I have a question regarding thermocouples; I would be very grateful for your help
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There is a thermocouple placed in an ambient room temperature environment (with a temperature of 25°C); it is connected to a P+F safety relay via compensation wires. The safety relay features internal compensation, and its output is sent to the DCS for display. What is the thermoelectric potential of the thermocouple at this time? What does the indoor DCS read? And write down the reason. I think there shouldn’t be a thermoelectric potential when the cold and hot ends are at the same temperature. Please advise, experts
Your understanding is clearly incorrect. Before we start production, the temperature transmitted by the heat sensors is displayed on the DCS to be between 25 and 35 degrees. The temperature in the server cabinets is usually around 26 degrees, so it’s impossible for these two values to cancel each other out and result in a certain temperature value
Based on the experiments I conducted recently, we placed thermocouples inside the cabinet area; a short circuit corresponded to around 26 degrees, while 0 mv corresponded to around 27 degrees. Under the conditions mentioned by the original poster, the value should be around 0 mv, as there is compensation calculation due to safety factors
The thermoelectrical potential is 0 mV; the DCS shows 25°C. It’s an exam question – I’m not sure if I answered it correctly:$
When the thermocouple is placed at room temperature, the temperatures at its cold and hot ends are equal; the output in mV is zero. Due to cold-junction compensation, the DCS displays 25°C.
:handshake, thank you all; I’m showing off a bit
The temperature of the thermocouple is obtained by adding the temperature corresponding to the potential value to the temperature of the cold end; therefore, with a potential output of 0, the temperature is the room temperature
Thermocouple temperature measurement is based on the following formula: E(t_measured, 0) = E(t_measured, t_cold) + E(t_cold, 0). Therefore, when the thermocouple is placed at room temperature, the temperatures of the cold and hot ends are equal, and no thermoelectromotive force is generated; in other words, the output of the thermocouple is E(t_measured, t_cold) = 0 mV； When the cold-end temperature is 25°C, there is a value for E(t_cold, 0); in other words, E(t_measured, 0) = E(t_cold, 0), which represents the mV value corresponding to 25°C. Therefore, the DCS displays 25°C.