Answer / ali abideen ,,

Generally in a transistor some amount of voltage is
required for turning it on. This voltage is the cut in
voltage. Up to this voltage the exists a nonlinearity in
its characteristics. Beyond this cut in voltage the char.
of transistor is linear. This nonlinear region lays between
0-4ma, beyond 4ma it linear. This is why 4-20 ma range is
used.

Answer / anil

starting with 4ma is to know DEAD or LIVE of the
transmitter,4 to 20 is used for linearty and according to
ohms law to covert 1 to 5 volt dc,

Answer / ravi vaya

Because 4 mA is called live zero of transmeter
If we set 0 mA at 0 pressure ya any ,we can`t deside inst.
is working ok .

And also mA is be a linear from 4 mA

Answer / lalatendu keshari prusty

4-20ma signals will travel along distance without dropping signal value and the internal circuit distinguish between connection break & minimum value.

Answer / mike betts

I am not sure why the range of 16mA i.e 20 - 4, but the 4mA
starting is so that you have a posative value of current for
all values that the loop current represents. This way, if
you have no current at all, the loop is open circuit for
some reason.

Answer / vanapalli rajesh

because 4-20ma is a linear range compared to other ranges

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