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

[plaamook]

I always turn my **** on. Always. Check the spg.
Good friend of mine nearly deaded himself twice not doing that at work. Ended up in the pot second time. Otherwise he’s a great diver. So there’s something.
I don’t have an octo coming off my 12. No point. But pony Reg has a spg and 2nd stage is hung like a standard octo because that Reg is set up for indi twins. So it’s v easy to grab.

Shut down drills...I had a funny thing where I was training for comm cert and we’re doing endless emergency drills. Anyway, they shut off the gas AND sent me off w and empty bailout. I switch over on the block and nothings there. Stuck in a helmet. That was funny...

 

[plaamook]

The challenge with "the classic" large single with attached 3 litre pony is remembering to turn both on. It *is* possible to have a separate SPG that's visible and your pony's regulator necklaced; your checklist on the boat will be to breathe from the pony reg and watch the gauge, then breathe from the main tank and watch that gauge. There's also the third "alternate air source" coming off your main cylinder.

One of the great things about twinsets and sidemount is you've just two regulators to worry about.

My personal system of choice for solo diving off the beach is sidemount every time. You don't need massive cylinders, those 8.5 litre long tins are sublime - 17 litres at 240bar is loads of gas. This means you've a single smaller tin to carry to the beach and go back for the other. You've then got full redundancy, ease of access to the valves, everything's in front of you and it dives amazingly well.

But even if you dive sm or indi twins/pony I still want to know why you’d need access to the valves.
You turn em both on and jump in the drink. Why do I need to reach the valves at that point?
I assume you’re thinking...in the event that you didn’t turn both on but...just do it. Like you remember keys and so on. I don’t know how people don’t turn em on and look at the spg on the spot when they set their kit up like clock work. Just don’t get it.
Takes all sorts I guess.

 

[DAJ]

Unfortunately today's diver can't swim

This level of the “today’s diver” is rather scary to me.

Also...why would I...or anyone... dive with a redundant supply switched off? Genuine question.

And their music is too loud and their style of clothes is bad and by the way, stay off my lawn...

Relax folks, it's all part of the evolvement of the sport and while you might see it as a detriment, newer divers might view it as progress. Like others, I became involved in diving in the late 70s and things were a bit different back then. In many ways, equipment, techniques, and overall education through the application of the Internet has been a positive change. Reflecting back on those early years, it's a wonder more of us made it this far as there were many trial and error dives that you basically figured out on your own unless you grew up around a large diving community.

To say every diver that learned to dive in the past 10-20 years can't swim or is scary to imagine diving is a rather short-sighted view. As with my department, after 30 years and all of the recent changes that I did not agree with, it became apparent that it was my time to retire and move aside and let the newer generations take control and move it forward. I dive the way I choose and others should do the same. While I am plenty capable of making any dive with the most rudimentary of gear, modern advances make it more convenient. I dive basically the same gear configuration regardless of how "easy" the dive is scheduled to be. The only real changes are wet suit versus dry suit and single cylinder versus doubles. All of the other gear is carried the same and in the same location so whether I am in my wet suit or dry suit, I know exactly where everything is located. If I am running doubles chances are I'm doing deco so my usual slung 40 with 28% is switched out with a 40 of 100%.

In regards to switching off a redundant air supply. There are some that subscribe to the practice that leaving the valve open can result in a free-flow or even a leak from the 1st stage that might not be noticed if diving alone or even with another. While I leave my valve open and can easily see/solve an issue as the cylinder is slung on my left side, others choose not to do so. That is fine as long as they understand and practice the procedures of getting that valve on when needed.

These days, this is what I dive most of the time. A 130 with a Y valve with double dip tubes. I sling an Al40, wear two computers along with a SPG. Essentially, three first stages and three second stages on every dive. Two lights, two cutting tools, lift bad, SMB, wet notes, and an extra mask. Major overkill for some but that it what I am accustomed to diving with and yes, I can swim it all to the surface without a problem.

 

[Wibble]

But even if you dive sm or indi twins/pony I still want to know why you’d need access to the valves.
You turn em both on and jump in the drink. Why do I need to reach the valves at that point?
I assume you’re thinking...in the event that you didn’t turn both on but...just do it. Like you remember keys and so on. I don’t know how people don’t turn em on and look at the spg on the spot when they set their kit up like clock work. Just don’t get it.
Takes all sorts I guess.

Know where you're coming from: if independent, then just leave the thing free flowing as you've got the other side to bail out with.

But... Why not turn off your reg so it can thaw out and then continue the dive? Or you've been a complete numpty and forgotten to turn it on before jumping off the boat (maybe time to give up diving?)

 

[BlueTrin]

But is a twinset redundant if the isolation valve is open? I understand the need for redundancy in hardware but does that redundancy still exist if we are creating essentially a single air source?

I guess what I'm really getting at is does the redundancy afforded by doubles do anything to mitigate user error? Does gear configuration reduce the chances, or in this case the consequences of a diver using up all their air because they are task saturated, narced, exerting themselves harder than they expected, or just not paying attention?

You should dive manifolded twinset only if you are able to isolate the tanks by closing them fairly quickly.

If you are unable to do this then you are just diving a big 24l cylinder.

Manifolded twinset has the advantage that if you close the left of right post you can still access all the air from the other regulator.

 

[BlueTrin]

But even if you dive sm or indi twins/pony I still want to know why you’d need access to the valves.
You turn em both on and jump in the drink. Why do I need to reach the valves at that point?
I assume you’re thinking...in the event that you didn’t turn both on but...just do it. Like you remember keys and so on. I don’t know how people don’t turn em on and look at the spg on the spot when they set their kit up like clock work. Just don’t get it.
Takes all sorts I guess.

You can feather the cylinder if the first stage leaks.

You open enough to get a breath and close. Because a sidemount tank valve is easy to reach this is fairly easily doable and allow you to access the gas in this tank.

You can do this with a stage as well for your deco gas on a twinset.

 

[Brett Hatch]

Do sidemount / independent doubles solo divers follow something like a rule of thirds? Meaning, turn the dive when you have 2/3 gas remaining in each bottle. This way, the worst case is that halfway through the dive, one bottle fails, and your remaining bottle will have as much gas in it as you've used so far, so should be enough to get you home safely.

With manifolded doubles, you could turn the dive at 1/2 gas remaining to achieve the same effect, which lengthens your dive time by a factor of 1.5.

Now, obviously, turning at 2/3 gas may be overly conservative, or turning at 1/2 gas may be insufficiently conservative -- that all will depend on the nature of the dive. But in the event that one source of gas fails catastrophically, manifolded doubles with an isolator gives you access to more gas than a sidemount / independent doubles / pony ever will. That is the main advantage over alternative configurations. Whether that advantage is worthwhile to you depends, of course, on a lot of things.

 

[John C. Ratliff]

Do sidemount / independent doubles solo divers follow something like a rule of thirds? Meaning, turn the dive when you have 2/3 gas remaining in each bottle. This way, the worst case is that halfway through the dive, one bottle fails, and your remaining bottle will have as much gas in it as you've used so far, so should be enough to get you home safely.

With manifolded doubles, you could turn the dive at 1/2 gas remaining to achieve the same effect, which lengthens your dive time by a factor of 1.5.

Now, obviously, turning at 2/3 gas may be overly conservative, or turning at 1/2 gas may be insufficiently conservative -- that all will depend on the nature of the dive. But in the event that one source of gas fails catastrophically, manifolded doubles with an isolator gives you access to more gas than a sidemount / independent doubles / pony ever will. That is the main advantage over alternative configurations. Whether that advantage is worthwhile to you depends, of course, on a lot of things.

My understanding of the “Rule of Thirds” is that you use it mostly for cave or wrecK diving, use 1/3 of the supply for the dive and turn it at 1/3. Now you have 2/3 of the gas remaining to return to the surface, and you surface with 1/3 remaining in the tank as a reserve. That’s the “Rule of Thirds” as I understand it.

SeaRat

 

[Wibble]

Do sidemount / independent doubles solo divers follow something like a rule of thirds? Meaning, turn the dive when you have 2/3 gas remaining in each bottle. This way, the worst case is that halfway through the dive, one bottle fails, and your remaining bottle will have as much gas in it as you've used so far, so should be enough to get you home safely.

With manifolded doubles, you could turn the dive at 1/2 gas remaining to achieve the same effect, which lengthens your dive time by a factor of 1.5.

Now, obviously, turning at 2/3 gas may be overly conservative, or turning at 1/2 gas may be insufficiently conservative -- that all will depend on the nature of the dive. But in the event that one source of gas fails catastrophically, manifolded doubles with an isolator gives you access to more gas than a sidemount / independent doubles / pony ever will. That is the main advantage over alternative configurations. Whether that advantage is worthwhile to you depends, of course, on a lot of things.

Sidemount rule of thirds…. Breathe both sides down to 2/3rds and turn the dive**.

Kaboom! One side explodes/whatever.

Now you have half of 2/3rds left, i.e. one third left. Leave the cave/wreck in an orderly fashion, do not stop to pick up your belongings. You live to dive again.


If the Kaboom! affected your buddy then you simply donate the longhose and unclip that cylinder and give it to your buddy. Now you both have one third so stop hanging around and get the heck out of there.




** other considerations may apply such as current direction, etc.

 

[Brett Hatch]

My understanding of the “Rule of Thirds” is that you use it mostly for cave or wrecK diving, use 1/3 of the supply for the dive and turn it at 1/3. Now you have 2/3 of the gas remaining to return to the surface, and you surface with 1/3 remaining in the tank as a reserve. That’s the “Rule of Thirds” as I understand it.

SeaRat

Right. And I should be clear, I don't do penetration dives. But for solo gas planning on indies / sidemount, it seems like a reasonable starting place to me. I guess the pony bottle version would be, if you have a 40CF pony bottle, turn the dive when you've drank 40CF out of your main cylinder.

Sidemount rule of thirds…. Breathe both sides down to 2/3rds and turn the dive**.

Kaboom! One side explodes/whatever.

Now you have half of 2/3rds left, i.e. one third left. Leave the cave/wreck in an orderly fashion, do not stop to pick up your belongings. You live to dive again.

Do you dive solo with sidemount / indies / a pony bottle? And if so, do you plan your gas with a rule of thirds, or something else? I am mostly interested in an answer for open water here, but cave/wreck pen is interesting as well, even though I don't do those dives.