Differential pressure - One of the most commonly used measurement method to measure pressure, Level, flow. As name indicate is measure a different in pressure thus give differential pressure,
Δ = p2 - p1
where, p2 = high pressure
p1 = low pressure
when pressure is applied to high side diaphragm deflexed and mA rises up on other hand if we provide pressure on negative side diaphragm deflexed again result in reduced mA signal which is common misconception when people think that applied pressure on negative side tends to increase mA signal which is totally wrong.
in figure it is a differential pressure transmitter which has two port high and low. In some case where transmitter used as a gauge pressure which doesn't have that low side of port which come in completely seal. On primary side connection are done. Pressure transmitter comes with 2 wire configuration which can be done very easily. Below that terminal test terminal used to measure mA current without breaking circuit which hs internal diode placed in such manner when you connect multimeter all current start flowing from multimeter blocking internal transmitter path. When remove multimeter diode gives way to pass current in main direction.
Different manufacture use different sensing technology to yield same result. Some of leading manufacture used technology are,
Emerson - Differential capacitance
Yokogawa - resonant element
Foxboro - piezoresistive technology
To know More in detail and cut view of pressure transmitter kindly go to video section and watch a video.