Problems to be noted in the process of verifying thermocouples in thermal experiments

       Problems to be noted in the process of verifying thermocouples in thermal experiments

       In the process of verifying thermocouples with a thermocouple verification furnace, the verification personnel can generally find timely and easily handle the inaccurate measured data caused by common problems such as unstable terminals, short circuit of thermocouples or strapping deviation from the geometric center, but Will forget some problems that affect the test results but are easily overlooked.

1. The length of the thermocouple

       JJG351-1996 "Working Cheap Metal Thermocouple" verification regulations clearly stipulate that the length of the surface thermocouple is not less than 750mm, the reason for the length of the thermocouple is to consider that the thermocouple must be wide enough after leaving the temperature measurement zone Temperature gradient zone. The thermoelectromotive force of the thermocouple is also generated in this area. To effectively prevent the heat from the hot end (measurement end) of the thermocouple from being transferred to the cold end (terminal), the basic method is that the cold end of the thermocouple must have a sufficient distance Keep away from hot end. Generally speaking, the error caused by the insufficient length of the thermocouple is negative, and the correction value is positive. The shorter the length, the greater the error. Therefore, the length of the thermocouple needs to be determined before the furnace verification.

2. Thermocouple wire bending

       The thermocouple wire is thin and soft, and easy to deform. When the plastic deformation of the thermocouple, such as folding and twisting, stresses the thermocouple wire, the thermoelectric characteristics of the thermocouple are changed, so that the accuracy of the thermocouple measurement results influences. Therefore, the thermocouple wire must be straightened before verification.

3. Influence of insulation resistance

       At a high temperature, the insulation resistance of a thermocouple drops sharply as the temperature rises, so leakage current will flow. This current flows into the meter through the insulation whose insulation resistance has dropped, making the meter indication unstable or producing measurement errors. Therefore, the insulation resistance test should not be ignored before the thermocouple is installed in the furnace. Only when the requirements of the verification regulations are met can the temperature tolerance test be performed.

4. Thermocouple wire is contaminated

       If the thermocouple wire is contaminated or even oxidized, the surface of the thermocouple wire will not be bright, dark or black. At this time, the thermoelectric characteristics of the thermode are very unstable, and the accuracy of the measurement data is poor. The electrode to eliminate the contamination layer.

5. Effect of response time

       The basic principle of temperature measurement by contact method is that the temperature measuring element should reach thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain time when measuring the temperature, so that the infrared thermometer can achieve thermal balance between the two. The length of the hold time is related to the thermal response time of the temperature measuring element. The thermal response time mainly depends on the structure of the sensor and the measurement conditions, and varies greatly. Therefore, in the daily verification process, it is necessary to select the appropriate heating rate and heat balance time according to different types of thermocouples.