950
5,000
4,950
20,000
CONDUC-
TIVITY
±2.5
mS/cm
±0.5% fs
±0.5
mS/cm
±0.03
SALINITY
N.A.
0.5 ppt
±0.5
ppt
N.A.
TEMP-
ERATURE
±0.5°C
±0.25°C
±0.1°C
±0.02°C
pH
±0.1
±0.1
±0.1
±0.02
DISSOLVED
OXYGEN
±1.0
mg02/l
±0.5% fs
±0.1
mg02/l
±2% fs
DEPTH
N.A.
±1.5% fs
±0.1% fs
± .25% fs
Information from manufacturer's product bulletins
fs = full scale
N.A. = not available
-------
TABLE B-4. HORIBA: U-7 MATER QUALITY CHECKER
Parameter
Method of Measurement
Range of Measurement
Temperature
Compensation
pH
Temperature
Dissolved
Oxygen
Conductivity
Glass electrode
Thermistor
Membrane type
Galvanic cell
4-electrode sensor
0 to 14 pH units
0 to 40°C
0 to 20 mg02/l
0-50.0 mS/cm
or
0-100.0 mS/cm
or
0-1000 mS/cm
Automatic
0 to 40°C
Not Required
Automatic
0 to 40°C
Not
Available
Indicator: Light-emitting diodes - 3 digits.
Power Required: Rechargeable battery (nickel-cadmium cell) or 117 Vac.
Cable Length: 2. meters with an optional length of 10 meters.
Weight: Instrument - 0.63 kg
Sensor - 0.82 kg
Shipping weight - 6.35 kg with case
44
-------
TABLE B-5. HYDROLAB: 6D SURVEYOR
Parameter
Temperature
Conductivity
pH
Depth
Dissolved
Oxygen
Method of Measurement
Thermistor
4-electrode a-c cell ;
pure nickel
electrodes
pH electrode,
reference
electrode, pair
Pressure transducer
Passive polarographic
cell
Range of Measurement
-5 to 45°C
0 to 1000 uS/cm
0 to 10,000 uS/cm
0 to 100,000 uS/cm
2 to 12 pH units
0 to 200 meters
0 to 10 mgOg/l
0 to 20 mg02/l
Temperature
Compensation
Not Required
Automatically
corrected to
25°C for
salinities up
to 35 ppt and
temperatures
between 0 and
45°C
standard
correction
0 to 45°C
Automatic
Automatic
0 to 45°C
Power Requirement: (Circulatory) 12 vdc.
Cable Length: 100 meters
Size: Length 53 cm, Diameter 17 cm
Weight: 7 kg
45
-------
TABLE B-6. MARTEK: MARK V DIGITAL WATER QUALITY ANALYZER
Parameter
Method of Measurement
Range of Measurement
Temperature
Compensation
Temperature
PH
Conductivity
Dissolved
Oxygen
Thermo!inear array
Combination pH
electrode with
silver-silver
chloride
reference
Platinized nickel
electrodes
Polarographic
gold-silver
electrode
-5 to 45°C
0 to 12 pH units
Not Required
Automatic
0 to 1000 uS/cm
or
0 to 100 mS/cm
0 to 20 mgO£/l
Not Required
Automatic
with in
situ
stirrer
Power Required: Regulated self-contained internal rechargeable battery pack
with built in charging circuit, external 18-36 Vdc, or 105-125 Vac, (210-250
Vac), 50/60 Hz.
Cable Length: 50 meters
Size: 5.72 cm x 38.1 cm
Weight: 0.9 kg
46
-------
TABLE B-7. PLESSEY ENVIRONMENTAL SYSTEMS
MODEL 9400 TELEMETERING SENSOR SYSTEM
Parameter
Method of Measurement
Range of Measurement
Temperature
Compensation
Temperature
Conductivity
Depth
Platinum
Resistance
Transducer
Inductive
Coupling
Technique
Bonded
Strain-gage
-2 to 35°C
0 to 60 mS/cm
0 to 300 meters
0 to 600 meters
optional
Not Required
Automatic
temperature
and pressure
correction
Automatic
Power Required: 150-250 mA constant current at a minimum of 33 V dc plus
cable drop;equipped with strip chart recorder.
Cable Length: 300 meters (moored)
600 meters (profiling)
Weight: 13.6 kg (in air)
9.1 kg (in water)
TABLE B-8. YELLOW SPRINGS INSTRUMENTS: MODEL 57 DISSOLVED OXYGEN METER
Parameter
Method of Measurement
Range of Measurement
Temperature
Compensation
Temperature
Dissolved
Oxygen
Thermistor
Polarographic
electrode
-5 to 45°C
0-5 mg02/l
0-10 mg02/l
0-20
Not Required
Automatic
from
-5 to 45°C
Power Required: Two disposable "C" size carbon zinc batteries (provide
approximately 1000 hours operation).
Size: 21 x 27.5 x 9 cm
Weight: 2.5 kg
Cable Length: 15 meters
47
-------
TABLE B-9. BECKMAN SELECT-MATE
Parameter
PH
Method of Measurement
Combination electrode
Range
0 -
of Measurement
14 pH
Power Required: 9 Volt battery operated (120/220V
Capability: Lab System Only
Size: 26 x 22 x 12 cm Weight: .
68 kg
units
Temperature
Compensation
Manual
- 50/60 Hz optional).
(1.5 Ibs)
TABLE B-10. ORION pH
METER
Parameter
pH
Method of Measurement
Combination electrode
Range
0 -
Power Required: 120 Vac, 50/60 Hz
Capability: Lab System Only
Size: 17 x 20 x 20 cm Weight: 2.
of Measurement
14 pH units
Temperature
Compensation
Manual
3 kg (5.1 Ibs)
TABLE B-ll. BECKMAN RS 7-C INDUCTION SALINOMETER
Parameter
Method of Measurement
Range of Measurement
Operating
Temperature
Salinity
The ratio of conduc-
tivities and salinity
is read directly from
tables, such as the
UNESCO International
Oceanographic tables.
0 to 49 parts per
thousand salinity
0 to 40°C
Accuracy ±1°C
Power Requirement: 120 Vac, 50/60 Hz
Capability: Moist sea atmosphere requirements of MIL-E-16400, and shipboard
vibration MIL-STD-167 Type 1.
Size: 51 x 41 x 28 cm
Weight: 13.6 kg (30 Ibs)
48
-------
GLOSSARY
The scope of this glossary is limited to the special area of knowledge
and word usage applicable to the text of this report. The definitions and
terms were researched from the sources of information listed at the end of the
Appendix. Most definitions have been quoted verbatim from these references.
accuracy: 1. The extent to which the results of a calculation or the
readings of an instrument approach the true values of the calculated or
measured quantities, and are free from error. 2. Conforming exactly to
truth or to a standard. 3. When applied to methods of analysis, a mea-
sure of the error of a method may be expressed as a comparison of the
amount of element or compound determined or recovered by the test method
and the amount actually present.
analog: Pertaining to data in the form of continuously variable physical
quantities.
automatic data processing (ADP): The utilization of electronic data
processing equipment (computer) to perform any variety of tasks involving
informational data.
baseline: A main line taken as or representing a base. The point or li
from which a start is made in an action or undertaking.
ne
baseline assessment survey: The planned sampling or measurement of parameters
at set stations or in set areas in and near disposal sites for a period
of time sufficient to provide synoptic data for determining water quality,
benthic, or biological conditions as a result of ocean disposal opera-
tions.
benthic: That portion of the marine environment inhabited by marine organisms
which live permanently in or on the bottom.
buffer: A solution selected or prepared to minimize changes in hydrogen ion
concentration which would otherwise occur as a result of a chemical
reaction.
calibrated accuracy: When applied to methods of analysis, the difference
between the indicated parameter value and its accepted value (known
value); determined by calibrating an uncontaminated sensor in the mea-
suring system.
calibration: 1. To determine, by measurement or comparison with a standard,
the correct value of each scale reading on a meter or other device, or
49
-------
the correct value for each setting of a control knob. 2. To determine
the settings of the control devices so that a system will operate or
perform within certain limits. 3. Periodic standardizations of equip-
ment and instruments.
cation: A positively charged atom or group of atoms, or a radical, which
moves to the negative pole (cathode) during electrolysis.
coastal waters: Ocean waters seaward to the territorial limits and waters
along the coastline (including inland streams) that are influenced by
the rise and fall of the tide.
conductivity: Measure of the ability of a solution to carry electrical cur-
rent and thus, a measure of ion concentration in water. High ion concen-
trations may render water unpalatable or even toxic to plants and animals.
continuous profiler: As applied to water quality measuring systems, implies a
system that makes uninterrupted measurements of parameters as a function
of the sensor package depth.
criteria: A standard on which a judgment or decision may be based. (See
standard,)
data: Any representations, such as characters or analog quantities, to which
meaning might be assigned.
data acquisition: The phase of data handling that begins with the sensing of
variables and ends with a magnetic recording or other record of raw data;
may include a complete radio telemetering link.
data base: A collection of information in machine-readable form.
data processing (information processing): Any operation or combination of
operations on data, including everything that happens to data from the
time they are observed or collected to the time they are destroyed.
digital: Relating to calculation by numerical methods or by discrete units.
dissolved oxygen (DO): The oxygen in sewage, water, or other liquid usually
expressed in parts per million (ppm). Oxygen has a low solubility in and
is nonreactive with water. Its solubility is dependent on water temper-
ature and partial pressure in accordance with Henry's Gas Law. Because
of this temperature dependence, DO tends to be critical during periods of
high temperatures since oxygen solubility is at a minimum while biologi-
cal activity, and its corresponding oxygen demand, is at a peak. Dis-
solved oxygen is one of the key parameters in assessing the quality or
degree of pollution in water. Its measurement is required for waste
water treatment, fisheries research, baseline studies, and enforcement
monitoring.
Eh: The redox potential measured in millivolts. Positive values indicate the
presence of surplus oxygen. Negative voltage indicates a reducing envi-
ronment, most commonly caused by the presence of sulfides. (See redox
potential.) 5Q
-------
electrode: The conductor by which current enters and leaves an electrolyte
when subjected to an externally impressed potential .
electrolyte: Any substance which, in solution or fused, exists as electri-
cally charged ions that make the liquid capable of conducting a current.
Seawater is an electrolyte.
electronic data processing (EDP): Processing data by equipment that is pre-
dominantly electronic in nature, such as an electronic digital computer.
error: 1. Any discrepancy between a computed, observed, or measured quantity
and the true, specified, or theoretically correct value of that quantity.
2. Error of measurement may be systematic or accidental. Accidental
errors are slight variations that occur in successive measurements by the
same observer. Causes are generally intangible. They may follow the law
of chance.
GEOSECS: The acronym for Geochemical Ocean Sections,one of the major programs
of the International Decade of Ocean Exploration, a multinational cooper-
ative study of the world oceans during the period 1970-1980.
hysteresis: An effect, involving energy loss, found to varying degrees in
magnetic, electric, and elastic media when they are subjected to varia-
tion by a cyclical applied force.
in situ: 1. In the original location (in the environment). 2. In the
natural or original position.
in situ system: Typically, seawater sampling equipment and various analytical
instruments which measure various parameters while on station. The data
from the instruments are displayed in real time on strip chart analog
recorders, and the same data are digitized on magnetic tape for subse-
quent computer analysis. (See pump-thru-system.)
in vitro: Pertaining to a biological reaction taking place in an artifical
apparatus (in glass or in a test tube, beaker, etc.).
in vivo: Pertaining to a biological reaction taking place in a living cell or
organism.
ion: Electrically charged atom or group of atoms.
linearity: The maximum deviation between an actual instrument reading and the
reading predicted by drawing a straight line between the upper and lower
calibration points.
mariculture: The cultivation of marine organisms by exploiting their natural
environment.
marine: Pertaining to the sea.
measurement: The process of determining the value of some quantity in terms
of a standard unit.
51
-------
meter sensitivity: The accuracy with which a meter can measure a voltage,
current, resistance, or other quantity.
metrology: The science of measurement.
monitoring: 1. The act of observing and recording laboratory and field
environment test conditions, test specimen responses, and performance
parameters. 2. The acquisition of data at approximately the same loca-
tion and at some fixed frequency. (See measurement.)
monitoring station: Particular point at which representative samples of a
body of water are collected periodically.
multiparameter capability: Ability to measure several constituents simul-
taneously.
nutrients: Substances essential to bacterial growth and function. Chief
among these are carbon, nitrogen, phosphorus, and sulfur, but a number of
trace elements have been shown to be essential to growth (K, Ca, Mg, Fe,
Mn, Zn, Co, Cu, and Mo).
ocean instruments: Devices which measure parameters of the marine environ-
ment, including oceans, estuaries, large fresh water bodies, the air
directly above these waters, and the sea floor below.
ocean: Those waters of the open seas lying seaward of the baseline from which
the territorial sea is measured, as provided for in the Convention on the
Territorial Sea and the Contiguous Zone.
ocean water: Water having between 20,000 and 40,000 mg/1 dissolved material
whose ionic ratios correspond closely to those specified in "Standard
Specification for Substitute Ocean Water," ASTM Method D1141-52 (1971).
operational: Ready for or in condition to undertake a destined function.
oxidation reduction potential (ORP): See redox.
parameter: A measurable biological, chemical, or physical characteristic,
e.g., temperature, pressure, currents, etc., the size of sediment grains
or fish.
parts per million (ppm): A ratio of pounds per million pounds, grams per
million grams, etc. Approximately equal to milligrams per liter, expres-
sing the concentration of a specified component.
pH (Hydrogen Ion Concentration): A term used to describe the hydrogen-ion
activity of a system. The pH of a solution is defined as the negative
logarithm of hydrogen ion concentration, and is thus an expression of the
acid or alkaline character of the solution. A solution of pH 0 to 7 is
acid, pH of 7 is neutral, pH over 7 to 14 is alkaline. pH is of major
importance in dealing with treatment of water and wastewater since it
affects taste, corrosivity, chemical reactions and biological activity.
Since certain wastes tend to be strongly acidic or basic, sharp changes
52
-------
or extremes of pH may indicate the presence of pollution in receiving
waters.
physical parameter: Most common, nonchemical characteristics of water,
including color, specific conductance, threshold odors, turbidity, and
dissolved and suspended solids.
pollution: The addition of sewage, industrial waste, or other harmful or
objectionable material to water at a concentration or in sufficient
quantity to result in measurable degradation of water quality.
precision: 1. The quality of being exactly or sharply defined or stated.
2. The degree of refinement with which an operation is performed or a
measurement stated. 3. Adapted for extremely accurate measurements.
4. Held to low tolerance in manufacture. 5. When applied to methods
of analysis, a measure of the reproducibility of a method when repeated
on a homogeneous sample under controlled conditions, regardless of
whether or not the observed values are widely displaced from the true
values as a result of systematic or constant errors present throughout
the measurements. Precision can be expressed by the standard deviation.
6. Narrowness of limits within which one may assume true value of
measured quantity—the narrower the limits, the better the precision.
prototype: A model suitable for use in complete evaluation of form, design,
and performance.
pump-thru-system: Typically water is pumped from known depths to the ship's
deck, where it is conducted to instruments which measure salinity,
temperature, dissolved oxygen, pH. In situ sensors measure pump depth,
temperature, and submarine light transmission. {See vertical profile.)
quality assurance: Testing and inspecting all of or a portion of the final
product to ensure that the desired quality level of product reaches the
customer.
quasi: Having some resemblance.
random error: An error that can be predicted only on a statistical basis.
(See error.)
range: 1. The difference between the maximum and minimum of a variable
quantity. 2. When applied to methods of analysis, the difference
between the smallest and largest of n observations is also closely
related to the standard deviation.
reagent: A substance, chemical or solution used in the laboratory to detect,
measure, or otherwise examine other substances, chemicals, or solutions.
redox potential: Voltage difference at an inert electrode immersed in a
reversible oxidation-reduction system; measurement of the state of oxi-
dation of the system. Also known as oxidation-reduction potential
(ORP). (See Eh.)
53
-------
reliability: 1. The probability that a component part, equipment, or system
will satisfactorily perform its intended function under given circum-
stances, such as environmental conditions, limitations as to operating
time, and frequency and thoroughness of maintenance for a specified
period of time. 2. The amount of credence placed in a result. 3. The
precision of a measurement, as measured by the variance of repeated mea-
surements of the same object.
repeatability: The degree to which an instrument or technique provides the
same result when exposed to identical input conditions.
replicate: One of several identical experiments, procedures, or samples.
rosette type sampler: A water sampling device consisting of a group of six or
more cylinders arranged in a circular or rose pattern frequently equipped
with a sensor package for making simultaneous measurements. Used to
obtain multiple samples on a single cast.
representative sample: Water sample whose measured values are characteristic
of the body of water from which the sample has been taken.
reversing thermometer: A mercury-in-glass thermometer which records tempera-
ture upon being inverted and thereafter retains its reading until
returned to the first position.
salinity: Total amount of dissolved salts (°/00) in seawater; commonly the
dominant factor controlling density of seawater. The measurement of
salinity (and temperature) can be used in the open ocean as tags to
identify water masses as they spread laterally between other masses, or
move along the ocean floor.
sampling: Obtaining small representative quantities of materials for analysis.
seawater: The water of the seas, distinguished from fresh water by its
appreciable salinity. The distinction between the usage of salt water
and seawater is not very sharply drawn. Commonly, seawater is used as
the antithesis of specific types of fresh water, as river water, lake
water, rainwater, etc ., whereas salt water is merely the antithesis of
fresh water in general.
sensitivity: The ability of the output of a device, system, or organism to
respond to an input stimulus. Mathematically, the ratio of the response
or change induced in the output to a stimulus or change in the input.
The degree to which a substance can be detected in the presence of inter-
fering components which have properties differing only very sliqhtly
from those of the substance.
sensor (primary detector; sensing element}: The generic name for a device
that senses either the absolute value or a change in a physical quantity
such as temperature, pressure, flow rate, or pH, or the intensity of
light, sound or radio waves and converts that change into a useful input
signal for an information-gathering system; a television camera is
therefore a sensor, and a transducer is a special type of sensor.
54
-------
stability: The measure of the length of time a measuring system, once cali-
brated, continues to measure the actual parameter value within the cali-
brated _ accuracy without the need for readjustment or recalibration.
Stability performance is based upon the measurement of standard cali-
brating solutions with an uncontaminated sensor.
standard: 1. An accepted reference sample used for establishing a unit for
the measurement of a physical quantity. 2. Constituting or affording
a standard for comparison or judgment.
Standard Seawater (normal water): Water whose chlorinity lies between 19.30
and 19.50 parts per thousand, determined to within ±0.001 per thousand,
which is used to calibrate or standardize salinity measuring devices or
methods.
standardization: 1. The adoption of generally accepted uniform procedures,
dimensions, materials, or parts that directly affect the design of a
product or a facility. 2. The process of establishing by common agree-
ment engineering criteria, terms, principles, practices, materials, items,
processes, and equipment parts and components.
standardized product: A product that conforms to specifications resulting
from the same technical requirements.
STD (salinity-temperature, depth recorder): An instrument for continuously
measuring the temperature, electrical conductivity, and depth. It auto-
matically determines water salinity from the temperature-conductivity-
salinity relationships.
synoptic: In general, pertaining to or affording an overall view.
system reliability: The probability that a system will accurately perform its
specified task under stated environmental conditions. (See reliability.)
thermistor: A resistive circuit component, having a high negative temperature
coefficient of resistance, so that its resistance decreases as the tem-
perature increases; a stable, compact, and rugged two-terminal ceramic-
like semiconductor bead, rod, or disk.
titration: A method of analyzing the composition of a solution by adding
known amounts of a standardized solution until a given reaction (color
change, precipitation, or conductivity change) is produced.
transient response: Rate at which a system responds to a step change.
transmissometer: A light-path device for measuring attenuation of light in
water due to scattering and absorption.
turbidimeter: An instrument for measurement of turbidity, in which a standard
suspension usually is used for reference.
turbidity: 1. The property in water that inhibits light penetration. Tur-
bidity is caused by suspended and colloidal matter, either from natural
55
-------
erosion or industrial and domestic waste. 2. Highly turbid water may
contain large amounts of organic material acting as food for microbes,
producing further growth and, consequently, increasing turbidity.
validation: The act of testing for compliance with a standard.
vertical profile: A graphical representation whose ordinate shows the varia-
tion of some oceanographic quantity along a straight line against hori-
zontal distance on this line as abscissa.
56
------- |