Gauge Pressure vs Atmospheric Pressure - Air System.
Channel: Ali Hassan Memon
Date: 27/12/2017

Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.

Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.

In a real, actual gauge, pressure is measured relative to the atmospheric pressure. If there was 1 atmosphere of pressure inside a container (so it's the same pressure inside the container as it is outside it), the gauge will not read 1 atm, but rather 0 atm, as the pressure inside the container would just be the same as the pressure outside. Relative to the outside world there would be no pressure in the container. This is gauge pressure.

Absolute pressure is technically what we think of when we say pressure - the force that the gas is applying per unit area of the container.

If the gas is applying 101,300 Newtons per square meter, then the absolute pressure would be 101.3 kPa. On the other hand, the gauge pressure would be 0 kPa, as 101.3 kPa also happens to be the pressure of the atmosphere outside the container.