Issues related to thermocouples and thermal resistors
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This post was last edited by wuhen714 on 2015-7-22 at 18:50. (1) What impact does using compensation wires as ordinary shielding cables in a thermocouple measurement circuit have on temperature measurement? (2) The measurement circuit of the thermal resistor (sorry, it’s a thermal resistance) uses a three-wire system; what happens when one of the wires in the common terminal is broken? What is the impact on temperature?
The compensation wire is used incorrectly; it depends on what type of thermocouple it is. Generally, there are two scenarios: 1. There is no temperature compensation, so the cold junction temperature cannot be compensated for. 2. The wire and the thermocouple are made of different metals; a temperature difference between the cold end and the end of the wire can generate a thermoelectric effect, which interferes with the measurement. The temperature difference when the two are combined is even greater. Secondly, thermocouples are not used in a three-wire configuration; it’s pointless to use them that way.
10 posts, 9 of which I can’t answer – I should read more books
(1) In a thermocouple measurement circuit, what impact does the misuse of compensation wires as ordinary shielding cables have on temperature measurement? If the temperature at the cold end of the thermocouple is the same as the temperature at the instrument’s terminal connections (the instrument has an automatic cold-end compensation function), it has no impact on the measurement ； If the cold terminal temperature is higher than the temperature at the instrument terminals, the displayed temperature will be low (the difference between the lower cold terminal temperature and the temperature at the instrument terminals) ； If the cold end temperature is lower than the temperature at the instrument terminals, the displayed temperature will be high (the difference between the higher cold end temperature and the temperature at the instrument terminals) ； (2) The thermocouple measurement circuit uses a three-wire system; what will happen if one of the wires in the common terminal is broken? What is the impact on temperature? I’ve never seen thermocouples use three wires; when a thermocouple is broken, it shows a value of zero or an open circuit status. Thermal resistors generally use a three-wire system......
The field thermocouple terminals (where temperature variations are significant) are connected, via compensation wires of the appropriate type, to a location where the ambient temperature is relatively stable (such as the control room). These compensation wires also generate a potential due to temperature differences; the thermocouple potential plus the compensation potential plus the zero-point compensation of the instruments in the control room equal the actual temperature at the measurement point.
Compensation wires used as ordinary shielded cables cannot provide compensation
Can a thermocouple also use a 3-core wire? Hearing about it for the first time. . . . Also, the original poster said that a shielded cable was used incorrectly; I think this could lead to inaccurate measurements, as there is no compensation in such a case. .
(1) This requires specific analysis, as there are many types of thermocouples. For example, if you are using a Type B thermocouple, using ordinary copper wire has no impact at all. It’s still accurate. It’s hard to say about the other divisions. (2) I guess you are asking about thermal resistance with this question. A wire at the thermal resistance common terminal was broken, causing the wire set to increase and the measurement value to become larger.
Hello, I would like to ask why the measurement value becomes larger when one of the common terminals of the thermal resistor is broken. What is the specific principle behind this measurement?
Hello, if the cold terminal temperature is higher than the temperature at the instrument’s terminals, then the displayed temperature is low (it’s the difference between the lower cold terminal temperature and the temperature at the instrument’s terminals). Does this mean that the displayed temperature is on the high side? Couple potential + compensation potential + zero-point compensation of the instrument in the control room = actual temperature at the measurement point (value taken from below); the compensation potential is equal to E(temperature at the wiring terminal) – E(cold terminal). If the cold terminal temperature is higher than the temperature at the instrument’s wiring terminal, then the compensation potential…
Good heavens, the original poster’s second question was about thermal resistors, but you all answered it as if it were about thermocouples. Am I not in the same dimension as you?