There are 3 main units that we use in our compressed air world:

  • Pressure
  • Volume
  • Capacity.

Consider them your new best friends. ?

Come with me and I’ll introduce you to each one of them. Don’t worry, they are very friendly.

Your new best friends

Here’s an overview of the 3 main units in compressed air land.

I’d like to think of pressure as ‘how strong’. Volume as ‘how much’ and flow / capacity as ‘how often’ or ‘how fast’.

You will see many different units being used, especially for volume and capacity. But it doesn’t really matter whether you use liters/second or CFM (cubic feet per minute) for example, or any other unit (coffee cups per week)… you can make up your own units if you want to!

Just remember that they all express the same thing: flow, volume or pressure in these examples.

Pressure

Pressure is ‘how strong’ compressed air is.

It’s how hard the compressed air pushes against an object. The piston in a cylinder for example.

You will often see it expressed as either BAR or PSI.

1 bar = 14.5 PSI

If we move a piston (by compressed air), the pressure pushes the piston out.

If we double the pressure of the compressed air, we double the outward force of the piston.

In this simple example, we doubled the pressure from 3 bar to 6 bar. This increases the force of the pneumatic cylinder from 1 kN to 2 kN.

Pressure to Force

Pressure is actually a ‘force per area’.

We can see this clearly in the imperial unit of PSI.

Pounds per Square Inch.

So at 100 PSI, we have 100 pound force on every square inch.

If our piston has an area of 4 square inch. The outward force would be 4 * 100 = 400 pounds of force.

1 bar is 0.1 N/mm2 (Newton per square mm).

So if we have compressed air of 7 bar, pushing against a piston with an area of 200 mm2, the resulting force will be 7 * 0.1 * 200 = 140 Newton.

Increasing pressure

Pressures can’t be added up.

If we have two air  compressors of 7 bar, we still have a 7 bar air supply.

We only doubled the volume of compressed air that we can deliver.

If we buy a new machine that needs a 12 bar compressed air supply, we have a problem!

You will need to buy a new 12 bar air compressor.

Types of pressure

There are two types of pressure:

  • Absolute pressure
  • Relative / Gauge pressure

This confuses people sometimes, but trust me it’s really simple.

The only difference is the reference pressure. What is ‘zero’!

You don’t notice it, but here on earth we live under a thick layer of air, the atmosphere. The air pushes down on itself, because of gravity, creating… compressed air. We live IN compressed air here on the surface of the earth.

The pressure? 1 ‘Atmosphere’ 🙂

1 Atmosphere equals around 1.01325 bar (let’s just say 1 bar), or 14.696 PSI.

So the only difference between absolute pressure and relative, or ‘gauge’ pressure is the reference pressure. For relative pressure, ‘zero’ is the pressure on the earths surface. For absolute pressure, ‘zero’ is the pressure in outer space, a vacuum.

The difference between absolute and relative pressure is 1 bar.

Simple as that.

In compressed air land, we normally talk about relative / gauge pressure.

If you see a gauge in your compressed air system, that’s gauge pressure (that’s where the name came from probably).

The only time you will need to use absolute pressure, is when doing calculations.

Volume

Volume is easy to understand. We use it often in daily life.

A carton of milk, a bottle of water, etc.

Volume is ‘how much’.

For example, in compressed air land, a 1000 liter air receiver.

We can add up volumes. If we install two air receivers of 1000 liter, we have a 2000 liter air buffer.

Capacity

Capacity or air flow is volume per time unit.

Some examples are:

  • liters / sec
  • cubic feet per minute (CFM)
  • m3 per min

Capacity is  ‘how often’ or ‘how fast’.

Where is it used in compressed air land? For example to express the output capacity of an air compressor. How much compressed air the compressor can deliver per minute (or second, or hour).

On the other side of the compressed air system, we use it to express air consumption of a machine or air tool. It’s the amount

of air that a machine needs per second, minute, or hour.

Capacities, or air flows can be added up.

If we have two air compressors of 7 bar. The first one delivers 30 l/s and the second one delivers 50 l/s. Now we have a total air supply of 80 l/s.

The same is true for air consumers. If we have two machines that both use 400 m3/hr, we have a total air consumption of 800 m3/hr.