Pressure is all around us, affecting us in a multitude of ways every day. From the moment you wake up, you’re interacting with different kinds of pressure in manners you might not even think about. Any time you grab or push something, you’re exerting pressure on it. Even just sitting in a chair is creating pressure. It’s a crucial building block of physics.
In this article, we’ll cover what exactly pressure is and the different types of pressure that you can measure.
What Is Pressure?
Pressure is the force applied perpendicularly to a defined (measured by a unit) area of an object. To put it simply, it’s force distributed over a defined area. There’s no directional quality to pressure, since it acts equally on all surfaces. For a given amount of force, the larger the surface area is, the lower the amount of pressure will be. This is why your hand may not leave a mark on a wall, but a much smaller nail will leave a mark when applied to the wall with an equal amount of force.
The traditional formula for pressure is as follows:
P = F/A
- P is the pressure.
- F is the magnitude of the force applied, which can be found my multiplying mass by acceleration (F = m*a); it’s common to use the acceleration of gravity (g) when talking about pressure.
- A is the surface area in contact, which can be found by squaring length (A = length2).
Note that the standard unit for pressure is the Pascal (Pa), with 1 Pa equal to one Newton per square meter. In the imperial measurement system, a common unit of measure is pounds per square inch (PSI).
It’s also important to note that all pressure measurements are, by nature, differential. In other words, the number you get when taking a pressure measurement will always be the difference between two things: the value you’re measuring and a starting point, known as a reference condition.
This differential property plays a particularly pivotal role in two classifications of pressure — absolute pressure and gauge pressure — both of which we’ll review in more depth below. That said, there are many different types of pressure, including but not limited to:
- Absolute pressure
- Gauge pressure
- Atmospheric pressure
- Differential pressure
- Vacuum pressure
Now, let’s review each type and its uses.
Absolute Pressure
Absolute pressure is the pressure of a system relative to a hard vacuum. In other words, absolute pressure uses a hard vacuum, which has virtually no pressure, as the reference or starting point, so measurement values for absolute pressure include the total pressure of a system — including the natural atmospheric pressure (which we’ll discuss later) surrounding it.
Engineers and scientists commonly use absolute pressure in their work. For example, oceanographers use absolute pressure to measure water depth and wave height, meteorologists use it to measure barometric pressure, and experts in the oil and gas industry use it to measure pressure in a well.
Gauge Pressure
Gauge pressure is the most commonly measured type of pressure and the type most people are familiar with. It’s the pressure of a system relative to the surrounding atmospheric pressure, so it’s equal to absolute pressure minus atmospheric pressure. In other words, gauge pressure uses atmospheric pressure as the reference or starting point and may change if the atmospheric pressure changes.
For example, measuring the pressure in a vehicle’s tires would be measuring the tires’ gauge pressure; if you see any sort of reading on your tire pressure gauge, that means the pressure inside the tire is higher than the natural atmospheric pressure. And when a tire is flat and your measurement reads zero, it means there’s no difference in pressure between the inside of the tire and the atmospheric pressure outside of the tire.
Atmospheric Pressure
Atmospheric pressure (Patm), also called barometric pressure, is the pressure exerted by the air in the atmosphere — it’s the ambient pressure exerted on us in any given location on Earth. Generally, we don’t feel this constant pressure, because our bodies can equalize atmospheric pressure given enough time to acclimate to it.
However, atmospheric pressure does change, primarily with altitude, though weather patterns and temperature can have an effect. Standard atmospheric pressure occurs at sea level, measuring at 14.7 pounds per square inch absolute (PSIA). (Note that atmospheric pressure is always expressed in terms of absolute pressure.) The higher you are above sea level, the lower the atmospheric pressure. For instance, Denver, Colorado, sits at 5,280 feet above sea level; the average atmospheric pressure at this elevation is 12.1 PSIA. And the further below sea level you go, the higher atmospheric pressure will be.
This altitude-based variance is why when we drive down a steep hill, we feel pressure on our ear drums: The pressure at the bottom of the hill is slightly higher than the pressure at the top, and our bodies can’t equilibrate quickly enough to balance out the rapid change in pressure. This variance also means that many pressure measurement tools must be calibrated to local barometric readings.
Differential Pressure
Literally speaking, differential pressure is the difference between two measured pressures. However, from a technical perspective, we know that all pressure measurements are differential. So why do so many people refer to differential pressure seemingly as its own, separate type of pressure? It’s a legacy phrase that some scientists and experts commonly use to describe a low-pressure type of measurement. Experts also commonly use it to refer to pressure sensors where there are two pressure connections. In practice, most of these low-differential-pressure sensors make measurements relative to ambient, atmospheric pressure. These low-pressure measurements are often used to calculate flow in a pipe.
Vacuum Pressure
Vacuum pressure, sometimes called negative gauge pressure, is a measurement that is less than atmospheric pressure.
All Pressure Measurement Relies on Calibration
Pressure measurement is an exact science where even small anomalies can have massive consequences. Different types of pressure require different tools for the job, and it’s important to ensure those tools are properly calibrated to provide accurate measurements. Want to learn more about pressure measurements and how to ensure your measurements are accurate and reliable?
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