My understanding on this, and I am a novice and not a cave diver, just always seeking to understand: the rule of thirds is not always conservative enough, and a rule of fourths might be better? I don't fully understand the path since I'm not a cave diver, that part was confusing. They turned to go back, and then turned again to try and get to a different exit?
My OW instructor is an experienced cave diver and she taught me the rule of thirds, just for my regular recreational diving. It just gives an extra cushion for your trip, and if your SAC has dropped during the dive (as you settle in or whatever) you can dawdle more once you get close to your ascent area.But always better to have that time extra at the end of the dive, just in case.
Disclaimer first: always get the right training, don’t use the below (or rest of SB) as a surrogate ...
with that out of the way, a few comments for non-cave trained divers to put the rule of thirds into context:
Let’s say that in a team of two, you swim into a cave using rule of thirds. At maximum penetration, ie the point you would turn around as prescribed by the thirds gas plan, you then both have 2/3 of your gas. Let’s say a catastrophic failure occurs just then and one of you loses all their gas. Then, on rule of thirds, only one of you has 2/3, the other has 0. You would just by the bare width of a hair both reach the exit with totally empty tanks ... if, and that’s a big IF, you do this dive in dreamy wonderland. That’s because at the time of failure you have exactly enough gas for the same consumption it took you to swim in without a single second deviation. Any slightest change, such as needing some time to deal with the problem, or thinking about navigation or the quite possible case that you or your buddy start pumping some adrenaline and your consumption goes up ... you’re not making it back. (In reality there are some tiny buffers such as time saved not picking up reels/jumps/... but for the sake of a simple in-out dive to illustrate the point here, it’s less than marginal). Hence it follows that even under ideal conditions, rule of thirds is very aggressive for a team of two. It does check out much better for a team of three. There are generally accepted rules in place for how to manage gas in a two person team, but thirds alone isn’t one of them.
Now if you add to this the fact that the team in the incident had several other things going against them:
- they were diving a siphon, so in the open water equivalent that’s like going with the current for the first half of the dive and swimming back against the current. Now the current in this cave (which in caves we call flow), is very minor, but it is there. So you can see that if you turn on thirds, and you would have that catastrophic loss at max penetration, there would be no way to make it out alive even under perfect conditions otherwise.
- they were spending significant time filming also on the way out and thus possibly slowing their exit more than their pace on the way in. Again in this setting the rule of thirds does not work in a two people team.
You might wonder why the example above on “catastrophic gas loss”, as this is fairly improbable and didn’t occur in the incident, but it’s the way cave trained divers ought to think about gas planning. In this incident there likely wasn’t a catastrophic loss, but their reserves still weren’t enough to cover the other things that went wrong ...
(Without wanting to risk an off topic discussion, but to close to loop to the open water example you referenced above ... well it depends on the environment in open water. If it is mandatory that you make it back to an upline, be it because of heavy commercial boat traffic or the risk of getting lost in open seas ... then gas planning works like in a cave. It’s different if you could surface at any time and survive, although possibly with some inconvenience).
So my rule of thirds is good for me, in my recreational only no deco dives that, in a catastrophic occurrence I could go to the surface in the emergency.
