d/p flow transmitter LP side give to pressure.what you
geting out put ? how you get(ma)?
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Answer Posted / carl_ellis
A loop powered transmitter's output can NEVER go to 3.0 mA
or 2.0 mA or 1.5 mA because this range is the electrical
current used to run the transmitter's electronics. A 2 wire
transmitter needs about 3.6 mA just to run its own
electronics. A 2 wire transmitter can only go as low as 0.1
or maybe 0.15mA below the nominal 4.0mA 'zero' to 3.9 mA or
3.85 mA.
But, 4.0 mA can be a negative number. There can be pressure
ranges of
-0.5 bar to - 0.8 bar
-0.5 bar to 0.0 bar or
-0.5 bar to +1.0 bar or
If the high side is left open to the atmosphere and a
pressure applied to the low side, the pressure will be
negative because a DP transmitter subtracts the low side
from the high side.
high side minus low side = pressure
atmosphere minus pressure = negative pressure
0 barg minus 0.5 barg = -0.5 barg
Just because the pressure is negative doesn't mean the
output has to be negative. The 4-20mA range just represents
a pressure range and that range can include a negative
pressures. The 4-20mA is interpreted as a positive or
negative value by the receiver, the
DCS/PLC/controller/indicator.
Carl Ellis
Measure First
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