As you may have learned early on, pressure is a primary variable in instrumentation and process measurement. You can measure many other variables using a pressure transmitter, like flow and level. This article will explain the difference between the main kinds of pressure: absolute, gauge, and differential pressure.
Absolute, gauge, and differential pressure
To explain pressure, let’s find a good practical example. Say you’re playing a video game on a console or your smartphone. Your fingers will apply force in an area. To calculate pressure, you need to know your force and your area. Next, we have absolute, gauge, and differential pressure. Below, you have a graphic showing the basic differences.
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.
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.
Differential pressure transmitter with metal sensor for measuring pressure differences
What is gauge pressure?
You can consider gauge pressure a form of differential pressure, 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 a vacuum or negative pressure. You even have divisions for vacuum, such as low, high and ultra-high.
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.
In industrial applications, you use an absolute sensor anywhere you have a vacuum, and it may surprise you how many such applications exist.
Now, you see how the difference between absolute, gauge, and differential pressure. You also have examples to help you connect the explanations with your daily applications.
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