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Thermocouples > 07 - Correlation of Electromotive Force (EMF)

Since the emf generated in a thermocouple depends on the chemical composition of the conductors and the temperature difference between the joints, that is, with each degree of temperature variation, we can observe a variation in the emf generated by the thermocouple, we can therefore construct a correlation table between temperature and the emf. For practical reasons, the drawing of these curves was standardized with the reference joint at a temperature of 0°C.

These tables were standardized by several international standards and drawn up in accordance with the International Thermal Temperature Scale of 1968 (IPTS 68), for the most commonly used thermocouples.

From these tables, we can construct a graph, where the millivoltages generated as a function of temperature are related, for thermocouples according to the ISA standard, with the reference joint at 0°C.

The relationship between the emf and the reference joint "E" versus the temperature "T" is usually approximated by an equation of the type:

E= A + BT + CT² +…

Where A, B, C…, are constants to be determined experimentally.

If the reference joint is at 0°C, then A = 0.

We can then see that this expression is not linear.

Thermocouple E.M.F. variation curve

Table 1: Usage limit of conventional thermocouples

TYPE	8 AWG	14 AWG	20 AWG	24 AWG
T	-	370 °C	260 °C	200 °C
J	760 °C	590 °C	480 °C	370 °C
E	870 °C	650 °C	540 °C	430 °C
K/N	1270 °C	1090 °C	980 °C	870 °C
R/S	-	-	-	1480 °C
B	-	-	-	1700 °C

Table 2: Usage limit of mineral thermocouples according to rod diameter

Ø	T	J	E	K	N
1,5	260 °C	440 °C	510 °C	920 °C	920 °C
3	315 °C	520 °C	620 °C	1070 °C	1100 °C
6	370 °C	720 °C	820 °C	1150 °C	1250 °C

Table 3: Usage limit of mineral thermocouples according to the rod material

Material	Recommended maximum temperature
Stainless steel 304	900 °C
Stainless steel 316	900 °C
Stainless steel 310	1100 °C
Inconel 600	1150 °C
Nicrobel	1250 °C

Table 4: Error limit according to ASTM E-230 standard

Thermocouple Type	Temperature Range	Standard (choose the largest)	Special (choose the largest)
T	0 a 350ºC	±1ºC ou ±0,75%	±0,5ºC ou 0,4%
J	0 a 750ºC	±2,2ºC ou ±0,75%	±1,1ºC ou ±0,4%
E	0 a 900ºC	±1,7ºC ou ±0,5%	±1ºC ou ±0,4%
K e N	0 a 1250ºC	±2,2ºC ou ±0,75%	±1,1ºC ou ±0,4%
S e R	0 a 1450ºC	±1,5ºC ou ±0,25%	±0,6ºC ou ±0,1%
B	800 a 1700ºC	±0,5%	-
T	-200 a 0ºC	±1ºC ou ±1,5%	-
E	-200 a 0ºC	±1,7ºC ou ±1%	-
K	-200 a 0ºC	±2,2ºC ou ±2%	-

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