why we use 4-20 mA to do calibration of the instruments.why
we cant use 0-20 mA?
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Answer Posted / kashif mahmood
1) This is called the live zero or elevated zero concept.
It makes us differentiate between the signal failure and
the zero reading of the transmitter. Let's say, LRV=0 mA
and the instrument cable is cut. How can somebody ascertain
that this is process zero(LRV) and not the cable fault.
whereas if LRV = 4mA and the cable is cut, the current
through the loop would be 0mA and one can find out that
this is the cable fault.
2) its the lowest linear possible range with minimum power
consumption.
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