Dual Bladders & DIR

[TexasMike]

LY,

Thanks. turns out that I'll get a chance to play with a Halcyon rig sooner than I ever thought. Iguana Don owns one of these and will be joining us for our Halloween dive. I'm looking forward to experiencing what you and the others are discussing.

 

[rcohn]

I just thought I'd play with some numbers. I'll note I am NOT a tech diver nor do I plan on becoming one.

The LP104 buoyancy is listed as -8.2 lb when filled to 2650 psi, however I know they like to overfill them, so to be duly conservative. I'll increase that to 3500 psi and (using a quick ideal gas assumption) I get 11 lb per tank. The ss backplates weigh 6 lb, which gives a -5.2 lb buoyancy and I'll thrown in another -5 lb for regulators, buckles, fins, etc. (which I think is pretty conservative). This gives a total of -32.2 lb. I'll assume stages and canister lights will be ditched, although considering the expense, I wonder just how eager most divers are to drop them in open water.

I don't know what shape most tech divers are in, but I probably couldn't swim up that load from a shallow dive let alone a deep technical dive. Of course, you still need to support the load on the surface, no sense in swimming up and then sinking right back down, so the only option will be to ditch the rig. This too will be very very expensive in the open water. Additionally most divers would be nearly exhausted (if they reached the surface at all) and DIR divers lack quick releases, making wriggling out of the backplate somewhat difficult, particularly when you must continually kick very hard to support the 32 lb load.

Unless someone can point out some gross errors in these numbers, it seems to be a pretty convincing argument that some form of redundant buoyancy is required with heavy tanks. Personally I would not want to depend on the drysuit, it is too easy to burp air out of the neck seal and lose buoyancy when over inflated.

Ralph

 

[Lost Yooper]

Ralph, it looks like your numbers are pretty close and is exactly why double steel tanks are not considered by many as such a great idea for wet suit divers. One has to factor in, however, the actual positive buoyancy inherent in a dry suit even when there isn't much air in the suit. I'm talking reality here, not theoretical, because you can never get all the air out. Even if you could, the suit and underwear are significantly buoyant which obviously helps in swimming up steel tanks.

This is my understanding of the topic and varifies my own experiences with steel tank diving.

TM, good luck on your dive. If you're like most, you'll fall in love with BP and find it difficult to go back to a regular BC. I suggest you have diversionary plan to fool ID in case you want to "borrow" it for a while.

Take care.

Mike

 

[TexasMike]

LY; already in the plan....probably yell, "Look! A beautiful female iguana!" as I point over his shoulder and start running the other direction.

Either that, or surface on the other side of the lake and go, "Backplate??? What backplate?" when I surface swim back to camp.

 

[rcohn]

Taking my previous analysis a bit further. When I dive in a drysuit with a single HP100 I use a 3 lb tank weight and 23 lb on my belt, which provide a -23.7 lb buoyancy. The buoyancy of the HP 100 is nearly the same (-8.9 lb full) as the LP 95 or LP 104. At the start of the dive I'm around 6.5 lb more negative than I will be at the end of the dive with a nearly empty tank. Since I do a lot of shallow shore dives where we swim in and want to stay down at the end of the dive I'm probably a couple of lb more negative than many divers would be. Regulator and other hardware must add at least -1 to -2 lb. Adding 2 lb for the hardware cancels the excess 2 lb needed for shallow diving. So at the end of the dive I have roughly 23.7+8.9-6.5=26.1 lb of ballast needed to balance the drysuit.

Now we showed before that the technical diver with double 104s must carry 32.2 lb negative buoyancy at the start of the dive, but I neglected to consider the end of the dive. In an emergency, when the last third of the air supply is used you will use up the 22 lb of gas (assumes air or nitrox not He) carried. I realize you probably wouldn't use all of it, but with Al stages you need extra ballast to cover for them, so I'll use all 22 lb. So, if I was diving I'd need 26 lb at the end but only have 32-22=10 lb. Therefore, I need to add an additional 16 lb of negative ballast. Thinking as many tech divers do, I would use non-ditchable ballast for "safety". Therefore, they don't really swim up 32 lb, they have to swim up 48 lb with full tanks. Lot's of luck with that load!

As LY pointed out the drysuit helps out here. My suit is 26 lb positive reducing the load to 48-26=22 lb. Add more than 10 lb to the suit (and possibly less) and it will burp out the neck seal, especially when ascending vertically. So at best, you must swim up 12 lb from depth. Any added tools, argon bottles, additional fixed ballast to cover multiple stages, cameras, etc. will add to this load. How do you "balance" a rig with double steel tanks?

I personally would choose to stick with a weight belt, but if you choose fixed ballast you need some form of reliable redundant lift.

Ralph

 

[Lost Yooper]

I'm not sure what to tell ya, Ralph. I can easily swim up my 95's with no air in my BC, and from my readings on the subject, so can those who wear 104's if they're properly weighted and using a shell dry suit with full underwear.
I'm not too sure I agree with only being able to get 10lbs of lift out of a dry suit. There's a bunch of guys out there who use just their suit for buoyancy control and plenty of others who have used their suits as a bail out to their wings. I'm pretty sure it's in excess of 40lbs, but that's just a guess. I suspect there's many variables involved like the type of suit and the type of seals on the suit.

Maybe someone else can jump in here for more discussion.

Mike

 

[rcohn]

26+10=36 lb of lift, so it's close to your 40 lb figure. It would vary with undergarments and suit size. I use DUI polartec 300s underwear (with a flex 50/50), thinsulate would be more buoyant. When I had to use an XL suit during training I needed 44 lb of lead to submerge with a full Al80 and I'm sure I was still underweighted

I dove on the Yukon with a single LP95 using the drysuit alone for buoyancy control (the last time I used only the suit) I had an annoying air bubble rolling around in the suit and I lost some air at the neck seal a couple of times.

Ralph

 

[Liquid]

Rochn, LY,

Please, let's get back to a very basic aspect of diving:

----> the BUDDY!!!!!!!!

We have a buddy to assist us. It is important to have a buddy. In tech dive with such equipment as stated above, It is even MORE important. One of the concepts about the lifting capacity of your BCD (wings are actualy a kind of BCD) in tech diving is, and please not this: In case of emergency a tech BCD MUST BE ABLE TO SUPORT THE NEGATIVE BOUYANCY OF AT LEAST 2 DIVERS!!!!!.

So if you are suposed to have at the deepest point of your dive, if dumping weights (prefarably,even if not dumping weights, so you can make the safety stops), a negative bouyancy that equals 30lbs, and the same with your buddy, you use a BCD with a 60lbs lifting capacity. Sounds to big!? I've seen BCDs with a lifting capacity of over 300lbs. looks like a darn Zodiac boat.

Note also that feeling the lungs does also contribute to achiving positive bouyance, and it can be used even if emergency ascents, as long as you are not HOLDING your breath. Besides, better have a pnemothorax on surface than deapth at the bottom.

 

[Lost Yooper]

Ralph, I've also tried using the suit solely for buoyancy control, and I couldn't stand it either.

Liquid, I absolutely agree with you on the buddy thing. Let me say that, in practice, if my BC were to totally fail on the bottom at the beginning of the dive, I would signal my buddy to alert him of the problem, probably use my suit for buoyancy control rather than kick up (even though I could) and we would probably surface. If, for whatever reason, the suit malfunctioned at the exact same time, I would probably make a buddy ascent, but with me doing my part by kicking until the air in the suit and BC expanded enough for me to take control of it. If everything went to crap, I would probably launch my bag and ascent line and swim up. Things would have to be going really badly for one not to be able to get up if your properly weighted and have a good buddy.

Getting back to wet suit diving (for which I'm not too keen on), I would do basically the same thing as above, but ditch the weight belt. Given a properly weighted rig (AL tanks, AL BP, and weight belt), a good buddy, and a lift bag and spool, a wet suit diver would also have to be having a really bad day not to get up safely. Therefore, I wouldn't see the need for double bladders for wet suit diving.

For those who care, I asked GI for clarity. For those who don't care, I don't want hear about it. Here it is:

Hey George,

I need some clarification.

You advocate the use of AL tanks with a wet suit regardless of an overhead environment or not, right?

I believe you also say that you should be able to swim up double steel tanks (even LP120's), AL stages, and whatever else with no air in your BC or dry suit, right?

Thanks,

Mike


Yes, all rigs must be balanced this way.

 

[Liquid]

LY-
with a buddy, you can just hang on to him, whilehe inflates his BC to suport your weight, that's the main point. In such a case, ofcourse, the dive is over. The great advantage of folowing this practice is that if you already got into decompression, you can make your deco-stops, safely, while hanging to your buddy. This makes, in my opinion, dual bladders, just a thing that may complicates things, like DIR says.

Another thing-
I don't belive G.I says aluminum should alwais be used. in overhead envirments (or at least in caves)he advocates Steel.