Twinsets, redundancy, and what problem are we solving...

[Bowers]

Does kaboom happen often?
Have you ever had kaboom happen to you? I never have.
Do you know anybody that kaboom happened to?
What causes kaboom?
What is kaboom anyway?

I had a dive buddy whos drysuit inflator hose blew an oring a few hundred feet into a cave with me. I have another buddy who bumped a valve nob and created decent leak but i was bot on that dive with him. I personally had a second stage free flow but after depressurizing and working with it i was able to fix the problem.

 

[John C. Ratliff]

I had a dive buddy whos drysuit inflator hose blew an oring a few hundred feet into a cave with me. I have another buddy who bumped a valve nob and created decent leak but i was bot on that dive with him. I personally had a second stage free flow but after depressurizing and working with it i was able to fix the problem.

I’m curious, was that dry suit inflator hose only hand tight on the first stage? I’m asking because we’re having a discussion on a different thread about hand tightening versus using a wrench and actually ensuring the hose is tightened on the first stage.

SeaRat

 

[Subcooled]

I'm probably overthinking this, but it has been bugging me...

Diving with a pony makes sense to me. Two separate air sources, each with it's own first and second stage reg and hoses etc provides redundancy in case of mechanical failure of any component and also mitigates the possibility of an out of air situation. One literally cannot use all their air without consciously switching air sources.

With a twinset, you still have mechanical redundancy, but unless you isolate your tanks from one another, it is still possible to find yourself in an OOA situation with no reserve gas available.

So, are twinsets considered redundant enough because the mitigate the possibility of mechanical failure alone? Is mechanical failure more or less likely than failure of the squishy part it is attached to? Are people discounting the possibility of human error while focusing on the possibility of mechanical failure?

What are people's thoughts on this?

• A pony bottle is a fine solution as long as it is comfortable to carry and contains enough reserve gas for your dive.
  • A twinset typically carries more gas, because the twin cylinders tend to be larger than a single cylinder + pony bottle.
  • A twinset, even one with very large cylinders, balances nicely because the cylinders are and remain of equal weight.
• I would thus say that a twinset is carried because of its capacity.
• It is true that you could suddenly run out of air with a twinset unless you follow your SPG. You can avoid this by shutting the manifold, but then you would need to switch regs constantly, just like one does in sidemount diving. This would affect gas sharing protocols. The correct solution to this is to learn how to monitor your gas consumption.
• Twinsets are typically equipped with a longhose, which makes sharing gas with a buddy a lot easier.

Twinsets can carry more stuff too, such as a suit gas bottle for drysuit diving and a battery canister for a heating vest/dive light. Hence, again, more capacity.

Please note that if a faulty compressor introduces carbon monoxide into the tanks (rare, but possible) then twin cylinders offer no redundancy - they have the same stuff inside. You would need a pony or stage cylinder that's filled somewhere else. One can mitigate this by analyzing the oxygen and carbon monoxide contets in ones cylinders.

 

[Bowers]

I’m curious, was that dry suit inflator hose only hand tight on the first stage? I’m asking because we’re having a discussion on a different thread about hand tightening versus using a wrench and actually ensuring the hose is tightened on the first stage.

SeaRat

I can’t say for sure but that buddy usually does a very light wrench turn to set hoses. It’s possible that step was omitted in this case.

 

[mdwalter]

If you want redundancy for hardware failures, use manifolded doubles. If you want redundancy for software failures (in your brain), use a pony. If you want both, use both. For solo diving, I typically either sidemount or dive manifolded doubles plus a stage. For OC solo sidemount I typically use two 72s or two LP85s and breathe 1/3 out, 1/3 back (and 1/3 reserve). For OC solo backmount, I typically use LP45s with and AL80 stage and breathe 1/2 the stage out and 1/2 the stage back (my "pony" is the 90cf of gas on my back with 2 fully redundant regulator systems - i.e. I'm bailing out to what many people start a dive with). This also make the changeover for repetitive dives easy; drop empty AL80, pick up full AL80 (move stage regulator over), ready for next dive.

 

[rx7diver]

If you want redundancy for hardware failures, use manifolded doubles. ...

Even better, maybe: "If you want redundancy for hardware failures, use manifolded independent doubles."

rx7diver

 

[HenryUK]

In a word, redundancy.

You don't know when you’re going to need it, but one day you will. A twinset means it’s alway there and you’ll be practicing— on every dive — that you can do the shutdowns. That it also has greater gas is a bonus.

In all the time I dived with a twinset I never needed to donate or do a shutdown for real. I’d put a tenner on it that if I didn’t have that redundancy it’s certain I would have needed it.

More subtle is that you don’t have gas anxiety. Just supposing you got caught in a net and had to cut your way out or got lost in a wreck. Just imagine the phenomenal stress that knowing your gas is going to run out very soon. On a twinset there’s plenty of gas.


Ponys are fine within their constraints. They’re small and you need to have them full but practice on every dive. A twinset on the other hand is always full on both sides.

Sidemount though…. You’ll have both sides filled and you’ll breathe from both. They’re easy to use, especially manipulating the valves. And they’re so amazingly stable. My choice for a solo dive.

I have a similar question (Planning a dive trip in a few months and wondering what I should do, bear with me. Twinsets or Stage/Pony Cylinder) and would appreciate your thoughts…

Beginning to think twinsets are the way to go…

 

[HenryUK]

Guys, you are way overthinking this. You are planning as if you are solo diving and tech diving at the same time, which is not necessarily the case. You talk about redundancy as if your wonderful, shiny new regulators are going to conk out on you at any time. Is that a remote possibility? Now, maybe, as these regulators have gotten very, very complex compared to the regulators of old. If you haven’t seen it, your should watch the Cousteau video about finding the Britannic. Yes, they used some rather advanced techniques and weren’t solo diving; they were tech diving with a triple tank system at 300 plus feet though. Why could they accomplish that? Well, they Mistral double hose regulator they used had six moving parts—that’s all. Take a look at the video:


Now, if you are diving solo using recreational rules (no decompression diving, no overhead environment, to caves, etc.), there is no reason for this redundancy, You can just swim to the surface. I have been diving this way for over 50, mostly solo. I usually have some redundancy, sometimes using double tanks, but not always. I have a Sherwood manifold that has twin posts, and a newer Scubapro manifold with twin posts. But this is so I can used my vintage double hose regulators with a single hose regulator that has LP and HP ports. But there are numerous times that I dive a small twin set with only a Mistral or Healthways Scuba regulator (both basically the same single stage design, with six moving parts). I have never had a failure. On several of my twin tank sets, I have a J-valve that I use with these regulators, just as I was taught to do in the U.S. Naval School for Underwater Swimmers in 1967.

Whenever you load up your gear with redundant gear, you are compromising your swimming ability. You have much more resistance to swimming in the water, which leads to more redundancy and a dive scooter (or two or three, according to the above photos ).

I have made a study of underwater swimming techniques, and those skills have deteriorated over the last two or three decades amongst most divers. Why? Well, the so-called “frog kick” is one reason, but the other is loading one’s self up with redundant, but very in streamlined, gear. Dry suits also have contributed to the loss of swimming ability of divers, again due to their excess drag.

Just a few things to think about.

SeaRat

Subtidal Clambed Survey006 by John Ratliff, on Flickr
How I dove in the 1970s, with a BCD built into the back of my wetsuit, and a CO2 vest for surface emergencies. I also used a full-face mask at times with my double hose reguator (one only). The weights are built into the Mar-Vel backpack. (Note also that I had hair on top back then. . )

John Photos003 by John Ratliff, on Flickr
Diving solo in Alexander Springs State Park, Florida in 1970, with only our PJ jump tanks and a Mistral regulator with a J-reserve.

JCR & Aqueon by John Ratliff, on Flickr
Studying swimming techniques and ability with the Aqueon in Lake Chalan, Washington, 1972. Note how streamlined I am.

fullsizeoutput_29b4 by John Ratliff, on Flickr
This is perhaps the most advanced scuba unit I own, which has a Scubapro twin post manifold, a Mk V/A.I.R. I regulator and a Mk VII/Pilot regulator. Talk about redundancy; but they are not isolated. And, I have about a 350 psig sonic reserve on the Mk VII. I also use this combo with a Mistral double hose regulator on the center post.

Wow, those are some photos… in fact I watched this documentary just last night (thanks YouTube) and was shocked at the lack of redundancy… when Cousteau entered the Britannic and said how dark it was ’only for a few seconds’ whilst waiting for the camera man and lighting to come in behind him… my thoughts were: what if that light failed? I completely agree that being streamlined is important, but I wouldn’t want to be 400ft deep (in a wet suit ) without a back-up light (the list goes on)… these guys were ground breaking (a.k.a crazy) but we can only thank them for their innovation (antics)…

 

[rjack321]

Sidemount means they'd need to actively drain both sides -- would they bother to check their gas when switching?

Which happened in MX in Grand Cenote about 3 years ago. @Norwegian Cave Diver has an extensive writeup of this double fatality.