Comparison of choices between thermocouples and temperature transmitters
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The signal transmitted from the thermocouple to the DCS is in millivolt form, and it has weak resistance to interference. Theoretically, a 4–20mA conversion should be performed on-site before sending the signal to the DCS; but why do design firms usually connect the thermocouples directly to the DCS cabinet? Especially on the boiler side, the compensation wire from the thermocouple to the DCS cabinet is nearly 200 meters long – isn’t this susceptible to interference? Why not install a temperature transmitter on-site?
It’s probably a cost issue; using a TC card is much cheaper.
Design institutes have different practices; economic factors are just one of them. Some design institutes use temperature-based controls exclusively, eliminating the need for separate junction boxes, which is very convenient.
Compensating cables also need to be purchased, and the cost isn’t much lower…… It’s a common issue; you should have raised your concerns during the design review
Basically, there’s no difference. It is easier to manage when the temperature is unified in the DCS cabinet room, and nowadays the DCS cards come pre-installed, which makes installation and management very convenient.
It’s probably due to cost considerations – going straight to the DCS TC card module. This saves the cost of one transmitter. Theoretically, the thermocouple MV signal is susceptible to interference, but if construction is carried out in strict accordance with the specifications, distances of four to five hundred meters are no problem.
Our factory is also facing this problem. We use multi-point thermocouples; each piece of equipment has four temperature measurement points, with three measurement sites at each point. There are a total of twelve thermocouples; the thermocouples at the front end go into the receiving cards, while the signals from the back end go into the interface cards in the cabinet, and communication is established via 485, thereby eliminating the need for compensation wires.
Reason why a temperature transmitter is not installed locally at the thermocouple:
The economy, right. You can save on the cost of transmitters.