Differential Pressure Transmitters
For differential pressure measurement
pressure ranges 0 … 1 mbar up to 0 … 70 bar
What is a Differential Pressure Transmitter?
(You can take this article as a Beginner’s Guide to Differential Pressure Transmitters )
Differential pressure (DP) transmitter, also called differential pressure transducer. Differential pressure transmitters measure the difference between two pressures.
Differential Pressure Transmitter converts pressure measurements,
into a proportional 4-20 mA or a 1 – 5 Vdc output signal,
that functions as the input to a controller, recorder, indicator or similar device.
These transmitters find application in the gas, water,
and process industries that need accurate measurements,
over a wide range of environmental conditions.
The most common and useful industrial pressure measuring instrument
is the differential pressure transmitter.
This equipment will sense the difference in pressure, between two ports and produce an output signal, with reference to a calibrated pressure range.
The industrial differential pressure transmitters are made of two housings.
The pressure sensing element is housed in the bottom half, and the electronics are housed at the top half. It will have two pressure ports marked as “High” and “Low”. It is not compulsory that the high port will be always at high pressure and the low port always at low pressure. This labeling has its relation to the effect of the port on the output signal.
With its 4 … 20 mA, 4 … 20 mA HART®, PROFIBUS® PA or FOUNDATION Fieldbus™ output signals,
combined with either intrinsically-safe or flameproof ignition protection
(by ATEX), the DPT differential pressure transmitter is
suited to applications requiring these features. The electronics of all
transmitters with explosion protection, even for the flameproof variant, are safe. Thus it is possible to make adjustments on the instrument in EX areas while the instrument is live.
The working principle of the differential pressure transmitters
If you are a beginner of DP transmitters, may you know what is pressure first?
Wat is pressure?
Pressure is experienced when a force is applied in an area.
This means that we can increase the pressure,
by either increasing the force or by reducing the area.
We also tend to think of force weight
(this is only true if gravity is constant).
As an example, the force produced by 1lb of weight due to gravity acting on 1 inch x 1 inch (1sq inch),
would produce a pressure of 1lb/sq inch, this is often written as 1 Psi (1 pound per square inch).
If the same 1lb was applying a force to only half the area, we would actually say the pressure was equal to 2 Psi.
In our day to day experience, we may pump our car tires to 26 Psi.
We also experience pressure in the form of weather (or atmospheric pressure).
This could be considered the force being applied by the atmosphere on our heads.
As the atmospheric pressure changes so does the weather.
High pressure usually referes to clear sunny days,
while low pressure produces cloudy ones.
If we refer to our car treys, two pressures are acting on the wall of the trey. The pressure of the atmosphere on the outside of the trey,
and the pressure we read on the gauge when we pumped the trey up.
So, how does a differential pressure transmitter work?
DPT series pressure transmitters represent the latest development in their class.
The digital sensor makes measuring pressure even more accurate than before.
Fully automated zero point calibration, AZ-calibration,
offers reliability in the most sensitive of applications.
Besides, it provides cost savings over the lifetime of a building,
as it makes the device completely maintenance free.
Applied pressure readings are acquired by the diaphragms,
which register pressure as surface deformation,
and translate that value into an electronic signal.
This signal is transferred to the differential pressure module,
which compares the two values and expresses the pressure difference,
as a value of pounds per square inch (psi), bar, or kilopascals (kPa). These diaphragms are all-welded to maintain container integrity,
in vacuum conditions and for wet leg applications,
such as level measurement in crude oil distilleries.
The Components of a Differential Pressure Transmitter
A differential pressure transmitter has three functional parts.
1) Direct Pressure sensing element (located in the lower housing).
The majority of industrial DP Transmitters,
are fitted with the diaphragm as the pressure sensing element.
This diaphragm is a mechanical device.
It is placed between the two pressure inlet ports.
The diaphragm will be deflected by the applied pressure.
This is clarified in Fig-7. This deflection is converted into an electrical signal. This is normally done by the sensors.
The commonly used sensors are (a) Strain Gauge (b) Differential Capacitance (c) Vibrating wire. The sensor output
is proportional to the applied pressure.
2) Electronic Unit: The electrical signal generated at the lower chamber by the sensor, is in the range of milli-volt only.
This signal is to be amplified to 0-5V or 0-10V range,
or is to be converted to 4-20mA for onward transmission to a remote instrument.
This upper housing is the transmitter part of the DP Transmitter,
which houses the Electronic Unit.
A DC output current is generated,
which is directly proportional to the pressure range of the Differential Pressure Transmitter.
The lower range is 4mA, and the upper range is 20mA.
This controlled current output is not affected by load
impedance variation and supply voltage fluctuations.
This 4-20mA output is superimposed,
with digital communications of BRAIN or HART FSK protocol.
The Components of a Differential Pressure Transmitter
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.
Yokogawa gauge pressure transmitters use digital DPharp sensor technology,
to get accurate, reliable readings to you quickly.
Our DP transmitters can totally work the same.
Differential pressure transmitters get the differential pressure of a closed system,
by obtaining the applied force at two measurement points,
and calculating the difference against pre-defined application parameters.
This article will explore how a differential pressure transmitter works in more detail:
What are the different types of pressure measurement?
Using the above example, we can illustrate 3 types of pressure measurement.
a) Gauge Pressure – (the pressure in the trey) or (difference between absolute pressure and atmospheric pressure)
b) Absolute Pressure – (the combined atmospheric and trey pressure)
c) Differential Pressure – (the difference between any two measured pressures)
Application of differential pressure transmitters
Differential pressure measurement is 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.
Industrial applications of Differential Pressure Transmitters:
There are unlimited industrial applications of Differential Pressure
Example Industrial Applications:
Closed Tank Level, Density, Filter health,
and DP Flow (Secondary Element) Often used in flow measurement,
where they can measure the pressure differential,
across a venturi, orifice, or other type of primary element.
Oil and Gas flow metering in onshore, offshore and subsea applications.
Water and effluent treatment plants.
It is largely used to monitor filters in these plants.
It is used to check Sprinkler Systems.
Remote sensing of Heating Systems for Steam or Hot Water.
Pressure drops across valves can be monitored.
Pump control monitoring.
Further applications of differential pressure transmitters,
include low-temperature reaction tank monitoring in the chemical sector,
and filtration monitoring for a range of flow systems.
Differential pressure transmitter with metal measuring diaphragm.
The differential pressure transmitter with metal measuring diaphragm,
can be used universally for the measurement of liquids, gases and vapours.
Typical applications are level measurements in pressurized vessels,
as well as flow measurements in combination with DP flow elements.
Further applications are pressure monitorings on filters,
as well as density and interface measurements.
Differential pressure transmitters can be used for reaction,
and process monitoring in a range of industrial sectors,
with high resistance to corrosive media and atmospheric moisture.
The diaphragms are fabricated from stainless-steel,
and are resistant to oxidization and gas adsorption in flue pipework for example.
Inconel diaphragms can be applied for improved resistance,
to salt water in marine hydrocarbon exploration.
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
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,
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.
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 instrument.
Operation of DP transmitters
The main assemblies of the DP transmitter,
are the electronics housing, sensor module and process flanges.
The electronics housing encloses the amplifier board
and the field wiring terminals as shown in the schematic of Figure.
The sensor module contains the pressure sensor,
two sealed fluid systems, an over pressure diaphragm,
and two isolation diaphragms.
The flanges provide the HI and LO port connections,
and also function as the outer wall of the pressure input chambers.
The electronic pressure sensor located at the upper part of the sensor module,
is mounted on a micro diaphragm that serves as a divider between the two fluid systems.
One fluid system corresponds to the HI pressure input,
and the other to the LO pressure input.
The isolation diaphragm of each system isolates the fluid
system from the input pressure.
When a differential pressure is applied across the HI and LO ports,
both isolation diaphragms will compress,
or retract in response to the change of differential.
The movement of these diaphragms causes similar pressure changes,
in each of the sealed fluid systems that are detected by the sensor.
If the differential pressure applied to the HI-LO ports,
exceeds the upper limits of the transmitter,
an overpressure diaphragm mechanism takes control of the situation.
The action of this mechanism prevents the over pressure from reaching the sensor,
thereby minimizing the risk of damage.
Implanted on the sensor’s micro-machined surface,
are four strain gauge resistors connected in a bridge configuration.
This circuit, which is powered by a constant current,
supply on the amplifier board,
produces a mill volt output that is equal to the difference between the two pressure inputs.
The output of the sensor circuit is wired to a high-gain, linear amplifier,
that converts the millivolt signals to a 4-20 mA current output.
Figure shows the transmitter output wired to a typical external loop circuit.
This circuit uses a 250-ohm load resistor and a 24 Vdc power source.
The 4-20 mA amplifier current,
flowing through the load resistor produces a1-5 V input signal,
for the external device.
The amplifier circuit includes a fine-gain and fine-offset potentiate
for performing minor calibration adjustments.
Transmitters also include internal coarse zero,
and coarse span switches for more extensive range conversion.
Easy configuration and operation for differential pressure transmitters
Service and configuration at the DPT-10 differential pressure transmitter,
is carried out using the optional display and operation module,
which can be fitted in four positions.
The operation menu has a simple and self-explanatory structure,
and has seven selectable languages as standard.
Alternatively the operating parameters can be set using the PACTware™ free,
and non-proprietary configuration software.
An instrument-specific DTM enables easy integration into
a corresponding Distributed Control Systems.
Technical data of differential pressure transmitters
Measuring range – Pressure -40 … 40 bar
Process temperature -40 … 120 °C
Process pressure -1 … 420 bar
Materials, wetted parts316L
Alloy C22 (2.4602)
Alloy 400 (2.4360)
Alloy C276 (2.4819)
Threaded connection ¼ – 18 NPT
Flange connection ≥ DN32, ≥ 1⅜”
Stainless steel (precision casting)
Stainless steel (electropolished)
IP 66/IP 68 (0.2 bar)
IP 66/IP 67
IP 66/IP 68 (1 bar)
4 … 20 mA/HART
4 … 20 mA
Ambient temperature -40 … 80 °C
Pressure Sensors from H&H.
H&H is a leading manufacturer of differential pressure transmitters,
for a range of industrial and research applications
– from the everyday to the cutting-edge.
With All-Welded diaphragms, automated on-board configuration,
and enhanced plug and play graphical interfaces,
measuring differential pressure is a safe and seamless process.
Our range of differential pressure transmitters includes:
SMT3151LT, a new type of differential pressure transmitter,
produced by advanced technology and equipment.
Continuous level, flow and differential pressure measurement;
SMT3151DP, smart differential pressure/high static pressure differential pressure transmitter. Output signal 4-20mA output+HART communication; Push Button Configuration;
HHXA-EJA210E, high performance flange mounted differential pressure transmitter HHXA-EJA210E,
can be used to measure levels of densities of solidifying or precipitating liquids;
HH-EJA118E, Differential pressure transmitter with two diaphragm seals for differential pressure and level.
If you would like any more information about our differential pressure sensors,
please send your inquiry now.
This article has covered the basic aspects of Differential Pressure Transmitters.
You can also have a look at our range of Differential Pressure Transmitters,
or contact us if you have a specific application that you would like to discuss.
Do you want to know more about differential pressure transmitter calibration procedure?
You can contact us for differential pressure transmitter pdf.
H&H Instrument offers kinds of differential pressure transmitters,
for differential pressure、level,and so on.
A wide variety of DP transmitters are available to you.
You can also choose from single flange, double flange, absolute pressure, low pressure,
high pressure to match your production conditions.
There are different styles of differential pressure transmitters.
H&H Instrument differential pressure transmitters products are exported,
and most popular in North America, Western Europe, India,Pakistan and South America.
You can ensure product safety by selecting from certified material, including ISO9001… certification.
Contact us Now: （You Can apply for a free sample）