A comprehensive introduction to thermocouples
Thermocouples are widely used. Thermocouples are often used in conjunction with temperature control boxes. The following describes the types of thermocouple applications.
Thermocouple working principle
First, the working principle of thermocouple:
A thermocouple is a temperature sensing element that converts a temperature signal into a thermoelectromotive force signal that is converted to the temperature of the measured medium by an electrical meter. The basic principle of thermocouple temperature measurement is that two homogeneous conductors of different compositions form a closed loop. When there is a temperature gradient at both ends, a current will flow through the loop. At this time, there is a Seebeck electromotive force between the two ends. Electromotive force, this is the so-called Seebeck effect. The homogeneous conductor of two different compositions is a hot electrode, the higher temperature end is the working end, the lower temperature end is the free end, and the free end is usually at a constant temperature. According to the relationship between thermoelectromotive force and temperature, the thermocouple index table is made. The index table is obtained under the condition that the free end temperature is 0 °C. Different thermocouples have different index tables. When a third metal material is connected to the thermocouple loop, 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, that is, not affected by the third metal access loop. Therefore, when the thermocouple is measuring temperature, the measuring instrument can be connected, and after measuring the thermoelectromotive force, the temperature of the measured medium can be known.
Second, thermocouple advantages:
Thermocouples are commonly used temperature measuring elements in the industry and have the following characteristics:
1 High measurement accuracy: The thermocouple is in direct contact with the object to be tested and is not affected by the intermediate medium.
2 Thermal response time is fast: Thermocouples are sensitive to temperature changes.
3 Large measuring range: Thermocouples can be continuously measured from -40 to + 1600 °C.
4 reliable performance, good mechanical strength.
5 Long service life and easy installation.
Third, the type and structure of thermocouple:
(1) Types of thermocouples
Thermocouples are K-type (nickel-chromium-nickel-silicon) WRN series, N-type (nickel-chromium-nickel-silica) WRM series, E-type (nickel-chromium-copper-nickel) WRE series, J-type (iron-copper-nickel) WRF Series, T-type (copper-copper-nickel) WRC series, S-type (platinum-iridium 10-platinum) WRP series, R-type (platinum-iridium 13-platinum) WRQ series, type B (platinum-iridium 30-platinum-iridium 6) WRR series Wait.
(2) The structure of the thermocouple:
The basic structure of the thermocouple is a hot electrode, an insulating material and a protective tube; and it is used in conjunction with a display instrument, a recording instrument or a computer. In the field use, according to the environment, the measured medium and other factors to develop a thermocouple suitable for various environments. Thermocouples are simply divided into fabricated thermocouples, armored thermocouples and special-type thermocouples; subdivided by high temperature thermocouples, wear-resistant thermocouples, corrosion-resistant thermocouples, high-voltage thermocouples, and explosion-proof thermoelectrics Even, thermocouple for aluminum liquid temperature measurement, thermocouple for circulating fluidized bed, thermocouple for cement rotary kiln, thermocouple for anode baking furnace, thermocouple for high temperature hot blast stove, thermocouple for vaporization furnace, thermoelectric for carburizing furnace Even, thermocouples for high temperature salt bath furnaces, thermocouples for copper, iron and steel, thermocouples for anti-oxidation, tungsten, thermocouples for vacuum furnaces, platinum-iridium thermocouples, etc.
First, the types of thermocouple probes commonly used thermocouple probes can be divided into:
(1) Standard thermocouple / probe (2) Non-standard thermocouple probe two categories.
The so-called standard thermocouple probe refers to a thermocouple probe whose national standard specifies the relationship between its thermoelectric potential and temperature, the allowable error, and a uniform standard index table. It has a display instrument that is compatible with it. Non-standardized thermocouples/probes are not as standard or order of magnitude as standard thermocouples/probes, and generally do not have a uniform indexing table, which is mainly used for measurement in some special occasions.
Standardized Thermocouple Probes Since January 1, 1988, thermocouples/probes and thermal resistors have all been produced in accordance with IEC international standards, and seven standard thermocouples/probes of S, B, E, K, R, J, and T have been designated. Designed a thermocouple/probe for China.
Second, the thermocouple / probe structure to ensure that the thermocouple / probe works reliably and stably, its structural requirements are as follows:
1 The welding of the two hot electrodes that make up the thermocouple probe must be firm; 2 The two hot electrodes should be well insulated from each other to prevent short circuit; 3 The connection between the compensation wire and the free end of the thermocouple/probe should be convenient and reliable; The casing should ensure that the hot electrode is sufficiently isolated from hazardous media. (3). Temperature compensation of the cold end.