Analysis of the causes of failure in the use of thermocouples
The qualified thermocouples are not qualified in use, and this phenomenon is rarely known and has not attracted people's attention. The failure of the qualified thermocouple in use is mainly due to the inhomogeneity of the thermocouple wire, the shunt error of the armored thermocouple and the improper use of the thermocouple. This article will explain the mystery.
1. Thermocouple wire inhomogeneity
1 thermocouple material itself is not homogeneous. The thermocouple is inspected at the measuring chamber and inserted into the thermocouple to determine the depth of the furnace by 300 mm 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 300 nm long filament from the measuring end. However, when the length of the thermocouple is long, most of the filaments are in the high temperature region when used, and if the thermocouple wire is not homogeneous and is in a temperature gradient, the portion will generate a thermoelectromotive force. This electromotive force is called a parasitic potential, and an error caused by a parasitic potential is called a homogeneity error.
2 The thermocouple wire is inhomogeneous after use. For the new thermocouple, even if the inhomogeneous performance meets the requirements, repeated processing and bending will cause the thermocouple to produce processing distortion, and the homogeneity will be lost, and the thermocouple in use will be at a high temperature for a long time. The deterioration causes a change in the thermoelectromotive force. When the locally deteriorated portion is in a place having a temperature gradient, a parasitic potential is also generated to be superimposed in the total thermoelectromotive force to cause a measurement error.
2. Shunting error of armored thermocouple
When measuring the furnace temperature with an armored thermocouple, when the temperature of the middle part of the thermocouple exceeds 800 °C, the insulation resistance will drop and the thermocouple shows an abnormal value, which is called the shunt error. Since the insulator of the armored thermocouple is powdered magnesium oxide, its insulation resistance drops by an order of magnitude for every 100 ° C increase in temperature at high temperatures. When the temperature in the middle portion is high, there must be leakage current, resulting in a shunt error in the thermocouple output potential.
3. Measurement error caused by improper use
Qualified thermocouples, if used incorrectly, can cause large measurement errors.
1 thermocouple insertion depth is not enough, the temperature measurement is low due to thermal loss;
2 The temperature at the reference end is not properly treated. For example, the flame spray in the furnace causes the reference temperature to be high, which may cause measurement errors.
It can be seen from the above discussion that the failure of the qualified thermocouple in use may be caused by the above three conditions.The distribution box is a massive parameter on the data. Generally, it constitutes a low-voltage forest. According to the electrical wiring, it is required to assemble the switchgear, measuring instrument, protective electrical appliance and auxiliary equipment in a closed or semi-closed metal cabinet or on the screen to form a low-voltage power distribution. box. The circuit can be switched on or off with a manual or automatic switch during normal operation.
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