The judgment method of thermocouple output failure
1) Accurately connect according to the wiring diagram of the meter. After the power is turned on, the meter first displays the thermocouple index number of the meter, then displays the meter range scale, and then the lower row of the meter displays the set temperature, and the upper row of the meter displays the measured temperature. . If the display on the top of the meter is not the temperature of the heating element, but displays "OVER", "0000" or "000", it explains that the output part of the meter is malfunctioning, and the following review should be made:
A) Remove the thermocouple from the output terminal of the meter thermocouple, and then use any wire to short-circuit the output terminal of the meter thermocouple. When the power is on, when the display value of the digital tube on the top of the meter is about room temperature, explain that the external connection of the thermocouple is open, and you should change the thermocouple of the same type. If it is still in the above-mentioned state, explain that the output terminal of the meter was damaged during transportation, and the meter should be replaced.
B) Remove the thermocouple of the above-mentioned faulty instrument, and replace it with the thermocouple connected to the same kind of indexing instrument that is running normally next to it. After power on, when the digital tube on the original faulty instrument displays the temperature of the heating element, explain the thermocouple If the external wiring is open, change to the same type of thermocouple. If it is still in the above-mentioned state, explain that the output terminal of the meter was damaged during the transportation process and the meter should be changed.
C) Remove the faulty thermocouple from the meter, use a multimeter to measure ohms (R) *1, use the two meter rods of the multimeter to measure the two ends of the thermocouple, if the resistance value displayed on the multimeter is large, explain The external connection of the thermocouple is open, change to the same type of thermocouple. Otherwise, there is a certain resistance value, explaining that there is a problem with the output terminal of the meter, and the meter should be changed.
2) Follow the wiring diagram of the meter to connect accurately. If the digital tube on the meter displays a negative value after the meter is powered on, explain that the thermocouple "+" and "-" connected to the meter are connected incorrectly. Only replace it again.
3) The wiring is accurate. When the meter is running, the temperature displayed by the digital tube on the meter differs from the temperature measured in practice by 30ºC~60ºC. The difference is even greater, explaining that the graduation number of the meter and the graduation number of the thermocouple are wrong. According to the corresponding relationship between the temperature (ºC) and the millivolt (MV) value of the thermocouple index number B, S, K, E, etc., the millivolt value (MV) occurs at the same temperature (ºC) ) The B index number is the smallest, the S index number is the second smallest, the K index number is the larger, and the E index number is the largest. It is judged according to this principle.
Analysis on the Reasons of Unqualified Thermocouple in Use
The thermocouple that has passed the verification is unqualified in use. This phenomenon is little known and has not attracted people's attention. The unqualified phenomenon of thermocouples in use is mainly caused by the influence of the inhomogeneous thermocouple wire, the shunt error of the armored thermocouple and the improper use of the thermocouple. The editor of the electrician learning network explains the mystery in this article.
1. The influence of inhomogeneous thermocouple wire
①The material of the thermocouple is not homogeneous. When the thermocouple is checked in the measuring room, insert the thermocouple into the verification furnace to a depth of 300mm according to the requirements of the regulations. Therefore, the verification result of each thermocouple can only reflect or mainly reflect the thermoelectric behavior of the 300nm long couple wire from the measuring end. However, when the length of the thermocouple is long, most of the wires are in the high temperature area during use. If the thermocouple wire is inhomogeneous and has a temperature gradient, the thermoelectromotive force will be generated locally. This electromotive force is called parasitic potential, and the error caused by the parasitic potential is called homogeneity error.
②The inhomogeneity of thermocouple wire after use. For a newly made thermocouple, even if the heterogeneous performance meets the requirements, it will lose its homogeneity if repeated processing and bending cause processing distortion of the thermocouple, and the thermocouple will lose its homogeneity when it is used for a long time under high temperature. The deterioration of the thermoelectromotive force changes. When the locally degraded part is in a place with a temperature gradient, the parasitic potential will also be superimposed on the total thermoelectromotive force, resulting in measurement errors.
2. Shunt error of armored thermocouple
When using an armored thermocouple to measure the furnace temperature, when the temperature of the middle part of the thermocouple exceeds 800 ℃, it will cause the insulation resistance to drop and the abnormal value of the thermocouple will appear, which is called shunt error. Since the insulation of the sheathed thermocouple is powdered magnesium oxide, the insulation resistance will drop by an order of magnitude for every 100°C increase in temperature at high temperatures. When the temperature in the middle part is high, there must be leakage current, which will cause a shunt error in the output potential of the thermocouple.
3. Measurement errors caused by improper use
If the thermocouple that has passed the verification is used incorrectly, it will cause larger measurement errors.
①The insertion depth of the thermocouple is not enough, and the temperature measurement is low due to heat conduction loss;
②Improper handling of the reference terminal temperature, for example, the flame spraying out of the furnace causes the reference temperature to be higher, which may also cause measurement errors. |
It can be seen from the above discussion that the unqualified thermocouple in use may be caused by the above three conditions.