Answer / nilesh karpe

Dp- transmitter workes on Differential pressure.the
pressure difference between low and high pr. side.for eg,
pressure on high side is 2Kg/cm2 & on low side is 1.5Kg/cm2
then D.P. will 0.5 KG/CM2.
in conterol terms the diaphragm element senses this 0.5
KG/cm2 & generate output curresponds to signal.The input of
transmitter is 24 V DC signal .& is converted to 4-20 MA
output according to the callibration range.

Answer / naga manikandan

If u r giving presssure to only LP side not HP side the mA
output will be -ve(i.e it will go below 4mA) and the
reading will be in -ve side.

Answer / jazz

Dp transmitters work by measuring the difference between the
pressure measured on the high pressure port (HP) and the low
pressure port (LP). If for example the pressure on the HP is
100 psi and the pressure on the LP is 75 psi, the DP
transmitter will measure the difference of 25 psi. Let say
for example that the dp transmitter range is calibrated from
0 to 50 psi dp, the transmitter will output a proportional
12 mA (representing 50% of the dp range) The 24VDC supply on
serves to give power to the transmitter electronics so that
the transmitter will output a proportional 4-20mA signal
corresponding to the pressure difference sensed by the
pressure ports.

Answer / 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

Answer / imran yousaf

when we apply pressureon LP side only , then there will be some outpur in the form of mA, it does not matter it ll' be either +ve or -ve bcs current have no polarity value.

Answer / shivu

yahh for DP-transmitter we will be having the two taps at
the side or at the bottom side ,but to know the working of
the transmitter or for the calibration purpose we have
give/apply the pressure at LP-side tapping & the HP-side it
should be kept open for the atmosphere ,,,,,,,,,hence by the
difference of the pressure we will get the MA-output.,,i,e
with respect to the applied pressure

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