Answer Posted / wilson

Most people has got the answer correct for "why 4-20mA and
not 0-20mA". Yes, it is to prevent undetected failure of
circuit or instrument.

As for "why not 3 to 15 or anything". I assumed the
question is not "why not 3-15mA" because not one use 3-
15 "mA", if really you have to ask that, you might as well
ask why not 2.314 to 33.776 mA.

3 to 15 is used, but in pneumatic system and the unit is in
PSI. i.e. 0.2 to 1.0 bar. The reason for 3PSI (or 0.2bar)
is the same as 4mA, to prevent undetected failure.

what will the sending unit in the fuel gauge utilizes?

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Programmable Logic control.

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