Air Pressure Measurements: What’s the Difference Between PSI, PSIA, and PSIG?

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What is PSI?

Come on, Fullbay, you may be saying, everyone knows what PSI is.

It’s true: most of our blog readers know that PSI is a unit of pressure measurement. But it’s not the only one out there.

Dude, Fullbay…we know that, too.

We know that you know. And we know that you know that we know. Just roll with us.

Even those who don’t work in technology or transportation industries are familiar with PSI. 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. That means they are ways to assess the amount of force exerted in an area, such as inside a tire or an engine.

But that’s where the similarity ends.

Various 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

If you’ve had a low or flat tire on a cold morning, you know that temperature can affect air pressure measurements.

There are still the same number of air molecules inside your tire no matter what the weather is like outside. 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.

We’re big proponents of proper tire pressure, by the way. Not only is it a safety issue, it also avoids unscheduled downtime.

What About Altitude?

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!

Here’s something else to keep in mind: 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’s the best at measuring 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.

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.

Lisa

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