Process Engineering Division – Instrumentation and Automation Section – Daily Topics – Topic No. 2021-02-02
Thread Content
Process Engineering Division – Instrumentation and Automation Section – Daily Topics – Topic No. 2021-02-02: Discussion question: 376. What is the principle of cold-junction compensation when using thermocouples for temperature measurement?
The principle of cold-junction compensation for thermocouples: When a thermocouple is used to measure temperature, it is necessary to keep the temperature of its cold junction constant (the end that performs the measurement is the hot junction, while the end connected to the measurement circuit through wires is called the cold junction). Only in this way can the electromotive force generated by the thermocouple be in a proportional relationship with the temperature being measured.
Cold-end compensation is used to stabilize the cold-end temperature, so that accurate measurements can be taken.
The principle of cold-junction compensation for thermocouples: When a thermocouple is used to measure temperature, it is necessary to keep the temperature of its cold junction constant (the end that is used for measurement is called the hot junction, while the end connected to the measurement circuit through wires is known as the cold junction). Only in this way can the electromotive force generated by the thermocouple be in a proportional relationship with the temperature being measured
The thermoelectric potential generated by a thermocouple is related to the temperature of its cold end. Generally, the temperature of the cold end of a thermocouple is not constant; it changes with room temperature. As a result, the thermoelectric potential also changes with room temperature, leading to measurement errors. Therefore, it is necessary to compensate for the temperature of the thermocouple’s cold end in order to reduce the measurement errors caused by variations in that temperature.
The thermoelectromotive force Et generated by a thermocouple is related to the temperature of its cold end. In general, the temperature of the cold end of a thermocouple is not constant; it changes with room temperature. As a result, Et also changes with room temperature, leading to measurement errors. Therefore, it is necessary to compensate for the temperature of the cold end in order to reduce the signal measurement errors caused by variations in that temperature. In thermocouple temperature transmitters, a cold-end temperature compensation circuit is designed; depending on the type of thermocouple, calculations are typically based on two criteria: a cold-end compensation potential of 25 mV at 0°C, and a change in ΔEt of equal to the change in the cold-end compensation potential when the temperature changes by Δt = 50°C.
The thermoelectric potential generated by a thermocouple is related to the temperature of its cold end. Generally, the temperature of the cold end of a thermocouple is not constant; it changes with room temperature. As a result, the thermoelectric potential also changes with room temperature, leading to measurement errors. Therefore, it is necessary to compensate for the temperature of the thermocouple’s cold end in order to reduce the measurement errors caused by variations in that temperature.
The thermoelectric potential of a thermocouple is a function of the temperature difference between its hot and cold ends; the temperature referred to usually is based on a zero temperature point. When the cold-end temperature is not zero, the thermoelectric potential of the thermocouple decreases. There are two methods for compensating the cold junction temperature of thermocouples. One method is to force the cold junction temperature to zero, for example by using an ice bucket – that is, placing the cold junction of the thermocouple inside an ice bucket filled with ice water. This method is usually used only in laboratories. Another approach is to use the properties of certain temperature-sensitive elements that change with temperature for compensating the cold junction temperature of thermocouples; thermistors are commonly used for this purpose. Generally, a thermistor is used to form a bridge, and the potential generated by the imbalance in the circuit due to temperature changes is utilized to compensate for the thermoelectric potential loss caused by the non-zero temperature at the cold end of the thermocouple.
The thermoelectric potential generated by a thermocouple is related to the temperature of its cold end. Generally, the temperature of the cold end of a thermocouple is not constant; it changes with room temperature. As a result, the thermoelectric potential also changes with room temperature, leading to measurement errors. Therefore, it is necessary to compensate for the temperature of the thermocouple’s cold end in order to reduce the measurement errors caused by variations in that temperature.
The principle of cold-junction compensation for thermocouples: When a thermocouple is used to measure temperature, it is necessary to keep the temperature of its cold junction constant (the end that is used for measurement is called the hot junction, while the end connected to the measurement circuit through wires is known as the cold junction). Only in this way can the electromotive force generated by the thermocouple be in a proportional relationship with the temperature being measured
The thermoelectric potential generated by a thermocouple is related to the temperature of its cold end. Generally, the temperature of the cold end of a thermocouple is not constant; it changes with room temperature. As a result, the thermoelectric potential also changes with room temperature, leading to measurement errors. Therefore, it is necessary to compensate for the temperature of the thermocouple’s cold end in order to reduce the measurement errors caused by variations in that temperature.