Answer Posted / carl_ellis
Answer 1 is correct. The ISA SP50 standard established
4-20mA worldwide as the analog control signal decades ago.
But what the answer 1 and the others miss is that using 1 or
2 mA at the low end would not provide sufficient power to
power 2 wire loop powered instruments.
2 wire loop powered instruments use about 3.6mA at 24Vdc to
power the electronics which makes the measurement and
controls the output circuitry. That's about 85mW, not a lot
of power.
1mA or 2mA of current would provide only a fraction of 85mW
available at 4mA for powering the instrument itself, and
reduce the functionality of 2 wire loop powered instruments.
Carl Ellis
Measure First
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