Updated: Nov 29, 2020

In this article, we will learn differential pressure (DP) or pressure differential transmitter widely used in the process industries to measure flowrate, level and pressure of the fluids.

Differential pressure:

One of the most commonly used measurement technique to measure pressure, level and flow are known as differential pressure or DP. As a name indicates, DP is used to measure a difference in pressure thus give differential pressure which is given by,

Δ = p2 - p1

where p2 = high pressure (Upstream)

p1 = low pressure (Downstream)

When a flow is passing through a pipeline, due to restriction like orifice plate, flow nozzle, Annubar, pitot tube, venturi tube, etc. it creates a pressure drop.

At a restriction due to pressure drop velocity will be high.

when pressure is applied to the high side, diaphragm deflexed and mA rises while on another hand if we provide 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.

(Source: Coulton instrumentation ltd.)

You can see in the above-mentioned schematic of the DP transmitter, in which it has two-part,

1. Primary Element

2. Secondary Element

The primary element consists of restriction like orifice plate, pitot tube, Annubar, flow nozzle, venturi tube, Dall tube, etc. while on the secondary side it contains DP sensing assembly.

Small capillary tube from the upstream side and from the downstream side is connected to sensing assembly.

Higher pressure on upstream and lower pressure on downstream is calculated. Difference between both then represented on the display.

In the second figure, again it is a differential pressure transmitter which has two port high and low.

On the secondary side, connections are made. Positive goes to positive and negative goes to negative. 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 the main direction.

Different manufacturers use different sensing technology to yield the same result. Some of the leading manufacture used technology is,

Emerson - Differential capacitance

Yokogawa - resonant element

Foxboro - piezoresistive technology

To know more in detail and cut view of pressure transmitter kindly go to the video section and watch a video.

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