BC or Drysuit for Buoyancy

[cummings66]

Originally Posted by it_mike

I still submit that if you are properly weighted (neutral with 500psi), than the bc is for surface flotation and to offset anything you've added during the dive.

Curiosity question, what gear config do you dive?

I dive a membrane suit, double steel 120's and double steel 100's, BP/W, Jet Fins and no weight belt. Normally a 200 gr thinsulate undergarment for warmth.

I know that for me with 18 lbs of air, near the end of a dive I'm a LOT lighter than I was at the start of a dive, so I use the Wing at the start of a dive to get me neutral instead of the drysuit. I can do it both ways, but 18 lbs of lift via drysuit is pretty roomy compared to the same from a wing.

That does mean that I have to adjust the wing during the dive, or the drysuit, but it's not that hard.

[Tassie Diver]

Originally Posted by SkuaSeptember

I fall into the DS for control category and here is the reasoning. At any given depth you require a certain specific volume of air to maintain neutral buoyancy. It does not matter if that air is in your BC, a lift bag that you hold onto or in your DS. If you are properly weighted at the surface - the lofting is just comfortable, the BC is empty and a full breath will float you while a full exhale will let you descend, then this is the balance that you want to keep throughout your dive. Hence no need to play with the BC. If you are wearing a shell suit, the only air space that will compress when you descend is in the lofting of your under garments. Keep the loft consistent and you will maintain neutral buoyancy. If you are wearing a crushed neoprene suit the compression of the material should be negligible to slight. If you are wearing a suit made of a more traditional neoprene, I would think that more of a case could be made for using your BC for control in conjunction with the DS...

+1!

I also use my drysuit for buoyancy control and am weighted for neutral buoyancy at the end of the dive.

IF I was diving with enough equipment to have negative buoyancy, I can see that it would be necessary to use my wing for buoyancy control. In that case, I would need to inflate my drysuit uncomfortably to provide a significant amount of extra lift.

Cheers

TD.

[RoyN]

I use my wing for buoyancy, and an inflation bottle to keep me from getting squeezed.

[it_mike]

Originally Posted by cummings66

Curiosity question, what gear config do you dive?

Well, that's the real planning point. You're right, if you're using the BC to manage neutral at the surface prior to the dive, you'll be managing it throughout the dive.

Doubles, ponies, steels...all effect the plan. I was responding to the general concept of drysuit buoyancy on a generic level, e.g. basic dive with AL80.

My gear config is task oriented; quarry, boat, shore, shallow, deep, open, or penetration diving, etc. Therefore it really depends, but the basic level concept stands. If you're properly weighted for a basic, single tank dive, bc empty on the surface, the only buoyancy change will be compression of your suit, ergo the buoyancy adjustment will be handled by reducing the squeeze.

Of course, (as I experienced again this weekend) if your suit leaks you will be managing your buoyancy via your BC.

[mitsuguy]

Originally Posted by Kingpatzer

Is the only need for air in your buoyancy compensator due to suit compression? If so, why is it that when diving in the tropics without a suit, starting neutrally buoyant, you'll still need to add air to your bc at depth?

On a dive where I live, tropics, no wetsuit, I dive with no ditchable weight (unless with students or tours, then I always add some to give away just in case), but anyways, I have a 2 lb Al plate and 4 lb spine weight... Diving an AL80, when I drop down, once I've made it to about 20-25 feet, I add a few small bursts of air, and then I'm neutral, at the beginning of the dive... As I descend further, I add more air, because the air that it took to be neutral is now compressed inside the wing and I need to increase the volume of the air, so, a few more quick puffs of air again...

the math is quite easy actually, assuming perfectly neutral at the absolute end of a dive, 0 psi in a AL80 tank, it's a 4.8 lb swing from full to empty. So, we need 4.8 lbs worth of air in our BC to offset a completely full tank, we need to add .078 cu ft of air to our BC (in saltwater)... Assuming we had the exact correct amount on the surface, then at 33 feet, we would only have .039 cu ft, then at 66 feet, we would only have .026, at 99 feet, .019 cu ft. So, to make up for this compression of the air pocket in your BC, you need to add air as you descend, even though there is no wetsuit compression...

edit: the 4.8 lb swing is just what I remember, but now that I am looking up the weight of air, it seems the actual weight of air is all over the place depending on who you ask...

edit again, its actually closer to 5.8 lbs full to empty (maybe 4.8 was 3000 to 500 psi was what I was thinking, but, I'm not redoing the math, cause its not important, just the theory is)

[cummings66]

This much I know. Yesterday I did a quarry dive where the bottom temps were about 48F and I decided to not wear my normal exposure garment. Instead I just wore the thermals that I wear under it which I use to soak up the odors so I don't have to wash the thinsulate as often. That meant I was going to be using the drysuit only for buoyancy control. That way I can puff it up and stay warm.

As always it worked as we've read above, and of course as I've said before too. You can do either or when diving and IMO neither is wrong. I dialed the exhaust valve in so that I was neutral and from that point on all I did was to add air until I was neutral or the valve exhausted. It was a pretty nice dive and I had a good time doing it. For me I decided that dive was to be a practice dive so all I did was practice skills, all of them.

Now, as some of us have said. With double steels that meant that I used quite a bit of air to become neutral and you could feel how loose the suit was on you. You have to pay lots of attention to managing that bubble but it's still pretty easy and that's because I practice that skill often which I think every drysuit diver should do if they use the BC for control.

I did not get cold at all, in fact I was a bit surprised by that but I did have a LOT of air in the drysuit.

An experienced diver can and should be able to do both methods because you never know what can happen.

[frozenwarp]

I used to use my DS for inflation but have recently swapped to to using my wing, one reason being when doing valve drills at the begining of a dive if i have a fair bit of air in my DS it tends to vent when i do my left post (even closed i tend to press my exaust valvewith my forearm)

[MSilvia]

Originally Posted by it_mike

...the basic level concept stands. If you're properly weighted for a basic, single tank dive, bc empty on the surface, the only buoyancy change will be compression of your suit, ergo the buoyancy adjustment will be handled by reducing the squeeze.

Compression is simply not the only source of buoyancy change. It will also change due to exhaling the gas you breathe, effectively gradually dumping weight from the full tank over the course of the dive. If you're neutral at the beginning of a dive with no gas in your BC and a full Luxfer AL80 on your back, you're going to be about 4 lbs buoyant at the end of the dive when you're trying to make your safety stop, assuming you have about 600 psi left. In order to offset that and be neutral at the end of the dive, you need 4 more lbs of weight at the beginning, which means you'll need enough additional gas to offset that weight, in addition to whatever else might be required for undergarment loft.