Bouyancy Control. BCD vs Drysuit or both?

[Coral Farmer]

I am still very new to diving and have to use a drysuit where I live. The subject has come up as to buoyancy control. Some of my friends only use their suits and some only put enough air in their suits to take the squeeze off and rely on their BCD for buoyancy control. Nobody has ever given me a reason for either way other than its just a personal preference. I have tried both ways and cant decide which way i like best. Are there any "Good" reasons to do it one way or the other? Does using the BCD make it any easier to control your trim ie. less of a air bubble moving around? Also would there be any advantage to a back-inflate vs jacket style BCD when diving a Drysuit? Anything else i am over looking?

 

[tbone1004]

A jacket could limit the bubbles ability to move and if it's too tight could prevent a fast vent if needed... but so could a tight weight belt or anything else, just that th ebands on the jacket are bigger.

It is largely personal preference, and also depends on conditions. If it's REALLY cold, you're going to want more air in your suit, which is why up in your neck of the woods a lot of the divers may use as much air as they can so they stay warmer. This can limit your maneuverability somewhat though. I use my suit just enough to control the squeeze, but that's my preference. I hate a big billowy suit...

 

[j.doe]

I find only putting in enough to prevent the squeeze is best for me.

1) it prevents task overload trying to control 2 different buoyancy systems.

2) as stated the bubble is smaller in the suit and extra air is confined and therefore easier to manage and counter weight.

3) I do NOT want to have to break a seal to vent air on my suit if I get too much air. I find that my dual pull / inflation dump can stay WAY ahead of any buoyancy issue where a shoulder vent on a dry suit, not so much.

4) I practice one buoyancy mechanism and am very good at using it despite what exposure protection suit I happen to be using.

5) Lastly for me I have had shoulder surgery, and therefore the less I have to use my shoulder the less sore it gets. Hence I like using the butt dump or hose inflation depending on my position in the water column.

Oh ya, also less air in the suit means that I have to hike to the beach with less weight I am not getting any younger these days.

 

[TSandM]

I was taught to use the suit for buoyancy, and found it very difficult. When I changed to putting only enough air in the suit to relieve squeeze and ensure mobility, and put the rest of my buoyancy gas in my BC, life got easier. The BC seemed much easier to empty, and much faster (which is true).

As I got more comfortable, I realized that I could put more air in the suit, and as a result, I would be warmer and more comfortable during the dive. But I had to stay ahead of the curve on ascent, because air in the suit takes longer to migrate to the dump valve, and dumps slower; if you get behind with a dry suit, there may be no salvage.

This week, we had a class of OW students in dry suits. We taught that they could put the air in their BCs or their suits, as they found it easiest to manage. The one in a good dry suit with a properly located dump valve decided it was easiest to put the air in his suit, because it would essentially "autovent" on ascent. The one in a poorly designed rental suit found her life easier if she kept the suit as tight as she could stand, and kept most of the air in her wing.

The bottom line? You do what works.

 

[Bubbletrubble]

If you're diving a single tank configuration and not going too deep (50-60 fsw), you'll find that adding enough gas to counteract squeeze is just about the same amount you'll need for buoyancy. At least that's been my experience.

I actually like the fact that I can fine-tune my trim by moving air around inside my suit.
FWIW, I find that my flexibility/maneuverability is better when I run a little extra gas in my suit.

I'd recommend diving the suit both ways to figure out what works best for you.
Personally, I like staying warm and being able to do hands-free ascents -- things that I can do easily by adding air to my drysuit only. On pretty much every dive I have a camera in my hands, so it's really convenient just to dip my right shoulder down a little to vent through my left upper arm exhaust valve during an ascent.

 

[Coral Farmer]

I have tried both and admittedly found the suit only approach to be the easiest so far. My dump is wrist mounted and found that on assent i just need to roll a little and stick my arm out a little for a good burp. Having said that i can see where in a situation where you get accidently get inverted you could get into a very positively buoyant position very quickly with no way to dump until you are right way up. I can see how the butt dump on a BCD would help alleviate this. I am still finding that the bubble is quite a handful, seems like every time i move just a little, it wants to move me even more. I suppose if i were to start using the BCD more then it would be easyer and i just have to learn to get used to the task loading. Perhaps I should also mention that I am using a shell suit so i probably want more air than with a Neoprene suit.

 

[fnfalman]

I prefer to use the suit as BCD. One less equipment to worry about.

However, with baggy suits that could be a real pain in the rear. So, gotta learn to work with your equipment for the best result.

As far as BC goes, I prefer to use my Halcyon Infinity rig because it doesn't have chest strap that would get in the way of the inflator valve.

 

[deadbilly]

I'll second bubbletrouble. When I dive singles, I mostly find that offsetting squeeze is enough to make me neutral, and I don't have to add to the wing at all. In doubles, I blow up the suit to be warm and comfortable and buoyancy is almost all managed with the wing.

 

[Bubbletrubble]

I have tried both and admittedly found the suit only approach to be the easiest so far. My dump is wrist mounted and found that on assent i just need to roll a little and stick my arm out a little for a good burp. Having said that i can see where in a situation where you get accidently get inverted you could get into a very positively buoyant position very quickly with no way to dump until you are right way up. I can see how the butt dump on a BCD would help alleviate this. I am still finding that the bubble is quite a handful, seems like every time i move just a little, it wants to move me even more. I suppose if i were to start using the BCD more then it would be easyer and i just have to learn to get used to the task loading. Perhaps I should also mention that I am using a shell suit so i probably want more air than with a Neoprene suit.

@Coral Farmer: As you already know, inversion of a drysuit diver doesn't necessarily lead to an uncontrolled ascent. After all, when a drysuit diver is hovering, no gas is entering or leaving the suit, right? A problem can occur, however, when the inverted diver alters his breathing (shallow breaths?), such that a slight movement toward the surface is initiated. This "upward" momentum can snowball quickly and result in an uncontrolled ascent.

As part of learning how to operate the drysuit, you should practice recovery from feet-first ascents. The technique that's normally taught is to tuck and roll in order to be able to position the exhaust valve at the highest point in the water column and dump air. Alternatively, you could open up your neck seal or wrist seal to vent in an emergency. Also, be prepared to disconnect the drysuit inflater hose if it's stuck open and that's what is causing the unintentional ascent.

With experience, you'll learn to manage the bubble in the suit better. It's hard to describe, but you'll figure out how to keep everything in balance. Body positioning really comes into play here. After about a hundred dives, you should be feeling pretty comfortable in it.

Have fun...

 

[MXGratefulDiver]

There is no simple answer that fits all occasions. Which is better depends on preference, to be sure ... but the question is why would someone prefer one method over the other. Several factors come into play.

First off, the type of suit. Trilaminate ("bag") suits tend to be less easy to manage than neoprene if you're using the suit for buoyancy, because they have extra fabric around the joints (to allow movement) and therefore can trap air in those places. Same goes for front-entry suit, as opposed to rear entry ... the former has to come over your head for donning, and to accommodate that there's usually a "fold" in the waist ... or extra material that allows the torso to extend for donning. How your suit is built determines where air will migrate and how easy it will be to manage it.

Then there's the matter of the bubble ... in a drysuit it travels the length of the suit. Air always wants to travel to the highest part of the suit, and as you move that is constantly changing. This is called "dynamic instability", and if you Google that term you'll find a nice article that explains what it means. For the newer diver, managing this travelling bubble requires effort. For the more experienced diver, much less so ... therefore the importance of this factor decreases with experience.

As several have already mentioned, a drysuit dump valve release air more slowly than a BCD dump valve. This can matter for more than just "inverted" divers ... it means that as you ascend, you need to stay on top of your buoyancy control. If there's a lot of air in your suit and you don't release it in a timely manner, you can reach a point where expansion is occurring at a faster rate than your dump valve is capable of equalizing it ... at that point you're going to the surface no matter what you do. Again, for the more experienced diver this isn't an issue.

This can also get back to the issue of suit type. People who use a full neoprene suit will find that their suit acts like a wetsuit in that it will compress and become less buoyant as you descend ... requiring more air be added to compensate for the suit compression. Conversely it will become more buoyant as you ascend ... requiring you to release even more air as the tiny bubbles in the neoprene material expand.

You dive on Vancouver Island, which is very cold water. That means you use a thick undergarment. Undergarments need loft to be effective ... and the more undergarment you wear the more air you need to put in the suit to create that loft. People diving thicker undergarments need more air in their suit ... and more weight to compensate for it ... than people diving in warmer climes and using a thinner undergarment.

And finally, there's mobility ... the amount of air in your suit affects it. Too little air creates suit squeeze, and can inhibit your movements. Too much air can create instability that makes it difficult to change your orientation (e.g. "head up" to look at something, or "feet up" to peek under a rock or take a picture). Photographers tend to use as little air in their suit as possible, because it makes changing their orientation easier. A friend of mine likes to go completely inverted sometimes to get the best angle for his shot. That's much easier to do with very little air in your suit than it is with a big bubble in there (since ALL the air is going to end up in your boots).

So the bottom line is that what works best for you will depend on the suit type, the thickness of the undergarment, your ability to anticipate buoyancy changes, and your diving style. Those variables will be different from diver to diver ...

... Bob (Grateful Diver)