Bouyancy and tanks question

[Craig66]

I hope that this question comes out right but here goes.

With an Aluminum tank one needs to add 5lbs to your weighting so that when the tank is empty (500) your weighting will be perfect as empty aluminum tanks are about 4lbs bouyant. As in they begin about 2lbs negative and end 4 pounds positive - a swing of about 6-7 lbs. Makes sense. Steel tanks on the other hand do not become positivily bouyant and so the extra weighting is not needed? A steel tank though will go through the same 7 or so pounds swing from -10 to -3 or so depending on the tank. My question is that that 7 pounds of negative bouyancy still needs to be accounted for based on your exposure portection and so why dont you need to add 5lbs or so to account for the swing

I hope that makes sense

Craig

 

[Jimmer]

Well like you said, an AL80 is about +4lbs empty, a steel HP100 is about -3lbs give or take. Since as you said, you set your weights for the empty state of the tank, you need 7 pounds less with the steel tank. The buoyancy swing is dealt with using your BC or drysuit really, but you need to be able to remain neutral on an empty tank for safety stops and the like.

Jim

 

[paulwlee]

Another way to see this is if you have a steel tank that swings from say -10 to -3lbs, then it is equivalent in the water as an AL80(-2~+4lbs) tank with 7lbs strapped to it.
Say with and AL80 tank you needed 20lbs total weight to be neutral with near empty tanks near the surface. Then if you switch to the above steel tank, you already have that 7lb inherent in the tank, so you only need to wear 13lbs.
All else is the same. (well except for subtleties of weight positioning and trim..)

 

[The Chad]

If you really want to nail your weighing just in a pool (or saltwater since your in Jersey) with your setup and a near empty tank, and see what it takes to get you down.

 

[Charlie99]

My question is that that 7 pounds of negative bouyancy still needs to be accounted for based on your exposure portection and so why dont you need to add 5lbs or so to account for the swing

Only the EMPTY BUOYANCY determines the amount of lead needed.


The key time of the dive that sets how much weight you need is when the tank is near empty. The amount of swing from full to empty doesn't affect the amount of lead needed. For example, if you had a 120cu ft tank that had +4 pounds buoyancy when empty, you would need the same amount of lead as with an AL80 that has +4 pounds buoyancy when empty.

The difference is that with the 120cu ft tank you would be about 9 pounds negative at the start of the dive, while with the 80 cu ft tank you would be about 6 pounds negative. Your BCD is used to compensate for both the weight of the air in the tank, and also for the compression of your wetsuit. The only time that the extra weight of the air becomes important in weighting calculations is when you are figuring out whether or not you need some sort of redundant buoyancy (such as a drysuit). You need some sort of redundant buoyancy if, with a full tank and a wetsuit compressed at depth, that you cannot swim back up to the surface.

 

[CompuDude]

If you really want to nail your weighing just in a pool (or saltwater since your in Jersey) with your setup and a near empty tank, and see what it takes to get you down.

Correct, although keep in mind there are buoyancy differences between fresh and salt water if you choose the pool.

Things are also considerably different in the open ocean with surge and water currents, compared to a placid pool. If you're new to diving, an extra couple of pounds may be in order to keep you from popping up when you start breathing faster due to stress or excitement. I say this after watching it happen with a new diver I dove with yesterday, who was perfectly weighted in a pool but could not keep down to save her life in a surgey location...