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ASTM Selected Technical Papers
Manual on the Use of Thermocouples in Temperature Measurement
By
Committee E-20
Committee E-20
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Subcommittee IV
Subcommittee IV
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ISBN-10:
0-8031-6652-4
ISBN:
978-0-8031-6652-3
No. of Pages:
260
Publisher:
ASTM International
Publication date:
1970

A thermocouple circuit is by its nature a differential measuring device, producing an emf which is a function of the temperatures of its two junctions. One of these junctions is at the temperature which is to be measured and is referred to as the measuring junction. The other junction is maintained at a known temperature and is referred to as the reference junction. (In a practical thermocouple circuit (see Section 2.3) copper wires are often connected to the thermocouple alloy conductors at the reference junction. The term reference junctions will be used to refer to this situation.) If these junctions are both at the same temperature, the presence of the copper “intermediate metal” introduces no change in the thermocouple's emf. If they are not at the same temperature the analysis of the circuit is complicated. Moffat [34] gives a helpful analytical method. The Seebeck coefficient (thermoelectric power) of many common thermocouples is approximately constant from the ice point to the upper temperature limit of the materials (see Section 3.1.1). For such thermocouples, an uncertainty in the temperature of the reference junction will reflect a similar uncertainty in the deduced temperature of the measuring junction. However this situation does not exist for all thermocouple pairs. Notable exceptions occur in the case of the high rhodium-in-platinum alloy thermocouples [35, 36]. In particular, if the reference junction of a platinum-30 percent rhodium versus platinum-6 percent rhodium (Type B) thermocouple lies within the range 0 to 50 C (32 to 122 F), a 0 C (32 F) reference junction may be assumed, and the error will not exceed 3 mV. This represents about 0.3 C (0.5 F) error in high-temperature measurements. [37]

1.
Moffat
,
R. J.
, “
The Gradient Approach to Thermocouple Circuitry
,”
Temperature, Its Measurement and Control in Science and Industry
 0091-9322, Vol.
3
, Part 2, Reinhold, New York,
1962
, pp. 33–38.
2.
Caldwell
,
F. R.
, “
Thermocouple Materials
,”
Temperature, Its Measurement and Control in Science and Industry
 0091-9322, Vol.
3
, Part 2, Reinhold, New York,
1962
, pp. 81–134.
3.
Zysk
,
E. D.
, “
Noble Metals in Thermometry, Recent Developments
,”
Technical Bulletin, Englehard Industries, Inc.
, Vol.
5
, No.
3
,
1964
.
4.
Burns
,
G. W.
and
Gallagher
,
J. S.
Reference Tables for the Pt-30 Percent Rh Versus Pt-6 Percent Rh Thermocouple
,”
Journal of Research, NBS-C Engineering and Instrumentation
, Vol.
70C
, No.
2
,
1966
.
5.
Stimson
,
H. F.
, “
Precision Resistance Thermometry and Fixed Points
,”
Temperature, Its Measurement and Control in Science and Industry
 0091-9322, Vol.
2
, Reinhold, N. Y.,
1955
, pp. 141–168.
6.
Williams
,
S. B.
, “
Triple Point of Water Temperature Reference
,”
Instruments and Control Systems
 0020-4404, Vol.
33
,
1960
.
7.
Thomas
,
J. L.
, “
Reproducibility of the Ice Point
,”
Temperature, Its Measurement and Control in Science and Industry
 0091-9322, Vol.
1
, Reinhold, New York,
1941
, pp. 159–161.
8.
Recommended Ice Bath for Reference Junctions
,” SAE Aerospace Recommended Practice No. 691, Committee AE-2, Temperature Measurement Sensing,
Society of Automotive Engineers, Inc.
,
1964
.
9.
Caldwell
,
F. R.
, “
Temperatures of Thermocouple Reference Junctions in an Ice Bath
,”
Journal of Research, NBS Engineering and Instrumentation
, Vol.
69C
, No.
2
,
1965
.
10.
Finch
,
D. I.
, “
General Principles of Thermoelectric Thermometry
,”
Temperature, Its Measurement and Control in Science and Industry
 0091-9322, Vol.
3
, Part 2, Reinhold, New York,
1962
, pp. 3–32.
11.
McElroy
,
D. I.
, “
Progress Report I. Thermocouple Research Report for the Period, November 1, 1956 to October 31, 1957
,” ORNL-2467.
12.
Morgan
,
W. A.
, “
Close Temperature Control of Small Volumes, A New Approach
,” Preprint Number 11.7-2-64,
Instrument Society of America
,
1964
.
13.
Feldman
,
C. L.
, “
Automatic Ice-Point Thermocouple Reference Junction
,”
Instruments and Control Systems
 0020-4404,
01
1965
, pp. 101–103.
14.
Muth
,
S.
, Jr.
, “
Reference Junctions
,”
Instruments and Control Systems
 0020-4404,
05
1967
, pp. 133–134.
15.
Roeser
,
W, F.
, “
Thermoelectric Thermometry
,”
Temperature, Its Measurement and Control in Science and Industry
 0091-9322, Vol.
1
, Reinhold, New York,
1941
, pp. 202.
16.
Claggett
,
T. J.
, “
External Thermocouple Compensation
,”
Instrumentation
 0020-4366, Vol.
18
, No.
2
, Second Quarter,
1965
.
17.
Baker
,
H. D.
,
Ryder
,
M. E.
, and
Baker
,
M. A.
,
Temperature Measurement in Engineering
, Vol.
1
, Section 7,
Wiley
,
1953
.
18.
Bauerle
,
J. E.
, “
Analysis of Immersed Thermocouple Error
,”
Review of Scientific Instruments
 0034-6748, Vol.
32
, No.
3
,
03
1961
, pp. 313.
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