Answer / priyabeet das

1st of all this is a live zero concept. now what is live zero? suppose the range is 0-20ma.so the Tx will indicate0ma as lower value.but when there is no i/p or there is some fault in process then also Tx will show 0ma as reading. so , seeing 0ma we cant decide whether it is for any i/p or not. that is why the lower LRV is chosen as 4ma.

Answer / suresh

This is Live zero concept. In case of airsupply failure or
Current loop failure the Control equipment (DCS/PLC) senses
below 3.5mA, considered as Loop Open circuit.

Answer / sudipta,iter

bcz 4 ma is the amount of current that occurs when there is
no input to the transmistter and coresponding pressure is
3psi.this is called as live zero concept.so by using thus
we can detect error easily .thats why we usese this value.

Answer / saravanan.m

FOR LIVE ZERO & ITS THE CORRECT LINERITY

Answer / oza paritosh m.

It is also useful for instrumentation guy to differentiate
between process zero and instrument zero.

Answer / abu maryam

For 4 mA & 3 Psi it is virtual zero to differ with actual zero (0 mA & 0 psi) to define if actual signal (mA & air )is found or not.
For 4: 20 mA
In my opinion the reason for use these values is most electronic cards work from 1: 5 volt and resistance (hart ) is 250 ohm
then V=I * R
R= 250 ohm and V= 1 volt so I= 1/250=,004 A= 4 mA
R= 250 ohm and V= 5 volt so I= 4/250=,02 A= 20 mA

Answer / dr

Why not more than 20ma??
More than 20ma continuously for 20Sec will make your heart
to malfunction.

Answer / prashant jadhav

If we look at Hazardous and power consumption point, 4-20 mA is the perfect range qualifying both points...Not able to produce large sparks and consumes least power...So it is made as a Standard signal..

Answer / rajesh j

4-20mA is because the least power consumption range with
linearity and to detect even when signal line opens

3-15psi is because of the minimum linear range for which
the control valve will respond to pneumatic signal

Answer / rafiq

these are standared things. its nothing but a i to p
converter. for 4 ma it will convert 3 psi and for 20 ma it
has 15 psi.

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