1. Two layers of metals with different thermal expansion coefficients are mechanically pressed together. When the temperature changes, its curvature will change. When the curvature reaches a certain degree, the circuit is connected (or disconnected) to make cooling (or heating) ) The equipment works.
Temperature is a very critical parameter that must be controlled in industrial production. It has a direct impact on the quality of industrial products, equipment and personal safety. The selection of temperature detection instruments should not blindly require high measurement accuracy, wide range, and high degree of automation, but should combine various factors such as the specific process in industrial production, the reality of the measured medium, and the economy. The principle to be followed is that the upper and lower limits of the measured temperature of the detection instrument should be greater than the fluctuation range of the measured medium temperature, the measurement accuracy meets the technical requirements of the production process, the use mode meets the observation needs of the measurement personnel, and it is convenient for daily maintenance and maintenance work, and on this basis , As far as possible to choose the most economical and affordable detection instrument.
Select according to usage
If it is only displayed on the spot, usually liquid glass, bimetal and pressure thermometer can be selected. If you need not only the function of measuring temperature, but also the ability to alarm when the measured temperature is close to the limit value, liquid glass, bimetal and pressure thermometers with additional alarm devices should be selected. If remote display is required, thermal resistance, thermocouple or temperature transmitter can be selected.
Select according to the measurement range requirements
The temperature of the medium to be measured is a very critical basis for choosing a suitable measuring instrument. If you are measuring room temperature, you can choose thermocouple thermometers, thermal resistance thermometers, pressure gauges, and bimetal thermometers. The characteristic of the organic liquid glass thermometer is that the indicator liquid is red, which is good for reading, but it cannot charge the contact. Therefore, when measuring the medium with a temperature lower than 100℃ and does not need to send a signal, the organic liquid glass thermometer can be preferred. The main advantage of the bimetal thermometer is its clear scale, vibration resistance and mercury-free, so when the temperature of the measured medium is lower than 300℃, it is best to choose a bimetal thermometer.
If the stability of the measured medium is lower than 150℃, you can choose a copper thermal resistance; if the temperature of the measured medium is within the range of 300℃ to 600℃, you can choose a nickel-chromium-copper thermocouple, but because of the copper Alloy wire is easily oxidized, so when it is used to measure the steam temperature exceeding 500℃, it is best to choose Ni-Cr-Ni-Si thermocouple. If the temperature of the measured medium is between 600℃ and 1000℃, you can choose Ni-Cr-Ni Silicon thermocouple; if the temperature of the measured medium is between 1000°C and 1300°C, platinum rhodium-platinum thermocouple should be selected. If the temperature of the measured medium is very high, you can choose a radiation pyrometer or an infrared pyrometer.
Select according to the measurement accuracy needs
If the required measurement accuracy is very high, you can choose platinum thermal resistance, platinum rhodium-platinum thermocouple or platinum rhodium-platinum rhodium thermocouple. If the required measurement accuracy is not very high, you can choose copper thermal resistance and nickel-chromium-nickel-silicon thermocouple.
Select according to the chemical properties of the tested medium
Most thermocouples are very stable in oxidizing or neutral media, but they are not suitable for working in reducing media for a long time; at the same time, platinum thermal resistors are not suitable for working in reducing media for a long time; when the temperature reaches 100℃ Copper thermal resistance is easy to be oxidized; thermistor is also very easy to be oxidized and deteriorated. For this reason, it should be prevented by installing protective sleeves, and appropriate protective sleeve materials should be selected according to the chemical properties of the measured medium. For example, for thermocouples: if the temperature is lower than 600°C, medium carbon steel, copper, lead, etc. can be used as the casing; if the temperature is lower than 1000°C, the austenitic stainless steel (heat-resistant corrosion) is usually selected. In addition, attention must be paid to the supporting use of secondary instruments, thermocouple compensation wires, and free-end temperature compensators. During installation, the accuracy of detection must be ensured, and at the same time, it should facilitate the maintenance and calibration of the instrument and avoid the lag of temperature measurement.