Thermocouple Armor Material

Correct use of armored thermocouples can not only accurately obtain temperature values ​​and ensure product quality, but also save material consumption of thermocouples, saving money and ensuring product quality. Improper installation, errors such as thermal conductivity and time lag are the main errors in the use of thermocouples.
1. Errors introduced by improper installation:

The installation position and insertion depth of the armored thermocouple cannot reflect the actual temperature of the furnace. In other words, the thermocouple should not be installed too close to the door or the heating place, and the insertion depth should be at least 8 to 10 times the diameter of the protection tube. The gap between the thermocouple protection tube and the wall is not filled with insulating material, which causes heat overflow or cold air intrusion in the furnace. Therefore, the gap between the thermocouple protection tube and the furnace wall hole should be blocked with insulating materials such as refractory mud or asbestos rope to prevent hot and cold air convection and affect the accuracy of temperature measurement. The cold end of the thermocouple is too close to the furnace body, causing the temperature to exceed 100℃. The installation of the thermocouple should avoid strong magnetic fields and strong electric fields as much as possible, so the thermocouple and the power cable should not be installed in the same conduit to avoid interference and errors. Thermocouples cannot be installed in areas where the measured medium rarely flows. When using thermocouples to measure the temperature of the gas in the tube, the thermocouple must be installed against the flow direction and fully in contact with the gas. 2. Errors introduced by insulation degradation:
If the thermocouple is insulated, the protection tube and the wire drawing board are too dirty or have too much salt residue, resulting in poor insulation between the thermocouple poles and the furnace wall, which is more serious at high temperatures. This will not only cause the loss of thermoelectric potential but also introduce interference.
3. Errors introduced by thermal inertia:
The thermal inertia of the armored thermocouple causes the indication value of the instrument to lag behind the change of the measured temperature. This effect is particularly prominent when performing rapid measurements. Therefore, thermocouples with thinner thermocouples and smaller protection tube diameters should be used as much as possible. When the temperature measurement environment permits, the protection tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the furnace temperature fluctuation. The greater the measurement lag, the smaller the amplitude of the thermocouple fluctuation, and the greater the difference with the actual furnace temperature. When using a thermocouple with a large time constant to measure or control temperature, although the temperature displayed by the instrument fluctuates slightly, the actual furnace temperature may fluctuate greatly. In order to accurately measure the temperature, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient and is proportional to the diameter of the hot end of the thermocouple, the density of the material and the specific heat. If you want to reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective way is to minimize the size of the hot end. In use, materials with good thermal conductivity, thin walls and small inner diameter protective sleeves are usually used. In more precise temperature measurements, bare wire thermocouples without protective sleeves are used, but thermocouples are easily damaged and should be calibrated and replaced in time.
4. Thermal resistance error:

At high temperatures, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance increases, hindering the conduction of heat. At this time, the temperature indication is lower than the true value of the measured temperature. Therefore, the outside of the thermocouple protective tube should be kept clean to reduce the error.