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emf (10). An example of such a table is provided in Table 8.6 for an iron/constantan thermocouple,
usually referred to as a J-type thermocouple. Table 8.7 provides the corresponding polynomial
equations that relate emf and temperature for standard thermocouples. Because of the widespread
need to measure temperature, an industry has grown up to supply high-grade thermocouple wire.
Manufacturers can also provide thermocouples having special tolerance limits relative to the NIST
standard voltages with uncertainties in temperature ranging from Æ1.0
C to perhaps Æ0.1
C.
Thermocouples constructed of standard thermocouple wire do not require calibration to provide
measurement of temperature within the tolerance limits given in Table 8.5.
1600
1400
1200
1000
800
600
Temperature [°C]
Electromotive force [mV]
Reference junction, 0°C (32°F)
Temperature [°F]
400
200
0
–40
–30
–20
–10
0
+10
+20
+30
+40
–300
0
500
1000
1500
2000
2500
–200
KP, EP
RP
SP
JP
KN
JN, TN, EN
JN, TN, EN
JP
TP
TP
Platinum (RN or SN)
Figure 8.19 Thermal emf of thermocouple materials relative to platinum-67. Note: For example, JP indicates
the positive leg of a J thermocouple, or iron. JN indicates the negative leg of a J thermocouple, or constantan. All
other notations are similar for each type of thermocouple. (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 337