Why does a transmitter output start from 3-15 psi or or 4 - 20 ma?

The transmitter output stance from what is known as “live zero”. This system has specific advantages

  1. The systems automatically alarms when the signal system becomes inoperative.
  2. The output areas is linear ( Ratio of 1 : 5).


The advantage is that it does not have to be biased to true zero. A “Live zero” gives the computer additional information, so that it can takes appropriate alarm action in case of a measurement failure, because it can discriminate between a transmitter operating, but transmitting a zero measurement and a failure, in the signal system.