Gauge Pressure & Absolute Pressure: Relation & Conversion

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  • 0:03 Feeling the Force
  • 0:53 Absolute Pressure
  • 1:52 Gauge Pressure
  • 3:18 Absolute Pressure vs…
  • 5:39 Lesson Summary
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Lesson Transcript
Instructor: Babita Kuruvilla

Babita has an electrical engineering degree and has taught engineering students and college students preparing for medical and dental college admissions tests.

In this lesson, we'll define absolute pressure and gauge pressure measurements. We'll also learn the equation by which these two measurements are related. We'll conclude the lesson with examples of absolute and gauge pressure measurements.

Feeling the Force

You're walking outside on a windy day and the wind is strong enough that you can feel its force. This feeling is caused by the air molecules that are bouncing off of you. We call this collective force of the air molecules per unit area the atmospheric pressure. Often, this pressure is measured in pounds per square inch (psi).

Have you ever wondered what this pressure would feel like if you were in space? That's a trick question because there's no atmosphere in space! There are no air molecules in space, so you wouldn't feel any pressure at all, which is why we call space a vacuum. In other words, there is absolute zero pressure in a vacuum.

We have two ways of measuring pressures, depending on what we use as reference. They are:

  • Absolute pressure
  • Gauge pressure

Absolute Pressure

One way to measure pressure is to use absolute zero pressure (like in space) as the base value against which other pressures are measured. The pressure measured relative to this absolute zero pressure in a vacuum is what scientists call absolute pressure. A pressure measurement below atmospheric pressure is called negative pressure, or vacuum pressure.

So you might be thinking: ''How is this possible? On Earth, how do we make measurements based on a vacuum?'' We use an absolute pressure sensor. Scientists have found a way to create a vacuum behind a diaphragm in the sensor. They do this by removing the air and then sealing the instrument so that it does not have direct contact with the outside. So, absolute pressure is also called sealed-pressure and is typically given in the unit of psia (pounds per square inch absolute). This is really helpful because even if the outside ambient pressure changes, we have an accurate method to measure the pressure.

Gauge Pressure

On the other hand, gauge pressure is measured relative to the atmospheric pressure. This means that the measurement values change along with changes in the atmospheric pressure. So gauge pressures change as the weather changes, and we can get different readings even though the absolute pressure reading would have remained the same.

When we check the air pressure in car tires, what we are measuring is the pressure inside the tire relative to the surrounding atmospheric pressure (i.e., the gauge pressure). This pressure is also known as vented pressure and is typically given in the unit of psig (pounds per square inch gauge).

Another instrument that measures gauge pressure is a mercury barometer, used specifically to measure the difference between the vacuum and atmospheric pressures. The tube is filled with mercury and immersed upside down into a container of mercury. A vacuum forms at the closed end of the tube, where the pressure is zero. At the open end, the atmospheric pressure presses against the mercury in the container.

When atmospheric pressure increases, the pressure on the surface of the mercury increases correspondingly. This causes more mercury to flow into the glass tube, so the level of mercury within the glass tube increases correspondingly. Therefore, the column of mercury will be higher and we can read the pressure value marked on the barometer that corresponds with the top of the mercury column.

Absolute Pressure vs. Gauge Pressure

Now, are these two types of measurements related? Of course they are because they both measure pressures. If we know one type of pressure and the atmospheric pressure, we can convert it to the other type using the equation:

Pabsolute = Pgauge + Patmosphere

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