Instruments: [Question of the Week] Week 28, 2011: Why is cold-junction compensation necessary when using thermocouples for temperature measurement?

The potential of a thermocouple is defined when the cold junction is at zero degrees. . However, the cold junction of the field thermocouple does not reach zero degrees. .

Answer: ① The magnitude of the thermocouple’s electromotive force is related to the temperatures at its two ends, and its temperature-electromotive force relationship curve is calibrated when the temperature at the cold end is 0°C. ②In practical applications, since the cold end of the thermocouple is exposed to the surrounding environment, it is affected by the ambient temperature; as a result, the temperature of the cold end cannot be kept at 0°C or fixed at a constant value. The thermocouple potential depends both on the temperature of the hot end and that of the cold end. Therefore, if the cold-end temperature varies freely, it will inevitably cause measurement errors. To eliminate this error, cold-end temperature compensation must be performed.

This post was last edited by denghl on 2012-12-21 at 21:21. Reply 1# Baghdad: Since the calibration of thermocouples is defined at zero degrees, cold-junction temperature compensation is required.

This post was last edited by denghl on 2012-12-21 at 21:26. The magnitude of the thermocouple’s electromotive force is related to the temperatures at its two ends, and the temperature-electromotive force relationship curve is calibrated with the cold junction temperature set at 0°C. In practical applications, since the cold end of the thermocouple is exposed to the surrounding environment, it is affected by the ambient temperature; as a result, the temperature of the cold end cannot be kept at 0°C or fixed at a constant value. The thermocouple potential depends both on the temperature of the hot end and that of the cold end. Therefore, if the cold-end temperature varies freely, it will inevitably cause measurement errors. To eliminate this error, cold-end temperature compensation must be performed

Since the cold end of the thermocouple is exposed to the space and affected by the surrounding ambient temperature, it is impossible to keep the temperature at the cold end constant at 0°C or fixed at any specific temperature during temperature measurement. Moreover, the thermocouple potential depends both on the temperature at the hot end and on the temperature at the cold end. Therefore, if the cold-end temperature varies freely, it will inevitably cause measurement errors. To eliminate this error

This post was last edited and replied to by denghl on 2012-12-21 at 21:29. Reply 1# Baghdad: Since the thermoelectric potential of a thermocouple is related to the temperatures at its two ends, its temperature-thermoelectric potential curve is calibrated with the cold-end temperature set at 0°C. In practical applications, since the cold end of the thermocouple is exposed to the environment, it is affected by the surrounding temperature; therefore, the temperature of the cold end cannot be kept at 0°C or fixed at a constant value. The thermoelectromotive force depends both on the temperature of the hot end and that of the cold end. Therefore, if the cold-end temperature varies freely, it will inevitably cause measurement errors; to eliminate such errors, cold-end temperature compensation is necessary.

This post was last edited by denghl on 2012-12-21 at 21:31. ① The magnitude of the thermoelectric potential of a thermocouple is related to the temperatures at its two ends, and the temperature-thermoelectric potential curve is calibrated when the temperature at the cold end is 0°C. ②In practical applications, since the cold end of the thermocouple is exposed to the surrounding environment, it is affected by the ambient temperature; as a result, the temperature of the cold end cannot be kept at 0°C or fixed at a constant value. The thermocouple potential depends both on the temperature of the hot end and that of the cold end. Therefore, if the cold-end temperature varies freely, it will inevitably cause measurement errors. To eliminate this error, cold-end temperature compensation must be performed.

This post was last edited by denghl on 2012-12-21 at 21:33. 1: The magnitude of the thermoelectric potential of a thermocouple is related to the temperatures at its two ends; the temperature-thermoelectric potential curve is calibrated when the temperature at the cold end is 0°C. II: In practical applications, since the cold end of the thermocouple is exposed to the surrounding environment, it is affected by the ambient temperature; therefore, the temperature of the cold end cannot be kept at 0°C or fixed at a constant value. The thermocouple potential depends both on the temperature of the hot end and that of the cold end. Therefore, if the cold-end temperature varies freely, it will inevitably cause measurement errors. To eliminate this error, cold-end temperature compensation must be performed

Thermocouple temperature measurement involves detecting the potential difference, that is, the difference in potential between the hot end and the cold end, on the assumption that the cold end remains at a constant temperature of 0 degrees Celsius. In reality, however, it is not possible to keep the cold end at 0 degrees Celsius or at a constant temperature, so compensation is necessary.

This post was last edited by denghl on 2012-12-21 at 21:35. Thermocouple temperature measurement is based on the Seebeck effect, also known as the thermoelectric effect. It involves connecting two different conductors or semiconductors in a closed circuit; when the temperatures at the two terminals (the hot terminal and the cold terminal) are different, a thermoelectrical potential is generated within the circuit. The temperature sensing end is generally referred to as the hot end, while the instrument’s wiring end is called the cold end (reference end). Since the thermoelectric potential calibration table for thermocouples is based on values measured at a cold end temperature of T=0°C. In actual use, the cold end of the thermocouple is located in the working environment and cannot be at 0°C; therefore, this temperature difference must be compensated for. Generally, the following compensation methods are available: (1) Cold-end temperature measurement calculation method. A compensation wire is used to move the cold end of the thermocouple to a location T0 where the ambient temperature is stable; after passing through the temperature at T0, a correction is made using a mathematical formula: E(T,0) = E(T,T0) + E(T0,0). The value of T0 can be determined using a calibration table. (3) Freezing point method: The cold end of the thermocouple is placed in an ice-water mixture; it is generally used in laboratories. (4) Compensation bridge method: This involves using an unbalanced bridge to generate a corresponding thermoelectric potential, in order to compensate for the changes in the thermoelectric potential of the thermocouple caused by variations in the temperature at its cold end. In a typical compensation bridge, the resistors of the three bridge arms are made of manganin, whose resistance value does not change with temperature. The resistor of the other bridge arm is made of copper wire, and its resistance value changes with temperature (suitable for temperatures ranging from -20 to 100°C); as a result, an unbalanced voltage is generated at the output of the bridge.