Diving Doubles in Salt water
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With this additional piece of data, I would move to a very strong recommendation for steel tanks, as you suggest you are considering. I suspect that, without any added weight, AND a SS BP, you would still be slightly positive. I say this as a diver whose absolute favorite set of tanks is double AL80s. I dove a set of double steel (HP) 120s this weekend in salt water, with an AL 80 and a AL40 as deco bottles. It was a challenge. I cannot imagine how much of a challenge it would have been with my AL80s.
ivobj
Contributor
I dive double Worthington X8-119 (basically the same as HP 120s), but I dive dry, with no buoyancy redundancy I would recommend lighter doubles, ALs or lighter steel doubles.
Steel X7-100s will weigh about the same double as AL 80s will but be about 12 pounds more negative when empty as the X7-100s are each -2.5 lbs empty compared to +3 each for the AL 80s (with valves).
When full, double X7-100s will be -10 lbs each, minus about 6-7 more pounds for the valve and isolator portion fo the manifold, for a total of about 26-27 pounds of negative bouyancy.
Consequently, the other side of the equation is to be sure you have enough wing to support the tanks along with any deco and stage bottles you may have along. If you are using X7-100doubles to just do two recreational dives is not really a big issue as a 40 pound wing will cover the tanks and the compression of a 3mm suit.
With stage and deco bottles, bottle and wing selection matters a lot more as the wing will have to support them when they are full and at their most negative in terms of buoyancy and you also need to ensure that you are negative enough to still hold a deco stop when every thing is empty and positive.
For example, Luxfer AL 40s are commonly used deco bottles as they are about 3 pounds negative when full and about neutral when empty (including the valve and reg). An AL 80 stage is about 2 pounds positive when empty and about 4 pounds negative when full. So you'd need about 7 more pounds of lift in the wing early in the dive and a 40 pound wing will probably be marginal at best.
If you use a steel deco bottle like an LP 45, they will be about 6 pounds negative then full and about 2 pounds negative when empty (but won't float tail end up like an empty AL 40 or AL 80) and your lift requirements will be greater.
And remember that with a wet suit, swimming up the tanks early in the dive with a wing failure will be very difficult, especially with stage and deco bottles attached - and this is where the 12 pounds less negative buoyancy of the AL 80s (and a team member) pays off.
So in effect, when considering doubles you need to consider the types of diving you will do as well as your entire configuration including steel versus aluminum or kydex back plate, wing, back gas tanks, any stage or deco bottles, and the exposure suit(s) you will use.
When full, double X7-100s will be -10 lbs each, minus about 6-7 more pounds for the valve and isolator portion fo the manifold, for a total of about 26-27 pounds of negative bouyancy.
Consequently, the other side of the equation is to be sure you have enough wing to support the tanks along with any deco and stage bottles you may have along. If you are using X7-100doubles to just do two recreational dives is not really a big issue as a 40 pound wing will cover the tanks and the compression of a 3mm suit.
With stage and deco bottles, bottle and wing selection matters a lot more as the wing will have to support them when they are full and at their most negative in terms of buoyancy and you also need to ensure that you are negative enough to still hold a deco stop when every thing is empty and positive.
For example, Luxfer AL 40s are commonly used deco bottles as they are about 3 pounds negative when full and about neutral when empty (including the valve and reg). An AL 80 stage is about 2 pounds positive when empty and about 4 pounds negative when full. So you'd need about 7 more pounds of lift in the wing early in the dive and a 40 pound wing will probably be marginal at best.
If you use a steel deco bottle like an LP 45, they will be about 6 pounds negative then full and about 2 pounds negative when empty (but won't float tail end up like an empty AL 40 or AL 80) and your lift requirements will be greater.
And remember that with a wet suit, swimming up the tanks early in the dive with a wing failure will be very difficult, especially with stage and deco bottles attached - and this is where the 12 pounds less negative buoyancy of the AL 80s (and a team member) pays off.
So in effect, when considering doubles you need to consider the types of diving you will do as well as your entire configuration including steel versus aluminum or kydex back plate, wing, back gas tanks, any stage or deco bottles, and the exposure suit(s) you will use.
halocline
Contributor
Are you sure you want to dive in a drysuit in FL ocean temps in the summer? Just curious, because there are other ways of dealing with the buoyancy. Given the money you're spending on the drysuit, you could get 2 sets of doubles, one AL80 set (or LP72s, neutral empty, great and cheap small doubles) and one big steel set for the manly dives.
I will dive my single tanks in my wetsuit during summer time an come this winter be nice and cozy for the cold in my drysuit, the drysuit can be worn anytime actually and just need to change the undergarments, they last a long time and keep me super comfy on deco dives, I can rent or borrow doubles for now until i can afford anything again. Besides diving doubles means more time underwater = colder for me, so a dry is a good investment for now.
I dive double Steel all the time in a wetsuit, as I rarely dive dry. LP 51's, LP 85's, and or HP 100's.
If you're worried about a complete wing failure, try (and practice occasionally if needed) using your lift bag as your backup for buoyancy compensation (if and only if it has an exhaust valve) so you can easily vent on the way up.
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