Some of the causes of errors in the use of thermocouples
What are the causes of errors in the use of thermocouples? In the case of
Proper use of thermocouples can not only accurately get the temperature value, to ensure product qualification, but also save the thermocouple material consumption, both to save money and to ensure product quality. Improper installation, thermal conductivity and time lag error, they are the main error in the use of thermocouple. 1, the improper introduction of the installation error:
Such as the location of the thermocouple installation and the depth of insertion can not reflect the real temperature of the furnace, etc. In other words, the thermocouple should not be installed too close to the door and heating place, insert the depth of at least the protection tube diameter of 8 to 10 times; Thermocouple protection between the casing and the gap between the wall does not fill the thermal material caused by the furnace heat spilled or cold air intrusion, so the thermocouple protection tube and the gap between the furnace wall hole application of refractory mud or asbestos rope and other insulating material block to avoid hot and cold Air convection and affect the accuracy of temperature measurement; thermocouple cold side too close to the furnace so that the temperature exceeds 100 ℃; thermocouple installation should be as far as possible to avoid strong magnetic field and strong electric field, so should not thermocouple and power cable installed The thermocouple can not be installed in the area where the measured medium is rarely flowing. When measuring the temperature of the gas in the tube with a thermocouple, the thermocouple must be installed in the direction of the flow rate and fully contacted with the gas. The In the case of
2, the error of the introduction of insulation error:
Such as thermocouple insulation, protective tube and cable plate dirt or too much salt residue caused by thermocouple between the furnace and the insulation between the bad, more serious at high temperatures, which will not only cause the loss of thermoelectric power but also the introduction of interference, The resulting error is sometimes up to Baidu. In the case of
3, the introduction of hot inertia error:
Due to the thermal inertia of the thermocouple, the indication value of the instrument is behind the measured temperature change, especially when the rapid measurement is made. So as far as possible the use of smaller thermal electrode, the protection of small diameter thermocouple. When the temperature measurement environment permits, even the protective tube can be removed. Due to the presence of measurement hysteresis, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the furnace temperature fluctuation. The greater the hysteresis, the smaller the amplitude of the thermocouple fluctuations, and the greater the difference from the actual furnace temperature. When the thermocouple with a time constant temperature or temperature control, the instrument shows the temperature although the fluctuations are small, but the actual furnace temperature fluctuations may be great. In order to accurately measure the temperature, you should select a thermocouple with a small time constant. The time constant is inversely proportional to the heat transfer coefficient, which is proportional to the diameter of the hot end of the thermocouple, the density of the material, and the specific heat. To reduce the time constant, the most effective way to increase the heat transfer coefficient is to minimize the size of the hot end The In use, usually use good thermal conductivity of the material, thin wall, small diameter protective sleeve. In the more precise temperature measurement, the use of unprotected casing of the bare wire thermocouple, but the thermocouple is easy to damage, should be promptly corrected and replaced.
