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I'm now to this forum, so forgive me if this has been done to death.

I dive on twin 7's (it works for me), but I'm now starting to do accerlerated deco and would like your suggestions as to what size stages I should carry.  Now I'm a small female type diver with pretty good gas consumption who thinks that an hour in the water is plenty so were not talking of excessively deep (50m max) or long dives/deco.  I started off thinking 2 x 3L (one for 80%, one for 40%) would be a good compromise, but now think that perhaps they should be 5L's. Not only that, but the steel vs Ali question pops up again.

Any opinions would be gratefully received.

Angie
 

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<font color='#000080'>Well, I think I'll leave the steel vs ali debate for someone else to butcher, or probably provide a link to.

As for size, I've been getting on nicely with 2 6l tanks I got on the cheap, not doing serious hangs but I like to balance things with one of air and one of 50%, or if I really need it, or sometimes when I'm diving solo will try both with 50%. Like indies for my deco gas!

If your sac is low enough, use 3s, but you might want to have a bit more in reserve, and get 5s. That is if you've got a decent source for 5s, I don't see them often. You'll also find that a 5l ally is going to be about the same size as a 7l steel.
 

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A short fat well off crap cave diver. Likes wrecks
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CE aproved no test or fill hassel Ali tanks are generaly heavier and bigger than steel tanks. Bob had a 5ltr Ali on a recent dive and it was about the same size as my 7ltr steel. In the water the ali's float as they become empty but that just meens you have to carry lead to compensate.

Twin 7s back gas is going to limit your dive profile. So worst case scinario a 50m max dive bassed on a lowish SAC of 15 you would get a max bottom time of 30mins and thats pushing it on a 240 bar fill. Using a fairley conservitave 20/80GF deco profile you will need 35mins of deco using 32% and 80%. (It would be better to get on the deco gas deeper to conserve and max the back gas and offer bail out.)

If you did do 30 @ 50 you would need to get 25mins out of your 3ltr of 80 and 10mins out of your 3 ltr of 32%

On A 10 sac for deco you would need 180bar of 80 and 180bar of 32% in 3ltr tanks to give you a 50bar reserve.

So bassed on the above worst case scinario dive you could get away with two 3ltr deco tanks. It even sounds like a nice little rig.


However the smaller the tanks the smaller the margen of error so I would always have a drop tank on the boat and be practiced in the yellow bag procedure of asking for more deco gas. Also consider that you might breath like a sparrow but you have to consider bail out for your buddy

ATB

Mark Chase
 
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