I’m reaching out for advice again, regarding the isolator issue~
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First, I’d like to thank all the experts. For a current project, it is required that all DCS signals be isolated using isolators. So, for the signals from thermocouples and thermal resistors that come from the field, should an 1:1 isolation approach be used (meaning the original signal remains unchanged both at the input and output), or should the inputs from thermocouples and thermal resistors be converted to 4-20mA signals at the output? Which approach is better?
Since it’s been requested, follow the requirements; otherwise there will be problems later. For thermocouple thermal resistance, an isolated temperature change safety barrier can be selected.
Any of these will work; there are no specific requirements. But we are *used to at least controlling and interlocking the circuits; the system receives 4-20mA signals, which means that field temperature sensors or safety gates with temperature conversion functions are used. Therefore, if unified management is adopted, all temperature signals can also be in 4-20mA format, although it is also possible to use mV or RTD signals – the receiver end of the system remains unchanged in either case
Signal isolation can only be achieved by using temperature variation; otherwise, it goes directly to the DCS.....
I think changing everything to 4–20 ma would mean that there would only be two types of cards needed: one for analog signals in the 4–20 ma range and another for digital signals. If one of these cards fails, it can be replaced immediately with a spare one, without having to worry about not having a suitable card or port available
For the signals from thermocouples and thermal resistors, in order to avoid accuracy losses during conversion, we generally do not use isolators and instead feed them directly into the DCS
There are advantages and disadvantages to using isolators, but overall, the advantages outweigh the disadvantages. Without an isolator, it is easy to damage the card channels, the entire card, or even cause more serious consequences. There are lessons to be learned from this.
We are also willing to make certain sacrifices: isolators are generally added at the upper parts of high-altitude structures and towers, while in indoor areas, reaction vessels, on the ground, at low altitudes, and in places where high precision is required, DCS is used directly; for the card components, point isolation is employed
Choose the latter option; it’s better to convert it to 4-20mA. In this case, the DCS cards used will be cheaper compared to those with thermal resistance or thermocouple input functions