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Buoyancy: Dry Suit or Wing?

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When diving in a Dry Suit should you control your buoyancy using your Wing (or Stab Jacket) or using the dry suit?


Comments from the thread...

Derek says:

Put enough air into your suit to stop squeeze and then use the Stab Jacket, the air is then in the correct place, no migrating air to give you the problems of an inverted ascent, no need for ankle weights, dumps at top and bottom,( I use a 'buddy') 


Mike says:

My personal preference is really depth dependent - which is also really kit dependent as well (ie amount carried) If I'm doing anything above 30m, I will use the DS for buoyancy, as I know  the auto-dump can 'excrete' the amount of air required with ease during my  ascent. Down below 30m (this cut-off is arbitrarily chosen by me based on  experience of using my suit) - then I know that buoyancy control by the  suit requires a lot more air, and to get rid of the amount involved  requires a much slower ascent than I would like to make, based on the  limited ability of my auto-dump. (Even allowing for the stops  ). Either that, or it's a much faster  ascent than I would like to make, and opening a cuff or my neck seal.... So with a deeper dive and more cylinders etc, I take off squeeze to  tolerable levels (varying that in terms of how cold it is as well), then  use the wing thereafter. It has a nice big DV, and I've never had a  problem with excess buoyancy there. On ascent, the air used in the suit should be taken care of by the  auto-dump - that's what it's there for, and I usually don't have to worry  about it. I will also tend to empty the wing, on my first stop, but  there's always some left there that needs emptying again at some stage.


Nigel says:

Get real guys. It is time to stop playing at diving and get the brains in gear. There is so much discussion on should you use you drysuit as your primary buoyancy device or the BCD/Wing but why? It's obvious.

The usual whinge is that *I was taught to use the dry suit*. But that was an exercise, a drill, you were learning how to get the air in (push the button, not too much) and out (raise your arm). Now grow up.

What is a dry suit? It is a big bag you put your temperature sensitive bits in to keep them warm and dry. OK it's person shaped but that's it's job. Warm and dry.

What's a Buoyancy Control Device? It's something for controlling buoyancy. They're actually a pretty neat idea. Try one some time. Let's examine a dive and compare what we use as buoyancy and how it works.

First get the weight right. That means you need to be neutral on empty so the last part of your ascent is not uncontrolled and abrupt. This means you start the dive with enough weight on you to match all the gas you carry.

Yes. I do mean all. I do count the pony, your reserves and the stages if you have them. You need to be neutral on totally empty. What good is it to heroically rescue your buddy from certain death and then bend him to blazes because, when he breathed the reserves of gas you claimed you carried for him and now you couldn't do the stops you both needed? Lacks finesse that does.

This means that when you step into the water you are overweight by the weight of all the gas you carry. Hence you carry buoyancy to match that weight. Example numbers are:

        200bar 10L air 2.4Kgs 
        230bar 12L air 3.2Kgs 
        230bar 12L 40% 3.4Kgs (oxygen is heavier than nitrogen) 
        230bar 20L air 5.4Kgs 
        300bar 20L 36% 6.8Kgs 
        300bar 24L 42% 8.3Kgs 
        

So what about a buoyancy check?

PADI teach a rather simple idea of take a deep breath and deflate the BCD and you should float eyes at water level. When you breath out you descend and start the dive. This weights you for about about 2Kgs negative so it works for a typical PADI rig. I think other agencies use something similar. Some people don't understand the phrase *deep breath* and take just a little one. Instantly underweight.

Now if you're diving a single 12L off you go. You descend with your BCD deflated by just breathing out and start adding air to something to control the rate of descent. Since this whole discussion is about dry suits you add air to that to keep it from pinching you in the folds as it deflates.

You don't go far. About a meter. If you are into serious diving that's the point where you do the bubble check. Either a graceful pirouette under the watchful eyes of your buddy or a personal stop and look up to see that you are not seeping gas from somewhere. You do do this don't you? Better to find a problem here, not at 40 meters.

Now let's consider a twin set. A little one, say 230bar by 20L and on air so it starts with 5.4Kgs of gas. The eyes level check will leave you three and a half Kgs buoyant at the end of the dive on empty so you need to do a real buoyancy check. Ideally you are at the end of a dive and you dump the suit and the wing and get somebody to pass you weights till you can hover just below the surface. Now work out how much weight of gas you are still carrying (only roughly) and add that much extra to your belt. Save away in your mind that you need 2.7% extra total weight in sea water (that's total: you+all the kit including existing weights). Fully kitted up I weigh in about 125Kgs so I need to add/subtract 3.4Kgs between the sea and the lake.

Now when you start the dive with this twin you are about 5.5Kgs negative with the suit comfortable and the wing flat so you must start carrying some buoyancy or the descent will be abrupt. Let's look at a drysuit. When you put it on it is comfortable and when you first get into the water it's OK. It is trapping the air in your insulating clothing so you stay warm but it is the trapped air that does most of the work. The insulating suit you bought is just a very sophisticated air trap. As you descend that air compresses and the suit closes in. You add air to restore the suit size and it stays comfortable. This restores the volume so you restore the buoyancy. Just taking the squeeze off the suit does not change things for buoyancy as you are just restoring the status quo.

For a wet suit to work you try to stop the water that gets in moving about. Hence one that fits you is snug and usually a pig to get on. The same is true of the air in a dry suit. You want it to get warm and stay warm and not go circulating between the nice warm skin and the cold outer membrane. The suit does not want to be loose or blown up like the Michelin man.

Right. Now down to the real question. Do we add more air to the dry suit as we come to the hover just above the bottom or to the wing?

We still have virtually the whole complement of back gas that we started with. Somewhere from 2.4Kgs in the single and up to 8.4Kgs with a big twin. We need to trim to neutral so we don't kick up the silt. Neutral is the point where we average spot on zero as we breath in and out.

Now we all learned to control our position in the water by breathing in the pool before ever we tried the open water and we probably have got the idea by now. We have about a litre of range as to where we put the average but we need to move about half a litre in and out. We also have a comfort zone that is much less. Neutral should be comfortable straining in neither direction.

So 2.4 to 8.4 Kgs means we need to stash between 2.4 to 8.4 litres of gas somewhere. Adding it to the dry suit works. It tends to make the thermals go loose and hence be less efficient at keeping us warm but we can put it all there if it's just 2.4Kgs. As the dive progresses we use the back gas and carry less in the suit.

The alternative is to keep the suit nice and comfortable and put the gas in the wing. The suit stays comfortable but not baggy. You don't have a big bubble of air moving around to emphasise any changes of attitude by rushing to your feet or to your shoulders. It is not pushing at the seals trying to get out. A wing is designed to have buoyancy gas in it. It stays where you put it.

Now my ScubaPro BCD carries about 8Kgs of lift which is masses when I'm diving a single but with the twinset I have a Dive-Rite Classic with 22Kgs of lift. Now that's overkill for me, but then so is the 130bhp motorbike.

The down side of carrying mixed buoyancy in both the suit and wing is that you have to manage them both. I confess to a shoulder auto-dump on the dry suit and it works brainlessly well on an SMB ascent although it needs a little thinking about on a *swim up the slope* style exit.

So what's the sum?

On a single you won't have problems unless you get your weight badly wrong either way. However when you learned diving you learned to control a BCD and when you progress to a twinset do you want to relearn it?

The drysuit is a tool to make diving more comfortable by keeping you dry. The inflators are crude, prone to failure and the dumps more so. The Wing/BCD is a precise tool to do the buoyancy job. Even the Buddy Commando, which is a bit of an agricultural implement in feel but is both squaddy and student proof, is better than a dry suit for buoyancy control. (Just don't get me started on suicide bottles.)

Buoyancy by drysuit is a skill like riding a mono-cycle. Clever but useless. If you work at it you can do it well and make it look easy. It works for a simple single and nothing else. It is temporary phase that people grow out of. You play at it to get the drysuit under control. When the drysuit works for you then you can forget it. It has no significance to real diving.



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