To calculate the amount of air in litres that are in the Cylinder.

WATER CAPACITY X GAUGE PRESSURE = LITRES AVAILABLE

The Water Capacity is the amount of water in litres that the cylinder will hold. This is more commonly referred to as the size of the bottle, i.e. 10 litre or 12 litre.

The Gauge Pressure is the amount of air in bars that is available.

Modern cylinders are now available in various sizes,

10 litre 232 bar

12 litre 232 bar

15 litre 232 bar

7 litre 300 bar

10 litre 300 bar

12 litre 300 bar

As can be seen the Water Capacity can be between 7 litres and 15 litres, and the working pressure is either 232 bar or 300 bar.

However as the cylinder is rarely filled to its full working pressure it will be safer to calculate available air using the actual gauge pressure.

10 (WC) x 200 (GP) = 2000 litres available

12 (WC) x 232 (GP) = 2400 litres available

WC - water capacity / GP - gauge pressure

To calculate the amount of air required for a dive.

We should assume that we use approximately 25 litres per minute (Lpm) of air on the surface. As we are surrounded by an atmospheric pressure of 1 bar, we would multiply the Lpm by the absolute pressure (pressure at depth) multiplied by the time given, i.e.

LPM X PRESSURE ABSOLUTE X DURATION = AIR REQUIRED

(Your reserve supply of 25 % should now be added.)

To calculate for breathing on the surface for 30 minutes

If we were to go down to 20 metres for the same time

As can be seen the only difference between the two examples is the PRESSURE ABSOLUTE.

It should also be noted that this is the procedure for calculating the air required for the BOTTOM TIME.

To complete the calculation you will need to calculate the air required for the ascent and safety / decompression stops.

We should consider the ascent rate as another dive
As the ascent rate is 10 metres per minute (mpm). The required time from a depth of 20 metres would be 2 minutes PLUS a 1 minute safety stop. This would mean that your actual ascent rate is 3 minutes.

Instead of using the actual dive depth to complete your calculations.

You would instead half the depth, and calculate the same as the other examples.

Example

We can now calculate how much air we theoretically need to carry out a particular dive, and we are also able to calculate how much air we have in the cylinder.

If we were to use the above example as our dive plan and we were planning to a 12 litre 232 bar, we would find that the air available is 2784 litres.

Our reserve air is 25% of the total amount of available air, therefore we only have 2088 litres of air available. We find that we are short by 312 litres, which means we could either use a larger cylinder or re-calculate our dive time.

There are 2 methods of re-calculating the new dive times.