Answer / praveen kumar gupta

4mA is the minimum operating current of controling device

Answer / vaseekaran

Let us keep the value as 0 to 20 mA,
The valve we measure in the output will be zero if the meter is in idle state and also it will show the same zero even the instrument is in fail condition.so we cannot get the exact and get confused if there is a problem in instrument while cross checking.this is the reason we are using 4 to 20mA.if the instrument fail then we can get only zero and if the instrument is idle we will get 4mA.so we can easily differentiate it.

Answer / raja

There is a long tradition dating from the early pneumatic
instruments for a control range from 25% to 100%. If you
want to know why, look at the pressure vs travel curve for a
flapper-nozzle. The early electronic instruments used the
same force-balance approach (many with a 10-50 mA range -
easier to get a higher force with the higher current) which
was eventually translated over into the pure electronic format.

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