Bouyancy-Wet vs. Dry?

[ebbdiver]

I've heard others describe the effect of squeeze on a thick wetsuit for cold water and the difficulty of bouyancy control. They say it's easier with a dry suit, but I'm not seeing the difference here. With a dry suit in cold water, you wear insulation under the suit for warmth. Well isn't this also a compressible airspace that would be the equivalent of a wet suit? If you're using dead airspace for insulation, wouldn't you need the same thickness for a given amount of warmth whether it was neoprene or polypropylene? Am I missing something?

 

[ArcticDiver]

I've heard others describe the effect of squeeze on a thick wetsuit for cold water and the difficulty of bouyancy control. They say it's easier with a dry suit, but I'm not seeing the difference here. With a dry suit in cold water, you wear insulation under the suit for warmth. Well isn't this also a compressible airspace that would be the equivalent of a wet suit? If you're using dead airspace for insulation, wouldn't you need the same thickness for a given amount of warmth whether it was neoprene or polypropylene? Am I missing something?

I've been a wet diver until this past weekend when I did the Dry Suit Certification. So, since everything is fresh maybe I can give a good answer.

There are different buoyancy wet suits as there are different buoyancy dry suits. Plus, you have the complicating factor of how much air you pump into the dry suit. The big difference is that the air bubbles in the wet suit compress as you descend where in a dry suit you add additional air to offset compression. Therefore, buoyancy remains roughly the same thoughout the water column.

In my suit, which is a shell suit I put enough air in the suit to keep the undergarment insulation from compressing. So, essentially I have a layer of insulation around me much like that in your house. In both cases it is the dead air space in the insulation that does the majority of the work.

As I descend, due to air compressibility, I must keep adding air to keep the dry suit from giving me the old squeeze and to keep the insulation from compressing. But because of the compressibility that is causing me to add air I get no more buoyancy from the additional air. In fact my buoyancy stays the same throughout the water column as long as I only add enough air to offset compressibility.

So, essentially from a thermal point of view the only function of the air is to keep the insulation from being compressed. As long as the compression remains the same there is no difference in buoyancy as you descend.

Now when you come back up all that extra high pressure air now expands as the pressure decreases and must be vented to prevent a runaway ascent. In my suit the automatic exhaust valve does the job.

So, I added a couple pounds because I wanted to play with the air settings. Now that I've got a bit of handle on the new suit I'll start taking off weight. In the end I think I'll probably be at just about the same weight as with the wet suit.

But, if you choose to use your dry suit both as environmental protection and to adjust buoyancy you will have to adjust it in much the same manner as your accustomed to with your BC. That requirement is beyond what you need to preserve insulation.

 

[DA Aquamaster]

In simple terms the difference is that with a dry suit you add air to the suit to offset suit compression and with a wet suit you add air to the BC to replace the bouancy you lost as the suit compresses.

Excessive weight with a wet suit means more air is required in the BC or wing to gain neutral boyuancy and this larger amount of air then requires more adjustment as it changes more as you decend and ascend. Excessive weight with a dry suit means either excesisve air must be used in the suit which can be hard to manage as it shifts in the suit or alternatively it means maintaining the appropriate amount of air in the suit and having to manage an additional volume of air in the BC or wing which increases task loading. Gross overweighting is something you want to avoid in a dry suit.

On the other hand, the lower limit to the amount of air that must be maintained in the suit is deterimined by the degree of suit squeeze (and chill to to compression of the undergarment) you can tolerate and/or the minimum amount of air required for proper function of the dump valve. Too little air in the suit can actually interfere with proper venting of the dump valve as it is squeezed against the undergarment in the suit.

It seems counter intuitive but a pound or two of extra weight above the absolute minimum required to get under water, and consequently a little more air in the suit, can often make a safe slow ascent much easier to obtain as the dump valve functions more efficiently. That extra pound or so of weight will also make the suit a little warmer as the insulation is less compressed.