Basic knowledge of temperature sensor

(2) Thermometer:

1. Thermal resistance: Thermal resistance is the most commonly used temperature detector in the middle and low temperature areas. Its main features are high measurement accuracy and stable performance. Among them, the measurement accuracy of platinum thermal resistance is the highest. It is not widely used in industrial temperature measurement and is made into a standard reference instrument.

① The principle and material of thermal resistance temperature measurement: The thermal resistance temperature measurement is based on the characteristic that the resistance value of the metal conductor increases with the increase in temperature for temperature measurement. Thermal resistors are mostly made of metal materials. At present, platinum and copper are the most widely used. In addition, materials such as rhodium, nickel, and manganese have been used to make thermal resistors.

② Composition of thermal resistance temperature measurement system: The thermal resistance temperature measurement system is generally composed of thermal resistance, connecting wires and digital temperature control display. Two points must be paid attention to: “The index number of the thermal resistance and the digital temperature control display meter must be the same; in order to eliminate the influence of the resistance change of the connecting wire, a three-wire connection must be adopted.”

2. Thermistor: NTC thermistor has the characteristics of small size, high test accuracy, fast response, stability and reliability, anti-aging, interchangeability, and good consistency. It is widely used in air conditioning, heating equipment, electronic thermometers, liquid level sensors, automotive electronics, electronic desk calendars and other fields.

3. Thermocouple: Thermocouple is one of the most commonly used temperature detection components in the industry. The advantages are:

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

② Wide measuring range. Commonly used thermocouples can measure continuously from -50 to +1600°C, and some special thermocouples have a minimum temperature of -269°C (such as gold, iron, nickel and chromium), and a maximum of +2800°C (such as tungsten-rhenium).

③ Simple structure and easy to use. Thermocouples are usually composed of two different metal wires, and are not limited by size and beginning, and there is a protective sleeve outside, which is very convenient to use.

(1). The basic principle of thermocouple temperature measurement

The conductors or semiconductors A and B of two different materials are welded together to form a closed loop. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, thus forming a large current in the loop. This phenomenon is called the thermoelectric effect. Thermocouples use this effect to work.

(2). Types of thermocouples

Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples.

Standard thermocouple refers to the thermocouple whose thermoelectric power and temperature are stipulated in the national standard, the allowable error, and there is a unified standard graduation meter. It has a matching display instrument for selection.

Non-standardized thermocouples are inferior to standardized thermocouples in terms of use range or order of magnitude, and there is generally no unified indexing table, which is mainly used for measurement in some special occasions.

Since January 1, 1988 in my country, thermocouples and thermal resistances have all been produced in accordance with IEC international standards, and seven standardized thermocouples, S, B, E, K, R, J, and T, are designated as my country’s unified design thermocouples.

(3). Temperature compensation of the cold junction of the thermocouple

Since the materials of thermocouples are generally more expensive (especially when precious metals are used), and the distance between the temperature measurement point and the instrument is very long, in order to save thermocouple materials and reduce costs, compensation wires are usually used to connect the cold end of the thermocouple (free The end) extends into the control room where the temperature is relatively stable and connects to the terminal of the instrument. It must be pointed out that the function of the thermocouple compensation wire is only to extend the hot electrode and move the cold end of the thermocouple to the instrument terminal of the control room. By itself, it cannot eliminate the influence of the temperature change of the cold end on the temperature measurement and cannot compensate. Therefore, other correction methods are needed to compensate the influence of the cold junction temperature t0≠0°C on the temperature measurement. When using the thermocouple compensation wire, you must pay attention to the matching of the models, the polarity can not be wrong, and the temperature of the compensation wire and the thermocouple connection end cannot exceed 100°C.