Question { 6802 }

why 4 to 20 ma? why cant some other range?

Answer

The 4 mA current has been selected for zero, so that we can distinguish the process zero condition from the supply failure condition. it is called the LIVE ZERO. If the process is zero then output will be 4 ma. If the instrument power is not available then it will give 0 ma. We can ensure the online  availability of the instrument for measurement.

The noise pick up in the signal cables may be in the order of say 3.5 mA and so 4 mA has been fixed as zero. As a convention the span to zero ratio is fixed as 5, so that the span output signal comes to 20 mA. (3 to 15 psi, 1 to 5 Volts,0.2 to 1.0 bar etc.)

For remote signal transmission the cureent signal is preferred to voltage because of the drop in the signal cable. The supply voltage is 24 V will make up for the loss in case of current signal transmission.