Compressed Air: Units

• Units of compressed air: A guide
• Pressure units
• Atmospheric pressure
• Compressed air pressure 
• Air compressor capacity

Estimated time to read: 2 minutes

Working with compressed air? You’re bound to see all sorts of different terms and measurements appear. What do they mean? What are they for? What do you need to know about them? It’s time to put your thinking cap on and look into it with us.

The good news: the International System of Units has a standard unit for measuring pressure. The pascal – which uses the symbol Pa – is used for everything from internal pressure and stress to tensile strength. This includes compressed air. So why do you see kPa everywhere? Simple. A single pascal is a very small amount of pressure. As such, compressed air is typically expressed in kilopascals – kPa – or megapascals – MPa.

1 kPa = 1,000 Pa

1 MPa = 1,000,000 Pa or 1,000 kPa

You may have noticed that we don’t just use the International System of Units when measuring air pressure. You’ve possibly seen measurements in bar. The idea is that we only use it to forecast the weather nowadays, but the reality says otherwise. For the sake of clarity:

1 bar = 100 kPa or 100,000 Pa

And, just to cover everything, we will admit that you might occasionally stumble across the odd imperial measurement:

1 psi = 6.8 kPa

‘psi’ stands for ‘pounds per square inch’.

Wondering how much pressure is a lot of pressure? Let’s start with atmospheric pressure. This is the general air pressure on the surface of the Earth:
100 kPa

Now you see why 1 Pa is such a small amount of pressure and why we typically measure in kPa or MPa.

There are two main specifications to look for with an air compressor. The first is the pressure, or ‘how strong’. The capacity is the amount of air, or ‘how much’.

Compressed air pressure is typically specified as overpressure. This means it is shown as the amount of pressure above normal atmospheric pressure. This is usually implicit but is sometimes clarified with an (e): kPa(e). For example:

The pressure of a mountain bike tyre –

35 – 45 kPa(e)

3.5 – 4.5 bar(e)

The difference between a small and a large compressor? It’s not usually the pressure, but the capacity. A large compressor is able to move more air than a smaller one. Filling a large tank? You’ll need an air compressor with a high capacity, or a lot of spare time.

The capacity of an air compressor is the amount of compressed air it can supply per unit of time. This is usually specified in one of three ways:

1.    l/min (litres/minute)

2.    l/sec (litres/second)

3.    m³/min (cubic metres/minute).

We will also give you the imperial measurements, just in case you come across them or have the question at a pub quiz:

1.    CFM (cubic feet/minute)

And just to be clear, 1 CFM = 28.3 l/min.

Take a look at how much compressed air your project will require to decide which air compressor capacity is right for you.

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