What are the measurement principles and differences between thermistor and thermocouple?

Thermocouples are one of the commonly used temperature detection components in industry. Its advantages are:

① High measurement accuracy. Because the thermocouple is in direct contact with the measured object and is not affected by the intermediate medium.

② Wide measurement range. Commonly used thermocouples can measure continuously from -50~+1600 ℃, while some special thermocouples can measure temperatures as low as -269 ℃ (such as gold iron nickel chromium) and as high as+2800 ℃ (such as tungsten rhenium).

③ Simple construction and easy to use. Thermocouples are usually composed of two different types of metal wires, and are not limited by size or opening. They have protective sleeves on the outside, making them very convenient to use.

1. Basic principle of thermocouple temperature measurement

Weld two different materials of conductors or semiconductors A and B together to form a closed circuit. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between them, resulting in a current of a certain magnitude in the circuit. This phenomenon is called thermoelectric effect. Thermocouples work by utilizing this effect.

2. Types and Structure Formation of Thermocouples

(1) Types of thermocouples

Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouple referred to refers to a thermocouple that has a national standard that specifies the relationship between its thermoelectric potential and temperature, allows for errors, and has a unified standard scale. It has a matching display instrument available for selection. Non standardized thermocouples are not as widely used or of the same order of magnitude as standardized thermocouples, and generally do not have a unified calibration table. They are mainly used for measurements in certain special occasions. Standardized thermocouple

Since January 1, 1988, all thermocouples and thermistors in China have been produced according to IEC standards, and the seven standardized thermocouples S, B, E, K, R, J, and T have been designated as unified design thermocouples in China.

(2) In order to ensure the reliable and stable operation of thermocouples, the structural requirements for them are as follows:

① The welding of the two thermoelectric electrodes that make up the thermocouple must be firm;

② The two thermoelectric electrodes should be well insulated from each other to prevent short circuits;

③ The connection between the compensating wire and the free end of the thermocouple should be convenient and reliable;

④ The protective sleeve should ensure sufficient isolation between the thermoelectric electrode and harmful media.

3. Temperature compensation of thermocouple cold end

Due to the fact that the materials of thermocouples are generally expensive (especially when using precious metals), and the distance between the temperature measuring point and the instrument is far, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the thermocouple to a temperature stable control room and connect it to the instrument terminals. It must be pointed out that the function of the thermocouple compensation wire is only to extend the thermoelectric electrode and move the cold end of the thermocouple to the instrument terminal in the control room. It cannot eliminate the influence of temperature changes at the cold end on temperature measurement and does not have a compensation effect. Therefore, other correction methods need to be used to compensate for the impact of cold end temperature t0 ≠ 0 ℃ on temperature measurement.

When using thermocouple compensation wires, attention must be paid to model matching, polarity cannot be connected incorrectly, and the temperature at the connection end between the compensation wire and the thermocouple cannot exceed 100 ℃.

Classification of temperature measuring instruments

Temperature measuring instruments can be divided into two categories based on temperature measurement methods: contact and non-contact. Generally speaking, contact temperature measuring instruments are relatively simple, reliable, and have high measurement accuracy; However, due to the need for sufficient thermal exchange between the temperature measuring element and the measured medium, it takes some time to reach thermal equilibrium, so there is a delay in temperature measurement. At the same time, due to the limitations of high-temperature resistant materials, it cannot be applied to very high temperature measurements. Non contact instrument temperature measurement is based on the principle of thermal radiation to measure temperature. The temperature measuring element does not need to be in contact with the measured medium, has a wide temperature measurement range, is not limited by the upper limit of temperature measurement, and does not damage the temperature field of the measured object. The reaction speed is generally fast; However, due to external factors such as the emissivity of the object, measurement distance, smoke and moisture, the measurement error is relatively large.

The application principle of thermal resistance

Thermistor is a commonly used temperature detector in the medium and low temperature range. Its main features are high measurement accuracy and stable performance. Platinum thermal resistance has a high measurement accuracy, and it is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.