Posted by TDI_2 on August 26, 2002 at 10:20:45:
In Reply to: BC lbs lift posted by Warren on August 25, 2002 at 19:00:15:
Think about this:
If you have only 20 lbs of lift capacity with your B/C, then you can only load 20 lbs of negative buoyancy onto yourself, and comfortably be able to get back to the surface. Your gear together with a steel tank will run at least 3 lbs neg in the ocean [almost empty tank, reg assembly, instruments, knife or shears, small light]. If you carry a heavy light too, then it will run closer to 4 to 5 lbs total.
That only leaves you with additional capacity for 17 to 15 lbs of lead weight.
Most divers need 20 to 30 lbs of lead weight on the West Coast if they are diving with a wetsuit.
If you do the math, 20 lbs + 3 to 5 lbs = 23+ lbs of lift required to offset, at least. For a night dive, 25+ lbs.
If you dive deep in your wetsuit, such as to 100 fsw, you will be at 4 ATAs ambient pressure, and the foam neoprene bubbles in your suit will be crushed to 1/4th their normal size as when near the surface, and then your suit is going to give you zero positive buoyancy, and all the weight of the lead and your gear will be upon your poor 20 lb lift capacity B/C. If that total exceeds 20 lbs, you will have to kick up to get back, if you can. If you cant, youre not coming back!
You can cut this buoyancy requirement down slightly by wearing a drysuit, since a drysuit allows you to vary the amount of insulation, and switch to argon for additional warmth with the drysuit.
Plus a drysuit can be partially used for inflation to gain positive buoyancy, but it is harder to control the expanding air on ascent. Its tricky and dangerous for most people to use their drysuits as B/Cs for buoyancy.
Your drysuit should be a backup device to your B/C and not a supplement for basic buoyancy, or else you are unsafe, and failure of either your B/C or your drysuit to hold air will result in overweighting at depth, and then again, you will have to kick up, if you can, and if you cant, then your not coming back!
With my drysuit I normally dive with 26 lbs of lead weight, since I like to wear a lot of insulation inside. So if you do the math, 26 + 3 to 5 = 29 to 32 lbs, and you therefore are a lot better off with a 35 lb B/C lift capacity out here in California.
For the record, I recommend a 45 lb lift capacity B/C, at least. Then you can add an air bubble to your drysuit, and be really warm, while neutrally buoyant at depth with the air bubble.
An alumimun tank is not the solution in a cold water environment either. You still need to wear about 6 lbs extra of lead weight to neutralize the positive buoyancy of an aluminum tank, or else after you have breathed most of the air out of that tank, you woould not be able to hold a 15 fsw safety stop.
A typical West Coast diver is already loaded up with lead weight to offset the exposure suit. Adding another 6 lbs to offset the aluminum tank only makes the diver heavier out of the water during the beach entry, or climbing back into the dive boat. Aluminum tanks are not a good idea in cold water environments.
People use aluminum tanks because they are cheap and easy to maintain. But that doesnt make them smart.
Clearly a 20 lb lift capacity B/C will not work on the West Coast, except possibly for a very petite diver in a drysuit with argon for additional warmth. And even then, maybe not.
So like Kendall said, if youre wearing a wetsuit and a steel tank, which is the most proper configuration for the West Coast, or if your wearing a lot of gear like in tech diving, or if you prefer to ride higher out of the water on the surface, then you will need more than 20 lbs of lift capacity with your B/C.
Reason #4 [in addition to Kendall's list of 1 thru 3] is if youre working with often overweighted dive students or novice divers, then additional capacity in your B/C is nice to have, precluding you from having to drop weightbelts during an assist.
Either way, wetsuit or drysuit, on the West Coast, in the cold water here, a 20 lb B/C lift capacity is inadequate, and probably unsafe even if marginally adequate. Dont make your dive a one-way trip!