E1C08 09/14/2010
14:53:58 Page 338
Thermocouple Voltage Measurement
The Seebeck voltage for a thermocouple circuit is measured with no current flow in the circuit. From
our discussion of the Thomson and Peltier effects, it is clear that the emf is different from the opencircuit value when there is a current flow in the thermocouple circuit. As such, the best method for
the measurement of thermocouple voltages is a device that minimizes current flow. For many years,
the potentiometer was the laboratory standard for voltage measurement in thermocouple circuits. A
potentiometer, as described in Chapter 6, has nearly zero loading error at a balanced condition.
However, modern voltage-measuring devices, such as digital voltmeters or data-acquisition cards,
have sufficiently large values of input impedance that they can be used with minimal loading error.
These devices can also be used in either static or dynamic measuring situations where the loading
error created by the measurement device is acceptable for the particular application. For such needs,
0
500
1000
1500
2000
2500
3000
Temperature [ F]
emf output [mV]
0
10
20
30
40
50
60
70
Platinum
10% rhodium
versus platinum
Platinum 13% rhodium
versus platinum
Tungsten/5%
rhenium versus
tungsten/26%
rhenium
Copper
versus
constantan
Chromel
versus
constantan
Iron versus
constantan
Chromel
versus alumel
Tungsten
versus tungsten/
26% rhenium
Figure 8.20 Thermocouple voltage output as a function of temperature for some common thermocouple
materials. Reference junction is at 32
F (0
C). (From Benedict, R. P., Fundamentals of Temperature, Pressure
and Flow Measurements, 3rd ed. Copyright # 1984 by John Wiley and Sons, New York. Reprinted by
permission.)
338 Chapter 8 Temperature Measurements
14:53:58 Page 338
Thermocouple Voltage Measurement
The Seebeck voltage for a thermocouple circuit is measured with no current flow in the circuit. From
our discussion of the Thomson and Peltier effects, it is clear that the emf is different from the opencircuit value when there is a current flow in the thermocouple circuit. As such, the best method for
the measurement of thermocouple voltages is a device that minimizes current flow. For many years,
the potentiometer was the laboratory standard for voltage measurement in thermocouple circuits. A
potentiometer, as described in Chapter 6, has nearly zero loading error at a balanced condition.
However, modern voltage-measuring devices, such as digital voltmeters or data-acquisition cards,
have sufficiently large values of input impedance that they can be used with minimal loading error.
These devices can also be used in either static or dynamic measuring situations where the loading
error created by the measurement device is acceptable for the particular application. For such needs,
0
500
1000
1500
2000
2500
3000
Temperature [ F]
emf output [mV]
0
10
20
30
40
50
60
70
Platinum
10% rhodium
versus platinum
Platinum 13% rhodium
versus platinum
Tungsten/5%
rhenium versus
tungsten/26%
rhenium
Copper
versus
constantan
Chromel
versus
constantan
Iron versus
constantan
Chromel
versus alumel
Tungsten
versus tungsten/
26% rhenium
Figure 8.20 Thermocouple voltage output as a function of temperature for some common thermocouple
materials. Reference junction is at 32
F (0
C). (From Benedict, R. P., Fundamentals of Temperature, Pressure
and Flow Measurements, 3rd ed. Copyright # 1984 by John Wiley and Sons, New York. Reprinted by
permission.)
338 Chapter 8 Temperature Measurements