This is a fairly simple calculation that is important to work out how long your air supply on your back will last you at given depth. It is vital to work out this data if you are planning "going deep".

This calculation will be based on a 12ltr @200bar on a 25 mtr dive of 40 minutes.

1. Calculate the total gas in your tank,12ltr x 200 bar = 2400lt

2. Deduct remaining gas post dive, 2400 - 600 = 1800lt

3. Divide remaining gas by dive time, 1800 / 40 = 45

Now we introduce a standard fixed requirement to the calculation and these are diving the depth dived by 10 and adding 1.

4. A dive depth of 25 mtrs/10 + 1 = 3.5

5. 45 / 3.5 =12.8

Your surface air consumption (SAC) for this dive would be 12.8 ltrs per minute. To work out a more accurate figure it is best to calculate it over at least 10 dives over varying depths, condtions, viz and work rates. This will give an average figure that will encompass all your diving types and will help you calculate gas usage.

SAC's vary from 10-12 for the experienced diver on a shallow warm water dive barely moving, right up to a trainee working hard, excited and new in the water wher a SAC may get well past 20. Not to worry though, the more you dive the better it gets and the longer you can stretch your dive.

How To Calculate The Buoyancy Of My Cylinder

We will calculate the buoyancy of a 15ltr steel cylinder with a normal air mix as used in the sea at 232 bar as this is the most common usage. Should you have requirement to work out something different let me know.

We will use Archimedes Principal (Eureka!) which states that buoyancy is equal to the weight of the water that the cylinder displaces when submerged excluding the weight of the cylinder itself.

We will also require certain fixed figures:

a. The density of steel 7.8kg/litre
b. The density of Salt water 1.03kg/litre
c. The density of air 0.0012kg/litre
d. The weight of a 15ltr 17kg

So with these we can work out the floatiness of our cylinder and weight ourselves accordingly.

First we need to find the volume of the cylinder which is: 17 / 7.8 = 2.18.
Add this to the volume of the cylinder: 15 + 2.18 =17.18

Now we need to find out the weight of the water: 17.18 x 1.03 =17.70

Now we need the weight of the air: 0.0012 x 232 x 15 = 4.20

Now we know that the cylinder when empty: 17.7 - 17 = 0.70

Soooooooo…..we can calculate that when full to the brim a 15 ltr steel cylinder charged to 232 bar with standard air its buoyancy will be:

17.7kg - 17kg - 4.2kg = -3.4kg

From this we can deduce that when filled and carried it will weigh over 20kg's, this is why we never leave them standing/rolling around. However when in the water it is only an extra 3.4kg, the trick that only comes with experience is judging a compromise on your weighting so you don't become overly buoyant towards the end of your dive; happy diving!