Minimum Lift Needed for Cold Water Diving

[OmarM]

Hey everyone, as the title says I am wondering what the minimum amount of lift in a BCD needed for cold water diving is! Im new to this forum, and new to diving, but I do have a degree in Physics which may be misleading me to thinking I know more then I do!

The way I see it, the primary determining factor is how much weight your carrying. Since your weight only needs to enable you to do two things (achieve negative buoyancy at the surface with a full tank, and neutral at the safety stop with an empty tank), the greater of these two requirements is the one well follow.

So the following numbers are my guesses and might be way off:
Me: I'm lanky and lean, so I'd say I am about neutral.
Uncompressed 7mm wet suit +hood, gloves and boots: +20lbs at surface. This becomes about +15lbs with compression at safety stop and pretty much neutral at a depth of 30m. (I am familiar with the 1:3 guideline, but I don't really know how much the neoprene in a wetsuit weighs
HP Steel Tank: -10 full to Neutral when empty
Miscellaneous accessories and other essential gear: Hard to ballpark but either way, this is logistically the same as ballast since it does not compress or change buoyancy throughout the dive.

So at the surface we would have a net buoyancy of +10lbs with no ballast. Ergo we need a minimum of 10lbs ballast.
At the safety stop we would have a net buoyancy of +15 lbs with no ballast, so the overall minimum is 15lbs.

So we add 15lbs to the BCD between regulators, knives etc and lead. The total kit then "weighs" 25lbs. If the BCD can float the kit at the surface with no one in it, then it can float it with a neutral diver in a compressed wetsuit at depth. So 25lbs seems like the minimum needed for cold water diving.

TLDR; 25lbs is the minimum lift needed by my calculations, which are unfortunately based on guesstimates which can best be described as shoddy.

 

[CT Sean]

I came up with basically the same conclusion for my 7mm stuff and an AL80 and went with a 25lb wing. I will say though that, locally, I only dive shallow so the 25 is always more than enough with 7mm suit etc.... I wouldn't do a dive to 99+ feet and find out whether it truly is enough to compensate for 7mm wetsuit compression.

 

[kmarks]

Estimating how much "minimum" weight you need is not an easy process. There are a bunch of attempts at wing/lift calculators on scubaboard, but even the best are mere approximations because there are just too many variables.

The actual mass of the neoprene in your suit is not important to the calculation because that is accounted for in the buoyancy approximations. If your suit is +20 at the surface, you can estimate that bubble compression will follow an inverse linear model with pressure at depth, i.e., at 10m/2atm you'll lose about 1/2 buoyancy, at 20m/3atm you drop to 1/3, etc. By the time you reach 30m/100 feet, you have lost about 80% of your suit's initial surface buoyancy, due to compression. There are some disagreements here, especially concerning the resistance of the neoprene itself, but this is a common general approximation. So a suit that is +20 at the surface, will be about +14 at 15 feet and +4 at 100 feet, and you'll want to account for that, which means 16 lbs for your "minimum" lift. However, you may find that you don't need all of your compensation to be done by your wing/BCD.

You will want to plan your carried weights to get you neutral at 15 feet with 500 psi left in your tank. If your tank is neutral when empty, your body is neutral, and your suit is +14 at 15 feet, you'll need to be carrying about 14 lb of weight between your regs, back plate/BCD (some jacket BCDs are inherently positive), lights, knives, reels, etc.

With a full tank, this means you'll start your dive carrying essentially 24 pounds (10 for the tank, 14 for other carried weight) and need about 4 lbs of lift on the surface to keep from sinking. You'll empty your BC, go 4 lbs negative, and start your dive. As you descend, you'll add air back into your BC as you approach your final depth (100 feet in this case). You'll need 20 lbs of lift to account for the 16 pounds you just lost due to compression and the -4 you were at the beginning to be neutral at depth. As you breathe down your air, you'll need less and less air in your BC/wing because your tank is becoming more buoyant. You'll also probably be increasing your depth as you go in most cases, which means your wet suit compression is becoming less of a factor, but maybe you are doing a square profile, hanging out on a training platform or whatever.

20 lbs isn't necessarily the minimum you need, though, for two reasons: 1) you may be able to control your buoyancy just fine the old fashioned way, by using your lungs to account for about 5-10 lbs, and 2) 20 lbs won't float your rig at the surface if you have all of your weight attached to the BC/BPW. There is a lively discussion about Balanced Rigs in the Basic Scuba Discussions Forum. If you are carrying all of your weight attached to a weight integrated BCD, you'll need enough lift in it to account for all of that, otherwise if you take it off at the surface, you'll never see it again. Also, if you take it off at depth, you may shoot to the surface. If you carry your ditchable weight on a belt (I do), you can try to "balance" your distribution so that your BC/BPW can float itself, fully loaded without a problem, and you are just about neutral between your belt and wet suit. That may not be possible with a heavy steel plate and heavy steel doubles, in which case you need the extra lift.

In most cases, how much lift you need is probably less than you think.

 

[OmarM]

Thanks so much for the replies folks! Some context, the reason I asked this question is that I am considering acquiring a BC with a 25lb wing. It seems to me like it will be enough lift, and that after fine tuning my weights I won't have any problems. Have I missed anything?

Also you Kmarks, you make an interesting point about being too positive if for some reason I need to remove my weight integrated BC at depth. I always imagined that if such a dire scenario arose, I would happily be willing to abandon my kit in exchange for personal safety, but as you said I may shoot up and we all know thats no good. So I ask, what sort of situations would require a diver to abandon his BC at depth?

 

[lowviz]

Hey everyone, as the title says I am wondering what the minimum amount of lift in a BCD needed for cold water diving is! ...//... TLDR; 25lbs is the minimum lift needed by my calculations, which are unfortunately based on guesstimates which can best be described as shoddy.

Don't be too hard on yourself. I'd say that is a reasonable first order approximation to the absolute minimum.

The problem (in coldwater diving) comes with having only one source of buoyancy. Assume that fails. Let's say you splash off a boat on a simple inshore dive. 60'. Rookie mistake, your wing isn't connected or your tank is turned off. You hit the bottom in a panic. You need to have options to overcome this. Abandoning your rig is not one of the more pleasant options.

As far as necessary buoyancy, I'd want a bit more than your minimum but your calcs are reasonable.

3 breaths off your main, 3 breaths off your safe second all the while watching your SPG. Get off the platform if the needle moves. Inflate wing/BCD and feel it inflate. Hand on mask, other hand on inflator or your secondary reg. I put one hand on mask and palm of same hand on reg. Splash, get a visual on the boat, signal a big "OK" (chances are nobody is looking, but you did your part) and then descend either down the line or free.

 

[OmarM]

The problem (in coldwater diving) comes with having only one source of buoyancy. Assume that fails. Let's say you splash off a boat on a simple inshore dive. 60'. Rookie mistake, your wing isn't connected or your tank is turned off. You hit the bottom in a panic. You need to have options to overcome this.

I think you just answered my second question about what scenarios would require a diver to abandon their BC at depth. What sort of options are there besides dropping the whole kit? Of course I could drop weights, but Im curious if it is common practice to carry any sort of backup buoyancy compensator. Seems like it could also be useful in the event that something punctures the wing, maybe in a wreck diving scenario.

 

[lowviz]

Yes, you want redundancy. Wings and BC's don't fail nearly as often as the diver does. Usually stupid stuff.

For coldwater diving the usual redundancy is BCD and drysuit. For warm water diving the 'redundancy' is having some acceptable way to get to the surface with a nonfunctional BCD. I suppose the first order approach to that problem is diving a balanced rig with a warm-water config and having real redundancy when diving all the necessary stuff that coldwater dictates...

 

[kmarks]

I think you just answered my second question about what scenarios would require a diver to abandon their BC at depth. What sort of options are there besides dropping the whole kit? Of course I could drop weights, but Im curious if it is common practice to carry any sort of backup buoyancy compensator. Seems like it could also be useful in the event that something punctures the wing, maybe in a wreck diving scenario.

A DSMB/sausage can serve as a backup buoyancy compensator in the most dire of situations. It'll give you something to cling to on the surface while you wait for the boat, and something to possibly help get you off the bottom if you are really stuck.

There are several reasons you might want to doff/don gear at depth, the most important would be entanglement. If you are swimming through and get stuck in a kelp forest, a fishing line, or netting on a wreck, you might need to slip out of your BC/harness to cut it free or untie it. Another possibility might be adjusting your tank should it start to slip or get bumped. Most rec divers will never have to doff/don at depth, but it's good to be prepared.

If your only option is to make an exit by ditching your whole kit, you are seriously up the creek.

 

[Rred]

The minimum lift you need, in cold water, warm water, hot springs or otherwise, is zero.

But you may want enough lift to ensure that you and all your gear can float indefinitely on the surface. If you are diving in open water where there may be wind-whipped wave spray, you may want more lift, so you can get your mouth and nose higher above the surface, because ocean spray has drowned people. That means enough lift to get your entire head and neck out of the water, and remember, any part of your buoyancy device that is above the surface? Has zero buoyancy.

But wait, you're not done yet. I had a problem with a buddy going out of air, and needed to lift two of us out of the water and comfortably above the surface. Yes, you may want enough buoyancy to keep two people up there.

But if you want a slim trim rig with minimal drag, you're back down to the smallest rig that will neutralize you and let you bobble around.

Strictly a personal decision, no rules to make it.

 

[johndiver999]

I know for a fact that a farmer John 7 mm with 3 mm hooded vest a 5 mm hood etc Can loose more than 20 lbs below 90 feet. I think I would put the wetsuit in a mesh bag and see what lead it takes to sink it. I would add this weight to the weight of air in the tanks and that would be the minimum capacity I would want. This assumes total loss of buoyancy in suit - not such a Crazy assumption.