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Beefcake
03-31-2009, 21:39
I was reading a DIR gear configuration article (I think it was linked directly from the GUE website; I can find it again if necessary), and it mentioned that steel tanks were preferable for technical / cave diving but that dual al80's were the preferred setup for open water ocean diving because of trim characteristics. Does anyone really do this? Right off, I would be concerned with the added weight necessary to offset dual aluminum tanks when empty. I'm asking purely out of curiosity. I have 4 good al80 tanks, but I had never thought of running them together (not that I am ready for duals anyway, just thinking ahead).

Rainer
03-31-2009, 21:42
I have a set of double AL80s (Luxfers). Not my preferred set of doubles from a weight perspective (a v-weight takes care of their buoyancy), but they are cheap, extremely easy to maintain, and they trim beautifully (they can also be broken down for later stage use, which is what I'll do with mine in the near future). They're fine for most T1 dives, and fine for deeper stuff once you add a stage or two.

ektess1
03-31-2009, 23:51
You use a steel backplate. That pretty much negates their postive bouyancy.
If you are diving wet, they are the only doubles that will not over weigh you.

mitsuguy
04-01-2009, 07:52
That was going to be what I said... here in Utila, the tech guys dive doubled Al80's typically... they also dive wet almost exclusively - typically a 3 or 5 mil, which is just fine for extended stays in 80 degree water... AL80's are cheap, very resistant to saltwater corrosion in comparison to steel, and, well, did I mention they are cheap... Double steel tanks would most likely overweight anyone diving wet unless you have a FJ 7 mil, then it would still be close... double steels usually run about -12 to -14 lbs underwater plus the backplate (-1 to -6 depending on al vs steel)... If I were to dive that configuration, I would be 2-3 lbs negative with no dumpable weight anywhere... with Aluminum tanks, you actually have to add a couple lbs, but they are only a slight bit heavier out of the water...

UCFKnightDiver
04-01-2009, 08:20
Its more like -8 to -12 when Full for the most part, There are a few steel tanks such as the Faber HP133 that are acctually positive when empty.

Gombessa
04-01-2009, 09:45
Its more like -8 to -12 when Full for the most part, There are a few steel tanks such as the Faber HP133 that are acctually positive when empty.

The buoyancy depends on the specific tank (HP, LP, capacity, how much it's filled, etc.) 160cf of air alone is already 12lb, 200cf is about 15-16lb. I calculate the buoyancy of my doubles LP80s as about -20lb (full and overfilled) even before adding manifold/bands.

Rainer
04-01-2009, 09:51
You're always going to be negative the gas. Not much you can do about that, except make sure you have sufficient lift to deal with it.

For those of us doing OW dives, also remember, as the gas requirements increase, we tend to have a greater portion of He in the mix (which is lighter).

Beefcake
04-02-2009, 15:00
Thanks for the perspective everyone. I guess since all I hear about is dual steel tanks, I hadn't considered the advantages. I was focused on the positive bouyancy of twin al80's as a bad thing, but that is an interesting point of not having enough ditchable weight with steel tanks. Anyway, as is evident from my other threads, I am a long way from being ready for any tech diving; this just helps in my planning for future equipment considerations.

Rainer
04-02-2009, 15:06
For these types of dives, ditchable weight really isn't a concern. No one is going to start ditching weight when they're dealing with mandatory decompression. Being over-weighted (which does NOT take gas weight into account), however, is a possibility with heavy steel cylinders and light exposure protection. AL80s certainly have their place in (DIR) diving.

Edit: P.S. They're cheap.

:)

MSilvia
04-02-2009, 15:08
I used them as a convenient transition from diving AL80 singles.

in_cavediver
04-02-2009, 15:50
I actually set a set of dbl AL80's up *after* years of diving with PST LP104 doubles. I use them in a wetsuit where the 104's would be crazy unbalanced. (I still don't carry weight with the 80's + SS plate)

CompuDude
04-02-2009, 19:00
I used them as a convenient transition from diving AL80 singles.

Ditto.

And my buddy took fundies in a set of Al80s. No problems with Al.80s. They're pretty much all you can get in some areas, as well.