Pressure Measurement: What Is the Difference Between PSI, PSIA and PSIG?
Pressure measurement is all about PSI. This is because pounds per square inch (PSI) is the most commonly used unit of pressure measurement in the United States. Even those who do not work in the technology or transportation industry are familiar with PSI. Pressure measurement is an important part of life in the 21st century. For example, before you drive or ride a bicycle, you need to make sure that your car tires or bicycle tires are inflated to the proper PSI. Now, various types of equipment include pressure sensors or meters to assist in monitoring and diagnostic operations. In addition, many occupations, from civil and mechanical engineers to meteorologists, to oil refinery pressure instrument technicians, will understand and use pressure measurement as part of their daily activities.
1. A Brief Look at Pounds per Square Inch — PSI
Pounds per square inch is a unit of pressure used most of the time in American households, commercial or industrial equipment. Other countries use different units to measure pressure. In scientific environments (physical laboratories, etc.), the pressure is usually measured in smaller units called Pascals (named after French physicist Blaise Pascal).
For reference, 1 PSI is equal to 6,894.76 Pascals. Pressure measuring instruments, such as pressure gauges and sensors, usually display measured values in PSI.
1 PSI =
27.7076 inches of water column
（Related articles about pressure unit conversion：A List of Common Pressure Units Conversion Formulas）
Two commonly used PSI variants are PSIA and PSIG.
2. PSIA – Pounds per Square Inch Absolute
PSIA is a term that describes the absolute pressure in PSI, including the pressure of the atmosphere. Absolute pressure is also sometimes referred to as “total pressure.”
The pressure of a vessel completely void of any air molecules would be 0 PSIA, while the average atmospheric surface pressure (at sea level) is roughly 14.7 PSIA.
3. PSIG – Pounds per Square Inch Gauge
PSIG is the term for pressure specified by a gauge or other pressure measurement device. It gives the difference between the pressure in a pipe or tank and the pressure of the atmosphere (atm). Gauge pressure is measured relative to ambient atmospheric pressure. A vessel completely void of any air molecules (at sea level) would be roughly -14.7 PSIG.
4. The Example of PSIA to PSIG Conversion
Note that PSIG is always lower than PSIA. The formula to describe this relationship is:
PSIG + 1atm = PSIA PSIA-1atm = PSIG (where atm is air pressure).
It is easy to calculate PSIA or PSIG or convert between the two. You can use the actual atmospheric pressure value for your location if it is available, or you can also use 14.7 psi (the approximate atmospheric pressure at sea level) as a standard value to convert PSIG to PSIA and vice versa. (Unless you live at high altitude or in a deep valley, the sea level value will work.)
In other words, since atmospheric pressure at sea level is 14.7 PSIA, you subtract the PSIA of 14.7 from an atm pressure of 14.7 to equal 0 PSIG (14.7 (PSIA) – 14.7 (atm) = 0). As an example, absolute pressure at sea level is 14.70 PSIA and absolute pressure at an elevation of 1,000 feet is 14.18 PSIA. At the higher elevation there is less pressure, so if an absolute pressure gauge is read at a 1000-foot elevation, its readings will be close to .5 PSI (14.70 – 14.18 = .52) less than those from a standard pressure gauge.
This picture may help you better understand the relationship between them.