Differential Pressure is Not Measured With Reference to a Specific Reference Pressure

7/25/2019 Differential Pressure is Not Measured With Reference to a Specific Reference Pressure

1/4

Differential Pressure is not measured with reference to a specific reference pressure (see Fg.4).

Unlike Gauge or Absolute pressure transmitters, Differential Pressure Transmitters do not attempt to fix the reference. Importantlyan increase in differential can be the result of increasing one of the pressures or decreasing the other.

An increase in differential pressure ould occur if P! became smaller ORif P" became larger. In a similar ay, a decrease in

differential pressure ould occur if P! became larger #$ if P" became smaller. The differential pressure measurement is notconcerned hether the loer of the to pressures is at a %acuum, atmospheric or some other pressure. It is only interested in thedifference beteen the to. Asshon in &ig'

The fundamental of differential pressure measurement is established.

(ot all Differntial Pressure transmitters, gauges, sensors, and transducers that measure differential pressure actually measure thedifference beteen to pressure as shon in fig'. )ome de%ices attempt to measure to gauge pressures and then mathematicallycalculate the difference. This method in our opinion is flaed, as it contains the uncertainty of four pressure measurements. If youould like to kno more about this sub*ect hy not contact one of our technical team.

3. Where is Differential Pressure (DP) Measurement used

Differential pressure measurement is largely used in domestic and industrial applications. It is often the basis of othermeasurements such as flo, le%el, density, %iscosity and e%en temperature. The most common being le%el and flo.

DP &lo rate measurement +&ig- is one of the most common applications for differential pressure transmitters. /y measuring thedifference in fluid pressure hile the fluid flos through a pipe it is possible to calculate the flo rate.

Differential pressure flo meters ha%e a primary and a secondary element. Generally speaking , the primary element is designed toproduce a difference in pressure as the flo increases. There are many different types of primary element, the most common beingthe orifice plate, %enturi, flo no00le and pitot tube.


2/4

The secondary element of the flo meter is the differential pressure transmitter. It is designed to measure the differential pressureproduced by the primary element as accurately as possible. In particular it is important that the differential pressure measurementis not affected by changes in the fluid pressure, temperature or other properties such as ambient temperature.

A good dp transmitter ill ensure that the differential pressure is measured accurately independent of other changing parametersand ill reliably transmit a signal to represent the differential pressure. In the case of a dp flo transmitter the output signal mayalso include s1uare root extraction. Although it is common these days for this function to be carried our in a flo computer of Dcssystem.

The output signal from and industrial DP transmitter is likely to be '"2mA, but it may also include digital communications such as3A$T, Profibusm &ieldbus, 4odbus '5- $TU or one of many other communication protocols. The ob*ecti%e being to pro%ide anelectrical signal for transmission to a remote process control instrument. )ee &ig-.

(o e ha%e disco%ered hat differential pressue is and ho it is used e can no focus our attention on the Differential PressureTransmitter itself

4. What is a Differential Pressure !ransmitter

The most common and useful industrial pressure measuring instrument is the differential pressure transmitter. This e1uipment illsense the difference in pressure beteen to ports and produce an output signal ith reference to a calibrated pressure range.


3/4

The industrial differential pressure transmitters are made of to housings +)ee &ig6. Pressure sensing element is housed in the

bottom half, and the electronics are housed at the top half. It ill ha%e to pressure ports marked as 73igh8 and 79o8. It is notcompulsory that the high port ill be alays at high pressure and the lo port alays at lo pressure. This labeling has its relationto the effect of the port on the output signal. This point is clarified in &ig : +Please see &ig :.

!ip "ou can see our range ofDifferential Pressure !ransmittershere.

At this stage, e ill try to explain the internal construction of the transmitter.

#. Differential pressure transmitter construction$

A differential pressure transmitter has three functional parts.

%) Direct Pressure sensing element (located in the lower housing).The ma*ority of industrial DP Transmitters are fitted ith diaphragm as the pressure sensing element. This diaphragm is amechanical de%ice. It is placed in beteen the to pressure inlet ports. The diaphragm ill be deflected by the applied pressure.

This is clarified in &ig:. This deflection is con%erted into an electrical signal. This is normally done by the sensors. The commonlyused sensors are +a )train Gauge +b Differential ;apacitance +c


4/4

&) 'lectronic nit$ !he electrical signal generated at the lower chamer * the sensor is in the range of milli+,olt onl* .This signal is to be amplified to 2-< or 2!2< range or is to be con%erted to '"2mA for onard transmission to a remoteinstrument. This upper housing is the Transmitter portion of the DP Transmitter hich houses the =lectronic Unit. )ee &ig: forfurther clarifications.

3) &+Wire 4+&-m /urrent !ransmitter$A D; output current is generated hich is directly proportional to the pressure range of the Differential Pressure Transmitter. Theloer range is 'mA, and the upper range is "2mA. This controlled current output is not affected by load impedance %ariation andsupply %oltage fluctuations. This '"2mA output is superimposed ith digital communications of /$AI( or 3A$T &)> protocol.

0. 1ndustrial applications of Differential Pressure !ransmitters$

There are unlimited industrial applications of Differential Pressure Transmitters.

#il and Gas flo metering in onshore, offshore and subsea applications.

?ater and effluent treatment plants. It is largely used to monitor filters in these plants.

It is used to monitor )prinkler )ystems.

$emote sensing of 3eating )ystems for )team or 3ot ?ater.

Pressure drops across %al%es can be monitored.

Pump control monitoring.

Documents

Differential Pressure is Not Measured With Reference to a Specific Reference Pressure