lift capacity calculation

[stargost]

I've looked through various thread but couldn't find the answer (which I'm sure must be somewhere). So here it is again:

How do you calculate the exact lift capacity you'd need (in you BC) based on diving conditions ?
Like your weight + "X" extra weight => "Y" amount of needed in your BCD/wing at max rec. depth (let's say 120ft)

For example:
- If you are diving in cold water, with 7mm wetsuit and stuff requiring 21lbs of weight, what would be the minimum lift capacity you'd need in your BC at 120ft to be neutral.

Thanks a lot

[Lee Taylor]

If your concern is not buying more BC than you need then I suggest using rental BC's of different lift until you dial in on the minimum lift that works for you and all your personal variations of personal size, weights, suits, tanks, etc.

[rstofer]

There's a 'sticky' spreadsheet right at the top of this forum.

Richard

[LeadTurn_SD]

Tobin at Deep Sea Supply https://www.deepseasupply.com/ is the pro at answering this question, but basically:

You need to determine how much buoyancy your exposure suits provides (this is the buoyancy that can potentially be lost), the buoyancy of your rig with full tank(s), and ideally your personal buoyancy (which is positive for most folks and can't be lost at depth).

One way to determine the buoyancy of your exposure suit is to get into neck deep water with your exposure suit and a mask on, have a friend hand you weights (hold them in a mesh bag), lift your feet, and see how much weight is needed to achieve neutral buoyancy at the surface (floating at eye level with a normal held breath). Or, you can stick the exposure suit in a mesh bag, add weights to the bag until it just begins to sink. Once you know the buoyancy of your suit, you know how much lift it provides, and how much lift you potentially can lose at depth... this lost lift needs to be compensated for by the BC. At the beginning of the dive, if properly weighted, you will also be starting at least several pounds negative (the weight of the air in your tank).

Your BC needs to provide enough lift to offset the lost buoyancy at depth, taking into account the buoyancy of your rig with a full tank and the ballast weight you are carrying. It also needs to provide enough lift to float your rig on the surface if you take it off (thus removing the lift your exposure suit provides).

In your example, if you assume your wetsuit lost "all" of its lift at 120ft, you are lean and have no "personal buoyancy".... using an AL80 that is about -2 lbs full, plus another -3 lbs for regs, -5 lbs if using a steel back plate, plus -21 ballast weights sounds like you'd need about 30-35 lbs of lift minimum at depth as a wild ballpark guess.

Others may be able to give a much better estimate, but you will need to know what tank (I just used an Al80), BC, and what the exposure suit buoyancy is....

Best wishes.

[stargost]

Sorry I missed the sticky thread , it was exactly what i was looking for ...

interesting to realize that some small size BC will actually not fit the requirements ...

Thanks all !