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Answer / tang
Because we want to know the "life zero".
Life zero is a condition where there's a signal when the process is at 0%. For an example if we use 0mA instead of 4mA, we might don't know either the process is really 0% or the measuring unit is faulty(that will also indicate 0mA).
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Answer / venkadesh
because the transistor operating region range is 4-20 ma,these in between range is transmitter working in safe level
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In 2 wire transmitter 1 wire is used for power and other is used for signal( 4-20)ma In 4 wire transmitter 2 wires are used for power and 2 wires are used for signal (4-20)ma . but my question is "what is the criteria or parameters we have to consider while selecting these transmitters for eg( distance, safety zones etc.
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