Differential pressure transmitter working principle

Differential Pressure (DP) transmitters measure the difference between two pressures. They use a reference point called the low-side pressure and compare it to the high-side pressure. Ports in the instrument are marked high-side and low-side. The DP reading can be either negative or positive depending on whether the low-side or high-side is the larger value. A DP transmitter can be used as a gauge pressure transmitter if the low-side is left open to the atmosphere.

Any difference of pressure across the cell causes the diaphragm to flex in the direction of least pressure. The sensing diaphragm is a precision-manufactured spring element, meaning that its displacement is a predictable function of applied force. The applied force in this case can only be a function of differential pressure acting against the surface area of the diaphragm in accordance with the standard force-pressure-area equation  F = P.A. In this case, we have two forces caused by two fluid pressures working against each other, so our force-pressure-area equation may be re-written to describe resultant force as a function of differential pressure (P(1)- P(2) ) and diaphragm area: F = P(1) – P(2) . A Since diaphragm area is constant, and force is predictably related to diaphragm displacement, all we need now in order to infer differential pressure is to accurately measure displacement of the diaphragm.


Where is Differential Pressure (DP) Measurement used?

Differential pressure measurement is largely used in domestic and industrial applications. It is often the basis of other measurements such as flow, level, density, viscosity and even temperature. The most common being level and flow.

DP Flow rate measurement  is one of the most common applications for differential pressure transmitters. By measuring the difference in fluid pressure  while the fluid flows through a pipe it is possible to calculate the flow rate.

Differential pressure flow meters have a primary and a secondary element. Generally speaking , the primary element is designed to produce a difference in pressure as the flow increases. There are many different types of primary element, the most common being the orifice plate, venturi, flow nozzle and pitot tube.

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

A good dp transmitter will ensure that the differential pressure is measured accurately independent of other changing parameters and will reliably transmit a signal to represent the differential pressure. In the case of a dp flow transmitter the output signal may also include square root extraction. Although it is common these days for this function to be carried our in a flow computer of DCS system /PLC .

The output signal from and industrial DP transmitter is likely to be 4-20mA, but it may also include digital communications such as HART, Profibusm Fieldbus, Modbus 485 RTU or one of many other communication protocols. The objective being to provide an electrical signal for transmission to a remote process control

Advantage Differential pressure Transmitter   :-

The biggest advantage of this type of technology are exceptional accuracy and stability. When you have a pressurized or closed vessel, we need to compensate for the pressure or vapor with the low-pressure cell of the transmitter. Depending on the needs, we can use a capillary or impulse line, but either way, we can have problems.

Disadvantage Differential pressure Transmitter 

A wet leg may leak or ice up, requiring a special installation to avoid these issues. A dry leg may create condensation in the line. These issues can make your measurements vary and demand extra maintenance. Hence, the capillary and seal system came to eliminate many of these problems. However, the solution also has its own problems!

Differential pressure Transmitter Technical specification:



ParameterMinimum Requirements
1ServiceWhite Oil – MS/HSD
2TypeVariable capacitance /Piezo resistance/ Resonant silicon sensor
3LocationAcross strainer / nozzle of tank for level measurement
4Accuracy0.065 % of span
5Over range protection150% of range
6Field IndicatorInbuilt, Digital in Engineering Unit (Kg/cm2)
7Output Signal4 –20mA along with HART/ Modbus
8Power24 V DC, 2 Wire
9ProtectionCircuit protected against Lightening & surges (Minimum up to 1 KVA of surges), Reverse Polarity
10Enclosure classEx-proof, IP 65 /NEMA 4 or better and Intrinsically Safe (IS)
11I.S. BarrierRequired
12Area ClassificationZone I & II, Gr. IIA/IIB, T3 as per IS2148
13ExecutionIntrinsic safe with active barrier in control room / Ex-Proof
14MountingAcross Strainer with bottom entry & coplanar style.
15Wetted Parts (M.O.C)SS 316
16Case MaterialPolyurethane coated Die cast Aluminium/ polyester powder coated Aluminium (suitable for use in corrosive environment)
17Range0-2 Kg/cm2 or as per site requirement
18Process Connection½” NPT (F)
19Electrical Connection½” NPT(F)
20ManifoldSS316 – 3 way suitable to process connection.
21Mounting KitNecessary accessories suitable for mounting across strainer
22Tag PlateMetal tag plate to be provided
23ApprovalsCCOE/PESO as per project requirements

Differential pressure Transmitter Inspection and Test Plan

This content provides you with sample transmitter inspection and test plan. This is draft ITP and you need to modify it to meet your PO requirements.

Transmitter Inspection and Test Plan TaskVendorTPI InspectionClient
Differential pressure Transmitter mill test reportHRR
Differential pressure Transmitter Performance/Function testHWR
Differential pressure Transmitter Visual / Dimension InspectionHWR
Differential pressure Transmitter Insulation resistance testHWR
Differential pressure Transmitter Documentation review prior to release(Final activity of Technical Inspection)HHR
Differential pressure Transmitter Pre-shipment InspectionHHR

H: Hold Point implies that relevant production activities shall not proceed until the continuation of work is permitted by Purchaser. (Notification req’d)

W: Witness point implies that Purchaser intends to witness the designated inspection feature. If Purchaser decides not to witness the relevant feature, production can proceed without permission of Purchaser. (Notification req’d)

S: Witness, but spot check basis. Initial operation will be witness point and subsequent operation will be witnessed at discretion of Purchaser considering the results of previous inspection. (Notification not req’d : Randam Inspection)

R: Review inspection & test records (Notification not req’d)

General Note for Transmitter ITP:

1) Differential pressure Transmitter Witness/hold point by Code Inspector, when applied by Code /Local Regulation, is to comply with its requirements and to specify in Vendor’s ITP.

2) Type of Inspection by Differential pressure Transmitter Purchaser may be changed according to equipment criticality and vendor QC evaluation at job stage.

3) When requested in the requisition, detailed Differential pressure Transmitter ITP shall be submitted by Vendor.

4) Monitoring/Observation of Differential pressure Transmitter Vendor shop daily routine works (i.e. Storage of materials, Adherence to approved procedures, Testing tool calibration check, workmanship, cleanliness and etc.) shall be done by Purchaser during inspection visit.

5)100% Transmitter inspection which is covered by the combination of witness / spot check / record review inspection & tests listed above and vendor’s original

QC activity shall be confirmed by Purchaser’s inspector prior to shipment.

6)Unless otherwise specified in Differential pressure Transmitter REQ., the sampling Q’ty of ” S (Spot Witness Check) ” shall be as follows:

3 to 20→3Pcs(all if total 2 and less) , 21 to 50→5Pcs, 51 to 100→10Pcs,101 to 200→20Pcs, 201 to 300→30Pcs, 301 to 500→50Pcs

Minimum 25 pcs or more of the commodity shall be arranged for H/W/S inspection and test at one time inspector’s visit as much as practical.

7) Final Differential pressure Transmitter Visual Inspection/Documentation Review/Pre-Shipment Inspection may be combined within one time visit.

Differential pressure Transmitter Installation hookup diagram

  • Hookup diagram for Liquid service Differential pressure Transmitter  
  • Hookup diagram for Gas service Differential pressure Transmitter  

Differential pressure Transmitter manufacturer list:

Invensis (Foxboro)
Emerson Process Control (Rosemount)
Endress + Hauser

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