It might not be something we always think about, but pressure measurement affects us every day. From tire pressure to industrial machinery, accurate pressure readings keep things safe and working properly. In the United States, a common unit for pressure is pounds per square inch (PSI). However, there are less common variations of PSI — pounds per square inch gauge (PSIG) and pounds per square inch absolute (PSIA). Simply put, these variations on the PSI provide more information about what’s called the reference condition for the measurement. For PSIG, that reference condition or starting point is atmospheric pressure; for PSIA, the reference point is a hard vacuum.
In this article, we’ll cover the difference between PSI, PSIG, and PSIA in more detail and discuss when you should use each.
What Is PSI?
PSI, or pounds per square inch, is the force of one pound distributed over one square inch of area. You’ll often hear it in reference to many household and commercial applications, such as the water pressure in pipes, the air pressure in tires, or the hydraulic pressure used in heavy equipment. Even our atmospheric pressure is commonly described in units of PSI.
It’s worth noting, though, that while the PSI may be a common unit of measurement in the U.S., the International System of Units (SI) defines pressure in a unit called the Pascal. It’s a much smaller unit than PSI: One PSI is equal to 6894.76 Pascals. Because it’s the unit that the SI uses, it’s standardized across countries and technical fields. In fact, in the world of metrology and calibration, many calibration certificates for measurement devices will state that the tool’s measurement is traceable back to the Pascal.
What Is PSIG?
PSIG, or pounds per square inch gauge, is the difference between the pressure you’re measuring and the atmosphere’s ambient pressure. (Like any measurement, PSIG — and PSIA and PSI for that matter — is differential by nature; they all require a starting point or reference point.) Perhaps the best example of PSIG is the pressure in a car tire. For example, a tire’s manufacturer might recommend inflating the tire to 35 PSIG. This means that the pressure inside the tire is 35 PSI higher than the ambient pressure outside of the tire; there’s a differential of 35 PSI between the pressure in the tire and the atmospheric pressure.
Alternatively, if the tire were to go flat, then it would have a pressure of 0 PSI. In other words, there’s no difference between the pressure inside the tire and the ambient atmospheric pressure outside of the tire.
The majority of industrial pressure measurements use PSIG measurements.
What Is PSIA?
PSIA, or pounds per square inch absolute, is the difference between the pressure you’re measuring and the pressure of a hard vacuum, which has essentially zero pressure. A good example of PSIA is the ambient atmospheric pressure of the Earth. Atmospheric pressure changes depending on elevation and other factors like weather patterns and temperature but at sea level, it’s 14.69 PSIA. In a literal sense, the ambient pressure at sea level is 14.69 PSIA higher than the pressure of the hard vacuum of outer space.
Other places where experts use PSIA as a unit of pressure include avionics and the oil and gas industry.
When Should You Use PSI vs PSIG vs PSIA?
For the majority of situations, you should use PSIG where you don’t need to account for atmospheric pressure and the focus is on the pressure in the system, such as the pressure inside a tire. Use PSIA, however, when you’re measuring atmospheric pressure or working with vacuum systems. And if you see PSI as the measurement listed for something, presume that it’s a gauge measurement, or PSIG.
The Importance of Reliable Pressure Measurement
PSI, PSIA, and PSIG are all similar units of measurement with slight differences. PSI is the broad unit for pressure, while PSIA denotes absolute pressure and PSIG denotes gauge pressure.
Regardless of which type of pressure measurement unit you use, your pressure measurement equipment has to be properly calibrated to provide accurate readings. Consistent calibration of pressure gauges, tire pumps, and absolute pressure sensors is vital to maintaining accuracy and safety. Want to learn more about pressure measurements and how to ensure your measurements are accurate and reliable? Check out these resources: