AL tanks and diving wet

[azstinger11]

Not at all unheard of. People vary. An instructor at our shop uses 38 with the 7 mil as did Bob DBF in this thread before he dumped the jacket. Another instructor I know at a different shop says he uses only 22 pounds with the same suit. The amount of weight you need to descend while wearing a 7 mil suit is the same whether it is in the form of lead, metal or anything else that is negatively buoyant.

Glad I don’t need anywhere near that for my 7mil or drysuit. That sounds backbreaking. It might only be 10-15lbs when you factor in tank weights and air but I'll always take lugging less pounds around.

 

[ColoDale]

Actually, the AL100 is about 1.5 lbs less positive. At 500 psi, Luxfer buoyancies are +3.4 lb (AL80) and +1.9 lb (AL100).

I don't have specs on buoyancy at 500 psi, only empty and at service pressure. I haven't calculated it based on size for 500 psi so I will take your word for it. Btw I have 3 different tables for tank buoyancies and they are all slightly different and depends, for Luxfers, if the are S series or not.

Maybe it's my perception as I take AL100s lower than 500 psi (not empty) here and don't use AL80s as much. I try to keep more air in the AL80s at the end since I know there is less. Perception cushion?
For me, I will not be neutral at 10-15 feet at the end of the dive with an AL100 the way I dive. 10-15 ft is not the plan btw. My buoyancy at depth will be good.
I don't want to go off task as this is an AL and wetsuit question and the difference either way is not alot. Differences or not, thicket wetsuit, I want a bigger tank.

 

[crofrog]

If you're diving wet, you should probably be diving with AL tanks, generally speaking, but any of these generalizations are risky.
The buoyancy between a Luxfer, Catalina 80s, the S80s, and C80s are all different in their starting and ending buoyancy by a few lbs. Just as the buoyancy between all the different steel tanks is different. For instance, an HP133 ends 1.45lbs positive and starts -9lbs an HP80 on the other hand, starts -8lbs and ends -2lb.

At the end of the day, if you're diving wet, you need enough ditchable weight to be able to swim up the tanks from the deepest point of your dive when you're at your heaviest point of the dive with full tanks and a 7mm wetsuit can have significant buoyancy loss. A suit with 22lb of positive buoyancy at the surface can provide six at 100ft. Diving AL tanks being less negative at the start of the dive allows you to use more ditchable weight and thus allows you to ditch weight in the event of a BC failure and swim the rig up.

 

[johndiver999]

If you're diving wet, you should probably be diving with AL tanks, generally speaking, but any of these generalizations are risky.
The buoyancy between a Luxfer, Catalina 80s, the S80s, and C80s are all different in their starting and ending buoyancy by a few lbs. Just as the buoyancy between all the different steel tanks is different. For instance, an HP133 ends 1.45lbs positive and starts -9lbs an HP80 on the other hand, starts -8lbs and ends -2lb.

At the end of the day, if you're diving wet, you need enough ditchable weight to be able to swim up the tanks from the deepest point of your dive when you're at your heaviest point of the dive with full tanks and a 7mm wetsuit can have significant buoyancy loss. A suit with 22lb of positive buoyancy at the surface can provide six at 100ft. Diving AL tanks being less negative at the start of the dive allows you to use more ditchable weight and thus allows you to ditch weight in the event of a BC failure and swim the rig up.

Not a bad idea, but it depends on how fat you are, what kind of steel tank you are using, how good of a swimmer you are and other factors. Aluminum tanks have been used with wetsuits (even thick ones) for decades. They are not necessarily a problem nor optimal.

For me, air capacity considerations would over-weigh the buoyancy considerations. Big steel tanks, on skinny people in freshwater with thin suits can be a problem and if they add in a metal plate on a BP/W it can get to be even more interesting.

 

[Boarderguy]

I dive wet with both steel and AL and adjust my lead based on which tank I grab. I'm still trying to figure out how a diver could not swim up their full kit when wet... total bc failure = ditch some weight and start kicking up. The majority of us could tread water in just swim trunks (with effort) with 10-15 lbs, why couldn't that same majority kick up 20-25 lb with fins on? 35-40 would suck but is doable especially as your wetsuit regains buoyancy. Drop one pocket and start working your way up to avoid an uncontrolled ascent. Drop the other once on the surface of you still are not floaty. Factoring in a tank's buoyancy is nice but unnecessary. If rather have the extra air and better trim from my steels than the floaty AL 80s.

 

[The Chairman]

I prefer a steel tank because I find getting into trim substantially easier. With an AL80, I have to put at least 2 pounds on the neck and try to get another 8 into my high back mounted trim pockets. I don't need any weight with most big steel tanks.

 

[jadairiii]

Steel tanks, aluminum tanks, blah, blah, blah....this will go on forever with everyone giving you their personal preference. With proper instruction on trim and buoyancy it should not matter one iota.

It all depends on what is available to me at the time. Wet suit, dry suit, no suit, whatever. Can you swim your rig if you have a catastrophic BC failure? (or at least ditch enough weight to do it).

You will find that most people that prefer steel tanks require the additional weight and, would hoover through an aluminum 80, so they need the additional gas for even a "standard" recreational dive.

 

[crofrog]

I dive wet with both steel and AL and adjust my lead based on which tank I grab. I'm still trying to figure out how a diver could not swim up their full kit when wet... total bc failure = ditch some weight and start kicking up. The majority of us could tread water in just swim trunks (with effort) with 10-15 lbs, why couldn't that same majority kick up 20-25 lb with fins on? 35-40 would suck but is doable especially as your wetsuit regains buoyancy. Drop one pocket and start working your way up to avoid an uncontrolled ascent. Drop the other once on the surface of you still are not floaty. Factoring in a tank's buoyancy is nice but unnecessary. If rather have the extra air and better trim from my steels than the floaty AL 80s

Have you ever swam up 25lb or 40lb?



Let's look at a "typical diver" they're 5'10" and 200lb diving in salt water and if they exhale fully they'll float with their head just awash, with 2lb of lead they'll be completely neutral in salt water with no exposure suit on.

If you now put that diver in a new high quality 7mm full suit, with a 7mm hood, 5mm gloves and booties they're likely around 27lb positively buoyant at the surface.

Then let's add a SS backplate, weighted STA they've added 12.5lb of SS to their system.

Item	Item Buoyancy	Buoyancy Sub-total
Diver	+2	2
Suit	+25	27
SS plate	-5	22
Weighted STA	-7.5	14.5
Regulators	-3	11.5

So our diver is left with about 11.5lb of positive buoyancy on the surface with no tanks.

For a AL tank

Item	Item Buoyancy	Buoyancy Sub-total
Diver, Exposure Suit and Rig	11.5	11.5
Luxfer AL80 @ 3000psi Luxfer AL80 @ 500psi	-1.5 3.4lb	10.5 (start of dive) 13.9 (end of dive)

So this diver in the AL80 needs to carry 13.9lb of additional weight to be neutral at the safety stop with 500psi in his tanks.

For a HP100

Item	Item Buoyancy	Buoyancy Sub-total
Diver, Exposure Suit and Rig	11.5	11.5
Faber HP100 @ 3500psi Faber HP100 @ 500psi	-8.4 -1.7	3.1 (start of dive) 9.8 (end of dive)

The diver in the HP100 needs to carry 9.8lb of additional weight to be neutral at the safety stop with 500psi in his tanks.


Now let's look at what the numbers look like at 100fsw where the wetsuit is likely only retaining about 25% of it's starting buoyancy.

Item	Item Buoyancy	Buoyancy Sub-total
Diver, Exposure Suit and Rig @ 100ft	-6.75lb	-6.75lb
Weight belt steel tank Weight belt AL tank	-9.8 -13.9	-16.55 (steel) -20.65 (AL)
Faber HP100 @ 3500psi Luxfer AL 80 @ 3000psi	-8.4 -1.5	-25 (steel) -22 (al)
Ditch weight belt	+9.8 (steel) +13.9 (AL)	-15.2 (steel) -8.1 (al)

So at 100ft with full tanks the diver in the faber hp100 ditching his weights would require them to swim up about 15.2lb and the diver on the AL tanks would need to swim up about -8.1lb. If we assume a full lung gives you about 5lb of positive buoyancy the diver in steels still needs to swim up ~10lb and the diver int he AL tank only needs to swim up about 3lb (30% of the weight of the steel tank)

 

[inquis]

So this diver in the AL80 needs to carry 13.9lb of additional weight to be neutral at the safety stop with 500psi in his tanks.

That would make them neutral at the surface. The wetsuit would have lost some buoyancy at the ~1.5 atm safety stop, so they would only need 6.1 lbs additional weight to be neutral there (using your assumptions about compressibility). For such a thick suit, it would be a good idea to take a few more lbs to control the tendency to "cork" the final ascent, so perhaps 9 or 10 lbs lead.

I don't mean to detract from your excellent discourse on weighting. Looking at things as you've described demystifies the concept and makes extrapolation to other gear very straightforward.

 

[inquis]

If you now put that diver in a new high quality 7mm full suit, with a 7mm hood, 5mm gloves and booties they're likely around 27lb positively buoyant at the surface.

The only addition I would make for others is to address, "how did you know the 25 lbs wetsuit buoyancy?" That obviously varies by the suit, but can be measured easily with a luggage scale, mesh bag/pillow case and enough lead/rocks/gold bars/whatever to sink it, and a hot tub/trash can/pool/whatever in which to sink it. Measure the bundle with the wetsuit+weight and then with just the weight. The difference is the surface buoyancy of the suit. (If you don't have a luggage scale, you can add weight until it just barely sinks.)

For the record, an XXL Bare Reactive 7mm (single-layer) suit is +16 lbs. An XL Bare Reactive 3mm is +7.6 lb.