why 4-20 mA in covection system?

Answer Posted / ranganadh

Long distance communication mA signal is preferable than
mV, V signals to avoid noise. It cann't be predictable with
0 mA, whether the loop was broken or the signal itself. So
to avoid loop break problems we consider 4mA as the lowest
signal. Moreover, it is recommended to put a 250 ohms
resistor at the input terminals of the instrument to get
1VDC for 4mA and 5 VDC for the 20mA and further these
signals will be amplified by instrumentation amplifier in
the instrument circuit.

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