EPA-600/2-77-220
NOVEMBER 1977
Environmental Protection Technology Series
THERMOCOUPLE READOUT INSTRUMENT
Robert S. Kerr Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental 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.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-220
November 1977
THERMOCOUPLE READOUT INSTRUMENT
by
C. G. Enfield and C. V. Gillaspy
Wastewater Management Branch
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
ROBERT S. KERR ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
ADA, OKLAHOMA 74820
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DISCLAIMER
This report has been reviewed by the Robert S. Kerr Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
ii
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FOREWORD
The Environmental Protection Agency was established to coordinate
administration of the major Federal programs designed to protect the
quality of our environment.
An important part of the agency's effort involves the search for
information about environmental problems, management techniques and new
technologies through which optimum use of the nation's land and water
resources can be assured and the threat pollution poses to the welfare
of the American people can be minimized.
EPA's Office of Research and Development conducts this search
through a nationwide network of research facilities.
As one of these facilities, the Robert S. Kerr Environmental
Research Laboratory is responsible for the management of programs to:
(a) investigate the nature, transport, fate and management of pollutants
in groundwater; (b) develop and demonstrate methods for treating waste-
waters with soil and other natural systems; (c) develop and demonstrate
pollution control technologies for irrigation return flows; (d) develop
and demonstrate pollution control technologies for animal production
wastes; (e) develop and demonstrate technologies to prevent, control or
abate pollution from the petroleum refining and petrochemical indus-
tries; and (f) develop and demonstrate technologies to manage pollution
resulting from combinations of industrial wastewaters or industrial/
municipal wastewaters.
This report contributes to the knowledge essential if the EPA is to
meet the requirements of environmental laws that it establish and enforce
pollution control standards which are reasonable, cost effective and
provide adequate protection for the American public.
William C. Galegar
Director
Robert S. Kerr Environmental Research Laboratory
ill
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ABSTRACT
An electronic circuit has been developed which acts as an electronic ice bath
for chromel-constantan thermocouples. The electronic ice bath is accurate to within
± 0.2°C from -25°C to +50°C. Simultaneously, the thermocouple output is scaled
and linearized such that the temperature can be read directly in °C with a sensi-
tivity of 100 mV/°C with 0 VDC at 0°C. Circuit diagrams and construction consid-
erations are included to allow the reader to construct a functioning unit.
IV
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CONTENTS
Foreword iii
Abstract iv
Tables and Figures vi
1. Introduction 1
2. Conclusions and Recommendations 2
3. Design and Theory of Operation 3
4. Calibration 8
5. Construction Consideration 9
References 14
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TABLES
1 Comparison Between Actual Temperature and Predicted
Network Temperature 10
2 Parts List 11
FIGURES
1 Schematic Diagram of Temperature Compensation Network 12
2 Pin Connection for Burr-Brown Model 4205 Multiplier 13
3 Scaling Buffer Amplifier 13
VI
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SECTION 1
INTRODUCTION
Remote temperature measurement is a necessary part of many research inves-
tigations . Air and soil temperatures are commonly recorded to help understand
the transport phenomena in heat and water flow. Several different types of trans-
ducers or detectors are in common use for this purpose. The most commonly
used are thermocouples and thermistors. The major drawback with thermocouples
is a lack of sensitivity and the requirement for a reference junction. On the
other hand, thermistors have a high output versus temperature but lack adequate
interchangeability.
At the Robert S . Kerr Environmental Research Laboratory field station in
Ada, Oklahoma, studies are being conducted to evaluate wastewater treatment
by land application. As a necessary part of this research effort, temperature
measurements are required in the assessment of physical, chemical, and biological
processes. To adequately monitor the environmental processes, an instrument
capable of monitoring multiple input thermal transducers with an accuracy of
± 1°C from 25°C to +40°C was required. Commercial units with adequate capabilities
were not readily available. For this reason, an instrument was developed to
meet the specific needs of the experimental studies.
This report deals with an electronic method of providing an accurate reference
junction for a chromel-constantan thermocouple, signal conditioning to linearize
the output of a thermocouple pair, and signal gain such that output is 100 mV/°C.
1
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
A temperature readout instrument can be constructed such that output from the
instrument is 100 mV/°C using a chromel-constantan thermocouple for a detector.
The accuracy of the instrument is ± 1° C over a range of -25 to 40° C for the de-
tector when the instrument operating temperature ranges from -25°C to +75°C.
An auxiliary output is available which assesses the temperature of the instrument
and is accurate ± 0.2°C from -25°C to +50°C with an output sensitivity of -100
mV/°C.
The instrument should be particularly useful for temperature measurements
in remote locations where a supply of ice is not readily available for use as a
reference temperature. The approach should be adaptable to other thermocouple
materials by changing values of RI and R2 in Figure 1. The two resistors are
used to give a calibrated output for the instrument. The range of temperature
could be expanded; however, the accuracy of the measurement will be reduced
because the actual output of the thermocouple was assumed to follow the binominal
equation T = a + bV + cV2. The approximation is adequate over a temperature range
of 100 Celsius degrees.
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SECTION 3
DESIGN AND THEORY OF OPERATION
The theory of operation of the thermocouple has been described in numerous
textbooks and is extensively discussed in a monograph by Dike (1) . The actual
design of. the included circuitry is based on the 1971 National Bureau of Standards
reference tables for thermocouples as reproduced in Omega Engineering's Tempera-
ture Measurement Handbook (2) . These data were fitted by the method of least squares
to the equation
T = a + b (V) + c (V) (1)
where T = temperature (°C)
V = thermoelectric voltage in absolute millivolts
a, b, and c are constants
The values obtained are 0.00113, 17.025, -0.2033 for a, b, and c, respectively.
Table 1 gives a range of values which might normally be expected for environmental
measurements and the predicted temperature based on Equation 1. Column 1 of
Table 1 is an assumed temperature. By determining thermoelectric voltage (V) ,
the theoretical network response (T) is calculated and presented as Column 2. The
error is then the difference in the two temperatures.
The actual circuit, Figure 1, is then based on Equation 1 and consists of four
sections:
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1) Preamplifier which boosts the thermoelectric voltage 1,000 times, so
that the magnitude of the voltage is in the range that can be easily manipu -
lated.
2) Squaring unit which is an analog functional block.
3) Summing unit which sums all values.
4) Reference unit which measures the absolute temperature of the reference
junction and provides an output voltage linearily proportional to the
absolute temperature of the reference junction.
The preamplifier, A i in Figure 1, is an instrumentation amplifier with a gain
of 1,000. The instrumentation amplifier differs fundamentally from an operational
amplifier. It is designed to be used as a closed-loop differential amplifier. The
necessary feedback networks are contained within the amplifier. Ideally, the instru-
mentation amplifier responds only to the difference between the two input signals
and exhibits an extremely high input impedance between the two terminals and each
terminal to the ground. The output voltage is developed single-ended with respect to
ground and is equal to the product of the amplifier gain and the difference of the two
input voltages:
E0 = K (E2-Ei) (2)
where EO K output voltage
E i = input voltage 1
Ea = input voltage 2
K = gain of the amplifier
with the instrumentation amplifier used, K = 100Kfi/Ro.
Thus, V™, = 1,000(E ,-E ).
TP1 v ref sens'
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where VTpl = voltage at Test Point 1
E - = thermoelectric voltage of reference junction
E = thermoelectric voltage at measuring junction
The multiplier's output is given by Equation 3
E0= (Xi -X2) (Yi -Y2)/10 (3)
with the input as described in Figure 2. As applied, the output observed at
TP2 = -(VTpl)2/l(h
The reference junction temperature is measured using a semiconductor. The
thermal effect on the foreword current of a P-N junction is expressed by Equation
4 as presented by Shea (3) .
I = Io {expECg/mKT) (V-IRJ]-!} (4)
5
where I = foreword current
I = reverse saturation current of the diode
o
g = electron charge = 4.8 x 10~ 10
K = Boltzmann's constant = 1.37 x 10~16
m = constant representing the effects of diffusion current flow
R = equivalent series internal resistance of diode
s
T = absolute temperature of diode junction
V = foreword voltage
Widlar (4) showed further, that by holding the current constant, the expression for
the foreword voltage drop across a diode could be written as
Vf = [1-CT/T)] VgQ = (V£oT/T0) = (KT/g)/n (TQ/T)
+ (n-1) (l+T/To) KT/g (5)
where V, = foreword voltage drop across the diode
V, = foreword voltage drop at T
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V = the extrapolated energy gap for the semiconductor material
° at absolute zero
n = constant
T = absolute temperature
T = absolute temperature at which V, is measured
Enfield and Tromble (5) measured the thermal characteristics of several IN4154
diodes and determined that the mean foreword voltage drop at 0°C with 1 ma foreword
current was 618 mV with a standard deviation of 15 mV. The mean thermal sensitivity
was determined to be 1.9 mV/°C with a standard deviation of 0.12 mV/°C. Based
on these experimental measurements a diode thermometer was designed which measures
the temperature of the reference junction and yields an output voltage at TPa of -100
mV/°C with OmVat 0°C.
Resistor Re functions with reference zener diode DZ1 to generate a reference
voltage for the circuit.
R? and Re form a voltage divider which maintains the base of transistor Qa
at a reference voltage (VR) .
Transistor Qi is in thermal contact with transistor Qa. Thus, the thermal
error caused by the change in the foreword voltage drop of Qz, in current flowing
through Rg, is compensated by an equal change in VR. This is optimized in the
circuit by causing the same current to flow through Qi and Qa. Thus, the current
which flows through diode (Di), the thermal detector, equals (VR-V, of
be
Q2)/Rs or approximately 1 ma. Amplifier AS acts as a single-ended summing scaling
buffer. Analyzing the circuit in its two parts, first analyze Figure 3, which is
one configuration for a scaling buffer.
The gain of the circuit converts the thermal sensitivity of the diode, approxi-
mately 1.9 mV/°C, to -100 mV/°C or a gain of approximately 53. With the components
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in Figure 1 the gain is adjustable from 40.2 to 54.1.
The output of the reference thermometer is zeroed at 0°C by summing a current
from the zener reference voltage into the inverting input of AS. The inverting in-
put acts as a virtual ground at V ,. , @ 0°C . Thus, to zero the output
= 0
Assuming V ,. , = -618 mV and V = 6 V, then
diode zener
618/Rn + (6000 - 618)/RT = 0 or R^ = 8.7 RM
With the values in the circuit, the range of R™ is 7.5 to 9-7 times RI i which
allows for sufficient variation in diodes.
The final stage of the instrument is merely a summing amplifier that combines
all the components and yields an output directly proportional to temperature with
zero at 0°C. To obtain an output sensitivity of 100 mV/°C, the following resistance
ratios must be maintained.
R5/Ri =1.703
Rs/Ra =0.02033
R5/Ris=l
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SECTION 4
CALIBRATION
There are three adjustments to be made prior to use of the instrument. The
following calibration procedure should be satisfactory:
1) Place the reference diode (D i) in an ice bath (0°C) . The diode should be
encapsulated to waterproof the junction.
2) Adjust RT2 for 0.000 VDC ± 2 mVDC at TP 3.
3) Monitor TP 3 for sufficient time to guarantee thermal equilibrium between
the ice bath and the reference diode.
4) Place the reference diode (D i) in a water bath or suitable reference
temperature device at approximately 100°C. Adjust RT 3 for an output sensitivity
observed at TPs of -100 mV/°C. This adjustment should be made as accurate as the
temperature is known.
5) Repeat steps 1 through 4 until no further adjustment is required. One
calibration cycle is usually sufficient.
6) With the thermocouple reference junction at the same temperature as the
thermocouple sensing junction, adjust RTi for 0.000 VDC ± 1 mVDC at TPi.
After the above calibration procedure has been followed, the instrument can be
placed in service.
8
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SECTION 5
CONSTRUCTION CONSIDERATIONS
Since the instrument described is for measuring temperatures, errors caused
by thermal drift of the components can be significant. Analysis of thermal drift
errors is beyond the scope of this report. For a sensitivity analysis, the reader
is referred to Tobey et al. (6) and Enfield and Hsieh (7) for a discussion of
the problems. In the circuit described, precision resistors with thermal coefficient
of 50 ppm/ C were used throughout the construction. Transistors Qi and Qz
were in thermal contact with each other as indicated by the dashed line around
the two transistors. DI was bonded to the thermocouple reference junction indicated
by the dashed square boxes in Figure 1, so as to maintain the references and
D i at the same temperature.
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TABLE 1. COMPARISON BETWEEN ACTUAL TEMPERATURE
AND PREDICTED NETWORK TEMPERATURE
ACTUAL
-25
-20
-15
-10
- 5
0
5
10
15
20
25
30
35
40
45
50
THEORETICAL
NETWORK RESPONSE
-24.6
-19.8
-15.0
-10.0
- 5.2
- .2
4.8
9.9
15.0
20.1
25.3
30.5
35.7
41
46.3
51.7
ERROR
0.4
0.2
<0.1
<0.1
0.2
0.2
0.2
0.1
<0.1
0.1
0.3
0.5
0.7
1.0
1.3
1.7
Temperature = °C
10
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TABLE 2. PARTS LIST
RESISTORS*
(In ohms unless specified)
Ro
Ri
R2
R3
Rf
Rs
Re
R?
Rs
R9
Rio
Rii
Rl2
Rl3
Ris
RIG
VARIABLE RESISTORS (20 Turn Trimmer)
RTi
RT2
RT3
CAPACITORS
SEMICONDUCTORS
DZi
Qi, Q2
INTEGRATED CIRCUITS**
A2, A3
MULTIPLIER
100
See manuscript (5.87)
See manuscript (492 K)
5 OK
5 OK
10K
820
3. OIK
3. OIK
3. OIK
2.74K
IK
7.5K
40.2K
16.2K
10K
47K 5%
100K
10K
100K
0-01 yf
IN4154
IN821
2N3904
3660K
3501A
4205J
5 All resistors 1% unless specified.
** Burr-Brown part numbers are listed for information only.
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I Ayft/A—__, | W* ,
-I5VDC
TP,
FIGURE I - SCHEMATIC DIAGRAM OF TEMPERATURE COMPENSATION NETWORK
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X, c,
X2 o-
Y, o-
Y2 o.
10
FIGURE 2- PIN CONNECTION FOR BURR-BROWN
MODEL 4205 MULTIPLIER
Rl
j VWV
E{ diode)
(E diode)
FIGURE 3 - SCALING BUFFER AMPLIFIER
13
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REFERENCES
1. Dike, Paul H. 1954. Thermoelectric Thermometry. Philadelphia, Pa., Leeds
and Northup Co. , 90 p.
2. Temperature Measurement Handbook. 1976. Stamford, Cn., Omega Eng.
Inc., 174 p.
3. Shea, Richard F. 1966. Amplifier Handbook, N. Y., McGraw-Hill, 1350 p.
4. Widlor, R. J. 1967. An Exact Expression for the Thermal Variation of the
Emitter Base Voltage of Bi-Polar Transistors. Proc. of Institute of Electrical
and Electronic Engineers.
5. Enfield, C. G. and John M. Tramble. 1970. P-N Junctions - A Tool for Tem-
perature Measurement. Waste Resc. Res. 6(3): 981-985.
6. Tobey, Gene E. et al. 1971. Operational Amplifiers Design and Applications.
N. Y. , McGraw-Hill, 473 p.
7. Enfield, C. G. and J.J.C. Hsieh. 1972. P-N Junctions Interchangeable Tem-
perature Transducers. SoilSci. 113:59-62.
14
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-220
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Thermocouple Readout Instrument
5. REPORT DATE
November 1977 issuing date
6. PERFORMING ORGANIZATION CODE
. AUTHOR(S)
Carl G. Enfield
Curtis V. Gillaspy
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
SAME AS BELOW
10. PROGRAM ELEMENT NO.
1BC611
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Robert S. Kerr Environmental Research Lab?- Ada,0
Office of Research and Development
U. S. Environmental Protection Agency
Ada, Oklahoma 74820
13. TYPE OF REPORT AND PERIOD COVERED
In-House
14. SPONSORING AGENCY CODE
EPA/600/15
15. SUPPLEMENTARY NOTES
16. ABSTRACT
An electronic circuit has been developed which acts as an electronic ice bath
for chromel-constantan thermocouples. The electronic ice bath is accurate to within
± 0.2°C from -25°C to +50°C. Simultaneously, the thermocouple output is scaled
and linearized such that the temperature can be read directly in °C with a sensi-
tivity of 100 mV/°C with 0 VDC at 0°C. Circuit diagrams and construction consid-
erations are included to allow the reader to construct a functioning unit.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. cos AT I Field/Group
Thermocouples
Temperature measuring instruments
09C
B. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)'
UNCLASSIFIED
21. NO. OF PAGES
21
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
15
* U.S. GOVERNMENT PRINTING OFFICE 1977- 757-140/6598
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