BCD use while diving drysuit ...

[300bar]

The BCD vs Suit is a "fairly " new argument.(I guess TEC.)
In the old days (standardsuit and brass helmet) the suit was used for BC.
I recently got some OLD Viking drysuits, complete with extensive manual the manual states clearly to use the suit for BC, but in those days there where no BCD's Jacket or wing, just horsecollars.

So when diving single, I only use my suit and proper weighting..

[TSandM]

I think there are two big differences between the BC and the dry suit, as far as using either as a buoyancy control device goes. One is that most BCs offer at least two (or more) options for venting gas, and therefore it is possible to get air out of them in any position. (And, as a DM, I love the fact that I can swim over a student who's getting light and pull their butt dump and stop a runaway ascent before it really gets going.) In a dry suit, once you are feet-up, you are toast until you can correct your position (unless you have ankle dumps, and very few suits do.)

The second is that the maximum exhaust rate of a BC is very high, and the maximum dump rate of a dry suit exhaust valve is slower. So, if you have a bubble which has expanded enough so that you can't effectively dump it through the dry suit valve as fast as it is currently expanding, again, you have few options. I know people write about pulling the neck seal open, but around here, once you got your hood out from over it and managed to grab the seal with thick gloves, you'd be on the surface anyway.

And the idea that you can use the dry suit alone depends on the tank you are diving. With my "generously filled" 95s or our 130s, I really CAN'T do all the buoyancy with the suit, as much as I would sometimes like to.

[RTee]

I use drysuit for warmth and comfort and my Wings...for buoyancy control.

In normal diving conditions and a properly adjusted exhaust valve setting, the suit should vent excessive air by itself on the way up as long as the individual maintains proper horizontal trim. Therefore the only thing left to do is to control the ascent is by dumping the applicable amount of air out of the wing or BCD as you normally do when not Drysuit diving,...by using the deflator valve of the hose or one of the purge valves. This is as simple as it can get. Normally you will naturally bring the left shoulder up when raising the BCD deflator valve to dump excessive air which then assist in getting excessive air exiting through the suit exhaust valve. I always do so, either diving single or doubles. Furthermore, knowing that my buoyancy is BCD oriented, then the drysuit should not be the problem if I start going up because it should not be problem unless I have a leaking or stuck inflator valve. In fact, if diving in a head down position (which I do not in a drysuit), I could say that dumping air from your BCD or wing should actually result in a descent (if caught early) rather than an uncontrolled ascent as you only have minimal air in your suit to start with. On the other hand, using your drysuit only for buoyancy could lead to an uncontrolled ascent as the only way to control it would be to perform the corrective actions to end up in a heads up position to then allow you to purge the excessive air out of the suit.