Mitigating Corrosion Under Insulation and Supporting the Longevity of Industrial Pipe Insulating Systems

# Gauge Pressure

Last updated: July 19, 2024

## What Does Gauge Pressure Mean?

Gauge pressure is the pressure relative to atmospheric pressure; it is positive for pressures above atmospheric pressure, and negative for pressures that are below atmospheric pressure. The atmospheric pressure adds to the pressure of any fluid that is not enclosed.

Gauge pressure is the additional pressure in any system relative to the atmospheric pressure, and is equal to the difference between the absolute and atmospheric pressure while also being zero-referenced against the ambient air pressure. The negative sign is usually omitted and is used to distinguish the negative pressure.

A value can be assigned to the word “vacuum” and gauge is referred to as vacuum gauge. The term gauge pressure is used when the pressure in the system is greater than the local atmospheric pressure. The gauge pressure scale was developed because almost all pressure gauges read zero when open to the atmosphere.

Gauge pressure should not be used for gas law calculations because it does not reflect the true pressure, but rather a pressure difference.

## Corrosionpedia Explains Gauge Pressure

A gauge is often used to measure the pressure difference between a system and its surrounding atmosphere. Gauge pressure is considered the most useful pressure measurement for many practical applications. Due to the importance of atmospheric pressure in many calculations, it should be added to gauge pressure when performing calculations.

Any change in atmospheric pressure due to a change in altitude or weather conditions greatly influences the gauge pressure.

For most working fluids that exist in a closed system, the gauge pressure measurements are most common. The gauge pressure is stated as a positive pressure when it is higher than the atmospheric pressure and as either a vacuum or negative gauge pressure when it is lower than the atmospheric pressure.

Gauge pressure can be calculated by the equation:

pg = ps – patm

where:

pg = gauge pressure

ps = system pressure

patm = atmospheric pressure

Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Absolute pressure is measured with reference to an absolute (and theoretical) vacuum. For example, an ordinary pressure gauge reading of zero does not mean there is no pressure; it simply means there is no pressure in excess of the local atmospheric pressure.

The conversion factor that needs to be used to convert between gauge pressure and absolute pressure is atmospheric pressure. By adding a pressure of one atmosphere to the absolute pressure, the corresponding gauge pressure can be determined.

Gauge Pressure = Total Pressure – 1 atm