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Thirds is Nonsense

Continuing on the theme of how math is on my mind these days I'm going to talk about how diving to thirds is bloody reckless. And how you can make 1 equal 3. Now, when I say thirds is silly I certainly don't mean to dive PAST thirds. That's just outright stupid. What I mean is that I hear an awful lot of people invoke thirds as though there is something mystical about the number 3; a talisman to keep them safe as cave divers. Even, and this is specifically why I made the accompanying graphics to illustrate, when people are diving in a team of two. As I have been given to understand it, the rule of thirds was written with a fairly specific set of parameters: team of three, outflowing cave system, no anticipated loss of visibility, no restrictions to have to navigate with the cumbersome burden of sharing gas, nor anything else that might slow the exit in any way. Outside of these parameters, thirds is not conservative enough. And I know you heard all that in your cave class, about how you might want to add some conservatism. Or sure as hell should have.

"Thirds" don't keep you safe. Appropriate gas planning keeps you safe. Yet the number of people I see discussing thirds as if it is the be-all and end-all of dive planning is alarming. Or hear them, here in Mexico, where everyone is using AL80s, not even taking a moment to discuss things; just reporting to eachother, "My tanks are full," and going on to assume they can use 1000psi/70bar without actually doing any simple arithmetic as a team. If you're in a team of three and you're not diving into a siphon or a mudhole, god bless. Take the additional half a moment to actually discuss it, gas match, then go to thirds and enjoy the dive. But even if every other condition or criterion is perfect, I want to talk about that team of two... which is what I see, to my horror, all-too-often.

So our two divers each use 1000psi when they both discover, at the same time, they have the exact same RMV and have both hit their turn pressure. We're speaking loosely here, and using pressure instead of the more appropriate measure of volume (and imperial pressure, at that), but this makes the mental image easiest. If they each used 1000psi, that means they each need 1000psi to go home safely. So when Diver 2's tanks explode, that isn't a big deal, because they do still have 2000psi. In theory - enough. But that's exactly enough and not one breath more. It assumes that if they could turn around without any interruption or delay they would be hitting the surface again at the exact moment the now-shared tanks deliver their very last breath. No extra time for gas-sharing procedure. No slowed travel as the two divers are in touch contact and kicking eachother. No lost visibility as they grind through a restriction that is slowing them down because it's tight and awkward while they're tethered together. No autonomic physiological response of an increased heart-rate and consequent increased breathing rate in either diver in the excitement of the moment, nor with the increasingly urgent fear of, "Oh my god, are we even going to make it?!?!?!" In short: all but hopeless. The chances of the divers actually making it out are shrinkingly slim. Ergo: thirds is nonsense. There is a way to improve our intrepid divers' chances. Increase them by three, in fact. Behold:

By dialling back usable pressure by 200psi we actually gain a 600psi safety margin. Having used only 800psi that means we have, in reserve, almost as much gas as a buffer as we need for the entire exit for one diver. 1:3. And that 200psi of dive time that you sacrificed off your third? What is that, an additional 5-10 minutes of swimming? Maybe less? So what are you going to see, another one or two hundred feet of cave? Seems to me that absolute certainty I and my buddy can go home safely is worth forgoing those extra few feet. Those few hundred feet of cave ain't going anywhere and if I'm still alive I can come back to see them later. Maybe with a third team member. Maybe after a stage class. Big, easy site you know super-well and aren't going very far in? Only dial back by 100psi and have 300psi in reserve. Know up front that you're going to be making a zero-viz exit through several restrictions out of a siphon? Go crazy and dial back by 300psi for 900psi in reserve, then you have even more than you used to get to your turn point. Gauges in metric? Dial back by 20bar to have 60bar in reserve. Thirds doesn't keep you safe. Having enough gas keeps you safe. Thirds is not always enough gas. Again, this all forgoes the additional layer of complexity of the necessity for gas-matching dissimilar tanks. It also ignores the all-too-frequent, potentially fatal misunderstanding that the person with the most substantial RMV should have the biggest tanks (that's a different story for another time). This is meant as a simple illustration that the simple "The Rule of Thirds" isn't so simple. Even more, this is meant as an entreaty: if you're in a team of two, don't dive to thirds. Just don't.

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1 Comment

Apr 25, 2021

Have been explaining this thirds rule to experienced cave divers only to be met with blank stares. It depends on team numbers. Thirds, I believe, was originally meant for boats dealing with fuel consumption. A third out, a third back, and a third in reserve.

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