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

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Cthippo

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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?
 

Wibble

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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.
 

rob.mwpropane

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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 I. 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.

This is so true. I've never had to donate or OOA but when I periodically check my psi in a set of doubles and it's WAAAAAYYYYYYYY more than I'll ever need for what I'm doing. It's a warm and fuzzy feeling. I would venture to say my air consumption went down after starting doubles just based on that feeling alone. You really can just sit back and enjoy the show.

Obviously gas planning for when shtf, practicing valve drills every dive, etc. but what a dream to end a dive with 35% or even 50% gas left most of the time. I've never heard the term "gas anxiety" but I like it. I don't have that anymore.
 

Marie13

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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 I. 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 dive SM exclusively. For a quarry bimble, LP50s are fabulous. Nice and light.
06E175C4-E213-4C27-BF80-09D72491BECE.jpeg
 

TravelGas

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There are several people still walking around thanks to my 30 cu ft. Pony bottle. I now have upgraded to a 40 cu ft Pony. On one of those rescues from over 100' FSW, the 30 did run out near the surface. The nearest chamber was hundreds of miles away with no way to get there.

I like redundant systems, they are like life insurance, yours or someone else's.
 
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Cthippo

Cthippo

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So, if I'm understanding this right, you are saying that that a large enough amount of gas is adequate to mitigate the possibility of an OOA situation and that no further steps are necessary? Or, to put it another way, taking enough gas means you don't need a second source of gas?

Is that correct?
 

Wibble

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...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?
Don't want to do a pony bashing session, but it's relevant to mention this.

Firstly define the pony. Where I dive -- the UK -- it's quite common for a PONY to be a 3 litre cylinder attached to the side of a 15 litre tank on the diver's back (for non-metric, that's a small cylinder on the side of a large one).

There's various problems with this:
  • The regulator needs to be available and re-stowable. If you can't re-stow it, you won't test it.
  • The PONY cylinder needs a SPG which you can read underwater. Button gauges simply don't cut the mustard
  • You need to be able to turn the valve on. What's the point of having a PONY if it's switched off
  • The 3 litre PONY with, say, 220 bar yields 660 litres of gas. That is simply not enough when you're breathing heavily and "deep", say 30m/100ft/4ATA, you'd get through that in a very few minutes with a high SAC (30 litres/min x 4 = 120 litres / min of your 660 litres)
  • At the very best a PONY is better than nothing for a fast ascent to the surface. Better than doing a Controlled Emergency Swimming Ascent, but not much better.
A twinset or sidemount means you've plenty of gas and you know it works as you're breathing it.

A larger slung pony cylinder is better than a silly little one, but they are dangle-tastic unless you sidemount them to keep them under control.
 
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Cthippo

Cthippo

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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?
 

Wibble

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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?

Yes. You practice closing this at the first sign of problems, i.e. if you suddenly get a cloud of bubbles around you, you calmly reach back and close the centre manifold to preserve at least half your gas (yes, a bit leaked out, don't be a pedant!) Then work out what to do; which side is leaking, so shut down one side and see if it abates; if not turn it back on and shut down the other side***.


***Most people who do the shutdown drills will inadvertently shut down both sides at the same time. Do it once; learn.
 
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