Emergency bailout strategies for recreational diving

[AfterDark]

I guess I'm see something in diving ID's w/o manifolds that other don't. I use them the same way as if they had iso-valves. I usually surface with 500 psi in both tanks on a solo dive or buddy diving 500psi in one and 1000 psi in the other in case my buddy needed it. I don't need to reach valves or remember which one is on or off or struggle to carry them when not on my back. The only drawback is I could loss half my gas without being able to shut off a valve, but since I can't reach the valves iso or not, it's the same for me.

 

[Akimbo]

I guess I'm see something in diving ID's w/o manifolds that other don't. ...

I wouldn't read too much into comments about manifolded doubles. If you go through a failure mode analysis, charting out every conceivable failure, occurring at different times of the dive and under different conditions, the slight advantage over independent doubles starts to emerge -- especially with multiple failures. Slow undetected leaks can be the most insidious. However, the probability of most of those failures is so remote that they start to approach having the cylinder explode on your back... OK slight exaggeration but you get the idea. They get more critical as hard or soft overheads enter the equation, but the likelihood doesn't increase.

IMHO, the one big advantage of progressive equalization is the reliable hard reserve, no matter how much you get distracted by events or crisis. Your SPG or computer can be 1000 PSI off, visibility so bad you can't read it, or you lose track of time observing a Nudibranch -- you can't miss the signal it is time to equalize and a way-point has been reached. IDs or a pony provide the same primary advantage, but not the redundancy or the way-point signals.

Your failure mode analysis should include what happens if your isolation valve leaks. Unlike an old 300-500 PSI J-valve reserve, you are not likely to misjudge your air consumption by 100%. Meaning if your isolation valve is leaking you will overshoot your estimated equalization point, but probably not before you become suspicious. You could also add an SPG to the regulator on the reserve/isolated side if you are concerned or you profile is critical -- you could see if the [-]online[/-] offline cylinder pressure was dropping or not. Dropping equals a leaking isolation valve.

In the end, you have to judge for yourself what is safe enough for every dive. Let's face it, for most recreational dives a capable diver will probably never even need an octopus, let alone a backup cylinder and whole regulator.

 

[rx7diver]

IMHO, the one big advantage of progressive equalization is the reliable hard reserve, no matter how much you get distracted by events or crisis. Your SPG or computer can be 1000 PSI off, visibility so bad you can’t read it, or you lose track of time observing a Nudibranch — you can’t miss the signal it is time to equalize and a way-point has been reached. IDs or a pony provide the same primary advantage, but not the redundancy or the way-point signals.

Akimbo,

What gives me pause about progressive equalization is the fact that the off-line tank is effectively a stand-by system, and stand-by systems make me a bit nervous, since they are being counted on to just work when they're needed. For example, suppose you breathe down the on-line tank and then attempt to equalize only to discover that you cannot (because you've somehow lost the gas that was is the off-line tank)!

Have you considered this? Maybe you were including this scenario when you stated, "Slow undetected leaks can be the most insidious."

Ronald

P.S. One reason why I prefer to sling my buddy bottle, instead of backmount it, is the fact that I can actively monitor this stand-by system.

 

[DaleC]

This is my concern with a blind sided system as well though I can see the easy fix being to just add an spg to that side. I prefer the ID value of complete seperation but I have also found having two distinct tanks a bit of a hassle on some charters. They tend to want to charge me for two fills even when I use half from each tank. I've thought of manifolding my ID's for this reason (ease of filling) but still diving with the isolator turned off. For my kind of diving I just don't want to trust to having to do "something" in order to preserve gas. I can also see progressive isolation as a way to avoid reg swaps but I've always found that to be a real world way of ensuring both regs are in performance condition. You wouldn't (or perhaps you would) believe how many divers I see who enter the water without even breathing their octo's once - and that's the reg I'm supposedly going to use... I think not.

 

[rx7diver]

DaleC, at this point in my life I've decided that for my deeper solo recreation diving I'll generally use smaller independent doubles if boat diving (say, from a six-pack or larger boat), and a single with slung buddy bottle otherwise (especially if I have a long and/or difficult shore entry). For my solo rec dives no deeper than 50 feet or so, I'll generally use only a single tank with no pony/buddy bottle. Part of my reasoning parallels some of what's been discussed in this thread. But part of my reasoning deals with other concerns as well. For example, I would really hate it if I fell while attempting a rocky and/or otherwise challenging entry/exit wearing independent doubles. And I would hate it if I were rolled by the surf while wearing independent doubles.

 

[Akimbo]

…What gives me pause about progressive equalization is the fact that the off-line tank is effectively a stand-by system, and stand-by systems make me a bit nervous, since they are being counted on to just work when they're needed....

That is the argument favoring a pony bottle that is not used unless there is an emergency. However those can also have slow undetected leak(s) and not be there when needed. So the next step is a set of isolation doubles AND an independent pony. Each backup adds additional options, failure points, complexity, and weight. And then there’s this guy:

 

[aquaregia]

Is there any concern about the rapid pressure change with progressive isolation? In my double 72s I realized that I was diving with my manifold closed once and when I cracked it slightly (to check) and equalized the temperature difference was really noticeable to the touch. I had about 500psi fly across in perhaps 5 seconds tops.

 

[Akimbo]

aquaregia: Concern, heck I snuggle up to the baby every time it warms up! Seriously, I don't think I understand your specific concern. Temperature differential, especially underwater, has no functional impact. So what if both tanks are not temperature stabilized after you stop hearing gas flow and the isolation valve is closed?

I can't imagine any metallurgical or differential expansion problem that isn't far greater at the filling station.

 

[aquaregia]

I suppose it's just that I was always urged to keep my fill rates under 600psi/min, but I suppose that was just to make sure I got a good fill.