A small problem that can not be ignored when detecting armored thermocouple

Usually when we detect armored thermocouples, we will care about the common problems such as unreliable binding posts, short circuit of armored thermocouples or binding violation of how many centers, but often ignore the following content:

First, the length of armoured thermocouple

It is considered that the armored thermocouple should have a wide temperature gradient region after separating from the temperature measurement region. The thermoelectric emF of armored thermocouple also occurs in this area. To effectively prevent the heat transfer from the hot end of armored thermocouple to the cold end (wiring end), the fundamental method is that the cold end of armored thermocouple should have a satisfactory distance away from the hot end. Generally speaking, the error caused by the insufficient length of the armored thermocouple is negative and the correction is positive. The shorter the length, the greater the error, therefore, before the furnace verification needs to determine the length of armored thermocouple.

Two, armored thermocouple wire bending

Sheathed thermocouple wire is thin and soft, easy to deform, when the plastic deformation of sheathed thermocouple wire folding, distortion and other plastic deformation of the thermal electrode wire stress, change the thermoelectric characteristics of sheathed thermocouple, so that the accuracy of the measurement results of the deformation sheathed thermocouple is affected. Therefore, the armored thermocouple wire must be straightened before verification.

Three, armored thermocouple wire is polluted

Armoured thermocouple wire is polluted, or even oxidized, will make the appearance of the hot electrode wire is not bright, dark and black, then the thermal power characteristics of the hot electrode is extremely unstable, the accuracy of the measurement data is poor, therefore, to clean the polluted electrode, eliminate the pollution layer.

Four, echo the impact of the moment

The basic principle of temperature measurement by contact method is that the temperature measuring element must reach thermal equilibrium with the measured target. Therefore, the temperature measurement needs to adhere to a certain time, in order to make the two reach thermal balance. Adhere to the length of the time, with the thermal response of the temperature component time. The thermal response time mainly depends on the structure of the sensor and the measurement conditions, with great difference. Therefore, in the daily verification process, according to different types of armored thermocouple to choose the appropriate heating rate, heat balance moment.

Five, the influence of insulation resistance

At high temperature, the insulation resistance of armored thermocouple decreases sharply with the increase of temperature, so the leakage current will occur. The current flows into the instrument through the insulation whose insulation resistance has decreased, so that the indicator of the instrument is unstable or the measurement error will occur. Therefore, the insulation resistance test should not be ignored before the armored thermocouple is installed in the furnace. Only when the requirements of the verification regulation are met, the temperature tolerance verification can be carried out.