How much lift?

[vjongene]

I was wondering if there is a good rule of thumb for choosing the size of a bladder or wing on a BC. Intuitively, the parameters would be as follows, assuming that the diver is weighted correctly, i.e. neutrally buoyant (or almost) with 30 bar in the tank at 3 m below the surface:

- compensate for the weight of the gas in the tank(s) at the start of the dive
- compensate for the decrease in the volume of the wet suit at depth, conservatively by compensating for the full volume of the wet suit
- if diving a dry suit, very little lift should be needed from the BC, because the volume of the suit will be more or less constant during the dive

If you add up these parameters, the amount of BC lift required during a "normal" dive should be very modest, even in cold water with a thick wet suit. A quick calculation would say no more than about 10 l if diving a single tank, with maybe an extra 2 l or so for each extra tank.

Talking to an instructor friend, he nevertheless argued that one never has too much lift, because of unforeseen circumstances. He cited two examples: having to bring to the surface a buddy with a BC problem (this happened to him: buddy's BC bladder had holes in it...), or wanting to bring a negatively buoyant object to the surface (also happened: found a weight belt loaded with 8 kg of lead). One additional consideration may be surface swims, where one wants to avoid any chance of sinking (this is very common where I dive - most of our shore dives start with at least 200 m on the surface). BTW, my friend argued against dumping the weight belt from the buddy with the BC problems, to avoid an uncontrolled ascent.

So what do SB members think? Better to calculate the lift based purely on the need to compensate for changes in buoyancy during the dive, or better to add a sizable margin of safety?

 

[lee08]

Never use your B.C. as a lift bag, if you let go of the object your lifting guess what? If your buddy needs to get bouyant on the surface, drop his weights. And as for the surface swim, try to take any b.c. and fill it full of air and see how comfortable it is to swim in. I have a b/p & w and I fill it with only enough air to keep me floating on the surface for long swims. For most rec diving applications I would say a 30 to 40lbs wing would be more than enough.

 

[Diver0001]

I was wondering if there is a good rule of thumb for choosing the size of a bladder or wing on a BC. Intuitively, the parameters would be as follows, assuming that the diver is weighted correctly, i.e. neutrally buoyant (or almost) with 30 bar in the tank at 3 m below the surface:

- compensate for the weight of the gas in the tank(s) at the start of the dive
- compensate for the decrease in the volume of the wet suit at depth, conservatively by compensating for the full volume of the wet suit
- if diving a dry suit, very little lift should be needed from the BC, because the volume of the suit will be more or less constant during the dive

If you add up these parameters, the amount of BC lift required during a "normal" dive should be very modest, even in cold water with a thick wet suit. A quick calculation would say no more than about 10 l if diving a single tank, with maybe an extra 2 l or so for each extra tank.

Talking to an instructor friend, he nevertheless argued that one never has too much lift, because of unforeseen circumstances. He cited two examples: having to bring to the surface a buddy with a BC problem (this happened to him: buddy's BC bladder had holes in it...), or wanting to bring a negatively buoyant object to the surface (also happened: found a weight belt loaded with 8 kg of lead). One additional consideration may be surface swims, where one wants to avoid any chance of sinking (this is very common where I dive - most of our shore dives start with at least 200 m on the surface). BTW, my friend argued against dumping the weight belt from the buddy with the BC problems, to avoid an uncontrolled ascent.

So what to SB members think? Better to calculate the lift based purely on the need to compensate for changes in buoyancy during the dive, or better to add a sizable margin of safety?

You're making it too complicated for a rule of thumb.

I'd say forget about the suit. Your BCD should have enough volume to float all of your gear with a bit extra (20% ?) incase you have to do something like hold your buddy's weightbelt on the surface.

Assuming a typical OW kit with drysuit, 10kg of weight and a steel tank with -3kg buoyancy at the beginning of the dive plus regs, lights, reels, slates, flippers and you name it. All in all I'd say you need 20kg minumum.

R..

 

[JimC]

Floating the rig at the surface is nice.
As is getting your head out of the water at the surface with a holed drysuit.

For me, thats 25-30# (12-14kg) lift for singles in a drysuit with heavy undies. I couldn't see anyone needing more than 35# (16kg) or so. Anything more is just extra drag IMHO.

If I need to lift something, I use a lift bag.

 

[Don Burke]

A Type I PFD has 32 pounds of buoyancy, so I would not go much below that for BC lift in regards to being on the surface.

Twin 130s would give you a weight change of about 15 pounds.

I use a 45 pound wing with my drysuit and doubles and if I had it to do over again, I would get a 40 pound wing for the doubles.

Using a BC as a lift bag is a pretty good way to get DCI.

A stuck inflator might change the instructor's feelings about too much lift.

 

[vjongene]

A Type I PFD has 32 pounds of buoyancy, so I would not go much below that for BC lift in regards to being on the surface.

Sorry, I don't know what a PFD is. Could you elaborate?

Thanks!

 

[Don Burke]

Sorry, I don't know what a PFD is. Could you elaborate?

Thanks!

Sorry, I was being a dumb 'murrican.

Personal Flotation Device

There are five general types, I through V, Type I is the open ocean SOLAS (Safety Of Life At Sea conference) type for use on vessels carrying passengers for hire.

My thought is that a BC that is going to be used as a lifejacket should be pretty close to lifejacket specifications for buoyancy.

 

[Scubaguy62]

My BC has a 40 lb wing and I hardly inflate it past 1/2 to float comfortably at the surface; I dive AL80s with a 3mm full suit and 14 lbs of lead.

 

[Lemonade]

- compensate for the weight of the gas in the tank(s) at the start of the dive
- compensate for the decrease in the volume of the wet suit at depth, conservatively by compensating for the full volume of the wet suit
- if diving a dry suit, very little lift should be needed from the BC, because the volume of the suit will be more or less constant during the dive

You are right on the money, but add ability to float your rig on the surface.

Talking to an instructor friend, he nevertheless argued that one never has too much lift, because of unforeseen circumstances.

Nah...
You should never use your BC as a lift bag.
A diver with failed BC should be able to swim up on his own unless he is grossly over weighted, which should not be in the first place.
Large bladders create drag and trap air, which tends to mess up your trim.
And the more unnecessary gear you have with you underwater, the less relaxed and enjoyable your dive is going to be.


Generally for a single tank 30 to 40 lbs of lift is sufficient even with 7mm neoprene suit.

 

[vjongene]

Thanks a lot for the feedback, guys! This helps a lot.

Now for the practical aspects: I have dived for the last three years with a ScubaPro Master Jacket, which gives me 61 lb (!) of lift and is one of the few BCDs that is actually certified as a PFD (now that I have learned a new acronym I may as well use it). I have to admit that I have never come even close to fully inflating it. I am switching to a BP/W setup. I ordered a rig with a 26 lb wing, and was wondering if this may not be enough for lake diving - it should be plenty for tropical seas in a 3 mm suit and an AL80. I dive singles only (a 15 l HP steel), with a two-piece 7 mm wet suit in summer and a dry suit in winter. The only piece of heavy gear I carry is a large torch. I know that I am not overweighted.

So, time to order a 40 lb wing for the lake?