Measuring Flow

Measuring Flow

Air pressure guage

Understanding Air Flow Measurements

If you’re buying an air compressor you’ll likely be confronted by lots of different air flow measurements that all measure (pretty much) the same thing. Here’s how to compare like with like.

You want to compare air compressors, but one delivers 32 CFM while the other offers 22 CFM FAD. Another states the figure as SCFM. And when you check your air tool it doesn’t mention CFM at all, but says it requires an air flow in l/s. The variety of ways in which air flow can be described can be bewildering. Here’s how to make sense of it all.

What Is CFM?

CFM is cubic feet per minute.

It measures the volume of air a compressor can deliver.

The longer your compressor is required to deliver its pressure, the higher the CFM you’ll want. The flow of air isn’t a constant, though, so CFM is a ‘theoretical’ figure which doesn’t take into account influencing factors like temperature and humidity. It’s a decent guide – but not an entirely accurate one.

Compressed air flow
Compressed air flow

What Is SCFM?

SCFM or standard cubic feet per minute is a more accurate figure as it bases air flow on a standard measure of atmospheric pressure, humidity and temperature.

Whilst there may always be slight variations in actual performance between two compressors which measure flow in CFM, SCFM should compare like with like.

What is (S)CFM FAD?

CFM FAD is an attempt to bring clarity to air compressor performance measurement.

FAD is ‘free air delivery’. It’s used because CFM and SCFM are measures of how much air a compressor can displace. They are perfectly reasonable measures but there’s a difference between how much air a compressor will displace and how much is actually discharged as a result of compression. So FAD is effectively a measure of how much useable air the compressor generates. Typically, this will be about a third lower than the CFM figure.

What is l/s?

l/s is litres per second, an alternative metric version of the CFM figure and one that’s commonly used on air tools. To convert l/s to CFM, multiply the l/s figure by 2.11888000328932.

Flow & Pressure

Remember that air flow is not the only critical measure in choosing your air compressor. Pressure is a vital element too and both are closely related. As you increase pressure, flow decreases and vice versa.

Find out more about flow and pressure considerations.

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