Quote:
| Originally Posted by nigelH The world's most obese diver would need masses of lead not masses of lift.
94lbs of lift is for people who don't understand diving. 50lbs (24Kgs) is realistic for any dive kit. By the time you are carrying twin 18s and two 15L stages so much of your gas is helium you need less lift rather than more. |
Nigel is probably correct but does not explain why. Human bodies are close to neutrally buoyant. A large body is surrounded by a large buoyant suit (wet or dry) that needs more weight to maintain neutral buoyancy. However, and I'm guessing here, I wouldn't expect the difference to be much more than 6kg between the typical small person and the typical very large person.
What you need adjustable buoyancy for is the weight of gas you are carrying. To be able to use all of the gas in your cylinders you need to be neutrally buoyant with empty tanks. Thus, with full tanks, you need extra buoyancy at the start of the dive. Using rough figures for air, your 12l tank at 232bar contains 3.3kg of air. Thus, when you start your dive (perfectly weighted) you need 3.3kg of buoyancy to maintain trim. At another extreme, you may have a triplet of 20 litre tanks and a couple of 12 litre stages. This amount of gas weighs 23.4kg. Now, say you are the careful type and would like to ensure you have sufficient buoyancy in the case of a drysuit flood you might need another 6kg of buoyancy. Thus, total buoyancy requirement is in the region of 30kg (66 lbs).
Now. There could be the case that you are carrying so much kit, wrecking bars, tools, torch batteries etc that you are negatively buoyant at zero bar gas without any extra weight. You need to add this to your buoyancy requirement. Even so, I must admit that a 90lb lift wing seems overkill for virtually any "normal" dive.
One corollary of this is a refutation of the concept of just using a drysuit for buoyancy adjustments. Even a single 15l of air has a potential buoyancy change of over 4kg. I would hate to dive with an extra 4 litres of air in my drysuit.
One final point. All the figures above relate to air and the same volume of helium weighs a lot less.
ATB
Ian