Answer / jose

Instruments have a nominal resistance of 250 ohms. With
that, the usual output of field instruments is 1-5 volts or
4-20 mA. If you put a 250 ohm resistor on a 1 volt output
the the result will be .004 amps or 4 mA. The same holds
true if you put a 250 ohm resistor accross a 5 volt output
you will get .020 amps or 20 mA.

Answer / vijay

usually there exsist linear relationship between this range
when you plot pressuer verses current graph. so depending
on application we can select o/p as 4-20mA or 3-15 psig.
also
for long distance transmission 4-20mA is good

Answer / rajesh mistry

1> Liner Scale
2> Live Zero as a electronics
3> Active signal for pneumatic equipment.

Answer / karthik

The Electronic circuits employed in all Instruments require
atleast 3 mA for its actuation so the lowest range is
limited to 4mA & the rest as said by JOSE is exactly correct

Answer / ravi kiran

After observation of current functioning engineers finalized
that at 4-20ma the current is having linear and constant
properties. Below 3ma and above 25ma the current properties
are different.

Answer / s

there are two reason live zero and other is linearity..
if we divide the 4-20ma in four part it will show more
linearity than 3-15ma

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