Thermocouple Thermometer - Temperature Measurement Based on Thermoelectric Effect
A pair of conductors of dissimilar materials that generate electromotive force in a circuit based on the Seebeck effect. A sensing element formed by joining one end of two conductors of different materials to realize temperature measurement by means of the thermoelectric effect.
Overview
A thermocouple is a temperature-sensing element and an instrument. It measures temperature directly and converts temperature signals into thermoelectromotive force signals, which are then converted into the temperature of the measured medium by electrical instruments (secondary instruments).
The basic principle of thermocouple temperature measurement is that a closed loop is formed by two homogeneous conductors of different compositions. When a temperature gradient exists between the two ends, current flows through the loop, and an electromotive force, namely thermoelectromotive force, is generated between the two ends. This is known as the Seebeck effect.
The two homogeneous conductors of different compositions are thermoelectrodes. The end at higher temperature is the working junction, and the end at lower temperature is the free junction. The free junction is usually kept at a constant temperature. Thermocouple calibration tables are established according to the functional relationship between thermoelectromotive force and temperature. Calibration tables are obtained under the condition that the free junction temperature is 0°C; different thermocouples have different calibration tables.
When a third metallic material is connected into the thermocouple circuit, the thermoelectric voltage generated by the thermocouple remains unchanged provided the temperatures at the two junctions of this material are equal. That is, it will not be affected by the third metal inserted into the circuit. Therefore, measuring instruments can be connected in thermocouple temperature measurement. After the thermoelectromotive force is measured, the temperature of the measured medium can be determined.
For thermocouple temperature measurement, the temperature of its cold junction (the measuring end is the hot junction; the end connected to the measuring circuit via leads is called the cold junction) must be kept constant, so that the magnitude of thermoelectric voltage maintains a proportional relationship with the measured temperature. If the ambient temperature of the cold junction changes during measurement, the measurement accuracy will be seriously affected. Taking measures at the cold junction to compensate for the influence caused by cold junction temperature variation is called cold junction compensation of thermocouples.
Appendix: Calculation method for thermocouple cold junction compensation Millivolt to temperature: Measure the cold junction temperature, convert it into the corresponding millivolt value, add it to the millivolt value of the thermocouple, and calculate the temperature. Temperature to millivolt: Measure the actual temperature and cold junction temperature, convert both into millivolt values respectively, subtract them to obtain the millivolt value, and then get the temperature.
