Drysuit or BC for boyancy control?

[Storker]

You dive a lot in a drysuit. Haven't you noticed?

Not really, I have to admit. Perhaps I haven't tried one of those DSs that suddenly lose buoyancy. My first suit was compressed neoprene, the one I have now is trilam.

 

[AJ]

I once had a 5mm compressed neoprene suit that quite suddenly... sort of ... collapsed I guess is the best way to describe it, if you go deep enough. It caused the suit to suddenly lose significant buoyancy.

This is exactly why I dive a trilam suit. No loss of buouancy going deeper by the suit material itself.

 

[Vegan Shark]

SI Tech, in their manuals for their valves, also says to use the drysuit for buoyancy, and the BCD as a backup.

Obviously both ways work, and it comes down to personal preference. I think using the suit for buoyancy is easiest--reduces multi tasking, requires no hands to exhaust since the shoulder valve vents automatically, and overall it just feels simpler.

I'm not a tech diver, and don't use stages, so can't comment on what's best for that type of diving. But, with double steel 14L tanks I have no problems using only my suit for buoyancy.

 

[REVAN]

A suit that was not as full all the time was significantly easier to swim, and I remember noticing that it felt like I was "lighter" in the water. That led to me being actually lighter - as I was able to drop the trim weight I had been carrying on my subsequent dives, since I had less air in my suit to offset with lead.

It sounds to me like you were just too heavy to use the suit for buoyancy control. If you have massive amounts of air, that can't be helped. But if you've got less than 6 pounds of gas (i.e. < 120 ft^3) and weight yourself appropriately, the BC is superfluous during the dive (unless you have an expanded neoprene DS or can't hold gas in the suit due to the design of the dump valve).

So, if you are diving with a lot of gas and will have large buoyancy shifts during the dive, use the BC to account for that. If you are diving like the other 95% of divers with normal amounts of gas, and using a DS with a non compressing material, you won't need to use the BC. I did a lot of diving in a trilam suit in the Pac NW with just a tank harness and no BC. The suit gave me all the buoyancy control I needed.

 

[TotDoc]

It sounds to me like you were too just heavy to use the suit for buoyancy control. If you have massive amounts of air, that can't be helped. But if you've got less than 6 pounds of gas (i.e. < 120 ft^3) and weight yourself appropriately, the BC is superfluous during the dive...

Like I had said, I never noticed anything problematic until I started tech diving. With my double cylinders, I was toting double the 120cuft of gas you mentioned with my cave pumped lp95s. When it REALLY became notable was when I added a negative rebreather unit and started running steel side mounted bailout, plus /minus stage drop bottles.

My point was never that it couldn't be done, or that it couldn't work... I had done it that way for a long time. My point was that it had always had some shortcomings, but they weren't noticeable until I started adding some complexity to the situation.

By running the suit as my buoyancy control all along, I had developed some muscle memory and habits that took some conscious effort to undo, and if I could go back and give myself advice on how to do things back when I started diving dry, I would have recommended running the suit with a little more squeeze, and gas in the wing for buoyancy control at depth.

I'm not telling anyone that they are 'doing it all wrong' if they use the suit only, even if they are doing tech dives. But I will tell you that if you continue to progress in your diving, you will come to a point that using the bc is strongly preferred, if not flat out necessary.

Since it's an easy transition to try, it may be worth giving it a go, just to see how it feels. It may save you some headaches in the future. Then again, if you stay in a single tank config, you may never know the difference, so dive however you like.

I just know that when I was taught, I was told the bc wasn't really necessary, and since I didn't know what I didn't know, it was easy to accept that as the best way. Now I know the flip side of the coin and figured maybe people should know why some folks do it another way.

 

[KenGordon]

While at some levels this is personal preference I think advice to use the drysuit for buoyancy is unhelpful. It assumes that controlling two air spaces is too hard for a novice dtysuit diver.

Initially people learn in pools, they use a BC to control buoyancy. Often they then use a wetsuit in open water. They use a BC to control their buoyancy. Next they learn to use a drysuit, often after quite a number of dives using a BC to control buoyancy. There is a choice to teach just how to stay warm with the least change in technique or to entirely change how buoyancy is done. Hmmmm, tricky choice...

Eventually, some divers will dive with lots of gas. Twin 12s mean a 6kg difference between the start of a dive and worst case holding a stop. Nobody really wants to do that with a drysuit, so it is taught to use the BC.

Why bother with the intermediate stage?

 

[REVAN]

While at some levels this is personal preference I think advice to use the drysuit for buoyancy is unhelpful. It assumes that controlling two air spaces is too hard for a novice dtysuit diver.

Let me try again, because I don't think I got the point across clearly enough.

Unless you are overweight in the water, there is no reason to add air to the BC. Doing so will just force you to the surface when you don't want to do that. If you are overweight because you have a lot of gas that's required for the planned dive, use the BC to make up for the extra weight. If you are not carrying a lot of extra gas, and you need to use the BC for additional buoyancy, that is probably a sign that you have too much lead which adds all kinds of unnecessary risk to your diving. Make the effort to get weighted right and you won't use the BC because there's no point to add air to it when you are already floating just right. There's only 4 pounds of air in a standard tank, which is about a half breath's worth of buoyancy shift. It's not worth bringing a second system online that needs to be manage just to accommodate that small bit of change.

If you get into tech diving, there are complexities that go along with that and you learn the skills and accept the risks that go along with the activity. If tech is not what you are doing and you don't need the added complexity in your dive, you are best to not introduce it in the first place. Any trapped air in a bladder in going to add an instability to your vertical control. Having two unstable systems to control is more task loading for the diver, and mismanaging the additional complexity may be the thing that gets you into trouble. Whenever possible KISS.

Proper weight management is one of the most important tasks a diver needs to address, and yet is is often glossed over by many divers. With a non compressing DS, as with diving in a bathing suit, you won't actually use your BC during the dive if you are weighted right (assuming you are using a "normal" recreational tank).

 

[chrisch]

...
Unless you are overweight in the water, there is no reason to add air to the BC. ...

Most divers are overweighted. Probably about 99%

 

[TotDoc]

Unless you are overweight in the water, there is no reason to add air to the BC

This IS the fundamental problem, but also the reason why you will always have the opportunity to add air to the BC as opposed to the suit.

EVERY properly weighted drysuit diver starts their dive relatively overweighted. Proper weighting of a drysuit is to be neutral with empty tanks at the depth of your last stop on the way to the surface. This strategy doesn't really change, even in tech diving, although there may be specific reasons to modify this 'rule' and sometimes, like when diving a rebreather with multiple stage and bailout tanks, is essentially impossible to implement due to the implicit ballast resulting from a gas supply that you won't breathe down during your dive. The reason every diver starts relatively overweighted is because proper weight is determined by the end state of our dive, not the beginning when our tanks are full. The weight of our starting breathing gas determines the degree to which we are overweight at the start of our dive. It also means that we remain relatively overweighted until near the end of our dive, so we have to manage any resulting problems from this the majority of the time we spend under water.

As opposed to free diving, weighting in scuba is a dynamic process. That's why divers use a bc in the first place. Buoyancy needs to be added somewhere to offset the weight of your starting breathing gas, and that excess gas needs to be dumped incrementally during the dive as your gas supply is consumed. The question becomes, then, where the best place is to store that gas supply dedicated to maintaining neutral buoyancy.

There are 3 options for this, which can be used in combination with each other: your lungs, your suit, or your bc. There are advantages and disadvantages to each.

As you pointed out, the weight shift from an aluminum 80 can be managed using your lungs alone. Although this is possible to do within the vital capacity of many divers, the comfort level in doing this varies significantly from diver to diver, especially with longer run times. This can lead to shallow breathing and co2 accumulation and/or poor SAC rates, so I would recommend against this as a solo method of buoyancy control in a drysuit.

The suit itself is a great place to stick some extra air, but this also comes with a downside. The advantage, of course, is that you can leave the wing fully emptied, and you now only have 1 airspace to manage. The downside is where that extra air must go, and the larger space that air has to move and expand in your suit versus your bc. In the case of a deflated wing and a negative starting tank, your tank will be pressed against your back. Since air has a tendency to rise to the highest point available in an air space, that means that the bulk of the air will need to go around your shoulders or into your legs, as the space under your tank on your back is physically compressed. Neither of these places is preferable, especially when changes in depth are involved. The reason why is that these are the 2 farthest points away from your center of balance while horizontal, so a relative change in buoyancy between these areas can result in changes to your horizontal trim. Also, having excess air in these areas have other more specific problems. At the shoulders, this increases your profile, which increases drag - from your other posts, this appears to be something you are familiar with. As the gas volumes carried increase, the drag from the excess gas in the suit to offset this weight increases significantly, to the point that it is quite obvious to a diver. Just because you aren't carrying enough gas to make this readily apparent doesn't mean the effect isn't there, it's just less obvious to you. The air in the legs isn't as much of a problem for drag, but it does become problematic while ascending, as there is no gas outlet on your lower body, increasing the risk of a feet first uncontrolled ascent.

Let's consider the bc, then, as a place to put gas. For starters, it's an area that divers are already used to using, so it's not a foreign or new concept. Second, it's a much smaller airspace, with a limited capacity for expansion, that is located closer to the center of balance on a horizontally oriented diver. Finally, it's very difficult to accidentally dump air out of your bc, as opposed to the shoulder dump on a drysuit that is over inflated, particularly when excess gas tends to hang around the shoulders, as I described above. The location and profile of the wing also tends to minimize the associated drag of air carried here, which is also beneficial.

As a sidebar here, I would argue that the buoyancy capacity of many bc systems marketed to recreational divers are significantly larger than they need to be, which can result in some inefficiency here. This is a gear problem that can be remedied by using a plate and wing configuration that utilizes an appropriately sized wing. Many people are pleasantly surprised when they see how small of a wing is really needed for single tank diving. If you're not sure how to figure this out, may I suggest contacting the folks at Deep Sea Supply. While they are clearly not the only folks making bc wings, they do make excellent gear at a reasonable price, and more importantly, they are willing to talk your ear off regarding how to optimize a balanced rig. Honestly, it might be worth a call just for the educational value. Oh, and for the record, I have no financial interests in DSS, I just appreciate the quality of their gear, the value of their customer support, and the quality of their advice. The owner is also an active contributor on this board.

So back to our discussion. You had said that with tech diving comes increased risk, and training to mitigate those risks. You also said you're a proponent of the KISS philosophy. As it turns out, so am I. I do believe that adding gear to a dive plan increases complexity, so even though you don't need to be doing technical diving to enjoy the benefits of wearing a drysuit, you are accepting the increased complications and associated risks of using that equipment. This holds true for drysuits, which is why training is available for their use.

You seem to indicate that using both the BC and the suit in tandem to control buoyancy is somehow more dangerous than using the suit alone, but I feel this is misleading. As I have already pointed out, having excess air somewhere on your person is a requirement when offsetting the weight of your breathing gas. If you are storing it in your suit alone, it has a much larger capacity to offset your trim or to expand to a much larger capacity than it would in an appropriately sized bc. That is where the danger comes from, especially since new drysuit divers are most likely to get into trouble due to expanding air in the legs and feet that needs to be redistributed to vent. The challenges of dumping gas from a wing and drysuit at the same time are minimal at best, considering that your inflator hose and air dump on your suit are both located around the left shoulder... That's why they are built this way! I also think the challenges of doing this are more mental than physical. Implying that new drysuit divers can't manage this is as laughable as the idea that it's impossible to teach new divers how to dive in neutral trim. You can naysay all you want, but when divers learn to do something right from the beginning, it's awfully easy for them to pick it up.

My whole reason for posting my initial response to this thread is that old divers have harder to break habits than new divers, since they have had a chance to build muscle memory for their chosen technique. Once habits are established, it takes some work to undo them. My whole point, from the beginning, has been that only one of these methods presented above will end up being problematic if you advance in your diving. So if it really doesn't matter either way to you, you might as well learn and practice the methods that will continue to serve you well, no matter where your diving takes you.

 

[Storker]

In the case of a deflated wing and a negative starting tank, your tank will be pressed against your back.

I would just like to point out that with the thick undergarments needed for really cold water, it's - IME - difficult not to have a negatively buoyant rig underwater, unless you want to carry a lot of weight on your belt. That, too, has its downsides, and personally I prefer having a rather moderate amount of weight on my belt and put the rest of my weight on my rig, making it negatively buoyant for most of my dive.

The biggest disadvantage with that strategy is - IMO, of course - that it feels really weird to have a neutral or slightly positive rig when I'm vacation diving in a WS.