Answer / sunil kumar

Before using 4 - 20 mA all most of the instruments were
using 0 - 20 mA but after some problem faced for
finding "live zero", it was changed to 4 - 20 mA. By live
zero I mean when the instrument is connected but with 0
process value, the current is 0 mA, & the same current
value is seen when there is a cable damage or no power. By
this it was difficult to find the live zero. After that it
was changed to 4 - 20 mA. 4 - 20 mA Why? why not 5 to 20 mA?
The reason is that most of the a to d convertor which is
capable of providing 1 - 5 VDC with 250 ohms the same
current could be used as per below.
4 X 250 = 1 V DC & 20 X 250 = 5 V DC.

Answer / gurpreet chahal

we use 4-20 ma to achieving maximum linearity ,and another
tning safety point of view no spark can genrate up to 20 ma.

Answer / therion

for eg. if we are monitor a flow tube and measure the rate
of flow through the tube or pipe then 4mA will represent
ZERO flow and 20mA maximum flow..if it drops to below 4mA
then it shows reverse flow (which is not good)

Answer / sandeep bisen

due to achieving maximum linearity in between 4- 20 ma.

Answer / harish - greenesol

actually for plc and for electronic systems we usually
supply in mamps. GENERALLLY FROM TRANDUCER WE USE TO TAKE
OP IN 4-20MA , for RTD also .

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