Sorry, I wasn't terribly clear about this. The reason for the question is because I'm thinking of getting a BCD which I can run twinsets on i.e. at the start of the dive I will possibly either be negatively bouyant and/or have no ditchable weight.
You need to be able to support yourself on the surface using only one source of bouyancy and in my case (assuming drysuit failure) it would be the BCD.
It will be dependant on tank weight mainly (the human body and bcd are generally neutrally bouyant) and I have no experience with twinsets. So if I wanted to twin my 15's (perish the thought) what sort of capacity will I need?
My current bcd (Mares Vector Chrome) does not state capacity but does state maximum permissible tank sizes - 1*20l or 2*10l - whereas the bcd's I'm currently looking at have Kg's quoted....
I know the DIR gurus fave Halcyon recommend 55lb wing for twinsets in drysuits and UK waters - do I need that sort of figure?
Either way, I'm after a rough guess from someone who has tried it I guess....
Using Archimedes principle, buoyancy is equal to the weight of the displaced water minus the weight of the cylinder.
The cylinder is made of 16.8 kg of steel, (which has a density of 7.8 kg/litre).
The steel therefore has a volume of 16.8 / 7.8 = 2.15 litres, so the total volume is 15 + 2.15 = 17.15 litres.
The density of salt water is 1.03 kg/litre, giving 17.15 x 1.03 = 17.7 kg
Air has a density of 0.0012 kg/litre.
The air in a full cylinder weighs 0.0012 x 232 bar x 15 litres = 4.2 kg.
The cylinder when empty has a buoyancy of 17.7 kg - 16.8 kg = 0.9 kg
The cylinder when full has a buoyancy of 17.7 kg - 16.8 kg - 4.2 kg = -3.2 kg
NB - these figures exclude valves, isolators, regulators, etc..
Well, there's that rule of thumb which says you should have at least as much lift in your BCD as you have weights to get you down.
I've always found that in full kit other than a suit (ie t-shirt in the swimming pool) I need no weight. Add a drysuit and nothing else, I suddenly need 11kg to get down. So the drysuit adds 11kg of buoyancy.
So in the event of a total drysuit failure, I would loose 11kg of buoyancy, so I need an absolute minimum of 11kg from my BCD.
Since my twin 7's hold 4kg of air, I would realistically need 15kg to be on the safe side.
I use the Buddy Tekwing, which supplies more than 20kg, so I figure I'm safe enough
So I would say, add the weight of air in the biggest twinset you're thinking of buying to the amount of weight you need to carry when drysuited, and make sure your BCD has that much.
Incidentally, if you want another way to work out cylinder buoyancies and gas weights, you can always use the one I wrote, which also takes what mix of gas you're using into account..
you need to remember that as you go deeper the volume of air in your bc decreases, the weight increases, i.e. gravity, so you need more lift. You will need a minimum of 50lb lift for twin 15's. anything smaller will be dangerous.
Remember your suit will compress at depth if neoprene, your thinsulate will lose bouyancy at depth all adding to your weight load.
You will get away with less lift until you hit 40msw ish, then you will need to hit the up button cos the cyls weight will drag you from there.
You have assumed the body is neutral, great if it is, personally I sink like a brick, even suited, or in the scud,
so take into acccount your own flotation when deciding what you need.
I don't follow...
I thought a big advantage of wearing a membrane suit was that it stayed constant buoyancy regardless of depth.
Why does depth make your thinsulate loose buoyancy? You add air to the suit so the thinsulate stays the same, is how I understand it..
No mention of membrane suit in my thread, personally I prefer a crushed neoprene suit. But yes certain thinsulates, compress badly at depth (one madein yorkshire especially), if you are adding air all the time to your suit it makes it a pain to try & combat the compression on these undersuits, you might as well try to useyour suit for bouancy, most undersuits are great, but some of these hightech ones need more weight to get you under then compress at depth. Using your suit for bouancy is fine on small twin sets, but not good with a set of 15's, you end up like the michelin man, so if you keep adding air to stop these undersuits compressing you end up the same, far better to get a good old fashioned undersuit, DUI or something of that ilk. and just keep the squeeze of the bits that matter with the suit, and use your wing for the job it was made for, it also gives you a more stable position in the water, rather than the air rush to your feet etc. when you invert.
Most undersuits appear normal till you hit about 40-45msw, then bamm your bouancy goes to rat***t, change the undersuit and its back to normal next dive...
Amazing.. I'd have thought that by keeping a constant volume of air in the suit, the thinsulate would stay uncompressed.
But then, I don't go any deeper than 35m, so it's not something I really worry about
Yourkshire undersuit..? Only one I know is Otter. Should I be glad I told them not to give me an undersuit with my latest drysuit?
Purely academic interest, really, since I use a small twinset and do bouyancy on the wing
Twin 15s... sure you wouldn't have been better off with the RB80 instead?
not Otter undersuits, but the one similar to a ferret..
Twin 15's have gone, just got 20's for some exploration I have planned.
RB80 would be nice, but the price is a bit on the ouch side of expensive.....