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DIR Information Network >>Buoyancy Compensators

How do I determine which lift capacity wing I need?

To best determine the appropriate wing, it is necessary to cover some basics of weighting and buoyancy. 

Proper weighting requires that a diver be able to maintain a safety or deco stop. This balance becomes critical during extended gas sharing scenarios, when a buddy team might find their cylinder(s) nearly depleted (and extremely buoyant). A simple test can be carried out in a confined shallow water dive site, where a diver can exhaust all the gas from their wing and with only 500psi (35 bar) in their tank be able to remain neutral at 10ft (3m).  If you can’t maintain neutral buoyancy, adjust weight accordingly.  Avoid the common temptation to over-weight. In an over-weighted situation, the diver’s variable buoyancy requirement will increase by whatever amount they are over-weighted.

Now that you are certain your weighting is correct, it is a simple matter to calculate your variable buoyancy needs.

Your required lift has little to do with the amount of weight you are wearing, but instead is tied to your variable buoyancy. A diver’s variable buoyancy is determined by the weight of the gas in their tank plus the loss of buoyancy from their wetsuit (or neoprene drysuit) due to compression at depth.  Divers wearing a trilaminate (shell type) or crushed neoprene drysuit do not experience such compression, making this calculation is even easier.  The average ¼ inch (approx. 5 mm) farmer john wetsuit looses about 9lbs (4 kg) of buoyancy at 100’ (30 m) and 80cf of air from a typical Al 80 weighs 6lbs (2.7 kg), so wearing an Al 80 tank and a ¼ inch (5 mm) farmer john wetsuit the variation in buoyancy from the start of the dive (at depth with a full tank) to the end of the dive (at 10ft./3m with 500psi/35 bar) will be 15lbs (6.8 kg).  Having weighted to be neutral at the end of the dive, gas equivalent to15lbs (6.8 kg) of lift will need to be added to the BC at the start of the dive (at depth) to be neutral.

Given that most divers would prefer to be more than just neutral at the surface, it is recommended that all divers double their variable buoyancy requirement to determine their minimum lift BC wing. In this case, the required bouyancy would be 30lbs (13.6 kg) of lift.

There are a few other factors that can increase your lift requirements.  One is a desire for your BC to float your gear without you in it.  It is not necessarily a requirement, but it can be of benefit-- particularly for divers that dive out of an inflatable or don their gear in the water.  You will determine this lift requirement by adding together the lift needed to float the specific tank you use and how much weight is on the system.

As with any change in one’s diving equipment, it is recommended that all divers make a few dives with their buddy, in controlled conditions, in order to determine how that change will affect the in-water performance, before returning to their normal diving regimen.

DIR Information Network >> Why would I want to balance "non-releasable" and "releasable" weighting? [Next]