Environmental Protection Technology Series
A Portable Device for
Measuring Wastewater Flow
in Sewers
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL
PROTECTION TECHNOLOGY series. This series
describes research performed to develop and
demonstrate instrumentation, equipment and
methodology to repair or prevent environmental
degradation from point and non-point sources of
pollution. This work provides the new or improved
technology required for the control and treatment
of pollution sources to meet environmental quality
standards.
'For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price $1
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EPA-600/2-73-002
January 1974
A PORTABLE DEVICE FOR
MEASURING WASTEWATER FLOW IN SEWERS
By
Michael A. Nawrocki
Contract No. 14-12-909
Project 11024EVF
Project Officer
Harry C. Torno
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D. C. 20460
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
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EPA REVIEW NOTICE
This report has been reviewed by the .Office of Research
and Development, EPA, and approved for publication.
Approval does not signify that the contents necessarily
reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or
commercial products constitute endorsement or recommenda-
tion for use.
ii
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ABSTRACT
A research and development program to develop a portable device which
is capable of measuring waste-water flow in sewers was undertaken by
Hittman Associates, Inc. for the Environmental Protection Agency under
Contract No. 14-12-909. This work consisted of an investigation of the
theoretical approach to be used, laboratory investigations and experi-
ments to develop design criteria, design and fabrication of two proto-
type units, and field testing and evaluation of these units.
Measurement of the cross-sectional area of flow was done by the use
of capacitor plates to sense the change in water level in the sewer pipe.
The method selected to measure the velocity of the flow involved the
timing of a heat pulse as it traveled down the pipe. Theoretical evalua-
tions and laboratory experiments were performed to prove the mode of
operation of the proposed gage.
Two prototype gages were fabricated. The overall accuracy of the final
prototype was, at best, +15 percent. Separately, cross-sectional area
of flow measurements were generally accurate to within five percent.
Velocity measurements were accurate to within 10 percent under ideal
conditions. The accuracy of the separate cross-sectional area measure-
ments were not affected by contaminants in the sewer. Scum deposits
on the walls of the gage significantly and adversely affected the accuracy
of the velocity readings.
This report was submitted in fulfillment of Contract Number 14-12-909
by Hittman Associates, Inc. under the sponsorship of the Environmental
Protection Agency. Work was completed as of April 1973.
111
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CONTENTS
Section Page
I Conclusions 1
II Recommendations 3
III Introduction 4
IV Phase I: Theoretical Analysis and 5
Laboratory Tests
V Phase II: Prototype Design 27
VI Phase II: Field Evaluation Results 42
VII References 53
IV
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FIGURES
PAGE
1 Geometry of the Capacitance-Cross-Sectional Area 6
Measuring Section
2 Typical Static Test Section with Capacitance Bridge 8
3 Normalized Capacitance vs Percent Area Filled 10
for 9 = 30° Plates
4 Normalized Capacitance vs Percent Area Filled 11
for Plates with Tapered Tops
5 Three Capacitor Plate Configurations Which Were 15
Evaluated for Use with Air Bubbles in Measuring
Velocity
6 Oscilloscope Trace of DC Signal Across Two 19
Exposed Electrodes as Velocity is Decreased
7 Optimum Placing of Thermocouples 24
8 Schematic of Prototype Sewer Gages 28
9 Electronic Instrumentation Package 31
10 Basic Body of Detector Section, Eight-Inch 33
11 Top View of Fully Assembled Detector Section and 35
Steam Reservoir and Pulsing Valve of Eight-Inch
Prototype
12 Side View of Fully Assembled Detector Section and 36
Steam Reservoir and Pulsing Valve of Eight-Inch
Prototype
13 Sewer Gage Installation 37
14 Detector Section Assembly, 24-Inch Prototype 38
15 Eight-Inch Prototype Test, Q = 4. 57 gpm 43
16 Eight-Inch Prototype Test, Q = 51.6 gpm 44
17 24-Inch Prototype Cross-Sectional Area Measure- 48
ment at High Flows, Quick Response Test
18 24-Inch Prototype Field Test of March 29, 1973 49
19 24-Inch Prototype Field Evaluation, March 16, 1973 51
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TABLES
PAGE
1 Major Variables Investigated for Their Effects 12
on the Dielectric Constant
2 Summary of Velocity Measurement Tests Using 22
Heat Pulse Method
3 Summary of Complete Sewer Gage Tests 25
4 Fabrication Costs for 8-Inch and 24-Inch 40
Prototype Detector Sections
5 Fabrication Costs for Prototype Steam Delivery 41
Assembly, Electronic Instrumentation, and
Steam Supply
VI
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ACKNOWLEDGMENTS
The support and technical guidance received from Mr. Harry C. Torno,
serving as project officer for the Environmental Protection Agency, is
greatly appreciated. The guidance received from Dr. H. R. Thacker of
the Environmental Protection Agency is also appreciated.
Major contributions to the design and field testing of the prototype gages
were made by Charles W. Mallory of Hittman Associates, Inc.
The cooperation of the Howard Research and Development Corporation in
providing field test sites for the prototype gages is gratefully appreciated.
vn
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SECTION I
CONCLUSIONS
Based upon the designs used for the second prototype gage developed
under this project, a sewer gage can be constructed which is capable of
measuring flow in partially filled sewers to within 15 percent accuracy.
This gage would use the principle of capacitance for measuring the
cross-sectional area of the flow and the timing of a heat pulse between
an upstream and a downstream thermistor to measure velocity. Further
refinement of some portions of the gage might produce accuracies of
within 10 percent.
Separately, the cross-sectional area measurements yield readings
within five percent of actual. Velocity measurements yield readings
within 10 percent of actual under optimum conditions.
The cross-sectional area measurements are not affected by contaminants
in the sewer. Scum deposits on the walls of the gage significantly and
adversely affect the accuracy of the velocity readings. These scum
deposits posed problems even though the prototype gages were tested
only in storm sewers. Thus, it is probable that gage fouling would be
an even more serious problem when the gage is used in sanitary sewers.
High flows are difficult to detect in the velocity measuring portion of the
gage. Also, the accuracy of the cross-sectional area measurements
deteriorates to 15 percent under high flows.
Further refinements in the shape of the capacitor plates will help to
increase the accuracy of the cross-sectional area readings at high
flows. The steam pulsing set up never functioned as planned. The
proper«functioning of this mechanism would enable a more accurate
determination to be made of the capabilities of the velocity measuring
portion at high flows.
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Low flows cannot be measured by this gage due to the requirements for
placing the heat sensing thermistors a minimum distance from the bot-
tom of the pipe.
An instrument which fits into a 24-inch pipe can be fabricated for
approximately $5000 on a one-time basis. This includes all the
peripheral equipment except the steam boiler and its heat source. A
complete gage which fits in an eight-inch sewer can be constructed
for approximately $4000 on a one-time basis. The steam supply
for either gage would cost an additional $620. On a limited production
basis, these costs might be expected to be reduced by up to 30
percent.
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SECTION II
RECOMMENDATIONS
Further work is not recommended on the use of heat pulses to measure
flow velocity because of the maintenance problems associated with
keeping the detectors clean. Recent information indicates that sonic
methods of velocity measurement may be a better approach than heat
pulse measurements. If this is not the case, several approaches could
be tried to improve the heat pulse measurement technique. These
would include greater temperature increases to overcome the thermal
resistance of coatings in the detectors, use of specially designed valves
to give shorter and higher magnitude heat pulses, use of single detectors
with timing initiated upon the injection of the heat pulse, use of retract-
able self-cleaning detectors injected into the stream by the pressure of
the steam pulse, development of steam boilers suited for unattended
field use and use of heat pulse measurements only for the calibration of
test sections, and using capacitance for flow measurement as discussed
above.
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SECTION III
INTRODUCTION
Accurate and reliable measurement of the flow of water in sanitary,
storm, and combined sewers is essential to virtually every water
pollution control and water resource program. A wide range of methods
of measurement currently exist (Ref. 1). Unfortunately, existing,
commonly-used instruments are usually severely limited in the range
of flows which can be accurately measured and are subject to fouling,
resulting in substantial missing data (Ref. 2). Consequently, Hittman
Associates was under contract to the U.S. Environmental Protection
Agency (EPA), Office of Research and Development, to develop a
portable device for accurately and reliably measuring the flow of
wastewater in sewers. This flow measuring device was to require
little electric current, place a minimal obstruction in the sewer pipe,
and be readily installed thorugh an ordinary manhole without special
preparation. The work on this project was divided into two phases.
In Phase I, the proposed gage was theoretically conceptualized and
laboratory tested under both static (still water) and dynamic (moving
water) conditions. A preliminary technical design and cost estimate,
based on the laboratory findings, was also prepared under Phase I.
In Phase II, two prototype gages were designed, fabricated, laboratory
tested and calibrated, and installed in a sewer outlet for a period of field
testing and evaluation. One prototype gage was constructed to fit in a
nominal eight-inch diameter sewer line while the other was constructed
to fit in a 24-inch diameter line. The field tests were conducted at a
sewer outfall where a well-calibrated weir existed immediately down-
stream. The weir measurements of discharge were supplemented by
cross-sectional area and velocity meter readings within the outfall itself.
This report constitutes the final and summary report for the entire
project. Included herein are the results of the Phase I analyses and
tests as well as the final findings of the field trials conducted under
Phase II.
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SECTION IV
PHASE I: THEORETICAL ANALYSIS AND LABORATORY TESTS
The volume rate of flow in a sewer is defined by the relationship:
Q = AV (1)
where:
Q = volume rate of flow, cfs
2
A = cross-sectional area of flow, ft
V = average or effective velocity, fps
Therefore, in order to determine the complete range of flows in a pipe,
from partly full, to full flow, to full flow under pressure, both the
velocity of the flow and its cross-sectional area must be measured.
THEORETICAL BACKGROUND FOR CROSS-SECTIONAL
AREA MEASUREMENT
The method utilized to measure the cross-sectional area of the flow
within a sewer pipe depends upon the unique properties of the electrical
capacitance of a sewer cross section, with the wastewater forming a
portion of the dielectric. If capacitor plates are incorporated in the
walls of a sewer pipe, the measured capacitance will increase as the
height, and thus the cross-sectional area, of the wastewater in the
sewer increases. This is made possible due to the difference between
the dielectric constant of air as compared to that of water. The ratio
of the dielectric constant of air to water is approximately 1:80 under
nor.mal conditions.
Ideally, referring to Figure 1, the normalized first order theoretical
capacitance function is: . ,
sine COS eo
x - - -
capacitance relative to _ uo cos , ,
norm maximum capacitance 1 + cos 0 Q ^ '
i - cos e
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Capacitor
Plate
Capacitor
Plate
Figftre 1. .Geometry of the Capacitance-
Cross-Sectional Area Measuring Section
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Equation (2) does not take field fringing into account and considers the
dielectric constant of air to be negligible compared to that of water.
For some values of fi in Equation (2), the response of the capacitance
readings to the filled area of the pipe is nearly linear. The effects of
field fringing will tend to increase the linear range of the response and
additional linearization can be accomplished by modifying the shape of
the capacitor plates near their edges. Thus, capacitance can be used
to determine the filled cross section of the pipe due to the difference
between the dielectric constants of air and water. Further, the relation-
ship between filled cross section and capacitance can be linearized for
maximum sensitivity over the entire range of flows by optimum sizing
and shaping of the capacitor plates.
STATIC TESTS: CAPACITANCE VERSUS CROSS-SECTIONAL
AREA MEASUREMENTS
In order to verify the theoretical predictions and arrive at an optimum
size and edge shape of the capacitor plates, an extensive laboratory
test program was conducted. This initial test program on the capacitance-
cross-sectional area test section was performed on an eight-inch diameter,
static, that is, stationary water, test section. Figure 2 shows one of
the initial static test modules, with the capacitance measuring bridge,
used during this phase of the laboratory test program.
The first aspect of the capacitance versus cross-sectional area mea-
suring portion of the gage to be investigated was the optimum size and
shape of the capacitor plates. Referring back to Figure 1, angle fi
was varied from 10 to 50 degrees in increments of; 10 degrees. At each
setting, the capacitance versus filled cross section response was mea-
sured for the entire range of water heights, from empty pipe to com-
pletely full.
Results of these experiments indicated that a 8 of 30 degrees produced
the most linear function of normalized capacitance; that is, capacitance
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Figure 2. Typical Static Test Section with Capacitance Bridge
8
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relative to maximum capacitance, versus percent of area filled.
Figure 3 shows this function for the 30-degree plates. Note the nearly
linear response of normalized capacitance versus percent area filled
function, except in the upper range.
In an effort to more nearly linearize this upper portion of the curve,
and thus provide for greater sensitivity in this range, experiments
were conducted on capacitor plates with tapered edges. The effect of
the tapers is dramatic, as is seen in Figure 4. Notice that the linear
portion of the curve has now been extended over the entire range. This
insures a maximum and consistent degree of sensitivity throughout the
range of filled pipe conditions.
The second aspect of the capacitance versus cross-sectional area mea-
suring portion of the gage which was investigated involved the optimiza-
tion of the materials to be used on constructing a prototype gage. The
original static test section was fabricated from Plexiglas because of
some of the inherent advantages of this material. Included among these
are its transparency, workability, and adequate strength for its proposed
application. However, unexpected difficulties with certain tests were
found to be caused by water absorption by the Plexiglas test section.
Consequently, a review was made of the physical properties of commer-
cially available materials which might be considered in the design of the
prototype sewer gage. This review concentrated on water absorption
characteristics. Certain materials, such as glass, exhibit negligible
water absorption but have undesirable mechanical properties. The best
material reviewed was a tetrafluoroethylene thermoplastic, commonly
known as Teflon. A new test section was designed and constructed
using this material instead of Plexiglas, which is a methyl methacrylate
thermoplastic. Further laboratory tests using the Teflon test section
confirmed its application for a sewer gage and completely solved the
water absorption problem.
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Teflon Sewer Pipe,
8" Static Test Section
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^^^^^••^^^•^^^^••^•^^^^^
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uu 10 20 30 40 50 60 70 80 90 100
Figure 4. Normalized Capacitance vs Percent Area
Filled for Plates With Tapered Tops
11
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The capacitor plates themselves are fabricated from 0.004-inch thick
brass plates. Some difficulty was experienced while taking capacitance
readings due to extraneous signals from nearby objects. This problem
was overcome by adequate electronic shielding of the device and lead
wires to the capacitance bridge, and by precise calibration of the test
section within exactly the same surroundings during each test. There-
fore, an adequate degree of shielding was determined to be a mandatory
requirement for the prototype instruments.
Following optimization of the size, geometry, and materials for the
capacitance-cross-sectional area measuring portion of the sewer gage
in pure water, a number of major contaminants in sewers were identified
as likely to affect the dielectric constant. Theoretically, the dielectric
constant of pure water varies from 88 to 73. 28 over the range of tem-
perature from 32° to 104°F (Ref. 3). Sand, rocks, and soil have
dielectric constants in the range of 10 to 15 (Ref. 4). The contaminants
listed in Table 1 were consequently investigated with respect to their
effect on the dielectric constant.
TABLE 1. MAJOR VARIABLES INVESTIGATED FOR
THEIR EFFECTS ON THE DIELECTRIC CONSTANT
Variable Probable Range
Temperature 1 to 35 C
Dissolved solids
NaCl 10 to 10, 000 mg/ H
NaHCOs 10 to 10, 000 mg/ H
MgSO4 - 7H2O 10 to 10, 000 mg/4
Ci2H22On 10 to 10, 000 mg/I
Some mixture of the above 10 to 10, 000 mg/ k
Suspended solids
Inorganic (clays) 10 to 10, 000 mg/£
Organic (starch) 10 to 10, 000 mg/ H
Some mixture of the above 10 to 10, 000 mg/ i
Selected mixture of dissolved and
suspended solids 10 to 10,000 mg/ JL
The dissolved and suspended contaminants tests had no significant effect
12
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on the cross-sectional area measurement up to 10, 000 mg/1 concentra-
tion. Temperature did have a noticeable effect upon the cross-sectional
area measurements. However, the effect of temperature was substantially
smaller than expected and was sufficiently small so that the observed
effect would not require temperature compensation in the prototype
instrument design, as originally contemplated. Tests were also con-
ducted upon the effects of combinations of all of the contaminants listed
in Table 1 on the dielectric constant. Results of these experiments also
showed no significant effect.
DYNAMIC TESTS: DEVELOPMENT OF VELOCITY
MEASURING SECTION
After verification, development, and testing of the capacitance versus
cross-sectional area measuring section on the static test stand, a spe-
cially designed dynamic test stand for flowing water experiments was
constructed. This test stand provided a constant head tank from which
flows up to 350 gallons per minute could be obtained in a four-inch test
segment. The water from the test section and overflow from the head
tank were constantly recirculated by a centrifugal pump. Flow to the
test section was controlled by two valves, one of the quick shut-off type
and one gate valve, the latter used for precise flow control. The four-
inch sewer gage test section itself could be mounted horizontally or at
any slope up to 10 degrees. This permitted experiments to be conducted
at any flow depth and at velocities ranging from 0 to 12 feet per second.
True flow was measured via a calibrated sump into which the sewer gage
test section discharged.
Initially, the cross-sectional area measuring section was mounted on
the test stand and a series of tests were run to determine its response
in moving water. After its performance under dynamic conditions was
proved, plans were formulated for the development of a velocity mea-
suring section. Five different methods for measuring velocity were
proposed, analyzed, tested, and evaluated. These are summarized in
the following subsections.
13
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Capacitance-Air Bubble Method
This method was the one that was originally conceived as being the most
promising for velocity measurement. Basically, the method again
depends upon the capacitance of the wastewater flow cross section and
the effect on this capacitance of the displacement of air bubble tracers.
Air bubbles can be detected due to the aforementioned difference between
the dielectric constants of air and water, and the consequent effect of
air bubbles on the dielectric of a capacitor. Theoretically, because of
this difference in dielectric constants, a bubble could be detected as it
rose and was swept downstream past a capacitor plate.
Static test stands were set up in which air could be bubbled past pairs
of capacitor plates. Three separate plate configurations, as shown in
Figure 5, were evaluated. Each configuration was evaluated in terms
of optimum size, location of the plates with respect to the top and/or
bottom of the pipe, and spacing between a number of parallel capacitor
plates.
Type (a), the "upright" configuration as shown in Figur.e 5, was found
to be the most sensitive in detecting air bubbles. Consequently, these
type (a) plates were mounted on the dynamic test stand with an air
sparger located upstream from the plates.
Great difficulty was experienced in detecting the air bubbles as they
crossed or surfaced at the capacitor plates in moving water. Flows
from approximately one-quarter full to full flow were tested with little
success. The distance of the air sparger from the nearest capacitor
plate, as well as the size of the bubbles, i. e., the volume of the air
pulse, was varied over the widest possible range. At low flows, up to
30 percent of the flow volume passing the capacitor plates was com-
posed of injected air, with little success in timing the speed of the air
pulse as it was swept downstream by the flowing water.
14
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(a)
"upright"
(b)
"longitudinal"
\
(c)
'square1
Figure 5. Three Capacitor Plate Configurations Which Were
Evaluated for Use with Air Bubbles in Measuring Velocity
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Failure of this approach was found to be due to the following:
(1) At other than full flows, surface perturbations of the water
accounted for a significant change in the capacitance readings.
This "background noise" masked the effect which a passing
air bubble had on the capacitance readings.
(2) At high flows, the large amount of entrained air in the flowing
water produced a two-phase flow situation (Refs. 5, 6, and
7). This entrained air made it difficult to distinguish between
this source of air and that air introduced in order to measure
velocity via capacitance reading.
(3) At low flows, the bubbles rose to the surface of the water
faster than they were swept downstream by the water flowing
past the capacitor plates. This rapid rise velocity thus
precluded their being detected downstream by the capacitor
plates before breaking the surface.
Inductive Method
Basically, this method utilizes a drive coil external to the pipe to
create an audio frequency magnetic field. The magnetic field, in turn,
induces an eddy current in any nearby conductor, such as the water in
the pipe. This eddy current can then be detected by sensitive pickup
coils located near the pipe invert. If the water is moving, the signal
detected will be out of phase with the stronger signal resulting from
direct coupling of the drive and pickup coils, the amount of this phase
shift being correlated with effective velocity.
For the four-inch dynamic test section, the drive coil was fabricated
from 100 turns of No. 24 gage enameled magnet wire, and the two pick-
up coils were each of 100 turns of No. 30 gage plastic insulated wire.
Optimization of this setup was achieved in the form of the location,
spacing, and shielding of the coils and other electronic equipment.
16
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Experiments performed on the four-inch dynamic test section failed to
demonstrate a detectable phase shift. When a number of subsequent
runs with all the factors optimized also failed, this velocity measuring
approach was abandoned. Failure of this approach was attributed to the
extreme smallness of the signal to be detected. Initial trouble was also
experienced with background "noise" and the extreme sensitivity of the
electronic equipment, but these problems did not, in themselves,
account for the failure of this approach.
Success could probably have been achieved by using larger coils and a
greater drive current. This approach was undesirable, however, due
to the requirement for minimum flow obstruction. The larger coils
would place too large an obstruction to the smooth flow of water through
the measuring section.
Magnetic Flow Meter Approach
The second approach for measuring velocity involved use of a large
coil to induce a DC field. Electrodes in contact with the water would
then, theoretically, detect a change in voltage which would be propor-
tional to a change in velocity.
Up to 50 volts at 1. 5 amperes of current were used to induce the field
in the drive coil. An additional refinement was added after the initial
tests in the form of a massive iron ring in order to further concentrate
the field. Results were negative.
Th.e identical setup was used in subsequent experiments, except that
alternating current was used. An AC signal of 1 KHz at 2 amperes was
used as a drive. It was hoped that either an amplitude change or a
phase shjlft of the AC signal could be detected; however, none materialized.
A number of inherent advantages of an AC signal over a DC signal war-
ranted its investigation in this connection. The greatest of these is the
17
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absence of "drift" of an AC signal and its noncorrosive or plating effect
on the exposed electrodes.
The magnetic flow meter approach will certainly work in a full pipe con-
dition if a large enough piece of suitable conductive material is used to
concentrate the field. Such instruments have, in fact, been used suc-
cessfully in a large number of applications (Ref. 8). The materials
used to concentrate the field could be an iron ring in the case of DC
or a laminated ring in the case of AC. However, the bulkiness of
this item again precludes its use where a portable, easily installed
instrument with a minimum of restriction in a sewer pipe is required.
Electric Current Method
A fourth attempt at velocity measurement involved applying a voltage
across the dynamic test section, between two electrodes in direct con-
tact with the water. Again, both direct and alternating current were
evaluated.
The use of alternating current produced negative results in that no
change in the amplitude, or a phase shift of the signal, could be detected.
The range of experiments performed using AC ran from use of normal,
60 cycle frequency to a very low frequency of 0. 01 cycles per second.
Also, additional tests were made using alternating current in the form
of square waves, with similar negative results.
Experiments using DC were run with a voltage of up to 50 volts at two
milliamperes. The change in DC voltage, as measured across a one
ohm resistor, was then observed as velocity changed. Here, an in-
crease in current was observed as velocity decreased, with the filled
cross-sectional area remaining the same. Figure 6 is a photograph of
a typical oscilloscope trace of the DC signal across a fully flowing pipe
as the velocity is gradually reduced. A DC signal between electrodes
placed along the length of the pipe showed a similar reaction, but of a
lesser magnitude.
18
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Figure 6. Oscilloscope Trace of DC Signal Across
Two Exposed Electrodes as Velocity is Decreased
19
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Although initially promising, further tests on this velocity measuring
technique pinpointed a number of developmental problems in using this
method in a practical instrument. One of these problems was the
inability to separate changes in the signal due to true velocity changes,
as opposed to random changes due to DC signal "drift. " A second
problem was the previously mentioned one of the plating phenomenon of
the direct current on the exposed electrodes. This, in itself, will tend
to degrade the DC signal over a period of time, especially in a sewage
medium, causing a loss of precision and erratic readings.
Heat Pulse Method
This method for velocity measurement proved to be the best of the ones
tried and was subsequently developed as the one for integration with the
cross-sectional area measuring section to make a complete sewer gage.
Basically, this method involves the tracing of a heat pulse as it is
swept down the pipe by the flowing water. The time of flight, as mea-
sured between strategically located thermocouples, would then give an
indication of the average velocity in the sewer.
Initial experiments were carried out in a still-water basin to determine
the heat conduction characteristics of water. A hot water pulse was
introduced at one corner of a water basin and thermocouple readings
were taken at a number of points throughout the basin for a half-hour
period. It was concluded that the heat dissipation through the water by
conduction was negligible when compared to the propagation of the pulse
down the sewer due to the velocity of the water. Thus, the velocity of
the heat pulse, and consequently the velocity of the water, could be
determined very accurately by timing the pulse as it passed between
two points.
Subsequently, a test section utilizing this concept of velocity measure-
ment was fabricated and installed on the dynamic test stand. The first
tests on this section were performed injecting hot water as the carrier
20
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of the heat pulse. Hot water was found to be a poor medium via which
to rapidly inject a high temperature heat pulse into the sewer because
of the relatively large volume of hot water required in order to produce
a noticeable temperature rise. This large mass of water being injected
into the pipe produces a localized increase in flow. The increased
velocity associated with this flow is sensed by the thermocouple probes,
giving erroneously high velocity values for the sewage flow. Conse-
quently, it was decided to use steam as the injection medium, the advan-
tage of steam being that a large amount of heat can be injected using a
small volume of steam, thus minimizing the localized flow variation
effects.
The standard pressure cooker used during the laboratory experiments
provided steam at pressures between 16 and 18 psi. Iron-constantan
thermocouples were utilized to detect the steam pulse because of their
sensitivity in detecting low temperatures.
Table 2 is a summary of the heat pulse-velocity test section experi-
ments, ranked from lowest to highest velocity. The development of
the heat pulse method of velocity measurement can be traced through
an analysis of Table 2. During the first series of experiments, it was
discovered that the factor most affecting the accuracy of the velocity
measuring section was the length of time during which the steam pulse
was injected, i. e. , the steam pulse duration. Tests 2, 3, and 4 in
Table 2 dramatically illustrate this point. In this series of tests, the
only variable was the length of time of steam injection. As the duration
of the steam pulse was reduced, the error in the velocity readings was
also seen to decrease. The high velocity readings with a larger steam
pulse were found to be caused by the momentum imparted to the flow in
the sewer by the injected steam. This increased flow rate, although
smalle^ than that caused by hot water injection, is nevertheless signifi-
cant, especially in the four-inch diameter pipe that was used for the
laboratory experiments. As the length of time of the steam pulse was
reduced, the error in velocity readings was accordingly decreased.
21
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Notice that steam pulses on the order of one-half of a second or shorter
in length produced significantly more accurate velocity measurements
than those of a longer duration. Pulses of a duration less than one-
quarter of a second were unobtainable in the laboratory dynamic test
program due to the use of a hand-operated steam injection valve. The
greater expense required to mechanize the steam injection and thus
obtain pulses of a shorter duration was not justified in the laboratory
test phase since the thrust of the laboratory experiments was to demon-
strate the basic concept and this could be done quite nicely with the
manual setup.
TABLE 2. SUMMARY OF VELOCITY MEASUREMENT
TESTS USING HEAT PULSE METHOD
Test
No.
1
2
3
4
5
6
7
8
9
10
11
Number of
Readings
7
5
1
1
4
5
8
5
9
6
11
Actual
Velocity
(fps)
0. 332
0.430
0.430
0.430
0. 553
0. 553
0. 963
3. 23
6.89
6. 89
7. 12
Mean
Measured
Velocity
(fps)
0. 360
0. 595
0. 543
0. 528
0. 614
0. 584
0. 936
3. 97
7. 51
7. 14
8.28
Deviation
From
Actual
+ 8.4
+ 38.4
+26. 3
+22.8
+ 11.0
+ 5.6
- 2.9
+22.9
+ 9.0
+ 3. 6
+ 16. 3
Steam
Pulse
Duration
(sec)
i
4
2
U
1
i
2
i
1
2
1
2
1
4
1
4
1
2
A second factor which was isolated as affecting the accuracy of the
velocity measuring section was the degree to which the thermocouples
protrude into the sewer pipe. Tests 5, 6, and 11 of Table 2 illustrate
the degree of refinement in the velocity readings which is possible by
having the thermocouples at the optimum height. In Test 5, the pipe
was flowing at approximately one-quarter full and the thermocouples
22
-------
were at the surface of the water. Test 6 was performed under exactly
the same flow conditions, but with the electrodes halfway between the
bottom of the pipe and the surface of the water. The effects of thermo-
couple placement are apparent. In Test 11, the thermocouples were
again at the surface of the water, with the pipe approximately one-quarter
full. The larger error in velocity measurement is again apparent.
The high velocity readings obtained when theeheat sensors were placed
too close to the surface of the water were due, in part, to the larger than
required steam pulses injected into the pipe. If the steam pulse was too
large, it was observed that waves were set up on the surface of the water.
These waves propagated downstream faster than the mean flow velocity.
The thermocouples sensed these waves, thus giving erroneous (too high)
velocity readings.
There is a trade-off between having the thermocouples protrude too high
into the pipe and thus be vulnerable to large objects flowing down the
sewer,and having them too close to the wall of the pipe and thus errone-
ously read what would perhaps be the slowest portion of the velocity pro-
file. The optimum height and location of the thermocouples in terms of
these two considerations were determined by experiment to be as shown
in Figure 7. Two thermocouples are used per cross section in order to
obtain a more accurate picture of the velocity profile across the pipe or
to easily switch from one thermocouple to another in case of damage.
They are placed 15 degrees off the bottom of the pipe to inhibit interfer-
ence with them by solids which may accumulate in the bottom of the pipe.
To summarize the results of the laboratory tests on the heat pulse
method: When all the experimental factors affecting the velocity mea-
suring device were optimized, the resultant error in the velocity mea-
suring section of the sewer gage was found to be less than 10 percent.
This is illustrated by Tests 2, 4, and 10 in Table 2. With mechanically
controlled steam pulses, it was postulated that this erorr could probably
be reduced even more and the majority of the remaining error could be
23
-------
Figure 7. Optimum Placing of Thermocoupl
es
24
-------
either electronically, physically, or mathematically calibrated out of
the gage to reduce the overall error in the velocity measurements to
within five percent.
DYNAMIC TESTS: COMPLETE SEWER GAGE
With the separate development of the cross-sectional area (capacitance)
and velocity (heat pulse) measuring portions of the sewer gage, the com-
bined sections were ready for testing on the four-inch diameter dynamic
test stand. First, the cross-sectional area measuring section was
tested and calibrated on the dynamic test stand in both the still and
moving water conditions. Then, experiments were performed on the
complete sewer gage, combining the capacitance readings and the velocity
measurements to arrive at a total flow. Results of these tests are sum-
marized in Table 3, listed from lowest to highest tested flow.
TABLE 3. SUMMARY OF COMPLETE SEWER GAGE TESTS
Test
No.
1
2
3
4
5
6
7
'8
9
10
11
12
13
Number of
'Readings
11
3
5
2
2
3
1
2
4
4
4
9
6
Actual
Flow
(gpm)
3.08
8.30
8.45
13.3
24.5
24. 5
24. 5
36. 1
63.5
63.5
108
270
270
Mean
Measured
Flow
(gpm)
4.46
8.78
10.. 10
12.4
35. 2
30. 3
26. 3
34.6
82.8
63. 3
123
294
280
Deviation
From
Actual
(%)
+44.8
+ 5.8
+ 19. 5
- 8.3
+43.6
+23.7
+ 7.4
- 4.2
+30.4
- 0.3
+ 13.9
+ 8.9
+ 3.7
Steam
Pulse
Duration
(sec)
1*
i
4
2
i
4
H
i
2
1
4
1
2
1
4
1
4
1
4
1
4
25
-------
As with just the velocity measuring section, experiments were again
performed on the variance of the flow measurements with the duration
of the steam pulse. Tests in series 5, 6, and 7 and series 9 and 10 in
Table 3 again illustrate the sensitivity of the measurements to the amount
of steam injected.
For all the complete sewer gage tests, the thermocouples were placed
near their optimum location; that is, approximately three-quarters of
an inch from the wall of the pipe.
After completion of the laboratory test program, is was concluded that
a portable sewer gage could be designed which would measure flow in
sewers to within an accuracy of approximately 10 percent, as Tests 2,
4, 8, 10, 11, 12, and 13 in Table 3 point out. Three factors would have
to be optimized in order to achieve this projected accuracy. These
factors are:
(1) Injection of a short, intense steam pulse
(2) Optimum location of the thermocouples with respect
to the periphery of the pipe
(3) Achievement of complete mixing of the steam pulse
26
-------
SECTION V
PHASE II: PROTOTYPE DESIGN
GENERAL DESIGN PARAMETERS
The principal objective of the first phase of this project was the proof
of the principal of operation and the development of design criteria for
a prototype sewer gage. This sewer gage was fabricated and field
tested in Phase II of the project. The gage was designed with the fol-
lowing requirements in mind:
(1) Capable of functioning under all conditions of flow, from
partially full, open channel type flow to full flow under
varying surcharge pressures
(2) Readily installed in existing pipes
(3) Minimum interference with pipeline hydraulics
(4) Neither influence nor be influenced by any contaminants
in the liquid
(5) Operate with satisfactory accuracy under all flow
conditions
(6) Applicable to a wide range of conduit sizes
(7) Rapidly installed through a standard manhole or in
other locations where the sewer is accessible
(8) Capable of being instrumented for remote readout
(9) Minimum of moving parts for easy maintenance
(10) Minimum power requirements
(11) Capable of being manufactured at a reasonable cost
Figure 8 is a schematic diagram of the major components of the proto-
type sewer gages. It was decided to construct two prototype gages —
one which would fit in a nominal eight-inch diameter sewer and the other
27
-------
CO
CO
Steam Supply Assembly
Output
Steam
Reservoir
r
Electronic
Instrumentation
Power Supply
1 ;
Electronic Instrument Package
Figure 8. Schematic of Prototype Sewer Gages
-------
to fit in a nominal 24-inch diameter sewer. This would allow not only
a comparison of the accuracy and sensitivity of the gage in both small
and relatively large diameter sewers, but would also enable an accurate
cost determination to be made for various size sewer installations.
The detector section is the only component of the sewer gage which must
be located within the sewer line to be measured. It was designed as a
hollow tube with an outside diameter slightly smaller than the internal
diameter of the sewer. It incorporates both the capacitor plates for
cross-sectional area measurement and the steam inlet ports and heat
sensors for velocity measurement. All the other components of the gage
are mounted external to the sewer; either within the manhole itself, as
in the case of the steam reservoir and pulsing valve which are bolted to
the top of the outfall end of the detector section, or outside the manhole
completely, as is the case of the steam boiler and heat source. The
electronic instrumentation package is enclosed in a waterproof container
and is sufficiently small so that it can be located anywhere that is most
convenient. This arrangement of the components provided that the
detector section was the only component which had to be fabricated
separately for the 8- and 24-inch sewer lines. All the other compo-
nents were designed and used for both prototype installations.
STEAM SUPPLY ASSEMBLY
The steam supply assembly was designed to deliver a pulse of steam
every minute to the steam exit ports located in the detector section.
The steam boiler used was a small, standard item with a maximum
working pressure of 100 pounds per square inch (psi). It delivers steam
at a constant pressure (pressures between 35 and 40 psi were used for
both the eight and 24 inch prototypes) to a steam reservoir. This steam
reservoir is no more than an insulated cylinder, 3. 3 inches in diameter
and 46. 8 inches long, mounted on top of the detector section. The reser-
voir was designed to release its entire contents of steam to the detector
section in pulses through a steam pulsing valve. The volume of the
29
-------
steam reservoir was calculated as adequate to heat one-half of the cross
section of a 24-inch sewer 1°F. These heating requirements were deter-
mined during the laboratory dynamic test program.
Automatic pulsing of the steam at one minute intervals was to be provided
by the steam pulsing valve located between the steam reservoir and the
detector section. No commercially available, automatically resetting,
steam pulsing valve could be located. Therefore, pulsing was accom-
plished through use of a modified pilot-operated valve. Complete suc-
cess at automatic steam pulsing was not achieved with this valve. How-
ever, the steam inlet pulses to the detector section were adequate for
field testing of the prototype sewer gages. Complete design and fabri-
cation of an exactly suited steam pulsing valve was not felt to be justified
during preliminary prototype development.
ELECTRONIC INSTRUMENTATION PACKAGE
The signals from the capacitor plates and heat sensing probes in the
detector section are fed into an electronic instrumentation package. This
package contains the electronic circuitry, power supply, and recorder.
Figure 9 is a schematic of the major components of the package.
As shown in Figure 10, the area signal is obtained from a counter whose
count is proportional to the reading obtained by the capacitor which mea-
sures the filled cross section. Concurrently, the "time of flight" or
velocity signal is obtained from a counter whose count is proportional to
the time of travel of the heat pulse between the upstream and downstream
heat sensors. The signals from the heat sensors are picked up by two
high gain amplifiers and fed to a start-stop circuit. The output from the
start-stop circuit gates the time counter.
The inputs to both the time and area counters are from a master oscilla-
tor. The oscillator provides a scale of 256 counts to full scale area and
also scales the time oscillator for a 256 to 1 count for time of flight.
30
-------
Therm
couples
Time
Counter
Gate
Time
Counter
Digital
Pipe
Capacitq
CO
Area
Counter
Gate
Count
Digital
Count
Time
Digital
to
Analog
Converter
Time
Digital
to
Analog
Converter
T
6 volt Battery
T
Analog1
A
Analog
Dual Channel Strip Chart Recorder
or
Visual Milliampere Meters
Figure 9. Electronic Instrumentation Package
-------
Outputs from both counters are converted to analog signals using a
digital to analog ladder network before being presented to the strip
chart, recorder, or visual meters. Both the visual meters and strip
chart recorder were used during the course of the field trials.
A divider circuit which would convert the area and velocity signals
directly to a flow volume was originally designed into the electronics
package. However, the output from the divider circuit was found to be
less reliable than the individual inputs of area and velocity. Thus, this
circuit was removed early in the field test program.
The entire electronic instrumentation package, including circuitry,
recorder, and six-volt battery, is contained in a 12 inch by 10 inch by
7 inch waterproof box. Its total weight is somewhat less than 30 pounds.
DETECTOR SECTION: EIGHT-INCH PROTOTYPE
Figure 10 shows the basic body shell of the detector section used for the
eight-inch prototype. A Teflon liner with the shaped capacitor plates
bonded to it is inserted inside the body. The Teflon then acts as a shield
between the capacitor plates and the wastewater flowing through the
detector section. The access holes near the bottom of the section are
for insertion of the heat sensors and knife-edge guards.
Fabrication of the completed eight-inch prototype was followed by a
period of laboratory checkout. The completed gage was installed on the
dynamic test stand and the steam and electrical lines were installed.
During this laboratory checkout, a number of varieties and ratings of
heat sensors were evaluated in terms of their sensitivity to the input
stream pulse and their stability against the experienced and expected
background temperatures. Thermistors with a 10, 000 ohm at 25 C
rating were selected as being the most suitable heat sensors for both the
upstream and downstream positions. After this was determined, final
calibration was performed on the electronics prior to installation at the
field test site.
32
-------
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Figure 10. Basic Body of Detector Section, Eight-Inch Prototype
-------
Figures 11 and 12 show the fully assembled eight-inch detector section
with the steam reservoir and pulsing valve attached. A rubber boot pro-
tects the electronic lead wires and also provides, via an air blowup
valve, a means of sealing the gage securely against the inner wall of
the sewer. The detector section is normally inserted into the sewer
line via a manhole or outfall structure as far as the upright reservoir
flange will permit. The steam pulsing valve and reservoir sits atop the
detector section within the manhole or outside the sewer if it is installed
at an outfall. Figure 13 shows how the major components of the entire
sewer gage system would be assembled for flow measurement.
DETECTOR SECTION: 24-INCH PROTOTYPE
Basically, the 24-inch detector section is a scaled-up version of the
eight-inch section with a number of important modifications which
resulted from the field testing of the eight-inch prototype. These modi-
fications increased the sensitivity of the instrument and bettered its
operational capabilities.
Within the eight-inch section, as shown in Figures 11 and 12, the Teflon
liner extended almost the entire length of the detector section. The
thermistors protruded through the shield and provided a one-foot dis-
tance between themselves for velocity measuring purposes. During the
laboratory checkout and field evaluation of the eight-inch section, it was
discovered that the direct coupling of the Teflon shield to the thermistors
produced considerable electronic background noise when the capacitor
plates were energized. The one-foot distance between the upstream and
downstream thermistors was also found to be at the limit'of detection of
the heat pulse by the downstream thermistor, especially at high flows.
Consequently, as shown in Figure 14, the capacitor plates and Teflon
liner are physically separated in the 24-inch prototype detector section.
The distance between the upstream and downstream thermistors has
also been reduced to eight inches to increase the detection probability by
34
-------
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Figure 11. Top View of Fully Assembled Detector Section and
Steam Reservoir and Pulsing Valve of Eight-Inch Prototype
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Figure 12. Side View of Fully Assembled Detector Section and
Steam Reservoir and Pulsing Valve of Eight-Inch Prototype
-------
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Figure 13. Sewer Gage Installation
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Figure 14. Detector Section Assembly, 24-Inch Prototype
-------
the downstream thermistor. A more powerful mainspring in the steam
pulsing valve and a simpler and finer-adjusted pilot valve pulsing sys-
tem bettered the operating characteristics of the gage during the field
evaluations in the 24-inch sewer.
COSTS
Costs of fabricating the eight- and 24-inch prototype detector sections
are given in Table 4. Table 5 shows the fabrication costs for the steam
reservoir and pulsing valve assembly and the electronic instrumentation
package which were common for both the eight- and 24-inch prototypes.
These costs do not reflect, of course, any costs of design. The cost of
the steam boiler and its related fuel supply are not included since the
unit used for this development work is not ideally suited for the applica-
tion. If commercial sewer gages are to be made available, this com-
ponent would have to be redesigned and scaled down.
The cost of the 24-inch detector section shown in Table 4 also reflects
the redesign of this section in the larger size. In effect, this produced
some cost savings since the overall length of the detector section, and
also the amount of Teflon needed for shielding the capacitor plates, was
reduced. Thus, a redesigned eight-inch detector section should cost
less than the $1625 cost of the initial prototype of this unit.
A complete sewer gage, say for a 24-inch line, would cost approximately
$5000, on a prototype basis, exclusive of steam boiler and heat supply.
On a semiproduction basis, this cost might be expected to be reduced by
up to 30 percent.
39
-------
TABLE 4. FABRICATION COSTS FOR
8-INCH AND 24-INCH PROTOTYPE
DETECTOR SECTIONS
Item
Aluminum body
Teflon shield
Thermistors
Rubber boot
Thermistor shields
Lead wire
Steam inlet diffusers
Capacitor plates
sealing
Assembly
8- Inch Section
Cost ($)
645
120
11
20
elds 96
10
asers 100
3 1
screws, bolts,
igs, and sealants 75
rdcoating, and
140
407
24- Inch Section
Cost ($)
1272
275
11
45
96
10
100
2
100
185
407
TOTAL
$1625
$2503
40
-------
TABLE 5. FABRICATION COSTS FOR
PROTOTYPE STEAM DELIVERY ASSEMBLY,
ELECTRONIC INSTRUMENTATION. AND STEAM SUPPLY
Item
Steam Reservoir and Pulsing Valve Assembly
End caps
Reservoir cylinder
Insulation jacket
Welding
Flange
Main pilot valve
Pilot valve spring
Miscellaneous piping, fittings, and valves
SUBTOTAL
Electronic Instrumentation Package
Circuit board and wiring
Electronic components
Dual channel recorder
Waterproof enclosure
Power supply
Fabrication and checkout
SUBTOTAL
Steam Supply
Steam boiler
Non-electric gas regulator for boiler
50 feet of steam supply hose
SUBTOTAL
Cost ($)
45
42
61
36
18
75
2
20
$299
277
171
171
75
4
1465
$2163
490
85
45
$620
TOTAL
41
-------
SECTION VI
PHASE II: FIELD EVALUATION RESULTS
EIGHT-INCH GAGE
The outfall culvert from a small, approximately four-acre lake served as
as the field test site for the eight-inch prototype sewer gage. The culvert
is a nominal 24 inches in diameter. Therefore, a simple adapter was
constructed for this culvert in order that the eight-inch prototype could
be inserted.
A well-calibrated, compound V-notch and rectangular sharp-crested weir
is located immediately downstream of this outfall culvert. Measure-
ments from this weir were supplemented with measurements of filled
cross section, velocity, and total flow at the gage itself.
The gage was tested for flows which ranged from near zero to up to 56. 1
2
gallons per minute (0. 125 ft /sec). Overall, the cross-sectional area
measurements via capacitance readings were found to be accurate to
within 10 percent of the actual filled cross section, although some less
accurate, scattered readings did occur. The cross-sectional area mea-
surements were also found to be more reproducible and trouble-free
than the velocity measurements. The velocity measuring portion of the
gage was generally troublesome and less accurate during the field trials.
This resulted in total flow measurements, i. e., the combined cross-
sectional area and velocity measurements, to be usually within 20 per-
cent of the actual measured flow. Figures 15 and 16 show two of the
tests which produced some of the most trouble-free results during the
field trials, one at a relatively low flow (0. 0102 cfs), and one at the
maximum flow tested (0. 125 cfs).
The eight-inch prototype did not function as smoothly as was hoped for
due to a number of development problems. Consequently, extensive
field test data could not be collected to substantiate the continued
42
-------
CO
0.013
ra 0.012
-------
0. 30
0. 25
CD
c*-t
0
0. 20
0. 15
0. 14
0. 13
0. 12
0. 11
0. 10
Measured
Q(Actual)
= 0. 125 cfs
10
Time, minutes
15
20
Figure 16. Eight-Inch Prototype Test, Q = 51.6 gpm
44
-------
reproducibility of the total flow readings. Most of these problems were
solved by the subsequent redesign for the 24-inch prototype. These
problems were:
(1) Coupling of the capacitor to the thermistor circuit via the Teflon
liner, as discussed in Section V, resulted in a large electronic
background noise which was superimposed on any signal which
the steam pulse might generate on the thermistors. The sensi-
tivity of the velocity measuring portion of the gage was conse-
quently reduced. This accounted for a fairly large portion of
the trouble experienced with the velocity measurements. The
problem was completely eliminated by physically removing the
thermistors from their Teflon shield portion of the gage in the
24-inch prototype.
(2) Water leaking underneath the edge of the rubber boot to the
inner parts of the detector section caused short circuits between
the capacitor plates and the aluminum body of the detector sec-
tion, and between the thermistor lead wires. The shorting of
the capacitor plates to the metal detector body, in effect,
caused the entire detector section to act as the cross-sectional
area measuring capacitor. The relatively small range of capaci-
tance change with water level within a generally high background
capacitance reading caused by this situation decreased the sensi-
tivity of the cross-sectional area measurements enormously.
Short circuiting of the thermistor lead wires also caused loss
of sensitivity in the velocity readings. The heat pulse signal
was often "lost" when water leakage occurred. This problem
was also completely solved in the 24-inch prototype by use of
more extensive potting of all electric leads in the detector sec-
tion, better insulation between the capacitor plates and the metal
bpdy of the detector section, and use of a tighter clamping and
sealing mechanism between the rubber boot and the detector body.
45
-------
(3) Erratic operation of the steam pulsing mechanism necessitated
the use of manual steam pulses throughout the field trials of
the eight-inch prototype. This caused variations in the steam
pulse duration and pressure, thus causing erratic velocity
readings. The functioning of this mechanism was improved,
although not rendered completely automatic during the field
trials of the 24-inch prototype.
These three problems all contributed to the low degree of accuracy of
the velocity measurements in the eight-inch gage. Complete elimina-
tion of the first two problems and partial fixing of the third in the 24-
inch prototype greatly improved the accuracy of the velocity readings.
A source of built-in loss of sensitivity which bears mentioning is the
complete loss of velocity readings at very low flow. This is due to the
placement of the heat-sensing thermistors at 15 of the bottom center-
line of the pipe, as shown in Figure 7, to avoid interference by any bed
load which may be present. At very low flows, the thermistors are thus
totally removed from the wastewater. Placement of the thermistor tips
with less than a three-quarter inch protrusion from the detector section
wall was tried in order to measure lower flows. No apparent deleterious
effect was noticed, so placement of the thermistors closer to the wall
was also done in the 24-inch prototype.
24-INCH GAGE
The necessary refinements which were identified during the field evalu-
ation of the eight-inch prototype were incorporated into the 24-inch
detector section described in Section V. Refinements were also made
in the steam pulsing mechanisms as described above. The 24-inch pro-
totype was field tested in the same culvert as the eight-inch prototype.
Since the culvert diameter is a nominal 24 inches, no special adapter
was necessary. Flows up to 7. 12 cfs were measured during the field
evaluation period.
46
-------
The cross-sectional area measuring portion of the gage was found to be
accurate to within five percent of actual flow area for flows up to
approximately three-quarters of full pipe flow. At the upper limits of
flow, the 24-inch prototype was found to measure cross-sectional area
to within 15 percent of actual area. Figure 17 shows this deterioration
in accuracy of the cross-sectional area measurements during a quick
response test.
This decrease in accuracy near the top range of flow is probably due to
the normal flattening of the capacitance versus filled cross-section
curve at the upper limits of filled cross section which was investigated
during the laboratory experiments in Phase I of the project. This
flattening reduced the sensitivity of capacitance readings to changes in
filled cross section in this range. Nominal 45 tapers were included on
the corners of the capacitor plates for the 24-inch prototype. These
tapers, however, which were adequate for the eight-inch prototype, are
probably insufficient for significantly linearizing the upper portion of
the response curve. Larger tapers might thus improve the accuracy of
the 24-inch cross-sectional area measuring portion in the upper flow
ranges.
The velocity measuring portion of the 24-inch prototype yielded better
results than its eight-inch predecessor. It was found to read within
ten percent of true velocity for flows up to half-full pipe flow. Over
half-full flow, the accuracy of the velocity measurements deteriorated
slightly. At flows at which the pipe was over 70 percent full, the steam
pulses could usually not be detected by the thermistors. Figure 18 is an
example of one of the short duration, steady state flow tests conducted
on the gage. The pipe was slightly more than half-full during the test.
The surprisingly good results obtained illustrate the accuracy of the
gage when most of its important physical parameters are optimized.
The only external parameter which was found to affect the accuracy of
the sewer gage was scum buildup on its walls where they are constantly
47
-------
2. 1
2. 0
Actual
Measured
1. 3
1. 2
1. 1
1. 0
0 10 20 30 40 50 60 70
Time, minutes
Figure 17. 24-Inch Prototype Cross-Sectional Area
Measurement at High Flows, Quick Response Test
48
-------
CO
t"-l
o
0.40
0. 30
0.20
0. 10
a 0. 30
S 0. 20
a
o •
i—I
0)
> 0. 10
1.4
(
1.0
Actual
— — — Measured
® ••
<§r
10 15 20 25 30
Time, minutes
35 40 45
Figure 18. 24-Inch Prototype Field Test of March 29, 1973
49
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in contact with the base flow. During the field trials, the 24-inch proto-
type was left in the sewer continually for a long duration operation and
maintenance test. This extended test lasted over one and one-half
months. About midway through the period, a loss in the accuracy of
the velocity measurements became apparent. The velocities measured
by the gage were consistently lower than the actual velocities. Inspec-
tion of the interior wall of the detector section revealed a buildup of
scum on the wall of the section which was continually immersed in the
base flow. This scum was measured to be at least one-quarter of an
inch thick and was composed primarily of algae and sediment. Figure
19 illustrates the dramatic effect this scum buildup had upon the accu-
racy of the velocity measurements. Once this scum was cleared from
the wall of the detector section, the velocity reading returned to within
an acceptable range of accuracy. The scum had no apparent effect upon
the cross-sectional area measurements.
DISCUSSION
Optimization of the design of the 24-inch detector section resulted in a
gage which is capable of measuring flows in partially filled sewers to
within an accuracy of 15 percent. Further optimization of the steam
reservoir/heat pulsing mechanism might further increase the accuracy
of the velocity measuring portion of the gage. The accuracy of the total
flow measurements might then be expected to approach 10 percent.
The use of capacitance to measure cross-sectional area was found to be
more accurate and trouble-free than the heat pulse sensing concept used
to measure velocity. Further refinement of the shape of the capacitor
plates should further increase the sensitivity of this method of cross-
sectional area measurement in the upper and lower ranges of flow.
The problems caused by the accumulation of wastes on the thermistor
probes of the velocity measuring mechanism may be a major problem
in the further development of this type of gage. Buildup of this scum
50
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en
<4-c
O
0. 30
0.20
0. 10
a 0.30
U-l
0. 20
o
_o
"QJ
0. 10
1. 4
CM
« 1.2
0)
1.0
Actual
Measured
Scum Cleaned off Wall
of Gage
I
10 20 30 40 50 60 70 80 90
Time, minutes
Figure 19. 24-Inch Prototype Field Evaluation, March 16, 1973
51
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was found to be a problem, even in the relatively "clean" stormwater
culvert in which the prototype gages were tested. Use of such gages in
sanitary or combined sewers thus require almost daily maintenance to
ensure accurate velocity readings.
A 24-inch sewer is probably the maximum diameter in which the present
prototype design will work to an acceptable degree of accuracy. The
large amount of metal in the vicinity of the capacitor plates, i. e., in
the body of the detector section itself, masks the capacitance readings
to some degree. If larger diameter sewer flow measurement is con-
templated, it is recommended that consideration be given to redesigning
the body of the detector section out of some nonconductive material, yet
one which will not absorb significant amounts of water.
52
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SECTION VII
REFERENCES
1. Replogle, J.A., Flow Meters for Water Resource Management,
Water Resources Bulletin, 6, 3: 345-374, May-June, 1970.
2. Wenzel, H. G., Jr., A Critical Review of Methods of Measuring
Discharge Within a Sewer Pipe, Data Devices Task Group,
USGS-ASCE Project-Research and Analysis of National Basic
Information Needs in Urban Hydrology, September^ 1968, 20 pp.
3. Hodgman, C.D., R. C. Weast, andS.W. Selby, eds., Handbook of
Chemistry and Physics, 41st Edition, Cleveland, Chemical Rubber
Publishing Company, 1959.
4. Westman, H. P., andJ.E. Schlaikjer, eds., Reference Data for
Radio Engineers, 4th Edition, New York, International Telephone
and Telegraph Corporation, 1956.
5. Anderson, R.J., andT.W.F. Russell, Designing for Two-Phase
Flow - Parti, Chemical Engineering, December 6, 1965, p. 139-144.
6. Anderson, R.J., andT.W.F. Russell, Designing for Two-Phase
Flow - Part II, Chemical Engineering, December 20, 1965,
p. 99-104.
7. Anderson, R.J., andT.W.F. Russell, Designing for Two-Phase
Flow - Part III, Chemical Engineering, January 3, 1966, p. 87-90.
8. Evans, R. L., Instrumentation in Wastewater Treatment Processes,
Rockville, Maryland, Instrumentation Development Engineering
Associated, Inc., undated, 18pp.
53
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
2.
i\ Accession No,
w
4. Title
A PORTABLE DEVICE FOR MEASURING
WASTEWATER FLOW IN SEWERS
7. Aathor(s)
Nawrocki, Michael A.
9, Organization
Hittman Associates, Inc.
9190 Red Branch Road
Columbia, Maryland
S. Report Date
6.
10. Project ',
//. Contrstct/G-rant "No.
14-12-909
13. Type < ••••;..ration Hittman Associates, Inc
• - ----._.* — _. . __ . _ -- *
U.S. GOVERNMENT PRINTING OfFICt 1974— 546-317/309
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