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Answer Posted / j jones
Sai Kiran.c is correct is saying that we don't start from 0mA because we need a "Live Zero" this is so that if the control loop goes open circuit we have an indication of this.
why do we use 4-20mA, not 1 or 2mA - 20mA, well simply just as in a pneumatic system we use 5-15psi, (0.2-1bar) it gives us the most linear output, and above that, if we were to use 1mA, any induced voltages from eddie currents in cables running alongside could cause a mA to be produced by the resistance of the cable, so our readings would be affected, and if we amplify the signal, the amplification has to be greater as we have to amplify 1mA more than a 4mA signal
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