Armored Wear-Resistant Thermocouple If not used carefully, it can also cause significant measurement errors. In response to 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., and points out precautions during use such as inhomogeneity of thermocouple wires, shunt error of armored wear-resistant thermocouple, selective oxidation of K-type thermocouple, K state, operating atmosphere, insulation resistance, and thermocouple degradation. This helps improve measurement accuracy and extend thermocouple life.
In existing temperature measurement systems, the commonly used temperature sensor—thermocouple—is often mistakenly considered 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 usage methods can cause large measurement errors, and even a qualified thermocouple may become unqualified due to improper operation. In reducing atmospheres such as carburizing, if not careful, K-type thermocouples can also exceed tolerance due to selective oxidation.
To improve measurement accuracy, reduce measurement errors, and extend thermocouple service life, 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 here introduces in detail the measurement errors caused by shunt error of thearmored wear-resistant thermocouple and related 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 taken out of the furnace and tested in the metrology room, and it passed. So what was the problem? Later, based on the on-site 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 means that when using an armored wear-resistant thermocouple to measure furnace temperature, if there is a temperature distribution exceeding 800°C in the middle part of the thermocouple, the insulation resistance decreases, causing abnormal thermocouple readings. 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, since 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, resulting in a shunt error in the thermocouple output electromotive force.
(2) Conditions for Shunt Error
Insert thearmored wear-resistant thermocouple horizontally into the furnace. Its specifications and experimental conditions are: diameter φ4.8mm, length 25m, middle heating zone length 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 depend on factors such as the type and diameter of the armored wear-resistant thermocouple.
