Posted by Kendall Raine on September 06, 2002 at 13:27:48:
A week ago Chuck asked that I follow-up and explain why there is an advantage to using aluminum tanks versus steel tanks with a wetsuit. He said he hadn't yet seen a cogent explanation. I'm not surprised. That's because discussions of the AL/AA issue in recreational set-ups often miss the fact that it's context specific. Ditchable v. nonditchable weight is that context and it's not black and white.
For comparability, I'll contrast a Luxfer 80 and a PST 80. I'll assume they both contain 80 cu. ft. of gas and that the empty buoyancy number for the PST 80 on the web site is correct. I don't know if it is (I do know some are wrong) and the precise number doesn't really matter. I'll also use a simple Boyle calculation for wetsuit compression. Chris and Wayne have made the case that this is too simplistic and that different wetsuits have different inherent buoyancy depending on the rubber used, it's life and the depth in question. If they are right, there are too many variables effecting wetsuit buoyancy to get hung up on a precise calculation here. The Boyle calculation applies to both steel and aluminum scenarios and the error terms cancel.
Buoyancy values used are:
Wetsuit = 16
AL Tank = -3 to 3
AA Tank = -8 to -2
AA Plate = -6
The logic is that a diver should weight themselves so as to be able to swim up from depth with a full tank and a total wing failure on the one hand and be able to hover with an empty tank at 20 feet. The full and empty tanks constitute the extremes of the static buoyancy (depth independent) spectrum. Wetsuit buoyancy is depth dependent and so dynamic.
At the surface, the diver using the AL has buoyancy 16-3-6=7 lbs positive. The diver using the AA has 16-8-6=2 lbs positive. Hence, to get neutral, the AL diver needs an extra 7 lbs of ballast and the AA needs 2. This may seem an advantage to the AA diver. Lets keep going, however. In the analysis the diver needs to be neutral at 20 feet with an empty tank. Using Boyle, the wetsuit's buoyancy falls from 16 to 11 at 20 feet. Hence, the AL diver has buoyancy of 11+3-6=8 lbs. The AL diver needs 8 lbs of extra ballast (let's assume it's a weight belt). The AA diver has buoyancy of 11-2-6=3 lbs of positive buoyancy. Again, the AA diver needs less additional ballast (again, in the form of a weight belt). Apparent advantage AA diver. But, at 100 feet, pressure is 5 times surface. Using Boyle, the 16 lbs of positive buoyancy is reduced to 3.2 lbs. Call it 3. The AL diver has buoyancy of 3-3-6-8=14 lbs negative. The AL diver needs to offset this with a wing. The AA diver has 3-8-6-3=14 lbs negative which needs to be offset by the wing. OK, what's the big deal? Both AL and AA divers now suffer total wing failure. Both lose the requisite 14 lbs of offsetting buoyancy. The AL diver drops the 8 lbs weight belt and is now 6 lbs negative. The AA diver drops the 3 lbs weight belt and is now 11 lbs negative. The AA diver has an additional 5 lbs of negative buoyancy in the form of non-ditchable weight in his tank which he needs to swim up. Insert any value you want for wetsuit buoyancy or wetsuit compressability and the relative differential still gives advantage to the AL diver. At 6 lbs negative, the AL diver becomes neutral around 1 ata. This obviously ignores gas consumption.
Does this mean that diving an PST 80 v. a Luxfer 80 will kill you if you have a wing failure? No, I'm not saying anything of the sort. I also recognise that the full tank at depth is an extreme case and that over time, the tank becomes less negative. I have used the same gear configuration for the AL and AA divers to illustrate the importance of the ditchable v. non-ditchable element. Remember I started this by saying the debate is context (ditchable v nonditchable) specific. The AA diver could avoid the AA plate and put another 6 lbs on his belt. If he did this, he would be negative 5 after dropping the belt. Hence, the advantage to the AL diver goes away if the AA diver understands this calculation and adjusts the ditchable/non-ditchable percentage accordingly. But then one advantage of the AA tank, taking weight off the belt, also goes away. Many people, by their gear configuration, don't seem to understand or care about this. Some arguments in favor of using wet steel start with the idea that you can take weight off the belt. Really, calling this issue a steel v, aluminum issue is inaccurate. It's a ditchable v. non ditchable issue.
Where the wet steel debate really starts to matter, and where it was born, is in OE and extended range diving when the gas loads are much greater. The need to stay neutral during deco with empty tanks requires divers to be heavily overweighted at the start of the dive. Diving double steel tanks, like 104's, wet, leaves the diver with a potentially fatal nonditchable negative buoyancy if there is a wing failure early in the dive. The wet suit compressability at depth just compounds this.