Answer Posted / bhagwan
We use 4-20 mA out put because this signal goes to the field (from control room to the field,) where it is then converted from 3 psi to 15 psi as output to control the flow , tempersture, pressure etc.
In the control room, the same signal is converted from 1-5 volts to read measure flow, temperatures,pressure and at the same time , it is sent back to out put card
to convert 4-20mA. to 3-15 p.s.I. to actuate control Valve.
As the distance beween Control Room & the field is very far, so the loses for transfering 4-20mA. are nill.
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