Absolute, gauge, and differential pressure!

Pressure

Absolute, gauge, and differential pressure

Now that we’re talking about pressure, let’s cover things related to this principle and its solutions. As you may have learned early on, pressure is a primary variable in instrumentation and process measurement. You can measure a ton of things using a pressure transmitter, like flow and level.

However, you may still have questions about this basic principle if you’re new to the field. Well, guess what? I’m here to simplify everything and make your life easier today. We all need to start somewhere, right? Yup, and I’ll start by explaining the concept of pressure. We’ll also go over differential, gauge, and absolute pressure. Trust me, you’ll want to stick around for this. You may even want to take notes.

differential pressure
Courtesy of Giphy.com

Before we begin, though, I’d like to show you a picture and ask you a question. Here’s the picture:

Courtesy of Endress+Hauser, Emerson Automation Solutions, and ABB.

Now, what differences do you see among these devices? They’re all different brands. Just kidding! They all measure pressure differently. If you said different brands, you get to sit in the front of the class, smarty-pants.

What is pressure?

To explain pressure, let’s find a good practical example. Suppose you have a bunch of tasks at work with hard deadlines. What do you feel when a deadline approaches and you know you won’t finish in time? Tada, pressure!

Okay, okay, now I’ll give you a real example. Say you’re playing a video game on a console or your smartphone. Your fingers will apply force in an area. Yeah, it really is that easy! To calculate pressure, you need to know your force and your area.

Pressure force

Next, we have absolute, gauge, and differential pressure. Below, you have a graphic showing the basic differences.

Courtesy of – First Sensor

What is differential pressure?

The name gives the answer. Basically, it’s the difference between types of pressure in an application. Usually, you’ll use a differential pressure (DP) transmitter to read the pressure difference between two points. Differential pressure shows up throughout the instrumentation world.

Courtesy of Auburn system

In applications like flow, a DP transmitter uses a primary element, like an orifice plate, to create a pressure change between one point and a later point. When you find the differential, then you can calculate the flow using the K-factor of the primary element.

Level measurement uses a closed pressure vessel. So you install a DP transmitter to measure the pressure at the bottom and the top of the vessel to find the difference. If you know the density of the product, then you can calculate the level.

We’ll discuss other examples of DP applications later. Let’s move on to gauge pressure.

What is gauge pressure?

You can consider gauge pressure a form of DP, using atmospheric pressure as one of your points. Of course, atmospheric conditions like altitude and weather differ in various areas, and these conditions will affect your sensor. You’ll need to put on your geography and meteorology hats for this one.

Anyway, when you have a process pressure higher than the atmospheric pressure, we call it positive pressure, and lower goes into vacuum or negative pressure. You even have divisions for vacuum, such as low, high and ultra-high.

Gauge pressure
Courtesy of First-Sensor

You’ll also notice a gauge transmitter looks a little different from a DP. It’ll have only one process connection or pressure port, because you’ll usually measure the atmosphere through a vent in the transmitter.

If you want to measure the pressure in a closed system, then you’ll use a gauge transmitter. Furthermore, you can measure level using a gauge transmitter. If you have an open tank, then you’ll install the transmitter at the bottom of the tank to do the measurement.

What is absolute pressure?

An absolute pressure transmitter uses a perfect vacuum as its reference. What’s the difference? Simple. A perfect vacuum never changes, no matter what the weather or area. Eliminating the atmospheric factor from your calculations makes your job easier and less prone to error.

absolute pressure
Courtesy of WIKA

In industrial applications, you use an absolute sensor anywhere you have a vacuum, and it may surprise you how many such applications exist.

Conclusion

Now, you see how absolute, gauge, and differential pressure differ. You also have examples to help you connect the explanations with your daily applications. You’re welcome!

If you have more questions, then ping us on hello@visaya.solutions or search for the topic in the search bar.

Cheers!

Read more about pressure devices below:

Save money and time on pressure monitoring – Most automated industries have a lot of pressure monitoring, where operators go out to check measurements, make notes, and inform their control rooms about process conditions. You know those operators spend a lot of time doing all that. Why should they spend all that time when it’s easier to send the information directly to the control room? Read more

Differential pressure transmitter – Before anything else, there’s something you should know: There are better technologies than a differential pressure transmitter on the market today that you can use for level measurement. However, you need to figure out your budget for your application. That way, you can figure out if a differential pressure transmitter will provide you with enough information and if the accuracy is enough for your application. Read more

DP level calculation – What information do I need to select the pressure cell for a differential pressure transmitter to do the level measurement? To do a DP transmitter level calculation, your meter setup will determine the math needed to find the level in the tank. You can find it using the Pascal equation for hydrostatic pressure: P = ρ*g*h Read more

Sources
First Sensor
MachineDesign

This article is part of our Book of Instrumentation. To learn more about Process Instrumentation check out the whole book here.

Related tags: Absolute pressure Differential pressure gauge pressure Pressure pressure application
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