Proper installation and use of thermocouple.

The correct use of thermocouple not only can accurately get the temperature value, ensure the product qualified, but also can save the material consumption of thermocouple, save money and ensure the quality of products. Incorrect installation, thermal conductivity and time lag error, they are the main error in the use of thermocouple.

1. Errors introduced by improper installation:

Such as the location of the thermocouple installation and insertion depth can not reflect the real temperature of the furnace, in other words, the thermocouple should not be installed too close to the door and heating place, insertion depth should be at least 8 ~ 10 times the diameter of the protective tube; The gap between the protective sleeve and the wall of the thermocouple is not filled with insulation material resulting in heat overflow or cold air intrusion in the furnace. Therefore, the gap between the protective tube of the thermocouple and the hole of the furnace wall should be blocked with insulation material such as refractory mud or asbestos rope so as to avoid the convection of hot and cold air and affect the accuracy of temperature measurement. The cold end of the thermocouple is too close to the furnace body so that the temperature exceeds 100℃; The installation of thermocouple should avoid strong magnetic field and strong electric field as far as possible, so the thermocouple and power cable should not be installed in the same conduit to avoid interference caused by error; Thermocouple can not be installed in the measured medium little flow area, when the thermocouple measurement tube gas temperature, must make the thermocouple against the flow rate direction installation, and fully contact with the gas.

2. Error caused by insulation deterioration:

Such as the insulation of the thermocouple, protective tube and wire plate dirt or salt slag too much resulting in poor insulation between the thermocouple pole and the furnace wall, more serious at high temperature, which will not only cause the loss of thermocouple potential but also introduce interference, resulting in error sometimes up to Baidu.

3. Errors introduced by thermal inertia:

Due to the thermal inertia of the thermocouple, the indicator value of the instrument falls behind the change of the measured temperature, which is especially prominent when making rapid measurements. Therefore, the thermocouple with thinner thermal electrode and smaller protective tube diameter should be used as far as possible. When the temperature environment permits, even the protective tube can be removed. Because of the measurement lag, the amplitude of temperature fluctuation detected by thermocouple is smaller than that of 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 the temperature is measured or controlled by a thermocouple with a large time constant, the temperature fluctuation shown by the instrument is small, but the fluctuation of the actual furnace temperature may be large. In order to accurately measure temperature, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient, and 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 effective way is to minimize the size of the hot end. In use, protective sleeves with good thermal conductivity, thin wall and small inner diameter are usually used. In more precise temperature measurement, the bare wire thermocouple without protective sleeve is used, but the thermocouple is easy to damage, should be corrected and replaced in time.

4. Thermal resistance error:

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