The laying method of thermocouples and the sources of temperature measurement errors

1, Method of laying thermocouples

When measuring surface temperature with thermocouples, except for portable ones, most of them are installed. The laying methods include surface laying and slot laying. When laying on a metal surface, the thermocouple measuring end is often welded to the metal surface and then extended along the surface (preferably along the isotherm) for a distance before being led out. For slotted thermocouples, they are finely laid on the surface of the metal being tested, welded with thermal thermocouple contacts, and then placed in ceramic or plastic tubes, extended for a certain distance before being led out; Ceramic tubes serve as insulation and protection. When used to measure high surface temperatures, aluminum oxide powder is added to the outside of the ceramic tube with a powdered adhesive, and water glass is used as the adhesive material to bond the thermocouple in the groove. When the temperature is below 400 ℃, use thermocouple wires coated with high-temperature resistant insulation layer or yarn coating. At present, thermocouple wires have been developed with outsourced fabrics that can withstand high temperatures of 8300 ℃. When measuring the surface temperature of non-metallic materials, the thermocouple measuring end cannot be welded to the surface, but sometimes a heat collector can be glued on and the thermocouple measuring end can be welded to the heat collector.

2, Thermal resistance changes caused by the installation of thermocouples

When laying thermocouples, the measured material is removed by slotting, and the thermocouple wire and non-metallic insulation layer are replaced. The equivalent thermal conductivity of the material will differ from that of the measured object. Without slotting, the installation of thermocouples increases the laying layer, and both situations will cause changes in the thermal resistance of the measured surface. The change in thermal resistance will lead to the destruction of local heat flux and temperature field, changing the original measured surface temperature. Although these changes are reflected in a small local area or at the measuring end of the thermocouple, they no longer make the measuring end feel the surface temperature of the thermocouple that was not originally installed, thus causing temperature measurement errors. Previous temperature measurement workers had limited observation in this area.

In addition, the surface emissivity of the thermocouple embedded part often differs from the surface emissivity of the measured material itself, which can also cause errors.

Therefore, when directly measuring the surface temperature of an object with a thermocouple, it is necessary to pay attention to the equivalent thermal conductivity of the thermocouple material and adhesive layer used, which should be as close as possible to the thermal conductivity of the measured material. The surface emissivity of the exposed part should also be as close as possible to the emissivity of the original measured surface to reduce errors caused by changes in thermal resistance.

When thin film thermocouples or thin film thermal resistors are installed on the surface to measure surface temperature, it can also cause changes in the local thermal resistance of the measured object.