Use the thermocouple properly, can not only get accurate temperature values, to ensure qualified products, but also can save the thermocouple material consumption, both to save money and also ensure product quality. Here are 4 means, which are major errors during the application of the thermocouples.
- Improper Installation Error
Such as thermocouple installation location and insertion depth does not reflect the true temperature of the furnace, in other words, the thermocouple should not be too close to the door and the heating place, the insertion depth should be at least 8 to 10 times of the protective tube diameter.
Thermocouple protection sleeve and wall spacing does not filled with insulating material so that the furnace heat overflow or cold air invades. Therefore the space between thermocouple protection sleeve and furnace wall hole should be built up by the insulating material,such as fire clay or asbestos cord, in order to avoid hot and cold the air convection, which effect the accuracy of temperature measurement .
Thermocouple cold end is too close to the furnace so that the temperature exceeds 100 ℃;
Thermocouple should be installed away from strong magnetic and electric field, so you can not put the thermocouple and power cables in the same lead pipe, in order to avoid the interference caused error;
Thermocouple should not be installed in an area where with little medium flows, when uses a thermocouple to measure the temperature inside of the gas pipe, you should install the thermocouple against the flow direction, and full contact with the gas .
2.Insulation Error
If thermocouple insulated, but protection pipe and strain plate with too much dirt or salt residue which causes poor insulation between the thermocouple interelectrode and furnace wall, more severe at high temperatures. it will not only cause the loss of the thermoelectric power but also pull in the interference, error resulting sometimes up on 100℃.
3. Thermal Inertia Errors
Due to the thermocouple thermal inertia, the instrument indicated value change lag behind the measured temperature change. when rapid measurement during this effect is particularly prominent. It is better to choose the thinner hot electrode, and smaller diameter thermocouple protection tube.
When the temperature circumstances permit, you can even take away the protective tube. Because of the exsistence of measure lag behind, temperature fluctuation amplitude which measured by thermocouples is smaller than the furnace temperature fluctuation amplitude. The bigger the measurement lags behind, the smaller the thermocouple fluctuations amplitude, the bigger the difference between the actual oven temperature.
When using a large time constant thermocouple to measure or control temperature, although the temperature fluctuation indicates on the instrument is small , but fluctuations in the actual oven temperature may be significant.
In order to measure the temperature accurately, you should select thermocouple with small time constant. Time constant is inversely proportional to heat transfer coefficient, but is proportional to the thermocouple hot end diameter, material density and specific heat.
If you want to reduce the time constant, besides increasing the heat transfer coefficient, the most effective way is to reduce the size of the hot end . Usually adopts good thermal conductivity materials, with thin wall, small inner diameter protective tubes.
In the higher precise temperature measurement, use unprotected sleeve bare wire thermocouple, but thermocouple will be easily damaged, should be corrected and replaced timely
4. Thermal Resistance Error
At high temperature, if the thermocouple protective tube has coal ash, dust attached to the above, thus thermal resistance increases, blocks the heat conduction, temperatures measured values will be lower than the real temperature. Therefore, the thermocouple protection tube should be kept clean to reduce errors.
