Aluminium/steel tanks: pros and cons?

[Bretagne]

If I understand correctly:
- due to thickest walls for Al, they have approximately the same weight but Al is more buoyant ->need to add more weight -> Al is finally heavier.
- easiest to have a good trim with steel
Am I right? what are the pros and cons for each?
Up to now, I have only dived with steel tanks.

 

[Boogie711]

AL - easier to transport on land due to lighter 'on land' weight. Also cheaper to purchase. That's about it. Because they go positive when empty, if you dive double tanks in a wetsuit they're a good option since you can swim them up in the event of a wing failure.

But even the dry land weight issue is a fallacy - you can get an E7-100 which holds more air than an AL80, is lighter on land, and is still negatively bouyant in the water.

Steel tanks can be overfilled with regularity, will last longer, don't need neck thread inspection during the visual (depending on the AL alloy used) and are negative in the water. However, they're quite often more expensive to purchase.

Steel tanks should also be galvanized as opposed to painted, especially if you dive salt water, due to increased corrosion issues... but that's just my opinion.

 

[MyDiveLog]

The only thing I'd add to Boogie711's comments are specifics. An AL80 tank is 3 lbs negative at the beginning of a dive and 3lbs positive at the end. Many people mistakenly believe that a Steel tank doesn't have this swing in bouyancy, but they are wrong; 80 cubic feet of air weighs about 6 lbs no matter what type of tank it is contained in.

My HP Steel tank is smaller than an AL80 because the steel, being stronger, doesn't have to be as thick. Unlike the AL80, the HP80 is neutral when it's empty (not all tanks are the same, so you'll need to check the bouyany ratings for any tank you look at). Being slightly smaller in volume it has slightly less bouyancy than an AL 80 (remember Archimedes principle?) but, for the most part, you are carrying the weight on the tank instead of your weight belt, as you would with an AL80. I have to add 5 lbs when I switch to an AL80.

Probably the real driver is cost: My experience is that AL80 tanks cost just under a third what a steel tank costs; an AL80 typically runs $95 on deal, where a steel tank will cost you $250 - $260.

 

[flw]

If you're in Europe then tha vast majority of cylinders are steel anyway, mainly Faber lightweights, which are fine, if the 15l tend to be a little bouyant. Not sure about France, but in the UK cylinders come in at between £100-170 dependant on size, Al will probably be more, certainly not less than steel.

 

[Warren_L]

MyDiveLog is correct - any tank will have a buoyancy swing dependent on the amount of gas you use, regardless of the type of tank you have. With the larger capacity tanks you then have the potential for larger buoyancy swings, and should adjust your weighting appropriate for your dive plan.