[SoB]

Thanks so far guys.

I already had the link to the calculater and for a steel 7ltr ali 7ltre we have the following;


Using Archimedes principle, buoyancy is equal to the weight of the displaced water minus the weight of the cylinder.
The cylinder is made of 7.5 kg of steel, (which has a density of 7.8 kg/litre).
The steel therefore has a volume of 7.5 / 7.8 = 0.96 litres, so the total volume is 7 + 0.96 = 7.96 litres.
The density of salt water is 1.03 kg/litre, giving 7.96 x 1.03 = 8.2 kg
Air has a density of 0.0012 kg/litre.
The air in a full cylinder weighs 0.0012 x 232 bar x 7 litres = 1.9 kg.
The cylinder when empty has a buoyancy of 8.2 kg - 7.5 kg = 0.7 kg
The cylinder when full has a buoyancy of 8.2 kg - 7.5 kg - 1.9 kg = -1.1 kg
7.0 litre 232 bar 8.8 kg (Greg Roach)
The cylinder is made of 8.8 kg of aluminium, (which has a density of 2.7 kg/litre).
The aluminium therefore has a volume of 8.8 / 2.7 = 3.26 litres, so the total volume is 7 + 3.26 = 10.26 litres.
The density of salt water is 1.03 kg/litre, giving 10.26 x 1.03 = 10.6 kg
Air has a density of 0.0012 kg/litre.
The air in a full cylinder weighs 0.0012 x 232 bar x 7 litres = 1.9 kg.
The cylinder when empty has a buoyancy of 10.6 kg - 8.8 kg = 1.8 kg
The cylinder when full has a buoyancy of 10.6 kg - 8.8 kg - 1.9 kg = 0 kg

So the Ali is neutral when full but positively bouyant at all other times which suggests that I need to carry extra weight to counter the +ve bouyancy at the end of a dive - where-as the steel is only just bouyant if completely emptied and just -ve when full suggesting that I could even get rid of some weight (well maybe not)?

So I'm still confused. Taking the points constructively though.

Why would you only use Ali for stages though?

Thank you,
Simon