Is this the correct way to calculate for thermocouples?
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The ambient temperature is 10 degrees, the control room temperature is 20 degrees, and the voltage measured on site is 4.3 mv. What will be the final result: 124 degrees or 117 degrees?
Ah, a K-type thermocouple should be used for Mu, and the result was 124 degrees.
This post was last edited by JianNeng on 2018-2-5 at 19:31; the result was 117 degrees, with E(x,0) – E(10,0) = 4.3 mV and E(X,0) = 4.697 mV
Under normal use, the cold-end temperature is not taken into account; based on the numerical values, it appears to be a K-type thermocouple. The voltage measured at the thermocouple terminals is 4.3 mV, while the voltage corresponding to 20°C indoors is 0.798 mV. Adding these two values gives 5.098 mV, which corresponds to a temperature of 124.35°C. For reference only
Nonsense. If it’s according to what you say, then what’s the need for a compensation wire? Just use the power cable to lead it to the control room – after all, the millivolt signal remains unchanged. At that amount, it’s considered a cold end. The thermocouple measures the temperature at the site; it does not represent the room temperature in the control room. This method of temperature measurement is a common approach used when dealing with faults in temperature instruments on-site; it is not intended to provide highly accurate temperatures. Field instrument technicians use this method by measuring the millivolt signal at the thermocouple terminals, estimating the temperature roughly, and then adding the atmospheric temperature at that location. Then check whether there is a problem with the thermocouple.
When I say to ignore the cold terminal temperature, I mean using a compensation wire to connect the cold terminal to the cabinet, with this connection path following the law of the intermediate conductor. The standard practice nowadays is to set a fixed value for temperature compensation in the indoor system (usually 1 mV). By connecting the thermocouple terminals to the indoor compensation wire on-site and measuring the mV value at the cold end of the thermocouple, adding this 1 mV compensation value gives the value displayed by the DCS
What is the basis for the law of the intermediate conductor? The temperatures at both ends of the conductor are the same. The temperature at the thermocouple terminals on site is 10 degrees, while the temperature in the control room is 20 degrees; there is a difference of 10 degrees between the two temperatures. How can the law of the intermediate conductor be applied in such a situation? The law of the intermediate conductor does not apply; your approach is incorrect. And the sum of the millivolt values you use – one being the millivolt voltage measured at the thermocouple on site, and the other being the millivolt value from the temperature lookup table in the control room – is adding these two values together accurate?
This post was last edited by liangjian2 on 2018-2-8 08:18. So why use compensation wires instead of ordinary cables? The compensation wire functions as an extension of the thermocouple’s cold end to the control room; therefore, in practical use, the temperature of the field environment is not taken into account. When adding the values, it is necessary to look up the mV value for each measured temperature and then add those values together; subsequently, another lookup is required to determine the total value℃
This post was last edited by beryue on 2018-2-8 at 15:49. 1. Connect a compensation wire to the DCS cabinet; the DCS cabinet serves as the cold end. The value displayed on the DCS is the actually measured temperature plus the temperature of the DCS cabinet (a temperature of the cabinet that represents the actual measured value is set as the cold-end compensation temperature in the configuration). 2. For on-site measurements, I usually measure directly at both ends of the thermocouple; in this case, the ambient temperature needs to be taken into account. 3. At least I have never done the thing of using ordinary shielding wire to replace the compensation wire. 4. The temperature calculated in practice is generally the temperature with the cold end set at 0°C.