Easily ignored when thermocouple calibration problem

Verification personnel in the process of calibrating thermocouple, for terminal sand, thermocouple, common problems such as short circuit or strapping deviating from the geometrical center to the test data is not accurate, generally can be found easily handle in time, but is easy to overlook some questions affect the test results.

1. The length of the thermocouple

JJG351 - 1996 'working with inexpensive metal thermocouple' verification regulation specified in the thermocouple length not less than 750 mm, for the length of the thermocouple is given by, because considering the thermocouple temperature measurement area after you leave to have enough wide temperature gradient zone. Thermocouple thermoelectric emfs is produced in the region, to effectively prevent the hot end of thermocouple ( Measurement end) The quantity of heat to the cold end ( Terminal) , the most basic way is to have enough of the cold junction of the thermocouple distance away from the hot end. In general error caused by the length of the thermocouple is not enough is negative, the revised is positive. The shorter the length, the greater the error is, therefore, before the furnace charging test to determine the length of the thermocouple.

2. Thermocouple wire bending

thermocouple wire is fine and soft, easy to deformation, when accidentally silk such as folding, twisted plastic deformation, make the hot electrode pair wire produces stress changed the thermocouple thermoelectric properties, so that the deformation of the accuracy of the thermocouple measurements. Therefore, must keep the thermocouple wire straight before test.

3. Thermocouple wire contaminated

thermocouple wire contaminated, even by oxidation, can make the hot electrode surface gloss accidentally, dark black, then the hot electrode unstable thermoelectric properties, the accuracy of the measurement data is poorer, therefore, to clean electrodes have pollution, eliminate pollution layer.

4. The effects of response time

contact method of temperature measurement is the basic principle of temperature measuring element to reach thermal equilibrium with the object being measured. When measuring temperature, therefore, need to keep a certain time and to make both reach thermal equilibrium. Keep the length of time, with temperature measuring elements related to the thermal response time. And thermal response time depends on the structure of the sensor and measurement conditions, the difference is great. So, in the process of daily check according to the different types of thermocouple to choose the appropriate heating rate, thermal equilibrium time.

5. The influence of the insulation resistance

thermocouple under high temperature, the insulation resistance rapid decrease with the temperature increasing, thus will lead to leakage current, the current through insulation insulation resistance had fallen into the instrument, the instrument instability or producing measurement error. Before loading furnace thermocouple, therefore, don't ignore the insulation resistance test, only when meet the requirements of verification regulation, temperature tolerance test can proceed.