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Answer Posted / v.r.m.kiran
We are using 4 to 20ma because
1. Distinguish between process Zero & Instrument Zero.
2. It is easy to convert 4 to 20ma to 1 to 5v(Digital
Signal) by using a 250 Ohms Resistor in series.
V = IR
V=4*250=1000 MV = 1V, 8*250=2000 MV = 2V, 12*250=3000 MV =
3V, 16*250=4000 MV = 4V, 20*250=5000 MV = 5V.
3. Now we are using digital Indicators for display. Olden
days there will be analog display meter in the transmitter
for indication(0 to 100)%. When we apply 4ma or while
checking the transmitter online pointer should indicate
zero then we say that instrument is working correctly but
it comes bellow zero we have to check our instrument & have
to calibrate.
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