Differential Pressure Transmitter – How Do They Work?
Differential pressure transmitters (DPT’s) are very useful instruments with a wide range of applications. They’re used in anything from monitoring a clean room’s air presure to make sure it has the right flow of filtered air to keep it clean, to ensuring a safety enclosure like a biosafety cabinet is performing within specified limits, to determining the amount of liquid or other materials inside of a tank or reactor. DPT’s do this by sensing the pressure at two different points and subtracting the two pressures, giving the differential pressure (or difference in pressure between the two points).
What Exactly Is Pressure?
To understand differential pressure, first let’s think about what pressure is. Pressure is the amount of force applied to somethig per unit area, where force could be in pounds (lbs) and area could be in square inches (in^2). Dividing the force by the area gives pounds per square inch (lbs/in^2), which is typically written as PSI. There are other units of measure, like using newtons (N) for force and square meters (m^2) for area, giving N/m^2, or newtons per square meter. For this article we’ll work in PSI.
Pressure increases when either the area that the force is applied to gets smaller, or the force gets larger. Think about someone who is 150 lbs stepping on your foot wearing work boots. If the area of the work boot’s heal is about 4 inches x 4 inches (16 in^2), then the pressure would be 150 lbs/16 in^2 = 9.375 PSI. It may hurt a little but likely wouldn’t be that bad.
Now think about getting that same amout of force, but decreasing the size of the area of the boot sole to that of a pointed stiletto heal. If the person weighs the same in either case (150 lbs), then the force hasn’t gone up, but the area has gone down a lot (say to about 0.5 inches x 0.5 inches = 0.25 in^2), resulting in the pressure (force per unit area) greatly increasing to 150 lbs/0.25 in^2 = 600 PSI. OUCH!. This results in 64 times more pressure just by decreasing the area the force is applied over from the size of a boot heal to that of a stiletto.
In the same way pressure goes up with increased force or decreased area, it also goes down with decreased force or increased area.
How Does A Differential Pressure Transmitter Sense Pressure?
Pressure is typically sensed using a diaphragm, a small device called a strain gauge mounted to the diaphragm, and some electronics to send and interpret the signal from the strain gauge. The diaphragm is typically a flexible disc made of thin metal. It may be made from other materials depending on the need for resistance to chemicals, humidity, etc. of what is being measured (I.E. if used in a chemical process, steam line, fresh air duct in an HVAC system and the list goes on).
The strain gauge and associated electronics have been set up to sense how much pressure is being exerted on the diaphragm by how far it has been stretched or compressed from its neutral state. Over the useful range that the diaphragm flexes over, there are defined correlations to what pressure is being exerted on the diaphragm to make it flex. For instance, if the diaphragm has no pressure applied to it, it doesn’t flex and the electronics will sense this and send a defined output saying the pressure is 0 PSI (or whatever value a neutral reading would be).
Basically, when the diaphragm flexes or contracts due to an increase or decrease in pressure, the strain gauge and electronic components sense the change in shape and correlate it to a pressure.
I’ll cover how a strain gauges works in more detail in another article soon.
Components Of A Differential Pressure Transmitter
DPT’s all have a few typical components. They have two diaphragms, one to sense pressure in one location, and a second for another location. The transmitter will subtract the two pressures that the strain gauge outputs and give the measurement of differential pressure, often in PSI.
The strain gauge itself provides a very weak signal, so some additional electronics are required to amplify and make sense of the signals. The electronics will use the signals to create an output interpreted as pressure that is either transmitted to a recording device, a remote display device, or will display the pressure on a screen onboard the transmitter.
Since DPT’s are sensing the difference in pressure between two things, IE, rooms, piping before and after a pump, ducts, etc., the transmitter will do some math and subtract the two pressure from one another, resulting in a differential pressure measurement.
In essence, there are two main components of a DPT:
1. Two diaphragms, each including a strain gauge, that sense two pressures, one higher (P1) and one lower (P2)
2. Transmitter that recieves the signals from the strain gauges and does math. It outputs a signal to a remote or onboard display or a recording device.
The transmitter will subtract the pressures from one another to get the differential -> DP = P1 – P2
Room Pressure Sensing
In many institutional buildings such as hospitals, as well as in industrial buildings like pharmaceutical and computer manufacturing facilities, differential pressure between some rooms is very important. For instance, when manufacturing inside a clean room, often the pressure inside the room is required to be higher than that of adjacent spaces. This is to keep any potentially contaminated air from entering the clean room and contaminating the process materials, IE, a pharmceutical product.
A DPT can be used to ensure that the clean room is positively pressurized by having the high pressure side (inside the clean room) monitored by one diaphragm of a DPT, and having the low side (adjacent space to the clean room, for example a entry corridor) monitored by the second diaphragm. The transmitter will subtract the measured pressure values from one another and indicate if the minimum differential pressure is being maintained. This gives confidence the clean room is indeed maintained as a clean space.
Safety for BioSafety Cabinet
A similar approach can be used for a balance enslosure, a fume hood or a biosaftey cabinet when doing weighing or chemisitry/biologics operations. The idea is to have an enclosed area that sees a certain amount of airflow (in a defined pattern depending on the application) and have a DPT monitor the pressure of the space outside the enclosure versus the pressure of the space inside the enclosure. This will give an indication if the correct amount of airflow is present to provide the right conditions for the operations taking place inside the enclosure, to protect the people working within them and also to protect the things they are working on.
Differential pressure transmitter used in level monitoring
When a DPT is used to sense the liquid level in a tank, one of the diaphragms will be inserted near the bottom of the tank. When liquid is added to the tank and begins to cover the diaphragm, the DPT begins to sense the pressure that the weight of the liquid exerts on the diaphragm. If the tank operates at atmospheric, or ambient, pressure, the second diaprhagm can be outside of the tank. If the tank operates under any other pressure, positive or negative, the second diaphragm needs to be inserted into the head space of the tank, above the liquid level at all times to maintain accurate readings.
Differential Pressure In Level Measurement
Differential Pressure Transmitters Are Very Useful!
Differentail pressure transmitters are a great way to measure many things. They help us to ensure critical air flows are present in rooms like clean rooms in hospitals and manufacturing areas that require very clean environments. DPT’s also help us to measure liquid levels within chemical, pharmaceutical, and other industrial areas. They may be the right choice in many applications, and gaining an understanding if what they do will help you select the right one for you purposes.
Thanks for reading, and please reach out to me by leaving a comment below if I can help you with anything at all.