Differential pressure Transmitter

You're asking for a schematic diagram of a differential pressure transmitter? Along with an explanation?

If this is what you need, I'm afraid that you won't get it here at CR4. That's too big a subject for a forum thread.

Vulcan,

If I were you, I wouldn't reply to such a question. looks as if the "guest" doesn't really know what he wants to know and is to lazy to try to study the subject,and phrase the question before posting and on top is far too arrogant demanding reply.

It is one of these anonymous posters, I hate it.

Wangito

Hi wangito,

I've changed my attitude towards these types of threads somewhat. Some threads I totally ignore if the question is extremely vague or too general in nature.

It is a reality that some people do not have access to good teachers, professors or instructors. So these guys come to us for help and I'm willing to give it to them (as long as they don't ask me to explain everything ).

With regards to arrogance, yes, sometimes the tone of the question comes across as arrogant (I want to know..., I need answer quickly..., explain in detail how...). I've come to see that these people only know how to speak or write English but do not know the niceties that they need to converse in a polite manner. It's somewhat like they say when you study to speak Chinese and they tell you to be careful how you pronounce certain words because they can be offending.

If the question is too vague or general, I ignore it, arrogant tone or not. If it's a request for information that I can give, I'll give it a try. Sometimes a reply can result in more information from the OP and it suddenly becomes interesting. You never know.

Cheers!

Differential pressure (dp) is the measure of a pressure drop across some known restriction, used in fluid flow measurements (air, water, steam, etc.) or across a filter system to detect plugging. I'm sure there are more applications.

The dp transmitter is connected across a known restriction (orifice plate, flow nozzle, etc), which is ordered and manufactured according to a process with known requirements.

The dp transmitter with a range capability fo handle the desired process is calibrated to the process (inches of water column, psi, etc.) representing the process flow range or filter differential.

The dp transmitter then will give an output (4 to 20 mA is common) sent to an indicating device (gauge, recorder, dcs, etc.).

What may give you some good diagrams and better explanation is if you google search dp transmitter.

You sound like you are comfortable with the theory of gauge pressure transmitters (psig sensors) but not differential pressure transmitters (psid sensors). If so, a differential pressure transmitter's (dp) diaphragm can flex due to pressure from high pressure comming from either. If pressure on the left is the same as on the right side of the diaphragm is equal the transmitter's output is zero (or usually set to 12 madc which is the middle of a 4-20 madc system.

Differential Pressure Transmitter

Most of the DP Tx measure positive differential pressure, so the zero point at 4 mA, shows a zero pressure difference on both sides of the diaphragm.

I came across a common mistake by Instrument Engineer over the Positive and Negative sides of the DPtx, they think that all measurements have to follow the rule of the higher pressure to the positive side and the lower to the negative side.

When I say positive side, I meant to say that when an increasing pressure is applied to this side, a rise in output of the transmitter.

However for a Boiler Drum Level Transmitter, which is using a DP Tx, the negative chamber is filled with condensate water from the top of the Boiler Upper Drum, giving a higher static pressure than the postive chamber. For this case, an Elevation is installed on to the DP TX to make the output to zero when there is an offset of higher negative pressure to the DP. Subsequently, a lower negative pressure will transmit an increasing output from the 4mA.

There is also the Suppression of the DP Tx, which is the opposite of the Elevation.

The Engineer tried to calibrate the Elevated DP Tx the normal way by injecting mmWG pressure into the positive chamber. He failed to get the output according to the Drum Level and thought the diaphragm is punctured.

We solved the problem by simply calculating the height difference of the top and bottom legs Drum level input and injecting the pressure (Relative to ambient) into the negative chamber in a decreasing manner to get the increasing output .

FOXBORO made famous their Differential Pressure Transmitter of pneumatic type, called the DP CELL. Rosemount has their Electronic Capacitance DP transmitter. Other makers are almost using the same design principles.

tijit_{digital is our name; portable is our game.}

One of the types of FLOW transducers used in hydraulics is a differential pressure type.

Until recently, I thought that, given a known orifice size, then the measured differential would derive flow but, it seems that there was also an LVDT stuffed up the transducer as well and a mean average between the two types of measurement produced nearly as reliable a result as a turbine flow meter (after correction for viscosity)!

First of all, let's drop transmitter all together. It applies only to application (mainly due to distance). In fact, every transmitter I've ever been involved with manufacturing is actually a transducer with voltage output that has a converter board added on to provide current output.

In regards to principle of operation, the delta-P sensor is used to measure the difference between two distinct pressure areas. As someone mentioned before, it could be used for measuring across a restriction in flow, it's also used in vehicles to measure the differential between intake air & exhaust gas or in an HVAC application it can measure the pressure drop from the opening of a door in order to prevent air conditioning from needlessly running.

The most popular design has been the capacitive design. My former employer, Kavlico Corp had the patent on the ceramic capacitive sensor for many years until its expiration. The design in differential applications used a dual diaphragm approach, ceramic for the air side & either a metal foil/plastics membrane for gaseous environments. With the advancements in MEMS technologies, HVAC type sensors have converted to using dual PRT sensing chips mounted on a board and ran through a differiatator chip for its output. The trend in this area is for frequency output instead of current or voltage.

Also as previously mentioned, theres not a positive/negative difference which I would call just plain bad labeling. Kavlico sensors are always labeled Hi/Lo and as I pointed out earlier, depending on your environments, the ports shouldn't be considered interchangeable.

Added note, I left out one important part with the dual diaphragm design. They are oil filled (silicon) for response time (no compression of air).

That's a question for an encyclopaedia, not an engineering forum.

dpt