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.
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 – PatmThe 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.
Gauge pressure can be measured for all fluids – air as well as liquids. An example is the mercury barometer, which indicates the atmospheric pressure. It used to be the only way to measure atmospheric pressure some decades ago. In this case, a glass tube which is closed at one end is filled with mercury and then placed upside-down in a container of mercury.
As the mercury falls under the pull of gravity, it creates a vacuum at the top of the closed end of the tube. When atmospheric pressure increases, the pressure on the surface of the mercury in the container increases and drives up more mercury into the glass tube; so the level of mercury within the glass tube increases correspondingly. You can then read out the atmospheric pressure as the value etched on the glass tube at the top level of the mercury column.
The pressure of a system above the atmospheric pressure is called gauge pressure. It can be calculated if the absolute and atmospheric pressures are known by using the formula:Pgauge = Pabs – PatmA practical example for the use of gauge pressure is when measuring tire pressure.
Another example is the mercury barometer that is used to measure the difference in the absolute and atmospheric pressures.