The bar is defined as 100,000 Pa, or 100 kPa. The bar is originally a British unit, and 1 bar roughly corresponds to the average air pressure at sea level (although 1 atmosphere (atm) is a better description). 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.However, Pascal is not the only unit used for pressure: Fleets and heavy-duty repair shops rarely use PSIA, unless isolating pressure in a system is necessary. That’s because it’s the best at measuring air pressure in tires, as well as coolant, fuel, and oil pressure in engines. Out of PSI, PSIA, and PSIG air pressure measurements, PSIG is the one most used in the transportation industry. PSI, PSIA, or PSG: Which One Should You Use? Of course, the actual numbers depend on how far up or down you travel and the difference in temperature. 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. Climates are typically colder at higher elevations sometimes the difference can be as significant as 5 degrees for every 1,000 feet. Here’s something else to keep in mind: when altitude change is involved, so is temperature change. The change could affect the pressure in a tire by as much as almost 5 PSIG! That’s not a lot…unless your trip takes you through an altitude change of more than 4,000 feet. As altitude goes up, atmospheric pressure goes down. What About Altitude?Īs if weather worries weren’t enough, altitude can also alter tire pressure. Not only is it a safety issue, it also avoids unscheduled downtime. We’re big proponents of proper tire pressure, by the way. 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. 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. Alternatively, air pressure measurements increase when the temperature goes up, and molecules move around more. However, the cold molecules move more slowly, decreasing the force they exert on the inside of the tire. There are still the same number of air molecules inside your tire no matter what the weather is like outside. If you’ve had a low or flat tire on a cold morning, you know that temperature can affect air pressure measurements. Altitude and Temperature Affects Air Pressure Measurements That’s because the gauge measures the pressure inside the tire compared to the atmospheric pressure outside of it. Taking that into account, our completely empty tire’s PSIG reading would be -14.7. The ambient pressure at sea level is about 14.7 PSIA, but ambient PSIG is always 0. PSIG: This is the term used for PSI in relation to atmospheric pressure. In a vacuum, if the tire in our example were completely empty of air, 0 PSIA would be the measurement. It refers to pressure in a perfect vacuum. PSIA: This designation applies to PSI Absolute. When you pump air into a tire, the molecules bounce around, exerting measurable pressure against the inside of 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. Although it’s a basic label, there are infinite ways to complicate it. PSI: This term is short for “pound-force per square inch,” typically referring to gas or liquid. Various factors can affect pressure, causing a need for air pressure measurements of different types. That means they are ways to assess the amount of force exerted in an area, such as inside a tire or an engine. PSI, PSIA, and PSIG-they’re all units of pressure measurement. That’s why it’s useful to understand the differences between PSI, PSIA, and PSIG. That calls for more than a basic formula. On the other hand, some fields require accurate air pressure measurements for a variety of applications. Just roll with us.Įven those who don’t work in technology or transportation industries are familiar with PSI. It’s true: most of our blog readers know that PSI is a unit of pressure measurement. Come on, Fullbay, you may be saying, everyone knows what PSI is.
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