Really stupid question... Why dive thirds with doubles?

[Cave Diver]

So I get that diving thirds or sixths or rock bottom or any similar gas management strategy is designed to make sure you have enough gas for you and a buddy to resolve an emergency and then get home together.

The basic assumption is that you are planning for your buddy to lose all of his gas at the worst possible moment. This makes lots of sense on singles. But why do this with manifolded and isolated doubles? Is it really possible for him to have a catastrophic loss of all of his back gas?

I have a better question.

Why not dive 1/3's?

 

[rjack321]

The basic assumption is that you are planning for your buddy to lose all of his gas at the worst possible moment. This makes lots of sense on singles. But why do this with manifolded and isolated doubles? Is it really possible for him to have a catastrophic loss of all of his back gas?

If you have a manifold, burst disk or neck ring go at max pen AND you have an instantaeous shutdown of the isolator (not possible but simplify your assumptions here). You now have exactly enough gas to exit yourself without sharing.

We plan of 1 major failure at the worst possible time. Buddy completely OOG (can happen) or something like a burst disk. Either one is shaving the line close enough. If you need more gas bring bigger tanks or bring stages, don't cut into 1/3rds.

This assumes some sort of flow assisting your exit. In no flow caves you can't dive 1/3rds with just 2 buddies (3 people have more resources). In siphons even more judiscious gas management is required.

 

[ucfdiver]

Silt-outs,navigational errors, and possible medical issues (cramp, or something worse requiring buddy assistance to the surface) are the best reason, a completely hypothetical and unrealistic 100% loss of back gas is the one most often quoted however.

 

[Reg Braithwaite]

Thanks everyone. To summarize, when diving doubles, conservative gas management plans like diving thirds do account for the unlikely possibility of a buddy losing all gas, but they also allow for managing other predicaments that would require additional gas to get home safely like becoming lost or getting silted out.

Great stuff.

 

[LiteHedded]

burst disk is much more likely to go at the beginning of the dive than at max penetration. but we plug or double those anyway.
if you have a neck oring go you should isolate it quickly and use the gas (exiting) while it's draining. it doesn't just disappear all at once magically.

in reality I know of three sets of doubles that fell out of moving trucks directly onto the manifold. none lost any gas. a manifold failing just does not happen.
we start with thirds and get more conservative (never less) depending on the system

 

[Tortuga68]

burst disk is much more likely to go at the beginning of the dive than at max penetration. but we plug or double those anyway

Not to derail the thread (that never happens here), but what are the Occupational Health & Safety implications of getting double-disked or plugged tanks filled? My LDS won't touch them

 

[LiteHedded]

none.
IMHO

 

[Cave Diver]

Not to derail the thread (that never happens here), but what are the Occupational Health & Safety implications of getting double-disked or plugged tanks filled? My LDS won't touch them

How do they know?

 

[MXGratefulDiver]

Yea I meant 2/3, but regardless, now you only have 1/3 and your buddy has the other 1/3. Dead.
You aren't going to have a zen like sac rate right after you lose ALL your gas. Its just not realistic. \

It really depends on the circumstance ... for example ...

- is there current, and if so which way is it going?
- are the divers involved experienced enough and calm enough to exit expeditiously ... or are they stressed to the verge of panic or inexperienced to the point of adding more problems during their exit?
- how much sight-seeing did you do on the way in (you for sure won't be doing any on the way out)?

There are a host of circumstances that will go into determining whether or not you make it out ... it's never as simple as saying "do this or you're dead".

And there are a great many overhead situations where diving thirds simply isn't enough (syphons, for example) ... which is why it's so important to understand the reasons why you apply the rules that you do, because you may find yourself in a circumstance where it makes sense to modify them during the dive (i.e. turn the dive early) ...

... Bob (Grateful Diver)

 

[boulderjohn]

It is not just logic that accounts for this rule. In the 1970s, cave diving deaths were analyzed to see why people were dying, and new procedures were developed in response to the identified causes.

The following is a quote from Basic Cave Diving: A Blueprint for Survival, by Sheck Exley:

Not surprisingly, the NSS has found that failure to allow at least two thirds of the starting air supply for coming out and handling emergencies has been a close second to not using a continuous guideline as a prime cause of cave diving fatalities. (p. 12)