The definitive guide to pressure transmitters

Pressure

The definitive guide to pressure transmitters

Today, you’ll learn all the things about pressure transmitters! We put together some of our best content into a handy guide that either a beginner or an experienced engineer can use.

As you know, pressure is a primary variable in instrumentation and control, mostly because it can help calculate other variables like flow and level. But before we wade into the sea of possibilities, let’s start with the basics.

PART 1: What is pressure?

No, it’s not what you feel while during an exam or interview. Well, that’s  mental pressure, but we want to talk about physical pressure, the force applied to an area:

Pressure force
Courtesy of Visaya Solutions

Yes, it’s as simple as that. The way you measure pressure in your process can vary depending on what you want to know. If you only want to know the pressure itself, then a gauge will show you the absolute or gauge pressure. If you want to use pressure to measure a variable such as flow, you need a differential pressure (DP) transmitter. Below you can see a graph with the three methods. Don’t worry if you don’t understand it; we’ll explain these methods now.

Pressure transmitters
Courtesy of First Sensor

Differential pressure

Let’s start with differential pressure, since you can think of the others as special cases of this one. Basically, DP transmitters measure the pressure difference between two points in your process. You have a high-pressure end and a low-pressure end. Suppose you have on the high side a pressure of 50 bars and on the low side a pressure of 10 bars. The differential pressure transmitter would give you a reading of 40 bars. That’s it.

differential pressure transmitter
Courtesy of Auburn Systems

To measure flow with DP transmitters, you need a primary element to create a pressure difference between two points. This element creates a restriction in the flow, forcing a drop in pressure and increase in speed as the fluid passes through the element. Using this differential pressure and the K-factor associated with the primary element, you can determine the flow.

DP transmitters do pretty much the same for level measurement, with one end at the bottom of the tank as your zero level, and the other at the top of the tank. Calculating the differential pressure with the fluid density will give you the level of the tank.

That’s the simplified explanation, anyway. We’ll go into further details later in this article, so let’s move on to gauge pressure.

Gauge pressure

As I mentioned before, you can consider gauge pressure a specific type of differential pressure, in which the low-pressure port of the DP transmitter vents to the atmosphere. Because of that, a pressure gauge has only one process connection or pressure point. When the pressure in the point is higher than the atmospheric pressure, we call it positive pressure. If it’s lower, then we call it negative or vacuum pressure.

gauge pressure
Courtesy of First Sensor

You can use gauge pressure to measure the level in an open tank. Because you have atmospheric pressure at the top of the tank, you just need to install a gauge transmitter at the bottom of the tank.

For this measuring method, you need to bear in mind that atmospheric pressure may vary with altitude, weather, and so on. These variances will influence your transmitter readings, so make sure you factor them into your calculations.

Absolute pressure

Absolute pressure transmitters use vacuum as a reference for one of the pressure points. Unlike atmospheric pressure, vacuum never varies, regardless of altitude or weather. Eliminating the atmospheric factors from your calculations makes your job easier and less prone to error.

absolute pressure
Courtesy of WIKA

For industrial applications, you can use an absolute sensor anywhere you have a vacuum, and it may surprise you how many applications can use it.

So those are the basic concepts of pressure measurement, and I’m sure you noticed that a couple of our examples measured flow and level. Let’s dig deeper into that now.

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PART 2: Pressure to measure flow

To measure flow with pressure, you need three essential components:

  • The primary element creates a restriction on the flow and thus a difference in pressure before and after it.
  • The installation structure – impulse lines, tubing, valves, and other mechanical bits – sends the pressure from the primary element to the transmitter.
  • The transmitter reads this pressure and transforms it into numbers you can read.

Seems like a lot at once, right? But again, don’t worry. We’ll break it down for you. As always, let’s start with the concept behind it all.

Bernoulli’s principle

If you studied fluid mechanics, you know this one as the base for many calculations. For those of you who aren’t as familiar with this guy and his studies, let’s introduce him to you.

Bernoulli was a Swiss mathematician in the 1700s. He studied hydrodynamics for a while and focused on the conservation of energy. After a few years he developed this principle, and we named it after him.

Bernoulli's principle
Courtesy of Anjung Sains Makmal 3

He found that the sum of all energies – static, potential, and kinetic – in a fluid running within a pipe is the same throughout the pipe. As you can see in the equation below, one of these energies is pressure! Yeah, that’s right. Thanks to this Bernoulli guy we can use DP transmitters to measure flow.

Bernoulli's principle equation
Courtesy of Emerson

This equation and the Bernoulli principle tell us that when fluid goes through a smaller space, its speed increases. Because the sum of the energies must stay the same, the pressure drops. With this pressure difference, we can calculate the flow of our fluid.

Reynolds number

I promise this is the last principle for this article. Besides Bernoulli, we had another important guy who found out something vital about fluid mechanics.

Osborne Reynolds became famous for his study of flow. We use his Reynolds number to predict turbulence and find out how fluid behaves on different scales. In instrumentation, we use the Reynolds number to scale out a new flow meter, finding out its range and applicability. Useful, huh?

Enough of principles! Let’s get a better idea of how it works.

Primary elements

Your process variables will dictate the primary element you use for your application. All of them create a pressure drop by restricting flow. The DP transmitters then measure the drop and use Bernoulli’s equation to calculate the flow. The equation to calculate flow with differential pressure is a little more complex than the Bernoulli equation:

Bernoulli equation
Courtesy of Emerson

Don’t panic! It looks scarier than it actually is. Most of these letters represent constants, which depend on your fluid and your primary element. Notice that the differential pressure (ΔP) sits inside the square root. That means you need to configure your transmitter to send the square root value instead of the linear one.

Now that we know what the primary elements do, let’s move on to the types available, or at least the three most popular.

Orifice plate

The orifice plate has the largest corner on this market. Orifice plates are easy to install, cover a wide range of applications, like gas, liquids, and steam, and come in different types. For instance, the conditioning orifice plate can resolve irregular flow profiles. It has the advantage of working in short pipes with straight runs, usually only two diameters before and after the sensor.

Orifice plate
Courtesy of AVCO

Pitot tube

This one got its name from the engineer who invented it, Henri Pitot, and it sees a lot of use in aviation. So next time you see a plane up close, look for a strange tube like this:

pitot tube
Courtesy of Keyshone

One side of the tube sits perpendicular to the flow. When fluid enters the tube, it applies a pressure we call total pressure. The downstream side of the tube only has static pressure. The difference between these two pressures is the differential pressure the transmitter reads. Again, with this differential pressure, you can calculate the flow of your fluid.

Venturi tube

Yes, yet another important physicist in fluid mechanics. The venturi tube works like the orifice plate, but instead of reducing the area where the fluid can flow, it uses a narrowed section of pipe to create the pressure drop. Connecting your DP transmitter to the upstream and downstream sides will give you the differential pressure.

Venturi tube
Courtesy of Leah Meter

Differential pressure meter installation and concerns

In the past you’d install each of the elements – primary, transmitter, and impulse line – by themselves. Doing so meant misalignment, seal pot level changes, and other kinds of problems could develop. To avoid these problems, vendors now make DP flow meters with primary elements integrated with the transmitters. In the image below you can see the difference between these two installations.

differential pressure different installations
Courtesy of Emerson

Also, calibrating a transmitter is fairly easy. However, a primary element may go years without a calibration, because removing it requires stopping your process and taking apart your structure. Nobody likes doing that.

So now that we know how to measure flow with differential pressure transmitters, let’s move on to level measurement.

PART 3: Pressure to measure level

Here we’ll follow the same steps we took when we talked about flow measurements, starting with the math and physics named after physicists and then moving on to the practical side.

Just as flow measurement uses Bernoulli’s equation, level measurement has its base in the Pascal equation. According to this equation, pressure (P) equals the liquid’s density (ρ) times acceleration due to gravity (g) times the liquid column’s height (h), or P = ρ * g * h.

Knowing your fluid’s density and the pressure on it, you can calculate the height of the liquid column and therefore the level on a tank. More complicated applications require a bit more calculation, but all start from the same principle.

level measurment sensor
Courtesy of The SensorsGuide

Now that we know the basics, we can move on to the practical examples.

Open tank

Level measurement on an open tank using DP transmitters is simple, because the pressure on the top of the tank is the atmospheric pressure. You only need to connect the high-pressure cell to the bottom of the tank and leave the low-pressure cell venting to the atmosphere. For this specific application, you can use a gauge transmitter instead of a DP since the reference pressure is atmospheric. Pretty simple, right?

Of course, sometimes connecting your sensor at the bottom of the tank may take some work or need a different setup from the usual. Check these out!

Standard setup

Here you have the transmitter installed at the tank’s zero level and the level connection filled with the same fluid as the tank.

simple setup for level measurement
Courtesy of Instrumentation Toolbox

With the transmitter at the zero level of the tank, you need to figure out how to calculate the minimum and maximum level. So here we go:

Minimum = level at 0% = HP (SGp * H) – LP (SG * H)

Maximum = level at 100% = HP (SGp * H) – LP (SG * H)

  • HP = high pressure
  • LP = low pressure
  • SGp = specific gravity of process
  • H = height

Here, the minimum will always equal zero because you have zero on both sides. On the 100 percent, you multiply the height of the liquid in the full tank with the gravity for the high pressure. For the low pressure, you’ll still have zero, so you can leave it.

Open tank with suppressed zero installation

We call this one suppressed zero because you install the transmitter below the tank zero level, generating higher values than the real value. First, let’s calculate the minimum and maximum for an installation without a seal pot. In this case, the process connection is filled with the product in the tank.

process product for level measurement
Courtesy of Visaya Solutions

We use the same equation, but we need to pay attention to the new height with the transmitter below the bottom of the tank.

Minimum = level at 0% = HP (SGp * h1) – LP (SG * H)

Maximum = level at 100% = HP (SGp * h1 + H2) – LP (SG * H)

  • HP = high pressure
  • LP = low pressure
  • SGp = specific gravity of process
  • H = height
  • h1 = height of suppressed zero
  • H2 = height of the full tank

Does that make sense? Now imagine that we need a seal pot on the suppressed zero setup, which means that you have another fluid connected to the low-pressure cell of the transmitter. Because the seal pot’s density differs from the process product, this difference will add to your math. Have a look!

zero set-ups for level measurement
Courtesy of Visaya Solutions

Then we have the following equations to calculate the maximum and minimum level of the tank:

Minimum = level at 0% =  HP (SGs * h1) – LP (SG * H)

Maximum = level at 100% =  HP [(SGs * h1) +( sGP * H2)] – LP (SG * H)

  • HP = high pressure
  • LP = low pressure
  • SGp = specific gravity of process
  • SGs = specific gravity of seal
  • H = height
  • h1 = height of suppressed zero
  • H2 = height of the full tank

Closed tank

Ready for the next step? Then let’s close the tank. Before we get into the calculations, I should point out some peculiarities of this application.

Impulse lines

In closed tanks, the product inside might generate gas or vapor either as part of the process or simply from being constrained. In these cases, you need the low-pressure side of the DP transmitter connected to the top of the tank. And that’s where impulse lines come in.

There are two different types of impulse line, wet leg and dry leg, depending on the product in the tank. If the product creates condensate, you have to use a wet leg solution. Otherwise, you can go for the dry leg.

wet leg dry leg types of impulse lines
Courtesy of Emerson Experts

Sounds good, but what exactly does dry and wet leg mean? The leg is basically the mechanical structure that connects the sides of a DP transmitter to the vessel. A wet leg has your impulse lines filled with liquid, although not necessarily the same liquid in both legs. Dry legs have columns filled with vapor, gas, or whatever may come from the process without condensate.

You can end up having issues with both installations. The dry legs, for example, may develop condensate, and the wet legs may clog or leak. We have an article discussing some solutions to these problems, but for now, here’s a quick rundown of possible alternatives.

Capillary systems

In trying to solve wet/dry leg issues, someone came up with the capillary system. They solve impulse line problems such as evaporation, condensation, leaks, and clogs.

The capillary setup consists of a remote seal system and a sensing diaphragm with oil-filled capillaries. A force deflecting from the diaphragm of the remote seal sends pressure through the oil. Then the transmitter captures that to show the process measurement.

Having a capillary system often means having a balanced system. That means you have the same length of capillary on each side of the transmitter and also the same remote seal. Theoretically, this helps avoid problems with temperature shifts and other issues. Of course, you know how it works in the real world.

Fancy options

Electronic remote sensors are another option. Different companies might call them different things, but they all do the same work. Instead of having a mechanical structure or remote seal system, you have a sensor that communicates digitally. Thus, all problems we have with impulse lines and capillary systems disappear, or so the sales reps say. But let’s get back to the math. Haha, you only thought you dodged it!

The basics

Our first scenario has an impulse line with a dry leg. We have only the standard gravity of the process liquid, with the transmitter installed at the same level as the zero level measurement. With me so far? Because right after the graphic comes the math!

zero level measurement
Dry leg and SGf filled with vapor -Courtesy of Visaya Solutions

Minimum = level at 0% =  HP (SGp * H) – LP (SGf * H)

Maximum = level at 100% =  HP (SGp * H) – LP (SGf * H)

  • HP = high pressure
  • LP = low pressure
  • SGp = specific gravity of process
  • SGf = specific gravity of fluid
  • H = height
Saved by zero

Next, we have a wet leg setup and the transmitter installed below the zero level, the suppressed zero model.

suppressed zero for level masurement
Courtesy of Visaya Solutions

Minimum = level at 0% =  HP (SGs * h1)

Maximum = level at 100% =  HP [(SGs * h1) + (SGp * H)]

  • HP = high pressure
  • LP = low pressure
  • SGp = specific gravity of process
  • SGs = specific gravity of seal
  • H = height
  • h1 = height of suppressed zero
  • H2 = height of full tank
Raising the bar

In the third example, we elevated the minimum level in a wet leg system.

minimum level in a wet leg system
Courtesy of Visaya Solutions

Minimum = level at 0% = HP (SGp * h0) – LP(SGs * H2)

Maximum = level at 100% = HP (SGp * H+h0) -LP(SGs * H2)

  • HP = high pressure
  • LP = low pressure
  • SGp = specific gravity of process
  • SGs = specific gravity of seal
  • H = height
  • h1 = height of suppressed zero
  • h0 = height of minimum level
  • H2 = height of full tank
Creeping into capillaries

Moving away from impulse lines, let’s see the math behind the capillary system.

capillary system and level measurement
Courtesy of Visaya Solutions

 

Minimum = level at 0% =  HP (SGp * h0) – LP (SGs * h)

Maximum = level at 100% =  HP (SGp * H) – LP (SGs * h)

  • HP = high pressure
  • LP = low pressure
  • SGp = specific gravity of process
  • SGs = specific gravity of seal
  • H = maximum level height
  • h0 = minimum level height
  • h = height of full tank
Electronic sensors

Last but not least we have the electronic sensors. In this case you go back to something like a dry leg equation. The diaphragm connects straight to the tank.

Electronic sensors for level measurement
Courtesy of Visaya Solutions

Minimum = level at 0% =  HP (SGp * H0)

Maximum = level at 100% =  HP (SGp * H)

  • HP = high pressure
  • LP = low pressure
  • SGp = specific gravity of process
  • H = height
  • H0 = minimum level

Tada! Now you know everything we know about pressure transmitters! If any experts out there think we missed something, then let us know and we’ll chat about it! Until then, let’s have a look at some options for you to consider.

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PART 4: 7 pressure transmitters on the market

Remember that there is no best or worst device. It will always depend on your process and its variables. You can have all kinds of reasons to choose a certain device, such as local support, international identity, and price. Here we present some devices from the most common vendors worldwide.

1. Deltabar FMD72 – Endress+Hauser

The Deltabar FMD72 from Endress+Hauser is an electronic device that can help you avoid problems such as clogs or leaks that might come up on your seal or seal pot.

Deltabar FMD72
Courtesy of Endress+Hauser

The transmitter has two sensors, one for high pressure and another for low. These sensors send digital values to the system to calculate your variable. Since the transmitter connects to the sensor with a cable, you can even install your transmitter in a different place for better visualization and setup through the local display.

Each sensor has an accuracy of +-0.005 percent and the system up to 0.07 percent. It also comes with different process connections and a transmitter temperature range from -40 to 125 degrees Celsius. Last but not least, you get a bunch of certifications for explosive areas and hygienic installation.

To find out more on the FMD72, click here.

2. Rosemount Wireless Pressure Gauge – Emerson

The all-stars from Emerson did a really nice upgrade of a simple manometer to an Industrial Internet of Things (IIoT) device. The Rosemount Wireless Pressure Gauge has a wireless digital connection built in that allows you to monitor your process information and health of the device.

rosemount wireless pressure gauge
Courtesy of Emerson Automation Solutions

It’s a nice balance between old and new to satisfy even the fussiest operators – traditional manometer and local indicator paired with wireless data transmission.

You have different options, such as gauge, absolute, compound, and vacuum. Moreover, the accuracy goes up to 0.5 percent with a measurement range up to 4000 psi. It uses wirelessHART and has a battery lifetime of up to 10 years.

Emerson has more on its old-and-new gauge here.

3. CPG1500 – WIKA

This device is another example of how to get manometers to the next level. WIKA has a digital pressure gauge which does way more than just show the pressure of your process.

The CPG1500 offers a digital display in which you can vizualise bar graphs, battery life, diagnostics, and setup. It has also a proprietary wireless protocol for the transmitter and datalogger setup, with Bluetooth as a nice bonus.

GPC1500
Courtesy of WIKA

It can also measure up to 1000 bar, with options like vacuum and absolute pressure ranges. It has an accuracy of up to 0.05 percent and a datalogger function that can register up to 50 measured values per second!

If you want to read more about it, check out this page.

4. OPTIBAR PC 5060 C – Krohne

The 5060 C gives you a flexible solution with good features. It offers good resistance and reliability in different processes with its built-in ceramic measuring cell.

pressure transmitters
Courtesy of Khrone

You can use the 5060 C from 25 millibars to 100 bar, and it also offers process temperature resistance up to 150 degrees Celsius. It even has seamless integration in HART, FOUNDATION Fieldbus (FF), and PROFIBUS PA. Plus you can set up the transmitter through the local display or with a field communicator.

Krohne can give you more dirt on its device here.

5. VEGABAR 82 – VEGA

The VEGABAR 82 has a high abrasion resistance with its built-in ceramic measuring cell and can measure vapors, gases, and liquids. It has a flexible measuring range from -1 to 100 bar and a temperature range from -40 to 150 degrees Celsius.

VEGABAR 82
Courtesy of Vega

It comes with many integration possibilities, with PROFIBUS PA, FF, analog, HART, or Modbus protocols. You can also get different types of wetted parts material, process connections, and approvals. Moreover, the display has built-in Bluetooth that lets you set up using your smart phone, tablet, or laptop.

You can find more data on the VEGABAR 82 here.

6. SITRANS P500 – Siemens

We already reviewed this product, so I’ll summarize. The P500 offers high accuracy, up to 0.03 percent, and stability, up to 0.05 percent. You can find a measuring range to fit your process, and it allows local configuration through its push buttons, so you don’t need a handheld to set up.

SITRANS P500
Courtesy of Siemens

Compared to the other pressure transmitters, its integration falls a little short. It has only analog and HART protocols, no digital protocols like FF or PROFIBUS available.

Check out the official site for more on the P500.

7. STD800 – Honeywell

We also reviewed the STD800. It has an excellent display with all the necessary process data and device diagnostics. Another great thing here is that you don’t need a field communicator to set it up; you can do it by using its three push buttons on top.

STD800
Courtesy of Honeywell

The STD800 gives you its standard accuracy as 0.035 percent and offers an option of up to 0.025 percent. When it comes to integration, it’s pretty flexible as well, in its analog, HART, FF, ad proprietary protocols. Furthermore, it has plenty of certifications for different processes.

Honeywell can tell you more about it here.

Conclusion

That’s it! You’re ready for next week’s quiz. Just kidding. Pressure transmitters are some of the most flexible devices in process automation, and you can install them for all kinds of measurement in the field. If you have more questions about pressure devices in general, drop them in the comments and we’ll get back to you!

Below you can check out an example of pressure application in the industry: