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By now, you should’ve heard of the acronym PSI, pounds per square inch, which is one of the most common units of measurement for pressure. PSI is used across all industries and applications to describe the amount of force is being exerted by something.

And so, that brings us to the different types of measurements, PSI, PSIA, and PSIG. This article will provide you with all the relevant information about them and their differences.

Table of Contents

Compressed Air Pressure Measurement

Pressure can be described as the force applied to an area. PSI is an indication of the maximum pressure an air compressor can produce, and in conjunction with CFM, cubic feet per minute, is one of the key measurements to understanding the performance of an air compressor. Especially when looking to purchase a new compressor, these two values together are some of the most important considerations.

It’s important to understand that the altitude and geographic location of your compressor affect pressure measurements, and so there are 3 different ways to look at PSI:

  • PSI
  • PSIA
  • PSIG
Photo by CEphoto, Uwe Aranas

Temperature can affect the air pressure measurements as you may know from checking your car tires in the morning on cold and warm days. Cold molecules move around more slowly and decrease the force they exert resulting in a decreased PSIG. On the contrary, if the temperature increases, molecules will move around more, and pressure will increase as a result. It’s estimated that around 1 PSIG is lost for every 10 degrees of temperature.

In terms of altitude, the greater the altitude the lower the atmospheric pressure. Though it’s not a lot, estimated at around .5 PSIG per 1000 feet, it’s still something to be aware of! But the important consideration is that as altitude changes, so does the temperature. So, a compressor at a higher elevation in a colder temperature will have to work harder to operate at the same pressure as a compressor at sea level in a warmer temperature.

Here’s an example of a pressure gauge readily available on Amazon!

What is PSI?

PSI stands for the pounds per square inch and is a measure of the force delivered by air compressors. For instance, a compressor could be rated 120 PSI which means it can deliver 120 pounds of pressure per square inch. PSI is the measurement in question, but what is 1 PSI equal to in other units of measurement?

These measurements include Pascals, Bar, Torr, inches of water column, inH, the terms absolute and gauge are generally universal. For the sake of demonstration, this article will use PSI. Let’s take a look!

1 PSI is equal to:

  • 0.068046 atmospheres
  • 0.0689476 Bar
  • 6894.76 Pascals
  • 51.7149 Torr
  • 2.03602 inHg
  • 27.7076 inches of water column

What is PSIA?

PSIA stands for the pounds per square inch absolute. Which is sometimes referred to as the total pressure, PSIA is the pressure relative to zero, or a perfect vacuum. Due to varying atmospheric pressure, gauge pressure, or non-absolute pressure, measurements are generally not precise, while absolute pressure is always definite.

The best example of an absolute referenced pressure is the measurement of barometric pressure. In order to produce an absolute pressure sensor, the manufacturer will seal a high vacuum behind the sensing diaphragm. Therefore if you hold open the process pressure connection of an absolute pressure transmitter to the air it will read the actual barometric pressure.

PSI vs PSIA

The key difference between PSI and PSIA is that is simply a measurement of pressure, while PSIA is the absolute pressure taking into account atmospheric pressure. This way, PSIA is capable of providing an accurate pressure value while PSI will not be so accurate because it doesn’t take into account the varying atmospheric pressure.

The difference in value between the two readings will be atmospheric pressure, which at sea level is equal to 14.7 PSI.

What is PSIG?

The most common pressure reference is gauge pressure which is signified by a ‘g’ after the pressure unit e.g. 30 PSIG. Gauge pressure is measured in relation to ambient atmospheric pressure. The term PSIG, gauge pressure, stands for pounds per square inch gauge and is used for PSI in relation to atmospheric pressure. The ambient pressure at sea level is 14.696 PSIA, but ambient PSIG is always taken at 0.

Gauge pressure is measured relative to ambient atmospheric pressure. A vessel completely void of any air molecules (at sea level) would be roughly -14.696 PSIG, or -14.7 PSIG, and ambient air pressure is always measured as 0 PSIG, regardless of whatever current barometric pressure is. For this reason, barometric pressure sensors are absolute; if a gauge sensor was used to measure barometric pressure, that sensor would be useless!

Changes in atmospheric pressure due to weather conditions or altitude directly influence the resulting gauge pressure from its sensor. A gauge pressure which is higher than ambient pressure is commonly referred to as positive pressure. If the measured pressure is below atmospheric pressure then it is called negative or vacuum gauge pressure.

Usually only one pressure port is on a gauge pressure sensor. The ambient air pressure is directed through a vent hole or a vent tube to the back of the sensing element. Outside air pressure is exposed to the negative side of the pressure sensing diaphragm via a vented gauge pressure transmitter. This makes sure that it always measures with reference to the ambient barometric pressure.

Therefore, a vented gauge pressure sensor will read zero pressure when the process pressure connection is held open to atmospheric air. A sealed gauge reference is very similar except that atmospheric pressure is sealed on the negative side of the diaphragm. High-pressure applications such as measuring hydraulic pressures usually adopt this, as atmospheric pressure changes will have only a slight effect on the accuracy of the sensor.

The pressure measured through a sealed device in which the zero point is set defines a sealed-gauge pressure. Whatever the pressure inside of the device was before sealing is the setpoint, which the manufacturer of the sealed pressure gauge decides.

PSI vs PSIG

People often query, do PSI and PSIG mean the same thing? Well, the answer is, yes, sometimes. PSIG is often the default meaning when a PSI unit is presented. For example, your tire pressure is gauge pressure. Likewise, the Kimray back pressure valve’s gauge reads in PSIG. 

PSIG vs PSIA

The simplest way to explain the difference between the two is that absolute pressure uses absolute zero as its zero point, while gauge pressure uses atmospheric pressure as its zero point. Due to varying atmospheric pressure, gauge pressure measurement is not precise, while absolute pressure is always definite.

How to Calculate PSIA & PSIG

An important fact to remember is that PSIG is always lower than PSIA. The formulas that are used to describe their relationship are:

PSIA to PSIG Conversion: PSIA to PSIG Formula

PSIA = PSIG + 1 atm

PSIA = PSIG + 14.7 PSI

Worked Example: 0 PSIG to PSIA

PSIA = 0 PSIG + 1 atm

PSIA = 0 PSIG + 14.7 PSI = 14.7 PSI

Therefore, 0 PSIG = 14.7 PSIA

PSIG to PSIA Conversion: PSIG to PSIA Formula

PSIG = PSIA – 1 atm

PSIG = PSIA – 14.7 PSI

Worked Example: 1 PSIA to PSIG

PSIG = 1 PSIA – 1 atm

PSIG = 1 PSIA – 14.7 PSI = 13.7 PSI

Therefore, 1 PSIA = 13.7 PSIG

Where:

PSIA = pounds per square inch absolute

PSIG = pounds per square inch gauge

Atm = Atmospheric pressure (14.7 PSI)

It’s extremely easy to convert between the two or calculate either PSIA or PSIG if you know the atmospheric pressure value for your location and add it into the formula. If you don’t know the atmospheric pressure value for your area, you can use the standard atmospheric value of pressure at sea level 14.7 (14.696) PSI.

PSIA vs PSIG: Which to Measure?

There are many different systems of pressure measurement, with absolute pressure and sealed gauge pressure being two of the most commonly used. There are many differences between these two measurements of pressure that have significant effects on their use and measurement.

Depending on why you are measuring pressure, determining whether you need gauge or absolute reference pressure is as important as selecting the pressure range itself, particularly for low pressure. If you get it wrong it could create huge errors in your measurements.

It is not always a straightforward decision but generally if you want to measure or control a pressure that is influenced by changes in atmospheric pressure, like the level of liquid in an open tank for example; you would typically choose vented gauge pressure as you are interested in the pressure reading minus the atmospheric pressure component.

On the contrary, if you intend to measure pressures that are not influenced by changes in atmospheric pressure, e.g. leak testing a completely sealed non-flexible container, you would use an absolute pressure sensor.

If a gauge pressure sensor was used instead of an absolute gauge to measure this container’s pressure, and the barometric pressure was to change, then the sensor’s reading would subsequently change, despite the fact that the pressure in the container has actually remained the same.

For further reading on this topic, and about compressed air systems as a whole, I’ve picked out this Compressed Air Operations Manual available on Amazon!

FAQs (Frequently Asked Questions)

How do I convert PSIG to PSIA?

To convert PSIG to PSIA, you must take the PSIG value and add 1 atmospheric pressure. This should be the atmospheric pressure value of your location, but if you cannot find out what it is you can use 14.7 PSI which is standard atmospheric pressure at sea level.

What does 14.7 PSIA represent?

14.7 PSIA, or 14.696 PSIA to be more precise, represents the value of standard atmospheric pressure (1 ATM) at sea level.

What does PSIA stand for?

PSIA stands for the pounds per square inch absolute. It’s a measurable pressure in relation to a perfect vacuum. Due to varying atmospheric pressure, gauge pressure, or non-absolute pressure, measurements are generally not precise, while absolute pressure is always definite.

What is the difference between psig and psia?

The simplest way to explain the difference between the two is that absolute pressure uses absolute zero as its zero point, while gauge pressure uses atmospheric pressure as its zero point. Due to varying atmospheric pressure, gauge pressure measurement is not precise, while absolute pressure is always definite.

What is the difference between psi and psig?

Though PSIG is often a default meaning of PSI, in simple terms, PSI refers to the amount of force exerted on an object per area of one square inch. PSIG, or pounds per square inch, gauge is a unit of pressure relative to the surrounding atmospheric pressure and the “g” in PSIG indicates that it’s a relative measurement.

Is PSI the same as PSIG or PSIA?

PSIG is often the default meaning when a PSI unit is presented. As an example, your tire pressure is read as gauge pressure. Likewise, the Kimray back pressure valve’s gauge reads in PSIG. PSIA is different from PSI or PSIG as it provides absolute pressure in relation to a perfect vacuum.

Is 0 PSI a Vacuum?

The pressure of a perfect vacuum is 0 PSI. You may notice when measuring a perfect vacuum that a pressure gauge will read -14.7 PSI. This is because it is a differential pressure gauge, and is measuring the difference between the vacuum and atmospheric pressure.


If you have any questions regarding PSIA vs PSIG vs PSI, please leave a comment below, with a photo if applicable, so that someone can help you!