| The figures show the temperature-emf curve (not
to scale) and a temperature measuring setup with a thermocouple and
a millivolt meter. Assume the cold and the hot junctions are at T1°C
and T2°C, respectively. According to the temperature-emf table
of the standard, the thermocouple generates emf of E1 mV at the temperature
T1 and E2 mV at T2. The millivolt meter receives the potential difference,
E2 - E1 which corresponds to T2 - T1. In order to obtain T2, we need
to add E1 to the potential difference, E2 - E1 for elimination of
E1. |
 |
An actual example may better clarify the above discussion.
Assume that we are using a Type E thermocouple to measure T2, which
is 550°C (1022°F) and the millivolt meter (more accurately
speaking, the terminals of the millivolt meter) is at room temperature
T1, which is 25°C (77°F). According to the temperature-emf
table of Type E, the thermocouple generates (with reference to °C):
41. 045 mV at 550°C
1.495 mV at 25°C
The potential difference is 39.550 mV.
The millivolt meter displays to 39.550 mV, which corresponds to 531.5°C,
not 550°C. We must make cold junction compensation by adding 1.495
mV to the potential difference, 39.550 mV. |
| M-System has flexible solutions to meet your specific
application and requirements. Consult our Signal Conditioners Data
Library. |
|
What
is cold junction compensation?
Temperature
should be measured with the cold junction at 0°C or 32°F. When a
thermocouple or its extension wires are connected to the terminals of a
device like a thermocouple transmitter the cold junction is at the room
temperature T1°C. If both temperatures of the hot and the cold junctions
are above 0°C, the device receives a lower emf than when the cold junction
temperature is 0°C. In order to measure the temperature accurately,
we need to add the emf value which corresponds to T1 to the measured emf.
To add this emf is called cold junction compensation. 