CEM Report Series
Report Status: Revision No. 1
Date: July 1982
No.: 5-Jnj. 7|6a^
TRANSMISSOMETER FIELD AUDIT RESULTS
JULY 198 2
Office of Air, Noise and Radiation
Division of Stationary Source Enforcement
Washington, D.C. 20460

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TRANSMISSOMETER FIELD AUDIT RESULTS
JULY 1982
Prepared by:
Robert Y. Purcell
Entropy Environmentalists, Inc.
Research Triangle Park, North Carolina
Prepared	for:
United States Environmental Protection Agency
Division of Stationary	Source Enforcement
Project Officer:	Louis R. Paley
Contract Number:	68-01-6317
Task Number:	29
Report Number:	5-272-7/82

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DISCLAIMER
This document was prepared by Entropy Environmen-
talists, Inc. under Contract No. 68-01-6317, Task No. 29,
and therefore, was wholly or partially funded by the U. S.
EPA. This document has not been subjected to the Agency's
required Peer and Policy Review. Therefore, this document
does not necessarily reflect the views of the Agency, and
official endorsement should not be inferred.

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Executive Summary
Auditing procedures were developed and applied to
approximately 100 transmissometers installed at fossil fuel
fired steam generators located within EPA Regions IV, V, and
VII. The results of the program indicate that the auditing
procedures can be effectively used to assess transmissometer
performance. In general, it was found that performance
reflected the level of operation and maintenance applied by
the source personnel.
This document contains descriptions of the auditing
procedures and the results from their application. The
results are discussed in light of: (1) Performance
Specfication 1 (for transmissometers; 40 CFR 60, Appendix
B) ; and (2) source operating and maintenance practices for
transmissometers. Recommendations are included which
address means for ensuring and sustaining acceptable
transmissometer performance.

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TABLE OF CONTENTS
Executive Summary
Section 1. Introduction	1
Section 2. Audit Program Description	3
2.1	Testing Program	3
2.2	Audit Methodology	4
2.3	Audit Analyses	6
Section 3. Summary of Results	9
Section 4. Discussion of Results	 18
4.1	General Operation	18
4.2	Monitor Specific Audit Procedure Results... 25
Section 5. Conclusions and Recommendations	28
5.1	Conclusions	28
5.2	Recommendations	29

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1. INTRODUCTION
Federal and State regulations that apply to stationary
sources emitting particulate matter require many of these
sources to install continuous opacity monitors
(transmissometers) . The quality of the opacity data
initially provided by a transmissometer is established
shortly after the monitor is installed; the appropriate air
pollution control agency certifies the acceptability of an
installed transmissometer based upon the results of a
performance test conducted according to the procedures
contained in Performance Specification 1, Appendix B, 40 CFR
60.
The quality of the opacity data provided by
transmissometers after the performance test is an important
issue, both for source personnel and for air pollution
control agency personnel, because use of the data is limited
by data quality. In the case of the agency, the opacity
data may be used: (1) to assess compliance with existing
opacity limitations, (2) to indicate particulate emission
levels through correlation of in-stack opacity measurements
with particulate performance test data, and (3) to provide
an indication of process operation and of particulate
pollution control equipment performance and maintenance, and
thus, to signal the need for corrective action by the
source, or inspections by control agency personnel. Source
operators, on the other hand, can use the opacity data not
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only as an indication of the performance of particulate
pollution control equipment, but also to maximize source
efficiency, and therefore, to reduce source operating costs.
To date, the current performance of installed
transmissometers has not been assessed; thus, the quality of
the data provided by these monitors following certification
is unknown. In response to the need for information
regarding the current performance of transmissometers, a
transmissometer performance auditing program was developed
and applied in the field. Initially, monitor-specific audit
procedures were developed for the most commonly encountered
opacity monitoring systems; these procedures are presented
in a separate report, entitled, " Field Performance Audit
Procedures for Transmissometers." After the development of
the audit procedures, an audit program was implemented in
EPA Regions IV, V, and VII. Approximately 100 audits were
conducted.
This report presents the transmissometer field audit
results obtained during the implementation of the field
audit program. Section 2 of this report describes the
auditing procedures applied. In addition, some of the
parameters audited are defined. Section 3 lists a summary
of the results afforded by the program and in Section 4 the
audit results are discussed. Section 5 presents conclusions
and recommendations derived from the audit program.
2

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2. AUDIT PROGRAM DESCRIPTION
2.1 TESTING PROGRAM
Audits of ninety-three transmissometers were conducted
at a total of forty-one sources in EPA Regions IV, V, and
VII. One monitor was located on a catalytic regenerator,
and the remaining monitors were located on coal-fired steam
generators.
The test site selection criteria for the audits were as
follows:
(1)	The test sites selected were to include represent-
atives of each monitor type.
(2)	Each monitor to be audited was to have been
installed for a minimum of one year.
(3)	Each monitor to be audited was to have previously
met the requirements of Performance Specifi-
cation 1.
The transmissometer audit program was designed to be
quick and simple, yet capable of providing an adequate
indication of transmissometer performance. A complete audit
can be performed by one technician with a basic
understanding of transmissometry. Materials required for
the audit can be transported in a small suitcase, and
include a jig used for performing calibration checks and a
minimum of three different neutral density filters with
opacity values traceable to the National Bureau of Standards
(NBS) .
3

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2.2 AUDIT METHODOLOGY
The transmissometer field audits were comprised of two
parts: (1) a general information survey, and (2) performance
of the monitor-specific audit procedures. The general audit
survey was used to gather information concerning maintenance
practices and performance history of the monitor. The audit
procedures were conducted to determine whether the monitor
had been operated properly and whether the calibra-
tion/accuracy of the transmissometer was of sufficient
quality to provide useful data.
The order in which the audit procedures are conducted
is slightly different for each monitor; however, each set of
procedures includes the same three basic elements. These
elements are:
(1) Monitor Component Analysis
a.	The fault lamp indicators on the monitor's
control panel are checked to determine if the
monitor is operating within the manufacturer's
prescribed limits.
b.	The stack exit diameter and monitor pathlength
are determined to verify the accuracy of the
monitor's preset stack exit opacity correction
factor.
c.	Various internal electronic checks are
performed using the controls located on/in the
monitor's control unit. These checks provide
further verification of the operational status
of the monitor.
d.	The control panel meter and chart recorder
responses are compared to the monitor's
internal span value, in order to determine the
accuracy of the control panel meter and
internal zero and span functions.
4

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(2)	Monitor Maintenance Analysis
The optical alignment and dust accumulation on
optical surfaces are checked to determine the
adequacy of the monitor mounting and
maintenance frequency.
(3)	Audit Filter Analysis
The calibration of the monitor is checked
using neutral density filters.
For the Lear Siegler, Esterline Angus, and Contraves
Goerz monitors, an audit device with an adjustable
retroreflector is mounted on the transceiver to simulate
clear stack conditions. Neutral density filters are then
inserted into the audit device to determine the
calibration/accuracy of the monitor at three different
opacity levels. For the Dynatron transmissometer, the audit
device must be placed in front of the monitor's
retroreflector, and the neutral density filters inserted
there. This methodology provides an incremental calibration
check, since the filter opacity is combined with the
effluent opacity.
5

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2.3 AUDIT ANALYSES
Each transmissometer field performance audit conducted
consisted of specific analyses and calculation procedures
for generating the audit results. The terms used in
identifying the audit analyses and computations are
described below, to facilitate an understanding of the audit
results.
(1) Stack Exit Correlation Error
Under normal circumstances, the cross-stack
optical pathlength of the installed transmissomter
is not equal to the diameter of the stack exit. To
obtain a true stack exit opacity value, the
measured opacity at the monitor location must be
corrected to stack exit conditions. This is
accomplished through the use of a pathlength
correction factor. The stack exit correlation
error is the percent error of the pathlength
correction factor (preset by the manufacturer)
used by the installed transmissometer when
compared to a calculated pathlength correction
factor. This calculated factor is generated
through the use of actual measurements,
blueprints, etc. The stack exit correlation error
is recommended to fall within a range of + 2%;
this error affects the stack exit opacity readings
exponentially.
(2) Control Panel Meter Correction Factor
Most transmissometers have a panel meter located
on their control or transceiver units. If this
meter is utilized by source personnel to monitor
opacity readings or to adjust an internal
transmissometer parameter, it must be accurate.
The accuracy of the panel meter is determined by
comparing the meter readings to the specified
value for the internal span filter. It is
recommended that the correction factor fall within
a range of 0.98 to 1.02 (+ 2%). If the panel
meter error is greater than 10%, there is probably
a different monitor problem causing the error.
This additional problem should become apparent
once the audit is completed.
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(3)	Reference Signal Error
The monitor reference signal is an internal
monitor output that indicates the electronic
alignment of the transceiver circuitry. Errors in
the reference signal directly affect the opacity
data obtained. It is recommended that the
reference signal be maintained within + 10% of the
specified value.
(4)	Zero Compensation Analysis
The zero compensation circuit of the trans-
missometer automatically adjusts the monitor's
zero to compensate for dust accumulation on the
transceiver's optical surfaces. Performance
Specification 1, Appendix B, 40 CFR 60 specifies
that the automatic zero adjustment should not
exceed 4% opacity (0.018 optical density, OD) .
This same criteria is used to evaluate the zero
compensation status (in units of optical density,
+ 0.018 OD) of the monitor during the field audit.
After cleaning, the zero compensation should
approach 0.000 OD.
(5)	Internal Zero and Span Errors
These errors are determined by comparing the
specified values that the monitoring system should
record with the actual values displayed on the
chart recorder. The zero and span errors should
fall within + 2% opacity. These specifications
mirror the recommendations of the monitor
manufacturers. These errors can occur due to one
or a combination of reasons: (a) the values for
the internal zero and span functions are wrong,
(b)	the internal electronics responding to the
zero and span functions are not working properly,
(c)	the signal from the transciever to the chart
recorder is being altered, and (d) the chart
recorder is not displaying the incoming signal
correctly.
(6)	Optical Surface Dust Accumulation Analysis
This analysis is performed to determine the amount
of dust (measured in terms of percent opacity)
found on the optical surfaces of the monitoring
system. To obtain a reliable assessment, this
audit analysis should be performed during
conditions when the stack opacity is relatively
constant. It is recommended that the dust
accumulation on the monitor's optics be no greater
than 4% opacity.
7

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(7) Calibration Error Analysis
This analysis is performed in accordance with the
procedures presented in Performance Specifi-
cation 1, Appendix B, 40 CFR 60. The performance
specifications require that the transmissometer's
calibration error be £ 3% opacity. The audit
procedures recommend the same specification.
8

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3. SUMMARY OF RESULTS
The following list is a summary of the results derived
from the performance of ninety-three transmissometer audits.
Table 1 presents a monitor-specific summary of results, and
Figures 1 through 5 display the average calibration error
results obtained for the monitors audited.
A.	Monitor Availability
1.	Eighty-nine of the ninety-three transmissometers
audited (96%) were operating during the time of the
audit.
2.	During the 30-day period prior to the performance
of the audit, the opacity monitoring systems
audited were on-line and operating an average of
95% of the time. Seventy-five percent of the
monitors were available for 100% of the time.
3.	There were no specific monitor components with a
history of failure.
B.	Monitor Operation
1.	Approximately 10% of the monitors audited had
inaccurate preset stack exit correlation factors.
2.	Several facilities (five monitors) had utilized
readings displayed on inaccurate panel meters to
adjust the monitor zero and span functions.
3.	Five monitors audited had been operated during
periods when fault lamps were activated.
4.	Several facilities (eleven monitors) used the
values obtained from the zero compensation circuit
(an indication of dust accumulation on just the
transceiver optics) to determine excessive dust
deposition for the entire monitoring system.
5.	Two sources (three monitors) had failed to maintain
the monitoring system within the manufacturer's
prescribed operating paramters.
9

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TABLE 1. TRANSMISSOMETER AUDIT RESULTS*

LEAR SIEGLER
CONTRAVES GOERZ
DYNATRON
ESTERLINE ANGUS
AUDIT
Number
Number
Number
Number
Number
Number
Number
Number
ANALYSES
Within
Outside
Within
Outside
Within
Outside
Within
Outside

Specs.
Specs.
Specs.
Specs.
Specs.
Specs.
Specs.
Specs.
lumber of Monitors Audited
58


27
5


3
MONITOR COMPONENT ANALYSIS








Fault Lamps^"
56
2
27
0
3
2
3
0
AGO Circuit^
57
1
NA
NA
NA
NA
NA
NA
Monitor Alignment
55
0
26
0
3
2
2
1
Stack Exit Correlation Error
51
7
26
1
2
3
3
0
Panel Meter Status^
0
555
22
5
4
0
2
1
Reference Signal Error
54
1
NA
NA
NA
NA
NA
NA
Internal Span Error
44
11
22
5
3
1
3
0
Internal Zero Error
53
2
25
2
3
1
3
0
MONITOR MAINTENANCE ANALYSIS








Zero Compensation Factor
47
B
NA
NA
NA
NA
NA
NA
Optical Dust Accumulation
41
12
22
3
3
1
3
0
AUDIT FILTER ANALYSIS**








Low Range (8.5% Opacity)
50
5
25
2
1
3
3
0
Midrange (18.5% Opacity)
49
7
22
5
1
3
3
0
High Range (42.5% Opacity)
46
9
19
8
0
4
2
1
See Section 3 of this report for definitions of terms and specification ranges.
The audit filter analysis for the Dynatron monitors utilized different neutral density filters:
Low range = 17.0% opacity, midrange = 56.5% opacity, and high range = 81.0% opacity.

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TABLE 1: FOOTNOTES
1. A fault light is "outside of the specifications" if the
light is illuminated on the control unit.
2. The automatic gain control (AGC) circuit is "outside of
the specification" if the circuit is not operating.
3. The monitor alignment is "outside of the specifictions"
if the transceiver alignment site indicates that the
measurement beam is not centered on the measurement
retroreflector. Three Lear Siegler monitors had
faulty alignment sights; therefore, they were not
included in the results presented.
4. The panel meter status is "outside the specifications"
if any of the meter readings were determined to be
greater than or less than 2% of the intended value.
Each type of monitor has a panel meter that
indicates opacity values; the Lear Siegler monitors
also indicate current and optical density values on
their panel meters.
5. Most Lear Sieger monitors audited had panel meters that
indicated accurate opacity readings; however, very
few of the panel meters indicated correct current
and/or optical density values.
11

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Range of
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11M i M 11
II II 1 1 II 1
i i i i 1 i i i i
ii ii | i i i i
ii ii | i i ii
1
0	10	20	30	40	50
Actual Opacity (% Opacity)
Figure 1. Observed Opacity Responses for 58 Lear Siegl
Monitors
12

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>1
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Expected Responses
I IIl| I l l I j IIlI|I I l i 11 I Ii|I li iJlI lI |l I I I 11 i I I | I
0	10	20	30	40
Actual Opacity (% Opacity)
Figure 2. Observed Opacity Responses for 27
Contraves Goerz Monitors
13

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21
100
-u
•H
o
(0
Qu
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>1
¦p
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± 3% Opacity
Error Band
Expected Responses
i i i i I i i i i 11 ii iIi ii i 11 iiii I I I I 11 II I I I I I 11 I II I I I I
Actual Opacity (% Opacity)
Figure 3. Observed Opacity Responses for 5 Dynatron Monitors
14

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¦p
•H
O
1
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m
a
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T3

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-







11 11 1 1 1 1 1 1 1

Range of
Measured Res
sponses 	

/
j /
/ /
/ /
/
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-



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//
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//
y
4
f
/' /
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Average
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ponses 	
4
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pacity
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 ' /
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1 II 1 | 1 1 II
l l l l | M l I
1 1 1 1 | 1 1 1 1
1 1 111 II II
ii ii | i i i i
1
0	10	20	30	40	50
Actual Opacity (% Opacity)
Figure 5. Observed Opacity Responses for All
Audited Monitors: 89 Monitors
16

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C. Monitor Maintenance
1.	Approximately 20% of the audited monitors did not
respond correctly to the internal zero and span
functions.
2.	For seven monitors, monitor zero and span
adjustments were made without cleaning the exposed
optical surfaces.
3.	Approximately 20% of the audited monitors had
excessive dust deposited on the optical surfaces.
4.	Four monitors were not aligned properly, and three
of these monitors were out-of-calibration.
D. Monitor Calibration*
1.	Approximately 10% of the audited monitors were	not
properly calibrated at low range levels (~	10%
opaci ty) .
2.	Approximately 15% of the audited monitors were	not
properly calibrated at the midrange level (~	20%
opaci ty) .
3.	Aproximately 20% of the audited monitors were	not
properly calibrated at the high range level (~	40%
opaci ty) .
4.	The monitor's response to the internal zero	and
span functions is not a good indication of monitor
calibration.
Before the calibration of a monitor was checked, all
exposed optics were cleaned. In addition, the monitors'
preset pathlength correction factor, whether it was
correct or incorrect, was used to correct the audit
filters for stack exit conditions. Thus, the calibration
error determinations exclude errors due to dust
deposition and inaccurate preset pathlength correction
factors.
17

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4. DISCUSSION OF RESULTS
4.1 GENERAL OPERATION
At the initiation of the transmissometer field audit
program, all of the monitors were believed to be functioning
properly; however, of the 93 monitors audited, only 89 were
fully operational. In addition, 22 of the operational
monitors audited (25%) had significant biases (>3% opacity)
in the recorded opacity data.
4.1.1 Stack Exit Correlation Analysis
An error in the pathlength correction factor affects
the monitored opacity exponentially. The most common error
in computing the optical pathlength is the use of the
flange-to-flange distance, rather than the stack/duct inside
diameter, for the transmissometer pathlength. Monitoring
systems adjusted for an incorrect pathlength ratio will
generate inaccurate opacity data; however, these data can be
easily adjusted to obtain true stack exit opacities.
Therefore, previously recorded opacity data which are
inaccurate due to an improper preset pathlength ratio can be
corrected and used for their intended purpose.
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4.1.2 Fault Lamp Analysis
The fault lamps provided a good indication of the
current operational status of the monitoring system. All of
the monitors with activated fault lamps (i.e., five
monitors) had calibration errors in excess of 10% opacity.
4.1.3 Panel Meter Analysis
The control panel meter analysis was not a good
indication of monitor performance, particularly for the Lear
Siegler monitors audited. The control panel meter errors
for the optical density and input scales were usually found
to be outside the recommended range. However, these scales
are not normally used by the sources, and thus, the accuracy
of these scales is not paramount. The opacity scale
correction factor was usually found to be within the
specified range; however, there was no correlation between
the panel meter factor and the calibration error
determination results. Even though the opacity scale on the
panel meter is usually accurate, the opacity responses
displayed on the chart recorder should be used during
monitor adjustments.
For cases where the panel meter (opacity scale) was
determined to be accurate and where the chart recorder
biased the data, a calibration curve can be constructed to
present the panel meter responses (accurate values) versus
the chart recorder responses. This curve could then be used
19

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to adjust previously recorded opacity
correct values. Seven monitors (8%)
offsets.
readings to reflect
had chart recorder
4.1.4	Reference Signal Analysis (Lear Siegler)
The reference signal for all of the Lear Siegler
monitors was within the recommended range; one monitor had a
reference signal outside the recommended range, but the
panel meter was not properly adjusted and the monitor was
calibrated correctly. The reference signal value did not
vary significantly for monitors which performed either well
or poorly during the calibration error determination;
therefore, no meaningful relationship could be found between
the reference signal and monitor performance.
4.1.5	Internal Zero and Span Analysis
The monitor zero and span checks did not appear to
provide a direct correlation to monitor calibration.
Seventy-two monitors had accurate zero and span responses;
however, twelve of these monitors had calibration errors
outside the recommended range. Also, seventeen monitors had
zero and/or span responses outside the recommended range,
but only ten of these (~ 60%) monitors had calibration
errors outside the recommended range.
A separate problem concerning zero and span adjustments
was identified at two sources (seven monitors); these
sources had adjusted the zero and span without cleaning the
optics. The zero mode is designed to simulate the amount of
20

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light that would be returned to the transceiver under clear
stack conditions. If there is any dust accumulation on the
transceiver window when the zero adjustment is made, then a
lesser amount of light is being returned to the detector,
and thus, an improper adjustment will be made. This
improper adjustment may or may not bias the recorded
opacity. The zero compensation circuit is automatically
reset during the zero adjustment, and thus, the circuit will
respond to the dirty optics as if they were clean.
4.1.6	Monitor Optical Alignment Analysis
Four monitors audited were not properly aligned; three
of the four monitors were out-of-calibration. For two of
these monitors (Dynatron), the source indicated that when
they were properly aligned, the monitors could not be
zeroed. An alternate alignment was determined off-stack
during calibration, and this alternate alignment was
duplicated when the monitor was installed on the stack; both
of these monitors were out-of-calibration.
4.1.7	Zero Compensation Analysis (Lear Siegler)
During the performance audit program, the results of a
comparison of the amount of dust deposition on the
transceiver optics and the corresponding level of zero
compensation showed that the zero compensation circuit
accurately indicates the amount of accumulated dust and
appropriately adjusts the measurements obtained by the
monitoring system. The zero compensation circuit of the LSI
21

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opacity monitor indicates the level of dust accumulation on
the transceiver measurement optics and adjusts the monitor
output to compensate for its effect. It does not indicate
the level of dust accumulation on the optical surfaces of
the retroreflector. In addition, the audit results showed
that the zero compensation levels for eight of the monitors
audited (Lear Siegler) were outside the specified allowable
range of 4% opacity.
4.1.8	Dust Accumulation Analysis
Quantitative assessments of dust deposition on exposed
optical surfaces were performed for eighty-five monitors;
sixteen monitors (19%) had excessive dust on the optical
surfaces. It is impossible to determine the correct opacity
levels recorded for times before the audit, since the dust
deposition does not occur at a linear rate. The audits also
showed that dust accumulation does not necessarily occur at
the same rate for the transceiver and retroreflector
measurement optics. The audit data indicate that dust
deposition on the monitor's optics is site-specific and
monitor component-specific.
4.1.9	Calibration Error Analysis
For those instruments which failed the calibration
error test, the calibration error increased with increasing
opacity. Execpt for two monitors, the monitors which failed
the the midrange calibration error test are a subset of
those monitors which failed the high range calibration error
22

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test. All of the monitors which failed the low range
calibration error test also failed the midrange calibration
error test.
For five of the monitors that were miscalibrated, the
sources had adjusted the monitor to read neutral density
filters without correcting the filter values for stack exit
conditions. In cases where this problem was the sole cause
of inaccurate data, the past data can be adjusted to reflect
the true stack exit opacities.
Section 60.13(c)(2), Part 40 of the Code of Federal
Regulations states that sources which entered into a binding
contractual obligation prior to September 11, 1974 to
purchase an opacity monitoring system shall demonstrate that
the monitor is capable of measuring emission levels
(opacity) within + 20% with a confidence level of 95%.
Within the confines of this auditing program, this
requirement was determined to mean that any installed
monitor should be able to measure the opacity values of the
audit filters within + 20%. The average calibration errors
for all the monitors audited were well within this + 20%
specification.
Twenty of the twenty-two monitors having calibration
errors > 3% opacity were biased high. Based upon this
experience, it can be postulated that opacity monitors which
are not performing well usually report high-biased opacity
data. Therefore, it is in the source's best interest to
keep its transmissometer well calibrated and maintained.
23

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4.1.10	Combining Duct Opacities
During the auditing program, thirty transmissometers
were audited which were installed in exhaust ducts that
transported effluent streams to a common exhaust stack.
None of these common stacks had an installed
transmissometer; thus, the sources had to determine the
stack exit opacity by combining the duct opacities measured
by the respective transmissometers. None of the combined
duct opacities were weighted according to the duct flowrate,
and a few sources only arithmetically-averaged the duct
opacities to obtain a stack exit opacity.
4.1.11	General Survey Analysis
The general audit survey addressed a variety of other
aspects of monitor operation (e.g., the accessibility for
maintenance, the supply of spare parts, etc.); for the most
part, these factors had little correlation with monitor
performance. However, one factor found to be important was
whether the monitor maintenance was performed by plant
personnel or by a monitor servicing contractor. Nearly all
(six of seven monitors) of the monitors serviced by
monitor-servicing contractors had calibration errors of £ 3%
opacity.
Monitor availability, as determined by reviewing the
previous month's data, was 100% for sixty-nine of the
ninety-three monitors audited; the average monitor
availability was approximately 95% for all of the monitors
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audited. The general survey also indicated that there were
no specific monitor components that had a history of
failure.
4.2 MONITOR SPECIFIC AUDIT PROCEDURE RESULTS
Several problems which were encountered during the
transmissometer field audits were peculiar to one type of
monitor. The problems were not viewed as design problems,
but were caused by source maintenance practices.
4.2.1 Lear Siegler Transmissometers
One problem observed at nine of the fifty-eight Lear
Siegler monitors audited was the accumulation of excessive
dust on the optical surfaces. These monitors have a zero
compensation circuit which is used by most sources to
monitor the dust accumulation and to determine cleaning
intervals. There were a few problems which were identified
during the audits with using the zero compensation circuit
as an indicator of dust accumulation. The zero compensation
addresses only the dust accumulation on the transceiver
optical surfaces, and thus, excessive dust accumulation can
occur at the retroreflector without the source being aware
of the problem. Also, some sources were not aware of the
specified limit of zero compensation, or of what that limit
meant. Performance Specification 1 (Appendix B, 40 CFR 60)
indicates that the zero compensation shall not exceed 4%
opacity. The value displayed on the zero compensation scale
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is in units of optical density, and some sources are not
aware that a zero compensation limit of 4% opacity equates
to an optical density limit of 0.018 OD.
The final problem identified which was related to the
zero compensation analysis was due to monitor calibration
practices. Zero adjustments were made without cleaning the
optical surfaces. When the zero is adjusted during
conditions of dirty optical surfaces, the zero compensation
is artifically reduced, leading to an incorrect optical
surface dust accumulation analysis by the zero compensation
ci rcuit.
4.2.2 Dynatron Transmissometer
Although only five Dynatron monitors were audited, some
problems were observed which may affect other installed
Dynatron transmissometers. Two of the monitors audited had
fault indications (measurement lamp output was outside the
specified limits) which were not noticed by the source. The
fault conditions were not noticed because the dust cover for
the control unit partially obstructs the view of the fault
lamp. The other problem which was observed was that two of
the monitors were not aligned according to the
manufacturers' recommendations. The source indicated that
the monitor could not be zeroed when aligned as recommended;
an alternate alignment was determined off-stack, and it was
duplicated on-stack.
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4.2.3 Contraves Goerz and Esterline Angus Monitors
No specific problems were observed that were
attributable to the source maintenance or set-up of the
twenty-seven Contraves Goerz and three Esterline Angus
monitors audited.
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5. CONCLUSIONS AND RECOMMENDATIONS
5.1 CONCLUSIONS
A. Monitor Performance
1.	Installed transmissometers provide accurate opacity
data, and they operate at a high level of avail-
ability.
2.	Monitors which are not properly calibrated almost
always introduce a positive bias to the reported
opacity data.
3.	The audit results obtained from performing internal
zero and span checks did not necessarily provide
valid indications of transmissometer performance.
Several of the excessive errors resulted because
the sources failed to rename the internal filter
values after the transmissometers had been
recalibrated. Nevertheless, while being but
marginally effective for auditing purposes, these
internal parameters can provide the source with
valuable information regarding the status of the
monitor's stability and calibration.
B. Monitor Operation and Maintenance
1. The auditing program revealed that the intensive-
ness of operation and maintenance practices was
directly related to transmissometer performance.
Those transmissometers which were calibrated and
cleaned with greater frequency provided opacity
data of higher quality. In addition, the audits
indicated that transmissometers maintained under a
service contract significantly out-performed those
maintained by the source alone.
2.	There are no generally applicable specific time
intervals for monitor maintenance that ensure
quality data; the maintenance intervals are
site-specific.
3.	Dust deposition was found to be site-specific and
component-specific, and the depositions do not
occur at a linear rate; therefore, opacity data
recorded over periods of time during which the
optics are dirty cannot be adjusted to reflect true
stack exit opacities.
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4. Incorrect pathlength correction factors were due to
the use of the flange-to-flange distance, rather
than the stack/duct inside diameter, for the
transmissometer pathlength.
5.2 RECOMMENDATIONS
A.	Monitor Performance
1. The source should ensure that the opacity
monitoring system provides high quality data;
inaccurate opacity data usually are high-based, and
thus, the source may assume that online particulate
control equipment is not performing efficiently,
and the source may report invalid instances of
excess opacity emissions.
B.	Monitor Operation and Maintenance
1.	The source must implement a comprehensive operation
and maintenance program for their monitoring
systems; this program may be implemented inhouse or
through a monitor-servicing contractor.
2.	Each source should perform a trial and error
analysis to determine the appropriate maintenance
intervals for each of its monitors. Initially, a
reasonable time interval for performing zero/span
adjustments, purge air filter cleaning/replacement,
and exposed optics cleaning would be every other
three months. The zero and span functions should
not drift more than 2% opacity between service
checks, and the total dust deposition between purge
air filter cleaning/replacement and optics cleaning
should not exceed 4% opacity for monitors with zero
compensation or 2% opacity for monitors without
zero compensation. If an initial time interval is
insufficient to provide the above results, halve
the time interval of interest.
3.	Monitor zero and span responses should not be used
as the sole indicator of monitor calibration.
4.	At a minimum, the source should adhere to the
manufacturer's recommendations regarding monitor
specifications.
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C. Monitor Calibration
1.	Care must be taken to ensure that the neutral
density audit filter values are corrected for stack
exit conditions before using them to calibrate the
moni tor.
2.	Sources should check that the preset pathlength
correction factor for the monitor is correct; the
monitor pathlength is the inside diameter of the
stack/duct at the monitor location, not the
flange-to-flange distance.
3.	No monitor adjustments should be made without prior
cleaning of the exposed optics.
4.	Monitor panel meters should not be used to provide
information during the adjustment of monitor
parameters; however, the panel meter should be
adjusted to display correct monitor responses.
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