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Answer Posted / 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.
| Is This Answer Correct ? | 78 Yes | 8 No |
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