Even those who don’t work in technology or transportation industries are familiar with PSI. At the very least, they understand that it’s a unit of pressure measurement. On the other hand, some fields require accurate air pressure measurements for a variety of applications. That calls for more than a basic formula. That’s why it’s useful to understand the differences between PSI, PSIA, and PSIG.
PSI, PSIA, and PSIG
PSI, PSIA, and PSIG—they’re all units of pressure measurement. They are ways to assess the amount of force that’s exerted in an area. Like inside a tire or an engine, for instance. However, that’s where the similarity ends. Different factors can affect pressure, causing a need for air pressure measurements of different types.
PSI—this term is short for “pound-force per square inch,” typically referring to gas or liquid. Although it’s a basic label, there are infinite ways to complicate it. For the purposes of this blog post, we’ll stick with the basics and continue to refer to the example of air in a tire. When you pump air into a tire, the molecules bounce around exerting measurable pressure against the inside of it.
PSIA—this designation applies to PSI Absolute. It refers to pressure in a perfect vacuum. In a vacuum, if the tire in our example were completely empty of air, 0 PSIA would be the measurement.
PSIG—this is the term used for PSI in relation to atmospheric pressure. PSIG is also known as Gauge Pressure. The ambient pressure at sea level is about 14.7 PSIA, but ambient PSIG is always 0. Taking that into account, our completely empty tire’s PSIG reading would be -14.7. That’s because the gauge measures the pressure inside the tire compared to the atmospheric pressure outside of it.
Altitude and Temperature Affects Air Pressure Measurements
Anyone who has had a low or flat tire on a cold morning knows that temperature can affect air pressure measurements. Whatever the weather, there is still the same number of air molecules inside the tire. However, the cold molecules move more slowly, decreasing the force they exert on the inside of the tire. Decreased PSIG is the result. Alternatively, air pressure measurements increase when the temperature goes up, and molecules move around more. Typically, those types of fluctuations only cause a 1 to 5 PSIG change. The rate is about 1 PSIG for every 10 degrees difference in temperature. Therefore, since the suggested pressure for drive tires is 75 PSIG (80 PSIG for trailer tires), you should check tire pressure and adjust it when they’re cold. Proper tire pressure avoids unscheduled downtime.
As if weather worries weren’t enough, altitude can also alter tire pressure. As altitude goes up, atmospheric pressure goes down. The rate of decrease is only about .5 PSIG per 1,000 feet. That’s not a lot unless your trip takes you through an altitude change of more than 4,000 feet. The change could affect the pressure in a tire by as much as almost 5 PSIG.
Plus, when altitude change is involved, so is temperature change. Climates are typically colder at higher elevations. Sometimes the difference can be as significant as 5 degrees for every 1,000 feet. That means driving from a relatively warm area at a lower elevation to a higher one might affect tire pressure by as much as 8 or 9 PSIG. Of course, the actual numbers depend on how far up or down you travel and the difference in temperature.
PSI, PSIA, or PSG: Which One Should You Use?
Out of PSI, PSIA, and PSIG air pressure measurements, PSIG is the one most used in the transportation industry. That’s because it is the one that best measures air pressure in tires as well as coolant, fuel, and oil pressure in engines. Fleets and heavy duty repair shops rarely use PSIA, unless isolating pressure in a system is necessary. However, now that you know the difference between PSI, PSIA, and PSIG, you can choose the best air pressure measurements for the job you’re doing.