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Gauge Pressure: Definition & Formula

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  • 0:03 What Is Gauge Pressure?
  • 0:43 Atmospheric & Absolute…
  • 1:20 Gauge Pressure
  • 2:05 Mercury Barometer
  • 2:54 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 the terms absolute, atmospheric, and gauge pressures and learn the equation that relates these three terms. We'll also see examples of gauge pressure.

What Is Gauge Pressure?

You are driving down the highway and notice that the car is slightly being pulled to one side. So, you take the car to your mechanic Mike, an expert mechanic, who immediately notices that the tires look slightly deflated. He uses a tire pressure gauge to check and says that the front tires are at 29 psi. He also checks the side of the tire and reads the ideal pressure for your wheels is 32 psi; so, he inflates all four tires to 32 psi. Mike advises you to check the tires regularly so that they are at their ideal gauge pressure. You drive away feeling safe to drive and that's when you start to wonder: What is gauge pressure?

Atmospheric & Absolute Pressures

At sea-level, the air above a surface has a weight due to the pull of gravity. This weight can be felt on the surface that it presses against ,and we know this pressure as the atmospheric pressure, denoted as Patm. So, if we keep going higher in altitude, there is less air above that level, and therefore, the weight decreases correspondingly. Eventually, we reach a point where there is no air (or vacuum). The pressure at this point is zero, so, the pressure measured relative to the pressure in a vacuum is called the absolute pressure, denoted by Pabs.

Gauge Pressure

The difference between absolute pressure and atmospheric pressure is what we call gauge pressure (Pgauge). It can be calculated if we know the absolute and atmospheric pressures using this formula:

Pgauge = Pabs - Patm

The gauge pressure is usually given in pounds per square inch (psi). So, when your mechanic measures the tire's pressure and fills air to 32 psi, he is measuring the tire's internal pressure that is in excess to the atmospheric pressure. The atmospheric pressure at sea-level is 14.7 psi. In the SI system, pressure is given in pascals, and so the atmospheric pressure is 101 kPa.

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