Reg setups: a Philosophy Question

[Nemrod]

Independent doubles and pony use (slung or attached to main tanks) was more common in prior times because there were no isolation manifolds in existence or at least widely available. The manifolds of those times could isolate regulators but not tank from tank. The pony or independent double rig was of course always isolated. A lot of newer divers don't understand that manifolds were not always isolating types. With an isolating manifoldthe argument for independent doubles is greatly dimenished but has no impact on the single tank diver who still may wish a redundant air supply, thus the pony/spare air arguments forever and ever.

An AIR II is a bad idea, if it fails you loose power inflation of the BC (no big deal) and your secondary for your buddy and if you keep an octapus as well for that possibility then you have now greatly incrased potential failure points. N

 

[munitor]

These all have something to do with various configurations...

Fault tree analysis and HFE in Rebreather design
http://www.rebreather.ca/Library/Underwater_Technology_Paper.pdf

Formal Risk Identification in Professional SCUBA
http://www.hse.gov.uk/RESEARCH/rrpdf/rr436.pdf

Performance of Diving Equipment - After accident analysis
http://www.hse.gov.uk/RESEARCH/rrpdf/rr424.pdf

Perhaps the most astonishing thing was that 2/3 of the tanks examined didn't meet air quality spec, mostly because too much water.

 

[two.crows]

Thanks for the excellent links, munitor !

As for the original question, when I picked up diving again after a long pause, I went through exactly the same thought process. After reading and discussing a lot, I decided to go with the DIR setup on a single cylinder initially (practical arguments overwhelmingly in favor IMO) and would sling a 40cf stage for extra redundancy "where required".

The thinking on "where required" is as follows: to manage risk reasonably, and given systems that have individual failure probabilities on the order of between 1/100 and 1/1000, you need to be able to survive two major system failures. Only the third failure may kill you, but the one-in-million/billion probability would be deemed acceptable. The major systems involved are: two second stages (high failure rates), one first stage (low failure rate), your teammate(fails when he gets separated) and the surface(fails when you can't ascend). It's then easy to see why in situations where you can't reach the surface because of deco obligation or overhead, you have to add a second first stage. And it's equally easy to see that when you dive with an instabuddy who gets lost while carrying your redundancy, this is equivalent to a major equipment failure. If you're diving a situation where a CESA is not a safe option, plus you can't trust your buddy to get himself lost in less than 1 of a thousand dives, that calls for a pony or stage as extra redundacy. However soooo many divers out there regard the limited capacity of the typical pony to be false security, that I have decided to listen to that advice. Plus the stage is more flexible, and can be put to good use elsewhere in a more advanced configuration. But that's just what I think.

Dive safe.

 

[TSandM]

Nice analysis, two.crows.

 

[ams511]

Thanks for the history lesson Pete, it puts things into perspective.

 

[munitor]

...If you're diving a situation where a CESA is not a safe option, plus you can't trust your buddy to get himself lost in less than 1 of a thousand dives, that calls for a pony or stage as extra redundacy. However soooo many divers out there regard the limited capacity of the typical pony to be false security, that I have decided to listen to that advice...

I think you had it right all along, and I consider small pony bottles to perfectly fill that redundancy gap in rig configuration that the 'instabuddy' doesn't satisfy.

Pardon my repeating this, since its undoubtedly elsewhere, but the formula for determining air consumed between two depths, assuming a constant ascent rate and SAC, is:

((a*a*c/66 + a*c) - (b*b*c/66 + b*c))/d

a = start depth of ascent in feet
b = finish depth of ascent in feet
c = SAC in scfm
d = rate of ascent in fpm

For an SAC of 0.7, assuming immediate switch to pony (backup reg on a necklace) that works out to 5cf to ascend from 90' at 30fpm with no safety stop but a very reasonable 30fpm ascent rate. A 6cf in such a situation is marginal, a 13cf should be more than adequate. As long as the protocol is immediate safe ascent this approach works. I think its interesting that the BSAC recommends a redundant pony bottle rig using a minimum 13cf bottle as part of the basic OW scuba configuration:

http://www.bsac.org/uploads/documents/Diving_Safety/safe_diving_06.pdf
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Alternate air sources (Air and nitrox diving)
The BSAC strongly recommends that all divers carry an Alternate Air Source (AAS) in the event that they may need to share air underwater.

Suitable AAS include an 'Octopus Rig' (a second 2nd stage fitted to the same 1st stage of the regulator, which includes the type fitted to a direct feed). In this case divers should be aware that a first stage failure on their regulator will affect both second stages and may render them in-operative.

A more strongly recommended alternative, however, is a totally independent air supply such as a 'Pony cylinder' (a small 2-7 litre [13-40cf] auxiliary cylinder attached to the main cylinder with its own regulator assembly) or separate regulators attached to each cylinder of a matched pair. If a manifold is fitted to the pair of cylinders, making them into a 'twin-set', it should allow the diver the ability to isolate each cylinder / regulator assembly should a failure occur. Auxiliary cylinders having a capacity of less than 2 litres [13cf] and BC mouthpieces are not considered adequate AAS.
------------------------------------

Maybe that's partly why the BSAC has a fatal accident rate for its divers no higher than for the aggregate US bunch, even though they spend a lot more time in cold, low vis, high current waters.

 

[H2Andy]

Maybe that's partly why the BSAC has a fatal accident rate for its divers no higher than for the aggregate US bunch, even though they spend a lot more time in cold, low vis, high current waters.

do we know what percentage of dive fatalities are caused by OOA situations?

i recall reading somewhere that most divers who die die with gas still in their tanks and fully functioning regs

(i could be misremembering that)

 

[Gilldiver]

What you may not be taking into account is the change in stress level any event that causes you to HAVE to go onto the pony will cause - out of air, failed reg, buddy problems.

Just about all divers SAC will be 2 to 3 times normal. So, a 13 is just too small. A 30 is about the best for most deeper sport diving as it will allow for a full deco stop - because the event that made you go onto the pony/stage may have also made you over stay your bottom time and put you into deco.

Finaly - in a real stress situation don't let some fear of a quick accent screw you. When everything goes south remember, I can't do anything about you dead on the bottom, but I can do something with you bent on the surface.

 

[H2Andy]

Finaly - in a real stress situation don't let some fear of a quick accent screw you.

i've always thought about this. in a recreational format, a quick ascent may bend you, but it won't bend you to the point of killing you.

holding your breath on the way up, however, will ... so it is imperative that you exhale the whole way up.

that's really my fear, not getting bent

 

[daniel f aleman]

You can never have too much gas. Figure what you need, then double it, and then get the right bottles to fulfill the specific needs of the dive profile.