Answer Posted / abu maryam

For 4 mA & 3 Psi it is virtual zero to differ with actual zero (0 mA & 0 psi) to define if actual signal (mA & air )is found or not.
For 4: 20 mA
In my opinion the reason for use these values is most electronic cards work from 1: 5 volt and resistance (hart ) is 250 ohm
then V=I * R
R= 250 ohm and V= 1 volt so I= 1/250=,004 A= 4 mA
R= 250 ohm and V= 5 volt so I= 4/250=,02 A= 20 mA

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