The issue of cold junction compensation in thermocouple temperature measurement
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A boiler uses thermocouple thermometers, locally installed integrated temperature sensors, and DCS current cards for temperature measurement; no compensation for the cold-end temperature is applied in this measurement process. Now, the issue of cold-end compensation needs to be resolved: should the temperature-induced current be set to zero, or should the DCS conversion value be adjusted by adding a fixed compensation value? Which approach is more reasonable? I would appreciate your advice!
It should be set on the temperature variation side; after the temperature variation comes the standard signal, and since the DCS processes it as a standard signal rather than a temperature signal, no further correction is needed. Furthermore, regarding changes in the ambient temperature at the site, applying a fixed compensation on the DCS is not scientific.
Which integrated temperature control system are you using? This usually has cold-end compensation built in. If the on-site temperature variation feature is not available, I think it would be more practical to use a Rosemount 644 temperature transmitter. The 644 manual states as follows: Total digital accuracy of the thermocouple measurement: Digital accuracy of +0.25°C (0.45°F) (cold junction compensation accuracy
No compensation is required; if an integrated temperature compensation system is not used, it is necessary to implement cold-section compensation on the cabinet side in order to transmit signals via compensation wires
The temperature-dependent output signal doesn’t need further cold-end compensation, right?
Integrated temperature transmitters all have cold-junction temperature compensation functionality; some also have linearization capabilities, so the current signal they output corresponds to the actual temperature being measured.
The last edit to this post was made by chensf123 on 2018-3-27 at 20:18. Thank you all for your replies; I would like to provide the following explanation: If an integrated temperature sensing system is not used on-site, that is, if temperature sensors such as temperature cards are used on the DCS side or thermocouple-based safety gates are employed… In such cases, the cold-junction extension method is typically used. By taking advantage of the fact that the temperature characteristics of compensation wires remain relatively constant within the 0–100 degree range, similar to those of thermocouples, the cold junction of the thermocouple can be brought into a control room where the temperature remains stable； And the control room temperature is used as the cold-end temperature for compensation. Many DCS manufacturers install a thermocouple (resistor) directly on the control room temperature card to serve as the thermocouple (resistor) for measuring the compensation temperature. What we purchase here is a domestic brand that cannot be found online; it has only two adjustment screws for temperature control, and the adjustment of the zero point range is achieved through these two screws. Not all intelligent models can be configured with many functions.
It’s just a knob for zeroing and one for adjusting the range
There aren’t many additional functions, but an built-in automatic compensation for the cold end of the thermocouple is a must, unless one is buying a very low-quality counterfeit product. The two adjustment screws are used to set the zero and range of the 4-20MA current.