Absolute pressure and gauge pressure are both measurements of pressure, but they differ in their reference points.
- Absolute pressure is the pressure measured relative to absolute zero pressure, which is a theoretical pressure at which all molecular motion ceases. In other words, absolute pressure includes atmospheric pressure in its measurement. Absolute pressure is typically measured using an absolute pressure gauge and is often expressed in units of pounds per square inch absolute (psia) or kilopascals absolute (kPa).
- Gauge pressure, on the other hand, is the pressure measured relative to atmospheric pressure. This means that gauge pressure is the difference between absolute pressure and atmospheric pressure. Gauge pressure is often used to measure the pressure of gases and liquids in industrial and engineering applications. It is typically measured using a gauge pressure sensor and is expressed in units of pounds per square inch gauge (psig) or kilopascals gauge (kPag).
Conversion of Gauge and Absolute Pressures
To convert gauge pressure to absolute pressure, atmospheric pressure needs to be added to the gauge pressure reading. Conversely, to convert absolute pressure to gauge pressure, atmospheric pressure needs to be subtracted from the absolute pressure reading.
To convert absolute pressure to gauge pressure, you can use the following formula:
Gauge Pressure = Absolute Pressure - Atmospheric Pressure
Where:
- Gauge Pressure: The pressure measured relative to atmospheric pressure, expressed in units of pounds per square inch gauge (psig) or kilopascals gauge (kPag).
- Absolute Pressure: The pressure measured relative to absolute zero pressure, expressed in units of pounds per square inch absolute (psia) or kilopascals absolute (kPa). Atmospheric Pressure: The pressure of the surrounding atmosphere at a given location, expressed in the same units as the pressure being measured.
Example
For example, if you have an absolute pressure reading of 100 kPa and the atmospheric pressure at your location is 101.3 kPa, the gauge pressure can be calculated as follows:
Gauge Pressure = 100 kPa - 101.3 kPa
Gauge Pressure = -1.3 kPa
This means that the pressure being measured is 1.3 kPa below atmospheric pressure.
Note that if the atmospheric pressure is not given, you can use standard atmospheric pressure values based on your location or assume a value of 101.3 kPa at sea level.