What is Pa, PSIA and PSIG, Bar and MPa and what’s their difference? Actually, they are all international measurement units for pressure, measuring pressure of gas, liquid or solid, but they are different at measure environment, application and most used regions, let’s discover all the differences and how to convert between them.

What is Pa?

In scientific environments (physical laboratories, etc.), pressure is usually measured in smaller units, called Pascal (Pa), which is the smallest basic unit of pressure in the International System of Units and is named in honor of French physicist and mathematician Blaise Pascal. The formula for calculating pressure is: P=F/S. F is the force applied to the surface of an object, and S is the area (square) of the object’s force-bearing surface.

Based on this, we can conclude that:

Ways to increase pressure include increasing pressure while the force-bearing area remains unchanged or reducing the force-bearing area while the pressure remains unchanged.

Ways to reduce pressure include reducing pressure while the force-bearing area remains unchanged or increasing the force-bearing area while the pressure remains unchanged.

Pressure is everywhere in life, such as diving. During diving, the deeper you dive, the greater the squeeze your body will feel. This is because the liquid exerts pressure on the side walls and bottom of the container, and the pressure increases with the depth of the liquid.

(water pressure when diving)

The characteristics of the internal pressure of a liquid are: the liquid has pressure in all directions from the inside; the pressure increases with the depth; at the same depth, the pressure of the liquid in all directions is equal; the liquid pressure is also related to the density of the liquid. The greater the density of the liquid, the greater the pressure. The size of the internal pressure of the liquid can be measured with a manometer.

(Pascal’s Law)

This principle also applies to gases. For example, the pneumatic tools commonly seen in our lives, and the air compressors that produce compressed air to drive them, are generally positive displacement air compressors (rotary screw air compressors and reciprocating piston air compressors). Their working principle is to reduce the volume of a certain amount of air, reduce the surface area it contacts, and increase its pressure; if the air compressor CFM is larger, and more air molecules are inhaled while reducing the volume of the gas, the pressure of the air will increase further. This is the principle of positive displacement air compressors compressing air.

By increasing the machine model, improving the working power, or multi-stage compression technology, these air compressors can extend the pressure range of compressed air from 29 to 14500psi or higher, which is approximately equal to 2~1000Bar (0.2~1000MPa), which is a very impressive number.

What is PSI ?

PSI (pounds per square inch) is a commonly used pressure unit in many different pressure measurement applications. It is commonly used in various industrial and technical applications, such as air compressors, tire pressure, tap water supply, storage and distribution of fossil fuels, and various testing experiments and measurement activities. For example, the sea level pressure under standard atmospheric pressure conditions is approximately 14.7psi.

While PSIA and PSIG are two different types of PSI, and the difference between PSIA and PSIG is that whether to take atm pressure(atmospheric pressure) into consideration when it comes to a PSI.    (PSI)

What is PSIG?

PSIG (pounds per square inch gauge) is the pressure measured by a pressure gauge or other pressure measuring device. Gauge pressure is measured relative to ambient atmospheric pressure, so it represents the difference between the pressure in a pipe or tank and atmospheric pressure, that is, it subtracts atmospheric pressure from the measured pressure to get the gauge pressure. For example, when measuring ambient atmospheric pressure at sea level, the PSIG value is always 0, while the gauge pressure in a completely vacuumed sealed bottle (at sea level) is about -14.7 PSIG.

Because PSIG is the result directly displayed to people by the pressure measuring device, in most cases, PSIG is generally assumed to be equivalent to PSI, and many pressure gauges on the market are marked with PSI instead of PSIG.

What is PSIA?

PSIA (pounds per square inch absolute) is also called total pressure or absolute pressure because it takes atmospheric pressure into account and measures the pressure of an object relative to an absolute vacuum. That is, the absolute pressure inside a completely empty, sealed bottle (in a vacuum) is 0 PSIA, while when measuring ambient atmospheric pressure at sea level, the PSIA value is always 14.7.

(PSIG to PSIA conversion)

How to convert PSIG to PSIA?

According to the above explanation, we can draw a simple conclusion:

1PSIA=PSIG+1atm

In the meantime:

1PSIG=PSIA-1atm

For example, if the barometric pressure at a location is standard atmospheric pressure, which is approximately 14.7 psi, a gas tank rated at 6 psi has a gauge pressure of 6 psig and an absolute pressure of 20.7 psia (14.7 + 6).

Note about incorrect conversion: 1 PSIG is equal to -13.7 PSIA, but it is wrong to conclude that 2 PSIG is equal to -27.4 PSIA; this is not how the PSIG to PSIA conversion works. Actually, 2 PSIG is equal to -12.7 PSIA.

What is atm?

To understand the relationship between PSIA and PSIG and relative to MPa and Bar, we first need to understand the ubiquitous third party when measuring these pressure values: atm. The generation of atm pressure can be explained from two perspectives.

Gravity: gravitational attraction of planet on the atmosphere causes air to press on the ground or objects, and these objects and the ground that support the atmosphere will be subject to corresponding pressure.

Molecular motion: The atmosphere is composed of a large number of gas molecules that move irregularly. The gas has strong fluidity, and these molecules will constantly collide with the surface of objects in the air. Each time the collision occurs, the gas molecules will give the surface of the object an impact force. The result of the continuous collision of a large number of air molecules is reflected in the pressure of the atmosphere on the surface of the object.

These two forces together form the atmospheric pressure. This concept was first proposed by physicist Torricelli in 1644. Later, Glick conducted the famous Magdeburg hemisphere experiment in Magdeburg, Germany, which gave people a deep understanding of atmospheric pressure.

According to standard atmospheric pressure (air pressure at sea level under standard atmospheric conditions): the more molecules there are in a unit volume of gas, the more times the air molecules collide with the unit area of the surface of the object in the same time, and the greater the pressure generated. Therefore, the size of atmospheric pressure is related to geographical altitude and temperature. The higher the ground is, or the higher the temperature is, the fewer gas molecules there are in a unit volume, the thinner the atmosphere is, and the smaller the atmospheric pressure is.

According to the international regulations of standard atmospheric pressure, we know that: 1atm = 101,325N/㎡= 101325Pa = 0.101325Mpa = 1.01325Bar = 14.7PSI.

A simple way to measure atmospheric pressure is to use a mercury barometer to calculate the value of atmospheric pressure by measuring the height of the mercury column.

What is Bar and MPa?

PSI actually belongs to the British system of units. Commonly used pressure units include psig and psia. Among them, psig refers to the pressure unit relative to atmospheric pressure, while psia is the pressure unit relative to vacuum.

PSI is the most commonly used pressure unit in the United States, especially in the American valve standard system. Even those who do not work in the technical or transportation industries are familiar with PSI. From civil and mechanical engineers to meteorologists to refinery pressure instrument technicians, they all understand and use pressure measurement as part of their daily activities.

However, psi describes the force in an area specific to one square inch. There are also many areas where imperial units are not commonly used. They may be more accustomed to saying MPa, or Bar. So sometimes we need to convert psia to bar or mpa.

The conversion of psi to metric units is:  1 psi = 6.895 kPa = 0.0689476 bar = 0.006895 MPa, 1bar is approximately equal to 14.5 psi, and 1 bar is approximately equal to 0.1 MPa.

1 Pascal = 1N/㎡ (Newton/square meter) = 1(kg·m/s²)/m² = 1kg/(m·s²)

1 bar = 100 kilopascals (kPa) = 10N/cm2 (Newton/square centimeter) = 0.1MPa

1MPa (megapascal) = 1000kPa (kilopascals) = 1000000Pa (Pascals).

1 standard atmospheric pressure = 101325 Newton/meter^2, which is the pressure generated by 101325 Pascals (Pa) = 760mm mercury column.

For example, an rotary screw air compressor with rated pressure marked as 232psi, the compressed air it produces exerts a pressure of 232psi per square inch, which is approximately 16bar, or 1.6MPa.

What are the common pressure unit conversions

1 psi = 6.895 kPa = 0.0689476 bar = 0.006895 MPa

(In everyday life, one can simply assume that 1 psig (pounds per square inch gauge pressure) equals 0.06895 MPa)

1 psi to atm = 0.06895 atmospheres

1 psi to Bar = 0.06895 bar

1 psi to Pascal = 6895 pa

1 psi to mmHg= 51.7149 Torr

1 psi to inch of mercury = 2.03602 inHg

1 psi to Inch of water = 27.7076 inches of water column

5 psi to bar 0.344735 bar

30 psi to bar 2.07 bar

35 psi to bar 2.41 bar

50 psi to bar 3.45 bar

60 psi to bar 4.14 bar

100 psi to bar 6.89 bar

150 psi to bar 10.34 bar

Conversion between other pressure units

1 bar (bar) = 0.1 MPa

1 millimeter of mercury (Torr) = 133.32 Pa

1 standard atmosphere = 760 Torr

1 kilopascal (kPa) = 1000 Pa

1 kilogram force per square centimeter (kgf/cm²) = 98.0665 Pa

Where are PSIA and PSIG used for?

There are many places in life where psi, psia and psig are needed as measurement standards. Psia is mainly used to measure liquid pressure, and psig is mainly used to measure air pressure. Common air pressure applications require the participation of air pressure equipment air compressors, and psig is also needed when considering what size air compressor do I need.

Air compressors compress the air available everywhere to obtain pressure and kinetic energy, and use compressed air as power to drive pneumatic tools such as jack hammer, air drill, nail gun, impact wrench, sandblaster; or use compressed air as a medium for transportation or to complete special tasks such as cement transportation, PET blowing, etc. The following are common compressed air psi application scenarios.

Traditional pneumatic tools, such as rock drills, pneumatic picks, pneumatic wrenches, pneumatic sandblasting, etc., generally have a pressure of 87~217.5psig, which is approximately equal to 6~15bar (0.6~1.5MPa).

Instrument control and automation devices, such as tool replacement in machining centers, generally have a pressure of 87psig, which is approximately equal to 6bar (0.6MPa).

Vehicle braking, door and window opening and closing devices, generally have a pressure of 29~87psig, which is approximately equal to 2~6bar (0.2~0.6MPa).

Air jet looms use compressed air to blow the weft yarn instead of the shuttle, and the pressure is generally 14.5~29psig, which is approximately equal to 1~2bar (0.1~0.2MPa).

In the food and pharmaceutical industries, compressed air is used to stir the slurry, and the pressure is generally 29~87psig, which is approximately equal to 2~6bar (0.2~0.6MPa).

For spray painting, the pressure is generally 43.5~87psig, which is approximately equal to 3~6bar (0.3~0.6MPa).

For bottle blowing machines, the pressure is generally 435~652.5psig, which is approximately equal to 30~45bar (3.0~4.5MPa).

For the start-up of large marine diesel engines, the pressure is generally 362.5~870psig, which is approximately equal to 25~60bar (2.5~6.0MPa).

For wind tunnel experiments, ventilation of underground passages, and metal smelting, the pressure is generally 870~1450psig, which is approximately equal to 60~150bar (6.0~10MPa).

Oil well fracturing, the pressure is 2175psig, about 150bar (15MPa).

CNG(compressed natural gas) compressing, the pressure is 1450~3625psig, which is approximately equal to 100~250bar (10~25MPa).

High-pressure air blasting coal mining, the pressure is 11600psig, about 800bar (80MPa).

Submarine sinking and floating, shipwreck salvage, submarine oil exploration, hovercraft ship inflation, the pressure is generally 21750~43500psig, about 1500~3000bar (150~300MPa).

Weapon system, missile launch, torpedo launch, the pressure is generally 29000~87000psig, about 2000~6000bar (200~600MPa).

(petroleum gas refining)

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