An idle thought becomes an idle question

[Twister_Ken]

Watched a scuba tank being filled the other day. Occurred to me that they must, logically, be heavier full than empty. Anybody know by how much?

 

[Toutvabien]

varies with size but for a 12 litre steel tank empty is about 3kg lighter empty than full.

 

[pvb]

Yes, 2.5 to 3kg for a typical scuba tank.

 

[prv]

Enough that you can feel the difference, certainly.

Pete

 

[sarabande]

so what happens when you start to reach the end of a tankful of air ? Do you gain an extra 3 kg of buoyancy ?

 

[prv]

so what happens when you start to reach the end of a tankful of air ? Do you gain an extra 3 kg of buoyancy ?

Yep.

Your buoyancy changes anyway as you descend and ascend (suit and various other compressible items are squeezed by the pressure but still weigh the same) so you compensate by adding and dumping air in your jacket to maintain neutral. Extra buoyancy from an empty tank is well within the tolerance of that so you don't really notice (although empty aluminium tanks can start to float slightly which looks odd).

Pete

 

[25931]

Yep.

Your buoyancy changes anyway as you descend and ascend (suit and various other compressible items are squeezed by the pressure but still weigh the same) so you compensate by adding and dumping air in your jacket to maintain neutral. Extra buoyancy from an empty tank is well within the tolerance of that so you don't really notice (although empty aluminium tanks can start to float slightly which looks odd).

Pete

In the days before the ablj we used to go down slightly overweight to make sure that we didn't ascend too fast.

 

[sailorman]

Totally dry air at sea level has a density of:
1.2 kg/m3
or 1.2g/L

http://en.wikipedia.org/wiki/Diving_cylinder

 

[prv]

Totally dry air at sea level has a density of:
or 1.2g/L

So 12l X 230bar X 1.2g/L = 3312g :encouragement:

You won't see all of that difference in practice because you don't fully empty a tank.

Pete