This article is about Different methods of pressure measurement widely used in the field of the instruments to measure field conditions in the industries.
To know the history of the pressure measurement click here.
Absolute pressure
Pressure can be measured with pipe or tank flooded with liquid or any other property. it use a ceramic cell. an electrically conductive material applied to ceramic cell compresses it against vacuum forming capacitor and thus measure absolute pressure. in the atmospheric environment absolute pressure indicated.
Gauge pressure
In gauge pressure gauge an opening is given so it can compensate against atmosphere to measure gauge pressure.
Hydrostatic pressure
In hydrostatic measurement liquid inside tank acts on the diaphragm with an increase in level pressure also increased. with this condition it easy to calculate the pressure inside tank which is given by,
p = ρ g h
where ρ is the density of liquid
g is the gravity force
h is the height of the tank
in an open tank, the pressure is continuously compensated in relation to ambient air. in atmospheric pressure, compensation pressure acts as gauge pressure. in such arrangement, a silicon chip used with resistance applied in form of whetstone bridge. when pressure applied oil used to transfer pressure to silicon chip where resistance change due to applied pressure.
Differential pressure
one of the most used measurement method to measure pressure. as the name indicate is measure a difference in pressure thus give differential pressure method,
Δ = p2 - p1
when pressure is applied to high side diaphragm deflexed and mA rises up on other hands if provide we apply pressure on negative side diaphragm deflexed again result in reduced mA signal which is a common misconception when people think that applied pressure on negative side tends to increase mA signal which is totally wrong.
In the 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 the port which come in completely seal.on the primary side, connections 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 such manner when you connect multimeter all current start flowing from multimeter blocking internal transmitter path. When you remove multimeter diode give way to pass current in the main direction.
Different manufacturers use different sensing technology to yield the same result. some of the leading manufactures used technologies are,
Emerson - use differential capacitance
Yokogawa - resonant element
Foxboro - piezoresistive
in future, we learn more about wiring and useful feature of the differential pressure transmitter.
Pressure transmitter
This is a transmitter used to measure only pressure not a difference in pressure. Which come with the simple design having directly mounted on top of the tank, bottom of the tank or in a pipeline where it uses the diaphragm to sense pressure.
it used as a level transmitter where we can use it as hydrostatic pressure measurement technique to measure level or pressure of the tank.
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