When thirds are not enough....

[rjack321]

Let's say two divers do a dive to thirds in a low flow system

Here's the logical error right here. Whether or not they recalculate for a second 600psi penetration is irrelevant.

Having 1/3rd reserve in a low flow system is not enough to exit with, period.

 

[BabyDuck]

what's worse is the 'rule of thirds' i hear discussed in threads about ow diving - a third out, a third to explore, and a third back up. NO! this is NOT the rule of thirds! if you can't stand to surface with a full third, so be it - putter around at the anchor line, go up really slowly, have a 20 min hang at 20 ft. (remembering rock bottom, of course.) but don't make up a fake-ola 'rule' about it.

 

[MarineResearch]

But everyone is forgetting about the air pockets trapped at the cave ceiling. Who needs a stinking reserve when you have so much free air available. I saw it in a recent cave diving movie. It must be true.

The rule of thirds is a convenient starting point for ideal conditions. I would rather have a shorter dive with extra air remaining than the alternative. If there is really something special that you want to see, you can develop a plan and go back later with extra gas.

 

[LiteHedded]

if you have a catastrophic gas loss at max penetration it's probably game over. let's be honest.

if you start going down these roads you can plan yourself right out of a dive. just something to think about

 

[Cave Diver]

if you have a catastrophic gas loss at max penetration it's probably game over. let's be honest.

Since we're being honest, how many people have had catastrophic gas losses at max penetration? I can't recall ever hearing of one which is why I think a lot of people don't plan for it. You have better odds of an asteroid blocking the entrance to the cave. (ok a slight exaggeration, but you get the idea).

It's more likely that the catastrophic gas loss will be someone who screwed the pooch finds themselves blowing through a ton of gas. Of course a lot of people rationalize "that will never happen to me because I don't do X, Y or Z" so they still don't plan for it so it's still game over.

 

[SuPrBuGmAn]

if you start going down these roads you can plan yourself right out of a dive. just something to think about

That may be for the better, if you didn't know how to plan the dive in the first place.

 

[LiteHedded]

That may be for the better, if you didn't know how to plan the dive in the first place.

agreed. rules of thumb like thirds can be used as an autopilot for people who don't want to think their dive through from the bottom up.

you have to look at the risks and decide what you're willing to accept and go from there.

 

[MarkJS]

Since we're being honest, how many people have had catastrophic gas losses at max penetration? I can't recall ever hearing of one which is why I think a lot of people don't plan for it.

Ken Fulghum & Sheck had a bit of a moment way back in Tenerife, if I recall.

HTH

 

[rjack321]

what's worse is the 'rule of thirds' i hear discussed in threads about ow diving - a third out, a third to explore, and a third back up. NO! this is NOT the rule of thirds! if you can't stand to surface with a full third, so be it - putter around at the anchor line, go up really slowly, have a 20 min hang at 20 ft. (remembering rock bottom, of course.) but don't make up a fake-ola 'rule' about it.

The esteemed DAN magazine committed this error this month. I forget the exact title of the article but its about deep dive "gas planning".

 

[Cave Diver]

Ken Fulghum & Sheck had a bit of a moment way back in Tenerife, if I recall.

HTH

So that's one example, from 1983. I still think the uncommon nature of this event is why more people don't take it under consideration.