How long can you dive on one cylinder?

You can (and should) work out on a reasonably regular basis how much you as an individual use. And your breathing rate will change with different kit, dives and even how you are feeling so it's important to keep track of different rates.

Your breathing rate is called your SAC Rate - or Surface Air Consumption Rate. The best and most accurate way to calculate it is during a normal dive and it's simple to do.  

1. Dive for a pre-decided time at a pre-determined depth and measure what you breath.

For example you might decide to dive at 10 metres for 10 minutes.  At the start of the 10 minutes check your SPG and write down how many bar you have in the your cylinder. (Do write it down, you won't remember it accurately otherwise!) dive for exactly 10 minutes staying at 10 metres and then write the depth down.

Once back on the surface grab a calculator!  

2. Now multiply the number of bar you used by the size of you cylinder to get what you used into litres.

If you used 30 bar and you are diving on a 12 litre cylinder then you would have used 360 litres of gas.  

3. Now divide the air consumed (360 litres) by the total time in minutes of the dive.

We used 10 minutes in our example so 360 divided by 10 gives 36. This gives you the consumption per minute.

4. Lastly divide this figure by the pressure (or depths in atmospheres) to give you the litres per minute you would have used at the surface.

In our example we were diving at 10 metres which is an atmospheric pressure of 2. So 36/2 gives you a rate of 18 litres per minute.

Our SAC is 18.

NOW we can work out how long a cylinder will last.

A 12 litre cylinder filled to 230 bar has 2760 litres of air in it. If you breathed from this at the surface that would last you 153 minutes (2760/18).

Take it down to 10 metres and it will only last half that time as the pressure is double (2760/18/2 = 76 minutes) . At 20 metres you divide the 2760 by 18 and then by 3. At 30 metres by 18 and thenby 4 and so on. 

Working hard

Obviously this is all calculated on an easy gentle dive. To work out how much you'd use if stuck in a current for example, simply do exactly as above but fin hard for the 10 minutes!

There are loads of factors that can affect how much you breathe.

The most common are bad trim and being over weighted which leads to over-finning.  Trim is about being streamlined and horizontal underwater.  Weighting is about... well I'm sure you know what that is.

If you think your weighting, trim or finning technique could do with a bit of help   then have a quick look at the PADI Peak Performance Buoyancy Course .  This is EXACTLY what this course is designed for.                                            

Other things can also affect your air consumption

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!