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14:54:1 Page 345
KNOWN Loading error should be less than 8:9 Â 10
À8 A.
FIND Input impedance for a voltmeter that would produce the same current flow or loading
error.
SOLUTION At 100
C a J-type thermocouple referenced to 0
C has a Seebeck voltage of E s ¼
5.269 mV. At this temperature, the required voltmeter impedance to limit the current flow to 8:9 Â
10
À8 A is found from Ohm’s law:
E s
I
¼ 5:269 Â 10
À3 V=8:9 Â 10
À8 A ¼ 59:2 kV
COMMENT This input impedance is not at all high for a modern microvoltmeter and such a
voltmeter would be a reasonable choice in this situation. As always, the allowable loading error
should be determined based on the required uncertainty in the measured temperature.
Multiple-Junction Thermocouple Circuits
A thermocouple circuit composed of two junctions of dissimilar metals produces an open-circuit
emf that is related to the temperature difference between the two junctions. More than two junctions
can be employed in a thermocouple circuit, and thermocouple circuits can be devised to measure
temperature differences, or average temperature, or to amplify the output voltage of a thermocouple
circuit.
Thermopiles
Thermopile is the term used to describe a multiple-junction thermocouple circuit that is designed to
amplify the output of the circuit. Because thermocouple voltage outputs are typically in the millivolt
range, increasing the voltage output may be a key element in reducing the uncertainty in the
temperature measurement, or may be necessary to allow transmission of the thermocouple signal to
the recording device. Figure 8.22 shows a thermopile for providing an amplified output signal; in
this case the output voltage would be N times the single thermocouple output, where N is the number
of measuring junctions in the circuit. The average output voltage corresponds to the average
Cu
Cu
Reference
junctions
Measuring
junction
1
2
Potentiometer
Copper connecting
wires
Thermocouple
wires
A
B
A
B
A
B
N
Figure 8.22 Thermopile arrangement.
(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.)
8.5 Thermoelectric Temperature Measurement 345
14:54:1 Page 345
KNOWN Loading error should be less than 8:9 Â 10
À8 A.
FIND Input impedance for a voltmeter that would produce the same current flow or loading
error.
SOLUTION At 100
C a J-type thermocouple referenced to 0
C has a Seebeck voltage of E s ¼
5.269 mV. At this temperature, the required voltmeter impedance to limit the current flow to 8:9 Â
10
À8 A is found from Ohm’s law:
E s
I
¼ 5:269 Â 10
À3 V=8:9 Â 10
À8 A ¼ 59:2 kV
COMMENT This input impedance is not at all high for a modern microvoltmeter and such a
voltmeter would be a reasonable choice in this situation. As always, the allowable loading error
should be determined based on the required uncertainty in the measured temperature.
Multiple-Junction Thermocouple Circuits
A thermocouple circuit composed of two junctions of dissimilar metals produces an open-circuit
emf that is related to the temperature difference between the two junctions. More than two junctions
can be employed in a thermocouple circuit, and thermocouple circuits can be devised to measure
temperature differences, or average temperature, or to amplify the output voltage of a thermocouple
circuit.
Thermopiles
Thermopile is the term used to describe a multiple-junction thermocouple circuit that is designed to
amplify the output of the circuit. Because thermocouple voltage outputs are typically in the millivolt
range, increasing the voltage output may be a key element in reducing the uncertainty in the
temperature measurement, or may be necessary to allow transmission of the thermocouple signal to
the recording device. Figure 8.22 shows a thermopile for providing an amplified output signal; in
this case the output voltage would be N times the single thermocouple output, where N is the number
of measuring junctions in the circuit. The average output voltage corresponds to the average
Cu
Cu
Reference
junctions
Measuring
junction
1
2
Potentiometer
Copper connecting
wires
Thermocouple
wires
A
B
A
B
A
B
N
Figure 8.22 Thermopile arrangement.
(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.)
8.5 Thermoelectric Temperature Measurement 345