Armored wear-resistant thermocouple If not used carefully, it can also cause large measurement errors. In response to the current problems, our Shanghai Automation Instrumentation Factory No.3 discusses in detail the main factors affecting measurement errors: thermocouple insertion depth, response time, thermal radiation and thermal impedance, etc., pointing out that thermocouple wire inhomogeneity, armored wear-resistant thermocouple shunt error, selective oxidation of K-type thermocouple, K state, operating atmosphere, insulation resistance, and thermocouple degradation are matters to be noted during use. This helps to improve measurement accuracy and extend thermocouple life.
In existing temperature measurement systems, the commonly used temperature sensor—thermocouple—is often mistaken as "just two wires, connect and done" due to its simple structure, but this is not the case. Although the structure of a thermocouple is simple, various problems can still occur during use. For example, improper installation or use can cause large measurement errors, and even a qualified thermocouple can become unqualified due to improper operation. In carburizing and other reducing atmospheres, if not careful, K-type thermocouples can also exceed tolerance due to selective oxidation.
To improve measurement accuracy, reduce measurement errors, and extend the service life of thermocouples, users are required not only to have operational skills in instrumentation but also knowledge in physics, chemistry, materials, and other fields. Based on years of practical experience, Shanghai Automation Instrumentation Factory No.3 introduces in detail the measurement errors caused by shunt error of armored wear-resistant thermocouples and the precautions.
Shunt error of armored wear-resistant thermocouple
(1) Shunt error
The armored wear-resistant thermocouple used in the carburizing furnace of Wazhou Group became inaccurate after only one week of use. To investigate the cause, the author visited the site but found no abnormality, so the thermocouple was removed from the furnace and tested in the metrology room, and it passed. So what was the problem? Later, based on the field installation characteristics of this thermocouple, research found that the above issue was caused by the shunt error of the armored wear-resistant thermocouple.
The so-called shunt error refers to the abnormal indication of the thermocouple when the middle part of the armored wear-resistant thermocouple has a temperature distribution exceeding 800°C, due to the decrease in insulation resistance. This phenomenon is called shunt error. According to the homogeneous circuit rule, thermocouple temperature measurement only depends on the temperatures at the measuring end and the reference end, and is independent of the intermediate temperature distribution. However, because the insulation material of the armored wear-resistant thermocouple is powdered MgO, its insulation resistance decreases by an order of magnitude for every 100°C increase in temperature. When the temperature in the middle part is high, leakage current will inevitably occur, causing a shunt error in the thermocouple output potential.
(2) Conditions for shunt error generation
Insert thearmored wear-resistant thermocouple horizontally into the furnace. Its specifications and experimental conditions are: diameter φ4.8mm, length 25m, length of the middle heating zone 20m, temperature 1000°C. In this experiment, the temperature difference between the measuring end and the middle part of the thermocouple is 200°C. If the measuring end temperature is higher than the middle part, a negative error occurs; conversely, a positive error occurs. If the temperature difference is 200°C, the shunt error is about 100°C. This cannot be ignored. The conditions for shunt error generation are related to factors such as the type and diameter of the armored wear-resistant thermocouple.
