Selection of instrument signal: FF bus or 4-20mA
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I recently reviewed the data sheet for instrument purchases, and the experts said that for instruments with interlocks, 4-20mA instruments should be used, while those without interlocks can use the FF bus. Do 4-20mA instruments only include control valves and flow meters, or do they also include pressure gauges, thermometers, and level gauges? Should level switches as well be equipped with 4-20mA instruments? I’m not quite sure; could someone who knows more about this please provide some clarification? Thank you
4-20 is still more convenient. Easy to use and simple
It is said that the adoption rate of 4-20mA/Hart currently exceeds 99%
The manager is being arbitrary – they won’t allow the use of 4-20 signals everywhere; instead, they insist on using FF signals. As someone in the process engineering field, reviewing instruments is a huge burden for me
It is recommended that you choose a 4-20mA signal; for combustible gas and fire alarm systems, the bus mode can be considered
For interlocking, 4–20mA is still the preferred choice. It’s an unimportant signal, and if there are many such signals, a bus can be used. It is best to calculate the costs of the following two options.
The project I am working on has also gone through this stage – whether to use 4-20mA for everything, or 4-20mA for the interlock functions and FF for the non-interlock functions. In the end, after decision-making by the leadership, FF was still used for the non-controlled parts, and a small number of controlled parts also adopted FF. It seems that the FF approach saves a lot of wiring work, but it is relatively more difficult to install, and there is a higher likelihood of problems arising later on. As mentioned above, if it’s similar to the control of pump groups without interlocks, and there are a large number of uncontrolled and uninterlocked sensors concentrated in one place, FF can be tried in small quantities; for everything else, it’s better to use 4-20mA
Control valves, flow meters, pressure gauges, thermometers, and level gauges can all use 4-20mA for transmission； Level switches cannot use 4-20mA; they are digital signals, not analog signals. The FF approach reduces a lot of wiring work; it is recommended for non-control instruments. It is not suitable for interlocks or safety instruments, as it makes it difficult to diagnose and address issues that arise later on.
Thank you for the detailed explanation; I’ve learned a lot
The FF bus, as a communication method, is difficult to diagnose when problems arise; 4-20mA HART has basically become widespread
Coincidentally, I ran into this problem today as well; a document mentioned that interlocking must use point-to-point hard wiring. In the case of FF, these general principles must be stipulated in an overall manner, as they cover a wide range of aspects