What would you do with my tanks?

[crofrog]

In what universe? When diving wet in warm water, AL80's buoyancy characteristics can be easily managed and may be a benefit for some. But in all other situations, AL80s have poor buoyancy characteristics. Apart from deco/stage bottles, aluminum tanks decidedly do not have good buoyancy characteristics for the majority of divers and uses. YMMV of course, but for me, I own 6 HP100s and 2 AL80s and I NEVER dive the 80s. Sometimes they will come up for VIP and I have not dived them since the last VIP. I'd sell them if I could, but here in South FL they are tough to sell. On the other hand, HP100s you can sell in a heartbeat because they are so popular.

Not really sure why owning a bunch of tanks is relevant but here you go, I own 3 sets of double 130s, 3 sets of double 80s, and 6 80's as single tanks or stages a few 40s, 2x 13s, and 2x 6cufts. In the past I've had a bunch of pst 120s, pst 100s, faber 100s, OMS 98s (lp95s), faber hp-80s, AL63s.

According to what I see (via Mfg specs and the tank specs posted on SB), my AL80s weigh a bit over 31 lbs empty. My HP100s weigh 32 lbs empty. That's a negligible difference. However, at the end of the dive, the buoyancy of the AL80s end up being about 6 lbs lighter than my HP100s. So, if properly weighted, I'd need to carry 6 additional lbs.

This I don't understand. How can you require LESS weight with AL80s than HP100s or other steels? AL80s end up positively buoyant by at least a few lbs. Most steel tanks remain negative, or only slightly positive.

Gotcha. OK. The way it was worded, it sounded like you needed less weight with an AL80 than with a steel. It sounds like what you meant was that you require less weight with an AL80 than the diver next to you would with the same tank. That makes sense.

You need more total weight with 100s than you do with 80s. 1cuft of air weighs 0.0766lb so 77.5cuft of gas weighs 5.9lb and 100cuft of gas weighs 7.7lb. So you'll need 2lb more weight when diving with 100s to account for the weight of the gas loss.

How this translates into the water is where things get a bit more complicated.

Tank	Weight full	Weight at 500psi
Luxfer 80	-1.7lb	+3.4lb
Faber F7-HP100 (3442psi SP)	-8.4lb	-0.6lb
PST 100 (3500psi SP)	-8.8lb	-1.3

Let's assume the diver needs ~10lb to make their wet suit and themselves numerally buoyant without anything else on at the surface...

So if you have -2 for a regulator, -2lb for a light and other stuff, and -2lb for a backplate you're looking at it.

	HP100	AL80
weight needed	10	10
regulator	-2	-2
light	-2	-2
backplate	-2	-2
tank empty	-1.3	+1.7
additional lead required to be neutral at the end of the dive	-2.7	-5.7
Negative weight with tanks full	-7.5	-5.1
Buoyancy after ditching weight belt	-4.8	.6

So at the surface you're about -4.8lb with the 100 after ditching your weight belt or +.6lb with the 80, however if you're diving wet you're wetsuit will loose a significant (almost all) of it's buoyancy at depth. So if your at 100ft and you're weight suit as lost 80% of it's lift. You're looking at being -13lb at depth with the HP100s vs about -7.4lb with an AL80. It's going to be way easier to swim up 7.4lb than 13lb considering it's about 54% of the weight.

Additionally the HP100 gives you about 3lb to move around the AL80 gives you about 6lb you can move around.

 

[Divin'Papaw]

Not really sure why owning a bunch of tanks is relevant but here you go, I own 3 sets of double 130s, 3 sets of double 80s, and 6 80's as single tanks or stages a few 40s, 2x 13s, and 2x 6cufts. In the past I've had a bunch of pst 120s, pst 100s, faber 100s, OMS 98s (lp95s), faber hp-80s, AL63s.

You missed my point entirely. It wasn't a "who has the most tanks" pissing contest. I was simply sharing anecdotal data that applies to many (but not all) recreational divers here in South Florida. I have ready access to AL80s and HP100s for single tank diving. I never dive the AL80s. That was my sole point. But thanks for sharing your impressive inventory.

With that said, many divers still dive, and love, the AL80. They are not "wrong". It is a personal decision.

 

[Pavao]

Gotcha. OK. The way it was worded, it sounded like you needed less weight with an AL80 than with a steel. It sounds like what you meant was that you require less weight with an AL80 than the diver next to you would with the same tank. That makes sense.

I actually never said “than with a steel”, those were your words, I said…

So, if I require less weight with AL80s, then beach diving is indeed easier

Which was a response to this attempt to say I’d be carrying more weight to offset

.
AL 80 … require more weight on the diver to offset making hikes to the beach and beach diving more difficult,

So, if I carry no weight with an AL80, any weight added, be it from a heavier steel 100, would be more weight carried than needed.

Although, if with no ballast weight, the steel puts you out of trim, you can really only move the tank further up (to a point). If that's not enough you would have to potentially add weight further up to balance it out. I don't use much weight with my steel tanks, and haven't noticed them doing what you describe.

Right, your thinking is in the right direction, adjusting tank on bc will have an effect on trim, but you’re correct, can only do that to a point. You are also correct in distributing weight to manipulate the balance, but what do you do when the tank alone makes you overweighted, attempt to fix one problem by making a second even bigger? If only I could just swap the tank for a more appropriate one

But bodies are made differently, and tanks are also made differently, so my observations may not apply.

And that has been my point all along, but for reasons beyond my understanding, some troglodytes take offense that AL80s works better FOR SOME PEOPLE.

 

[crofrog]

You missed my point entirely. It wasn't a "who has the most tanks" pissing contest. I was simply sharing anecdotal data that applies to many (but not all) recreational divers here in South Florida. I have ready access to AL80s and HP100s for single tank diving. I never dive the AL80s. That was my sole point. But thanks for sharing your impressive inventory.

With that said, many divers still dive, and love, the AL80. They are not "wrong". It is a personal decision.

and then you missed the rest of the points

 

[Boarderguy]

 

[Divin'Papaw]

and then you missed the rest of the points

I understand them fully, I'm just over the topic.