Gauge pressure vs. absolute pressure?

just wiki it. Anyways I will give you a oneliner from wiki itself-

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

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

• Differential pressure is the difference in pressure between two points.

It's just a matter of defining your 'zero point'.

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.

Assuming you are in air at sea level and you have an open container the pressure inside and outside will be the same - so a pressure gauge will read zero. That's gauge pressure PSIG

But there is 1 atmosphere of pressure inside the container - so 1atm of absolute pressure.

Now pump the air out of the container to give a vacuum, you have zero absolute pressure and -1 atmosphere of gauge pressure. Although negative gauge pressures are rarely used as they are confusing