Differential Pressure Transmitter Explained
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Simply put, a differential pressure transmitter is a device that measures two opposing pressures in a pipe or vessel.
To understand how a differential pressure transmitter works, we need to know some principles of pressure.
The pressure is the result of force applied onto a unit of area.
In the case of fluid and gas, this area would be the inside of a pipe, tank, vessel or other sealed housing.
Since pressure is a physical quantity, it can be measured.
The pressure is measured in one of the following five units.
- Pascal
- Bar
- Standard atmosphere or atm
- Torr
- Pounds per square inch or psi
The most commonly used measurements of pressure in an industrial fluid and gas processing environment are bar and psi.
One bar is the equivalent of the earth’s atmosphere at mean sea level.
A pound per square inch is the force of one pound on an area of one square inch.
You will most likely use these units of measurement in your work as they are commonly used and almost every manufacturer of pressure measurement devices calibrate them to these units of measurement.
Delta-P or DP is the industry-standard term for the measured differential pressure.
Delta, meaning change in, and P meaning pressure.
In addition, most people will simply call a differential pressure sensor a DP sensor or transmitter.
A DP transmitter is a pressure-measuring gauge or an electrical device that uses two elements to measure the differences of pressure in a sealed container such as a pipe.
DP transmitters will have
- A primary element
- A secondary element
- An electronics housing
The primary element will produce a difference in pressure as the flow in the pipe increases.
Different types of primary elements include
- Orifice plates
- Venturi tubes
- Pitot tubes
- Flow nozzles
- Laminar flow elements
- Wedge elements
The secondary element will measure the difference of pressure produced by the primary element as accurately as possible.
The primary element is sometimes called the high side, and the secondary element is called the low side.
The elements in a DP transmitter will have sealed diaphragms and one of several ways to convert the pressure applied onto the diaphragms into an electrical signal.
Sensors such as differential capacitance, vibrating wire or strain gauges are used to produce the electrical signal that will be sent to the electronics housing, where it is processed, converted and amplified into a current or voltage output that can be read by the control system.
The current output is commonly a 4-20 milliamp signal, or in some cases a 0 to 5 or a 0 to 10 volt signal.
The electrical output generated by the DP transmitter electronic module is linear and proportional to the actual measured Delta-P.
Depending on the range of measured pressure, a 4 milliamp signal would equal a measured Delta-P of 0 psi and a 20 milliamp signal would equal the maximum measurable value. In this case, we’ll say that the maximum Delta-P is 100 psi.
One very common way of measuring flow using a DP transmitter is for the primary element to introduce a constriction in the pipe.
Using a restrictive element on the primary side will create high and low pressure.
When the flow increases, more pressure drop is created.
As we remember, the secondary element will measure the difference in pressure created by the primary element.
These measurements are sent to the electronics housing where the signal processing is done.
By using Bernoulli’s equation, which states that the pressure drop across the constriction is proportional to the square of the flow rate, the Delta-P is then converted into units of flow.
These units of flow can be gallons per minute or cubic meters per hour. Using DP transmitters as a method to measure flow works best on fluids that are relatively clean and of low viscosity.
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This chanel deserve more than 361 k subscribers
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I wished you have shown in the schematic at 6:00 how the piping connection is done to the secondary element (impulse lines)
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Thanks a lot, Nikolai! The RealPars team wishes you a splendid new year as well.
Very useful to understand the regulator principal in compressed cylinders
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The thing that always gets me on these transmitters is un screwing the housing to get to the wires. They are usually made of aluminum and gull & seize up.
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This is great do you have courses for fluid flow analysis; control valves, tanks, orifice, pumps etc?
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Thanks for upload.please do more videos on field instruments please please
Hey!
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I have being watching many videos, they are great explanations. Could you make a video explaining the difference of an SCADA and a control through an HMI please
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Supera explained... please do video on control valve and actuator types
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Nice video!! Can you guys explain to me the formula how to calibrate a pressure transmitter and set up the value LRV and URV on a hart calibrator for a close vessel and a open vessel thanks.
Hey!
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Detailing of manifold valve that connect at the bottom of the transmitter would be helpful.
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Does the paid subscription to your site offer more in-depth instruction of the topics offered here? Thanks.
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It indeed does, our free videos on TH-cam do not follow a specific start to finish structure. Whereas our course library is specifically designed in a way where it provides all the tools needed for beginners to evolve into an advanced level through our hands-on courses.
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can u kindly explain or share link, for the function of DPTransmitter connected to VFD of Pumps in Chilled Water Flow network (Closed Loop)
Hi there,
Thanks for the topic suggestion, I will definitely go ahead and forward this to our creator team.
Happy learning!
Would you please clarify what did you mean by " .....and one of several ways to convert the pressure applied onto the diaphragms into an electrical signal " ?
The process pressure can be converted into an electrical signal through a detected change in capacitance, by sensing the pressure using compression of a special ceramic substrate, or by deflection of a piezoelectric bridge. These are all physical methods of converting pressure forces into electrical signals.
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The best educational channel, but I think the pressure transducer that you explained in the introduction to the video is the flow transformation, isn't it?
Differential pressure transmitters can be used for flow measurement, level measurement, and differential pressure measurement. The masnifold shown on the pressure transmitters shown in the opening segment are most often associated with flow measurement, but can be used in all three applications.
Which type flow transmitter is most reliable and gives most correct values & why? please make a video
Thanks for your topic suggestion, Samir! I will happily pass it on to our course library.
Great video thank you for uploading. Just one comment: around minute 3:14 when it says “the low increases” ... doesn’t that need disambiguating? When the flow’s speed increases, right? Because the flow’s mass rate stays constant....
Btw, if the goal was to measure dP across a strainer... wondering how it would work...
There are many types of sensing elements that measure the fluid pressure upstream of the sensor (high side) and downstream of the element (low side). This differential pressure can be correlated to the flow, and the relationship is "flow is proportional the square root of the pressure differential". If I simply want to measure the pressure drop across a filter or strainer, for example, I just need to position the high-side sensing tap upstream of the filter housing or strainer and the low-side sensing tap downstream of the filter housing or strainer.
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Sir, can you please provide the different types of valves and purpose in industry
Hey Kishor!
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Good
upload please more videos like,ph meters,ultrasonic flow meters,rota meters,and dozzing pumps
Hey Asad!
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Please make videos about : gas header pressure " upstream " , gas pressure " down stream ". Well head pressure and temperature .. and Riser and test Riser Pressure
Hi Mohamed!
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Any measuring instrument for mean effective pressure sir
If you are referring to mean effective pressure, which is the measure of an engine’s capacity to do work independent of engine displacement, then a very small pressure transducer is used that measures the cylinder pressure. Mean effective pressure determined by calculation over the entire cycle of the engine (compression, ignition, exhaust). These are typically small electromagnetic gauge pressure sensors. A good example can be found at: new.abb.com/products/measurement-products/diesel-engine-monitoring/pressure-transducer-pfpl203
What is the working principle of DP transmitter
mmH2O is also a commonly used unit.
i want to learn DP transmitter programing.can u educate me.because i'm doing some boiler job most of the time i face problems with a DP transmitters about the programing.
Hi Hansajith,
Thanks for your comment and feedback! I will happily pass this on to our course developers for possible future video courses.
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very nice video, but you are making a confusion between a DP transmitter and a DP Flow transmitter...and almost completely ignore DP Level transmitters :) appreciate the time and work though :)
I really need to get back to work. I have never worked on piping in my life and will never use this information.
How the change in capacitence converted into electrical signal
Hi Vundru,
Thanks for your comment, and great suggestion!
I will happily go ahead and pass this on to our course developers for a possible future video course.
What If DP is showing negative value?
There are many instances where a negative value is quite normal. A common range for a DP cell is -150 inH2O to +150 inH2O. This allows for situations where the transmitter is installed what is called a zero elevation situation (wet leg, transmitter positioned below URL).