Dual-bladder or single-bladder? The whys and wherefores...

(Disclaimer: I'm posting in the Advanced forum instead of the Technical forum because the question I'm asking applies to solo divers as well as tec divers. Mods, please feel free to move if necessary.)

Here's the scenario: I dive SM, often solo, and I live just outside Atlanta, where the summers are legendarily hot and humid. Thus, drysuits are not really a tenable option year-round, and in the summertime, I often dive wet, even when the bottom temps are about 50F/10C. For all this SM diving, I commonly use a double-bladder Hollis 100 with the Edd mods.

I'm also taking a Tec40 class and looking to continue down the tec path, meaning (a) I need redundant buoyancy, and (b) I'll also need to carry more tanks with special blends. It's certainly possible to sling multiple tanks SM, but diving a BM twinset in a tec scenario would make efficient use of unused space and have the extra benefit of reducing underarm clutter.

Thus, I'm vaguely considering buying a bp/w setup so I can do BM doubles, but @tbone1004 pointed out in another thread that there's some controversy about whether double-bladder wings are good ideas.

So let's talk about it. What are the up- and downsides of double bladders?

Oh...and before you post, please note that "OMG it's gonna kill you" or any other similarly hyperbolic post will be summarily ignored. Let's keep it calm and logical. (Yeah, I know...this is ScubaBoard...but I'm still asking.)

 

I will only dive double steels with redundant lift. My Nomad LTZ is dual bladder.

 

I would rather a 50lb lift bag vs dual bladder for most open water diving. That applies to anything where the MOD of my breathing gas roughly equals the bottom. Worst case you lose everything and end up on the bottom, then use the lift bag for your ascent. Sure it's annoying, but is what it is. Cave diving is a bit more complicated, but you have to ask what the risk of a true catastrophic loss of lift is, and more importantly what caused it. Was it a puncture or a pinch flat? The dual bladder likely won't do anything. If it was an inflator or dump valve like what happened to @The Chairman right before he broke his leg, then the dual wing will help and is much better than a drysuit IMO.
Is it worth the hassle, compromise on the rig itself? well that's a different discussion..... I don't think so which is why I don't have one, but I'm not morally opposed to them like some are.

 

It depends. If you can't get yourself to a "balanced rig" then you need redundant lift. That can come from a drysuit, dual bladder wing, liftbag, whatever.

 

when was the last time you dove a balanced rig? I can honestly say I have never in a cave since I use pretty thin undergarments... I never wear lead, but I chose to go to bigger backgas tanks instead of extra stages. If I was in Mexico diving with all AL80's, then it would be a cakewalk. With the new smCCR I may actually be going to all AL80's as well because of it, but then I have to wear lead :-(

 

When the elbow was broken on my bladder, I just went old school with no bladder. Of course, those were AL80s and I had to weight myself to get neutral. I did rely on my sausage for any surface float.

 

Primadonna.

Wintertime I'm carrying ~15kg (>30#) distributed between my rig and my belt. See how much I cry for you.

 

Double bladder wings have declined in popularity because many divers concluded that they aren't any safer than single-bladder wings. Compared to a bag, the upside is that you can finish the dive with a normal amount of buoyancy and trim control rather than having to fuss with something that's an awkward shape and that is tending to pull you into an awkward orientation.

Some divers use two wings, instead of one wing with two bladders. Regardless, there are two possible ways to configure the two bladders. Each poses problems.

Method one. Each wing's power inflator is connected to an air supply

The problem here is that it is difficult to diagnose problems with a power inflator that is dribbling air into the wing. It is possible to have the secondary wing inflate slowly over the course of the dive and not realize that there is a problem until it is impossible to control ascent. Problems venting the wing must be solved quickly and the automatic (and usually correct) response is to try to exhaust trapped air from the primary wing. There is some added risk of a runaway ascent with this configuration that must be factored in when evaluating the safety benefit of being able to control a runaway descent due to a wing rupture.

Method two. Secondary wing power inflator isn't connected

The problem then is that it potentially takes longer to connect the power inflator to the secondary wing (or to inflate the wing orally) than it would to deploy a lift bag from a pocket or other reasonably accessible location. The usual approach is to inflate orally, but when a good deal of the wing capacity is in fact needed, this takes time and concentration. Once a lift bag starts to fill from a regulator that has the purge button depressed, it fills quickly.

 

If you dont ever use the second bladder it may become unusable on its own from the pressure of the other bladder. Especially true if once used and than not for a long time the bladder inside sticks to itself making it unusable. Using 2 needs 2 hoses and is another FAILURE POINT as many look at it. I recently had someone ask the same thing thinking if he had a dual 40# bladder he could if needed get 80 lift from it. nO NO NO . His next thought was to alternate usage of them. did n ot appear to be a good stratagy either. then it was he could getting a small dual bladder and use the second only when on the surface to reduce the moving bubble. He finally gave up on the idea, especially when he was thinking about having second airs on both. As redicuous as it sounds I kinds wish that we had the old horse collars as a backup

 

I personally do not dive a dual bladder and rely upon my drysuit or liftbag for redundant lift. I believe that with a dual bladder it should have its own LP hose and be connected at all times. I also believe it should have a shutoff on it also. In the event of a stuck open inflator and the quick added buoyancy, you do not have time to unplug the hose just to find it was the other inflator leaking before you are in an uncontrolled ascent.

The redundant bladder will need to vigilantly maintained and inspected so that it really is functional when and if you have a bad day and need it. Many people just ignore them and just assume nothing could ever cause them to fail.