This is one of those 'pieces of string' questions, but one most new divers ask - and most older ones should but often don't know the answer to. So here we will look at how you work out how long your cylinder might last you - so calculators and bits of paper at the ready folks!
But first let's go over the physics behind some of it.
Firstly how much air you use will depend on how hard you are working and what depth you are at.
The more you are finning the more gas you will consume. Exactly the same as walking versus running. The harder your body has to work the harder you will breathe.
In diving bad trim and being overweighted also add to the breathing requirement and therefore using more air (or gas).
Next there is depth, or rather pressure. As we dive deeper the pressure on us increases. And no I'm not talking super-deep depths here. By the time we are at 10 metres the pressure on our bodies has doubled. At 10m we will use twice as much air doing absolutely nothing as we will at the surface.
So if, for example we breathe at a rate of 10 litres per minute (lpm) sitting down at the surface - like now as you read this - you will breathe at a rate of 20 litres per minute at 10 metres if you were doing the same thing.
For this calculation the formula is simple: 2 x the rate at the surface. After that for every further 10 m add 1 to the multiplying factor. So 20m is times 3, 30m x 4, 40m x 5 and so on.
Stress, exertion, the cold and your equipment all contribute to your air consumption.
Stress has a major influence on how much air you use. It NEVER ceases to surprise me how much more air I use if I;m on a course as a student than when I'm just diving. Wreck penetration, I love it but my air consumption goes up. Even diving a site I've not dived before can change my breathing rate. So always be conservative with your SAC rate.
The colder you are, the more air you will breathe as your body tries to keep your core warm. Make sure that you have enough thermal protection before and during a dive. Treat yourself to a new undersuit, an extra thermal vest or a proper pair of undercut socks! Not only will you be warm but your air consumption will be better too!
Good equipment, especially a decent set of fins can change your breathing rate enormously. They will save energy and therefore air.
If your equipment is in poor shape or leaking, it will increase air loss and your overall all consumption. Make sure you equipment is in good working order and appropriate for the dive that you are undertaking.
So, how long does a scuba cylinder last underwater?... now where is that piece of string!
HAPPY DIVING EVERYONE!