Thermocouple and compensation wire Q&A: Regarding distance constraints and their impact on accuracy.
I will answer about the effects when the distance from the thermocouple to the measuring instrument is long!
In power plants and large chemical plants, the distance from the temperature measurement point to the instrument room can be several kilometers. What are the issues in such cases? A: Due to the principle of thermocouples, the thermoelectric voltage generated is determined solely by the thermocouple materials used and the temperatures at both ends, and the characteristics do not change based on the diameter or length of the thermocouple wire. Therefore, it does not matter if the measuring part is thin and the middle part is thick, and there are no issues with measurement accuracy even with thin film thermocouples. However, there are constraints on the instrument side that measures the thermoelectric voltage, specifically a limitation on the input resistance to the instrument, which must be within a certain number of ohms. For this reason, in large-scale plants such as power stations, the diameter of the compensation wire is increased to facilitate long-distance transmission. For a certain manufacturer's temperature controller, the allowable signal source resistance for the thermocouple is set at 100 ohms or less, while the allowable wiring resistance for temperature measuring resistance elements is set at 10 ohms or less per wire. *For more details, please refer to the PDF document or feel free to contact us.*
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