Thermocouples are commonly used temperature measuring components in temperature measuring instruments?

Thermocouple is a commonly used temperature measuring element in temperature measuring instruments. It directly measures temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium through electrical instruments (secondary instruments). The appearance of various thermocouples often varies greatly due to their needs, but their basic structures are roughly the same. They are usually composed of main parts such as hot electrodes, insulation sleeves, and junction boxes, and are usually used in conjunction with display instruments, recording instruments, and electronic regulators.

The basic principle of thermocouple temperature measurement is that two conductors with different compositions form a closed circuit. When there is a temperature gradient at both ends, current will flow through the circuit, and there will be an electromotive force - thermoelectric force - between the two ends, which is called the Seebeck effect. Two homogeneous conductors with different compositions are thermoelectric electrodes, with the higher temperature end as the working end and the lower temperature end as the free end, which is usually at a constant temperature. Create a thermocouple calibration table based on the functional relationship between thermoelectric potential and temperature; The calibration table is obtained under the condition of a free end temperature of 0 ℃, and different thermocouples have different calibration tables.

When a third metal material is connected to a thermocouple circuit, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged and will not be affected by the connection of the third metal to the circuit. Therefore, when using thermocouples for temperature measurement, a measuring instrument can be connected to measure the thermoelectric potential, and the temperature of the measured medium can be determined. When measuring temperature with a thermocouple, it is required that the temperature of its cold end (the measuring end is the hot end, and the end connected to the measuring circuit through a lead is called the cold end) remains constant, so that the magnitude of its thermoelectric potential is proportional to the measured temperature. If the temperature changes at the cold end during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures to compensate for the impact caused by temperature changes at the cold end is called normal cold end compensation for thermocouples. Use dedicated compensating wires to connect with measuring instruments.

Calculation method for thermocouple cold junction compensation:
From millivolts to temperature: measure the cold junction temperature, convert it to the corresponding millivolt value, add it to the millivolt value of the thermocouple, and convert it to temperature;
From temperature to millivolts: Measure the actual temperature and cold end temperature, convert them into millivolts, subtract them to obtain the millivolt value, and then obtain the temperature.

Proper use of thermocouples can not only accurately obtain temperature values and ensure product quality, but also save material consumption of thermocouples, which not only saves money but also ensures product quality. Incorrect installation, thermal conductivity, and time lag are the main errors in the use of thermocouples.

Temperature is one of the important parameters that need to be measured and controlled in industrial production processes. Thermocouples are widely used in temperature measurement, with many advantages such as simple structure, convenient manufacturing, wide measurement range, high accuracy, low inertia, and easy remote transmission of output signals. In addition, as thermocouples are active sensors that do not require an external power source for measurement, they are very convenient to use and are often used to measure the temperature of gases or liquids in furnaces and pipelines, as well as the surface temperature of solids.