The difference between thermocouple and thermal resistor and application selection method

Temperature measuring instruments are often used in daily work. Thermal resistors and thermocouples are both temperature measuring instruments. Should thermal resistors or thermocouples be selected for the same temperature measurement location?

Structure of thermocouples

There are three shapes of thermocouple front joints. According to the type, wire diameter and application temperature of the thermocouple, gas welding, butt welding, resistance welding, arc welding and silver welding can be used for welding. In industrial applications, in order to facilitate installation and extend the service life of thermocouples, the method of adding sleeves is usually adopted

The sleeves are generally divided into protective tube type and armored type.

Thermocouples with protective tubes are used to insert the core wire and insulation tube of the thermocouple into the protective tube. The protective tube can maintain the mechanical strength of the thermocouple while preventing the core wire from oxidation and corrosion.

Armored thermocouples

The measurement principle of armored thermocouples is the same as that of thermocouples with protective tubes. It replaces the insulating tube (ceramic) with a slender metal tube (called a sleeve) and uses magnesium oxide (MgO) powder as the insulating material. Because of its thin outer diameter, it is easy to bend and is more suitable for measuring the temperature of the back of an object and narrow gaps. In addition, compared with thermocouples with protective tubes, its response speed is more flexible. The outer diameter range of armored thermocouples is very wide, and it can be stretched into various sizes from 8.0 mm ф to 0.5 mm ф. The thinner the core wire is stretched, the lower the upper limit of the common temperature. For example, the upper limit of the common temperature of the K-type thermocouple with an outer diameter of 0.5 mm ф is 600 ℃, and 1050 ℃ when the outer diameter of 8.0 mm ф is 8.0 mm ф.

Thermal resistor structure

There are three types of component shapes of thermal resistors, and ceramic package type is currently the main one. Ceramic package type is used for thermal resistors with protective tubes and armored thermal resistors. The bare wire diameter of ceramic and glass package platinum wire is about tens of microns, and the bare wire diameter of mica plate type is about 0.05 mm.

Distinguishing

Although they are all contact thermometers, they have different temperature measurement ranges. Thermocouples are used in high temperature environments because the output thermoelectric potential in the medium and low temperature areas is very small (it can be checked by looking up the table). When the potential is small, the requirements for the counter-interference measures and the secondary table are very high, otherwise the measurement will be inaccurate. Moreover, in the low temperature area, the relative error caused by the change of the cold end temperature and the ambient temperature is very prominent and difficult to be fully compensated. At this time, at medium and low temperatures, the temperature measurement range of the thermal resistor is generally 200~500, or even lower.