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Gauge and absolute pressure
Intermediate
Gauge pressure is relative to the atmosphere.
Absolute pressure = gauge pressure + atmospheric pressure. Tyre gauges read the gauge value.
A tyre at 220 kPa gauge has 321 kPa absolute.
Gas laws need absolute pressure.
In more detail
Gauge pressure is the pressure relative to atmospheric pressure, and absolute pressure is measured from a perfect vacuum. Absolute pressure equals gauge pressure plus atmospheric pressure.
Real-world uses
- Tyre gauges show gauge pressure, which is zero when the tyre matches the atmosphere.
- Engineers use absolute pressure in gas law calculations.
- Tyre gauges show gauge pressure, which is zero when the tyre is at atmospheric pressure, so the reading excludes the air around it.
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Frequently asked questions
Why do tyre gauges read zero when open?
They measure gauge pressure, which is the difference from the atmosphere.
Which pressure is used in the gas law?
Absolute pressure, since the gas law requires true pressure.
Why do tyre gauges read zero in air?
Gauge pressure is the pressure above atmospheric, and in open air the tyre is at atmospheric pressure.
Related topics in Fluids & Elasticity
Atmospheric pressure · Barometer · Manometer · Buoyancy · Archimedes' principle · Floating and sinking