Why are thermocouples rather than thermal resistors commonly used in power plant steam systems?
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I noticed that in most power plants, dual thermocouples are used for measuring the temperature of steam pipes. In terms of temperature measurement units, thermal resistors can also be used; they are easier to work with compared to thermocouples. Are there any other considerations?
Let’s look at the long-term temperature range for steam operation instead.
Setting aside the differences in performance between thermal resistors and thermocouples, it might actually be related to the design habits of the design institutes – they prefer to stick to previous designs as it’s simpler!
This is related to the temperature measurement range. Power plants are typically high-temperature environments. According to API551: the recommended operating temperature for thermal resistors is -200~450 ； The recommended operating temperature for thermocouples is -270~1815°C (the range varies depending on the type). Generally, factories use 400°C as a threshold; thermoresistors are used for temperatures below this value ； Thermocouples are preferred in low-temperature areas, as resistance thermometers provide more accurate measurements there, whereas thermocouples have a low thermoelectric potential at such temperatures.
The temperature range for which thermocouples can be used is wider than that of thermal resistors
I used to think the same way too; supercritical thermal power plants have temperatures of four or five hundred degrees, but I saw that the steam in nuclear power plants’ secondary loops is only at 300 degrees, and yet thermocouples are still used there, which seemed a bit strange to me
For convenience’s sake, everyone chooses thermocouples； Either they didn’t think much about it when making the choice, as the thermocouple has a wide temperature measurement range anyway. However, in low-temperature regions, the accuracy of thermal resistance is superior to that of thermocouples; therefore, thermocouples are not used in cryogenic conditions.
There are many situations in power plants where thermocouples must be used; if one has to choose just one type to use, then it has to be a thermocouple； Power plants are subject to strong electrical interference; thermocouples have better interference resistance than thermal resistors ； Recommended by some power plant design manuals or specifications.
Thermocouples have better interference resistance than thermal resistors； I think this claim is incorrect; setting aside the range of measurements, the most basic example is that high-voltage motor windings are equipped with thermal resistors rather than thermocouples.