Thermocouple accuracy out of spec

This post was last edited by Sai Xin on 2022-3-31 09:27. We are a company that manufactures thermal resistors and thermocouples. Based on our experience, the following situations may lead to accuracy deviations in the products: 1. The products are defective from the start, with accuracy levels that exceed the specified requirements； 2. The insulation performance of the product is very important. For armored thermoresistive thermocouples, if the insulation performance is poor, that means there is moisture inside, and this will gradually cause the measurement results to be lower than the actual value ； Therefore, the insulation performance of the product is an important step in the inspection upon arrival at the factory. 3. For armored thermoresistors, if film elements are used and the temperature variations are large, deviations can easily occur. Since the thin-film element is made by sputtering platinum onto a ceramic base and then sealing it, the deformation caused by thermal expansion and contraction cannot be released properly, which makes errors more likely to occur. 4. When thermocouples are used in high-temperature oxidative environments, their Ni-Cr electrodes are relatively prone to oxidation; chromium oxide is likely to form, causing the color to turn green. foreigners refer to this phenomenon as \"Green Rot.\" This also leads to a decrease in the potential difference between the two electrodes of the thermocouple, resulting in lower measurement values. Eventually, this causes the wire of the thermocouple to become brittle and break ； Foreigners have conducted a lot of research on this; I have two English-language papers, but it’s pointless to publish them separately ； Feel free to contact me if needed. The above is just a starting point; those of you who are experts on site likely have more experience in applying these methods.

Thermal resistors are used less frequently on our end; K-type thermocouples are mainly employed for temperature measurement, typically to gauge the temperature at the exit of the radiant section of pyrolysis furnaces (under conditions of around 850°C). Due to the erosive effect of the high-temperature pyrolysis gases, the insulation surrounding the thermocouple can sometimes be worn through, so when dealing with such issues related to the thermocouples, they are usually removed for inspection… and only reinserted once no problems are found Thermocouples are prone to measurement errors after rapid cooling and heating; subsequently, to determine whether there is a problem with the thermocouple, it is swapped with a thermocouple from an adjacent pipe section...

Ultimately, it’s still a matter of quality – you get what you pay for.