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

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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.
Your planning is to have enough gas to safely finish the dive should Kaboom! happen.

Things to consider
  • an elevated breathing rate — it’s stressful!
  • any decompression obligation
  • depth - affects gas consumption rates
  • where is your first stop; is that a decompression gas or is it the surface
  • any additional time required for the peril; overhead, return to shot line
  • Contingency reserve
This gives you an idea of the additional time required, thus the additional gas required to get you out of there.

You then need that amount of gas in both cylinders such that if either Kaboom! fails, you’ve enough in the other side.

This could be two thirds, it may be more or less.
 
Do sidemount / independent doubles solo divers follow something like a rule of thirds? ...
@Brett Hatch,

My solo diving is recreational, whether shallow or moderate-depth (or deeper depth). So, no planned deco, and no overhead for me, solo.

For me, I bailout to the surface. For my shallow dives, if my regulator ceases delivering gas, I ascend directly to the surface. For my moderate-depth dives I use a Y-valve, and if my regulator ceases delivering gas, I switch to the other regulator and ascend directly to the surface.

I monitor my SPG and gas plan constantly to ensure that I do not run out of gas. My gas plan always ensures I have enough gas to ascend safely, directly to the surface, leaving a safety margin.

When I was doing deeper solo dives, I wore independent doubles and used a gas plan that allowed me to safely ascend on one cylinder if something put the other cylinder out of commission. If something causes a delay at depth when diving deeper, you can very quickly go into mandatory decompression, so my gas plans planned for this.

rx7diver
 
...These days, this is what I dive most of the time. A 130 with a Y valve with double dip tubes. ...

View attachment 724885
@DAJ,

Is that one of those rare-as-hen's-teeth Beuchat DIN Y-valves? And you're using it on a HP cylinder (since TP5250)? If so, then schweeeeeet!

rx7diver
 
Yes it is. I have a couple of them that I use and two extras that are euro models that I keep for spare parts if needed. The cylinders are HP 130s

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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?
Having not yet read many other replies other replies:

I personally chose the sidemount path because:
  • Complete redundancy. Independent tanks and independent regs. (You can technically do that backmount, having the tanks and regs separate.)
  • Reduced weight on my back, while climbing in and out of water. A single 80AL tank on my back, climbing into a boat with waves was enough to occasionally give me a slightly sore back.
  • While side-mount has a decent learning curve, long-term I just love it for some unknown reason.
When I dive, I currently do either 2x 80, or 1x80 and 1x 19 (sidemount, with offset sizes). The 19cu should be enough to handle normal solo-emergencies at my normal depths; typically 30 to 60 feet, but rarely up to 100.

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.
I had always assumed 19cu should be enough for up to 130ft with safety stop, assuming no severe panic. That said, if you're rescuing others, I can definitely see a larger (30 to 40) coming in handy.

My friends who own 30 and 40cu tanks seem to never bring them on dives though. The tank you have on you, is always better than the larger tank you left behind. I suppose it could just be their own personal laziness. I should sometime try a 30 or 40cu, to see if it's "inconvenient" enough.
 
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.
I get where you are coming from but i believe there is more to it than that. The rule of thirds is used in cave and technical diving as a 1/3rd in, 1/3rd out and the other 1/3rd is for your buddy in case they need to share on the way out. It works out in sidemount that the same rule also gives you the 1/3rd for your exit even if an entire side is lost.
I personally would use the bad reg side by feathering whenever possible so that i have more than enough gas to exit.
Also in the case of manifolded doubles, if i use half my gas then loose a tank valve o-ring and require isolation, i now only have 1/4 my gas left (not a common situation but we are mentioning “what ifs”).
To the OP’s question (yes i know im a month late to the party) the human error is not the primary consideration at higher levels of diving. Training and hard set rules are supposed to mitigate negligently draining tanks. However i have always considered sidemounts “passive redundancy” to be superior to twinsets “active redundancy” because even when the human factor creates problems (not checking guage or not noticing free flows) sidemount will give you a second chance and twinsets will kill you.
And to the old guys saying to just swim to the surface and considering pony bottles to be for those who lack skill… if you are at 100’+ then your likely hood of swimming up WAY to fast is crazy high. Thats why lots of divers got bent back in the day and agencies created slower ascent concepts and redundant air… i like knowing i can freedive to 100’ but i’d still rather bail onto a pony than CESA after a long day of diving.
 
I get where you are coming from but i believe there is more to it than that. The rule of thirds is used in cave and technical diving as a 1/3rd in, 1/3rd out and the other 1/3rd is for your buddy in case they need to share on the way out. It works out in sidemount that the same rule also gives you the 1/3rd for your exit even if an entire side is lost.
I personally would use the bad reg side by feathering whenever possible so that i have more than enough gas to exit.
Also in the case of manifolded doubles, if i use half my gas then loose a tank valve o-ring and require isolation, i now only have 1/4 my gas left (not a common situation but we are mentioning “what ifs”).
To the OP’s question (yes i know im a month late to the party) the human error is not the primary consideration at higher levels of diving. Training and hard set rules are supposed to mitigate negligently draining tanks. However i have always considered sidemounts “passive redundancy” to be superior to twinsets “active redundancy” because even when the human factor creates problems (not checking guage or not noticing free flows) sidemount will give you a second chance and twinsets will kill you.
And to the old guys saying to just swim to the surface and considering pony bottles to be for those who lack skill… if you are at 100’+ then your likely hood of swimming up WAY to fast is crazy high. Thats why lots of divers got bent back in the day and agencies created slower ascent concepts and redundant air… i like knowing i can freedive to 100’ but i’d still rather bail onto a pony than CESA after a long day of diving.
The agencies are training to the lowest common denominator, and since scuba courses have been depleted of hours, that is pretty low. For those divers who lack the skill in the water, then the redundancy you mention is a very good thing. But excluding overhead environments, diving with this much redundancy can cause other problems.

First, you have twice as much equipment to monitor. Here human factors comes into play, and the more equipment one has, the more possibility of a human error. It was mentioned above that a pony bottle needs a SPG (submersible pressure gauge). Why? This increases by a factor of two the pressure gauges one needs to monitor. If a cylinder is checked for pressure (we used to use a pressure gauge to check pressure before the dive, as during the dive we had none), three is very little possibility that the cylinder will loose pressure during the dive.

Second, there is an issue with streamlining. I dive a high current river, and any extra equipment adds drag. In pristine environements without current, this probably is not much of a problem, but in a river, it can make quite a difference.

Yes, I’m one of the old guys you mention, but let me tell you of the course that Midge Cramer set up through NAUI and Oregon State University for college credit. It was a 3 credit hour course, which lasted a term (3 months). During that time, Midge had the students constantly in the water, and had hour-long lectures on diving physics, decompression theory, etc. So students getting their Basic Scuba card at the end of his course were assured to be competent divers.

You state, “…if you are at 100’ + then your likelihood of swimming up WAY too fast is crazy high…”. Not if your are properly trained. 100 feet is 4 atmospheres absolute, meaning that the air density at 100 feet is 4 times denser than at the surface. That also means that upon a CESA, the air in one’s lungs expands to four times its original volume. In addition, you have residual air in your lungs even upon full exhalation. Let’s say that I have a complete regulator malfunction, and there is no air to breathe upon exhalation. And, instead of turning to a pony bottle, I decide to simply swim to the surface. I know that the air in my lungs will expand as I ascend. I therefore have no problem simply going up with my mouth open, and as expanding air becomes a lungful, I can exhale the excess (yes, there ill be excess).

I did this as an experiment in my local pool; simulate complete loss of air supply in the deep end (16 feet deep, as this was a competition diving pool) by turning off the air during a doff, exhaling and trying to inhale without air pressure in the regulator. I then headed up, but not to the surface. I simulated a depth of 60 feet, and decided to swim horizontally the sixty feet up towards the surface. I actually experienced some air expansion, as the depth of 16 feet allowed an increase of my lung full of air. I had no trouble swimming in a slow, controlled manner, that sixty feet to the surface. What you describe the “…likelihood of swimming WAY too fast is crazy high,” is actually the perception that the ascent will be not a Controlled Emergency Swimming Ascent, but rather a panic ascent. That’s because the ascent will be by someone who lacks the training and water skills to accomplish that ascent, and that probably accurately describes you in that situation too. There is no reason to panic, as swimming to the surface as a real way of handling this situation.

Let me share a part of diving history, JY Cousteau’s words in his book, The Silent World. On page 180 of the hardback copy he talks about Frederic Dumas‘ diving course, which is pretty detailed but taught by example. Here’s what Captain Cousteau describes:
At the end of the course the honor students swim down to a hundred feet, remove all equipment and return to the surface naked. The baccalaureate is an enjoyable rite. As they soar with their original lungful, the air expands progressively in the journey through lessening pressures, issuing a continuous stream of bubbles from puckered lips. Cousteau, J.Y. with Frederic Dumas, The Silent World, Harper & Brothers Publishers, New York, Copyright 1953, page 180.

So not only is a CESA possible from 100 feet, Jacques Cousteau described it as an “enjoyable experience” for highly trained divers. What we have now is divers who are not highly trained, have minimal water skills, and are not relaxed in the water. Agencies responded with equipment “redundancy” rather than training. Carry this tank and simply switch, rather than not carrying a spare tank and relying upon water skills. The equipment manufacturers and dive shops jumped on this, as they could sell more equipment, and make more money.

SeaRat
 
Well after reading this entire thread I'm guessing that no one here is going to like my solution for redundancy but it's what's worked for me for many years and I'm sticking with it.... Keep in mind that I dive mostly solo, cold water, drysuit and on 32% so pretty much keep all my dives within 110ft.

I have HP Steels in both 102 and 120cf. I mostly use the 102's. I use a back mounted 19cf Pony fully ON prior to and during my entire dive. The Pony has a fully redundant dedicated regulator and SPG....and is basically a buddy on my back with an always full 19cf tank.

I have an Aqualung Air Source that serves as an octo if I ever need to share my primary tank(which would be unlikely in a solo scenario) but it also gives me a back-up to my main tank in case of a 2nd stage failure.

I have an Air Integrated Oceanic Pro Plus 3 because I'm old, it has BIG numbers and I don't trust radio waves to tell me my air pressure. I also have a back up hose mounted US Divers "Puck" computer. If the Oceanic ever failed, then my dive is over anyway and I'm headed to the surface with my eyes on my puck and and my right hand on my Pony 2nd stage.

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Well after reading this entire thread I'm guessing that no one here is going to like my solution for redundancy but it's what's worked for me for many years and I'm sticking with it.

Your solution is fine with me and I have used very similar rigs many times. Humans can endlessly debate an "optimum" solution but that does not mean the other options are bad... unless maybe you want to be this guy:

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Being too overburdened to have fun is an automatic fail.
 
Your solution is fine with me and I have used very similar rigs many times. Humans can endlessly debate an "optimum" solution but that does not mean the other options are bad... unless maybe you want to be this guy

Everybody's n-dimentional terrain map of suitability is different (including "n".) Solutions will differ. That may be inconveneient, but is not necessarily a bad thing.
 

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