Gas Management:

To work out how much we have in a 12 litre twin set:

2 x 12 litres = 24 litres

now multiply the total size in litres (24), by the fill pressure (in our case 300 bar):

300 x 24 =  7200 litres.  This is the amount of gas we have available to us.

 Bottom Time                                                                                                                                                                      

Now to work out how much we breath at depth.  We know our highest surface air consumption (S.A.C) is 25 litres per minute, and the dive is to 50 meters depth.

Divide the depth of 50 meters by 10 and then add 1:                  (50 ÷ 10 = 5) +1 = 6.  This is our absolute pressure in bar, or 'Pbar'.

Multiply our S.A.C  of 25 litres by our 'pbar' of 6:                      25 x 6 = 150.  This is how many litres we breath per min at a depth of 50 meters.

Now, multiply this total by our bottom time 21 minutes:                        150 x 21 = 3150 litres.  This is how much gas/air we have used up from the surface, to the point of (but not including) the ascent.

Total gas used so far: 3150 litres