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Understanding Pressure Measurements

Aug 13, 2021 ·

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There are many tests that vehicles undergo during the manufacturing process, including pressure measurements and tests on a range of parts and systems.

Having a knowledge and understanding of different pressure measurements and how they are used can be helpful when pursuing a career in the automotive industry. Attending a trade school like Universal Technical Institute (UTI) can help give you experience working with different pressure measurements and equipment.1

Keep reading to learn the distinctions between pressure measurement units and the applications they’re used for.

Units of Pressure Measurement

There are a few different ways to measure pressure, which is the amount of force exerted in an area.

The most common unit used for measuring air pressure in the United States is pounds per square inch (PSI). Knowing what PSI means and how it is used is important for various applications in the transportation industry.

For household, industrial and commercial equipment, PSI is used most of the time. Some of the applications it’s used to measure include tire pressure, test and measurement activities, and fuel storage and distribution.

While PSI is probably the unit you’re most familiar with, different factors may affect pressure and call for different kinds of measurements:

  • PSI: PSI typically refers to measuring pressure for a gas or a liquid. The example of air in a tire is the easiest to picture. Molecules will bounce around inside the tire, which exerts measurable pressure against the inside, and from that you can take a reading.

  • PSIG: PSIG stands for “pounds per square inch gauge.” This means the pressure is relative to atmospheric pressure. PSI and PSIG can mean the same thing at times — often when a PSI unit is measured, PSIG is the default meaning (tire pressure is considered gauge pressure).
  • PSIA: PSIA stands for “pounds per square inch absolute.” This is referencing pressure relative to a full vacuum or zero. For example, if a tire had no air in it whatsoever, the measurement would read as 0 PSIA.

It’s also possible to convert a unit of measure in PSIG to PSIA. Considering PSIG is relative to atmospheric pressure, you can go between the two by adding or subtracting this measurement. The average pressure of the atmosphere at sea level is 14.7 PSI. To get a PSIG reading from PSIA, you simply subtract the 14.7 from PSIA. If you’re switching from PSIG to PSIA, you do the opposite and add 14.7 to your PSIG reading.

What Affects PSI and PSIG Pressure?

There are a few things that can affect the PSI and PSIG readings, regardless of how many molecules are inside a space.


Regardless of the number of air molecules there are, different weather and temperatures can affect how those molecules move and how much force they create. Colder molecules move more slowly, which decreases the amount of force they exert and results in a decreased PSIG reading.

Molecules move around more if the temperature rises, which will result in a higher reading. However, these fluctuations will typically only cause a 1 to 5 change in PSIG. It is important, however, to check and adjust tire pressure when it’s cold.


When altitude increases, atmospheric pressure goes down. The average decrease in PSIG is only about 0.5 per 1,000 feet, but this can be a lot depending on where you’re traveling. Altitude change is also related to temperature change, as going from a higher altitude to a lower one will result in a temperature increase.

Pressure Measurements on Vehicles

PSIG measurements are most commonly used in the transportation industry, since they can measure air pressure in tires accurately, as well as other pressure systems in or near a vehicle’s engine.

During engine manufacturing, engine test stands are used to examine different engine variables to ensure things are working properly. PSIG or gauge pressure sensors are used to complete these tests, which include:

  • Fuel pressure: An internal combustion engine relies on fuel to run, so monitoring the pressure of the fuel line ensures there is efficiency and control during the process of delivery.
  • Oil pressure: The oil in your vehicle’s engine is responsible for lubricating moving parts and helping things run smoothly. A gauge is used to measure if the oil is effectively moving through passages in the engine and not facing resistance.
  • Coolant pressure: Coolant helps keep your vehicle’s engine operating at an appropriate temperature and prevents overheating. It’s important that your coolant doesn’t experience too much expansion, because hoses, lines and gaskets could be affected and damaged.

These pressure tests are the most common, though there are more in-depth tests that might check for things such as pressure decay or restrictions of airflow to the engine. These can help ensure there are no system leaks or unused power capacity in the engine.

Learn Pressure Testing at UTI

Does the idea of working in a hands-on field sound interesting to you? It’s a great time to explore the potential the automotive field has to offer — total auto technician employment is expected to be 743,800 by 2031.47

At UTI, you can enroll in the Automotive Technology program and get the education and experience you need for an entry-level career in the industry.1 Courses can teach you the skills you need to know, including how to perform different pressure tests on various systems.

Request more information today to take the first step toward an exciting future.

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1) UTI is an educational institution and cannot guarantee employment or salary.

2) For program outcome information and other disclosures, visit

47) The U.S. Bureau of Labor Statistics projects that total national employment for Automotive Service Technicians and Mechanics will be 743,800 by 2031. See Table 1.2 Employment by detailed occupation, 2021 and projected 2031, U.S. Bureau of Labor Statistics,, viewed October 13, 2022. UTI is an educational institution and cannot guarantee employment or salary.

Universal Technical Institute of Illinois, Inc. is approved by the Division of Private Business and Vocational Schools of the Illinois Board of Higher Education.


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