Thirds is Still Nonsense

About a year ago I wrote about a dive planning tactic I both teach and use which turned out to be surprisingly controversial. Honestly, I’d thought it seemed sort of obvious to the point of mundane; but it turns out that I am an idiot who doesn’t know what they’re talking about. The short form of THIS article is that when diving in a team of two I always dial back from using full thirds to ensure there is a safety buffer of usable gas to swim out of the cave. Whether I’m an idiot or not I have continued to both teach and plan dives this way regardless of gear configuration. Over the last week, however, while teaching a student sidemount cave techniques, something occurred to me about how this method of gas reserve is primarily applicable to backmount divers. It is assuming a total loss of breathing gas which, albeit remarkably unlikely, does exist as a possibility. On sidemount, however, the de facto isolation of breathing supplies renders a total loss of breathing gas an even more remote eventuality. In principle, in sidemount, should a diver lose one regulator, they would switch to their other tank, having reserved enough gas in that tank to make it out of the cave still independent, without having to gas share at all. So a diver descends with 3000 psi/210 bar in a pair of AL80s. After using 1000 psi/70 bar the diver turns and, what do you know, BANG! The left regulator erupts into a torrent of bubbles. When the team confirms that there is no fixing whatever the problem is, the diver still has 2000 psi / 140 bar to swim out of the cave, which is exactly how much they need. Exactly how much. No room for slower swimming. Very little time to try to fix the problem. No safety buffer for getting caught in a restriction for a minute or two. No wiggle room for when the diver starts glancing at their dwindling gas supply and starting to worry that the needle is getting awfully low, wondering if they’re actually going to make it, and starting to breath heavier with increasing dread. Therefore, shaving things awfully goddamn close. It’s also worth considering that, in sidemount, some of the places we’re moving through don’t necessarily disqualify the ability to share gas… but they can sure make the exercise pretty bloody difficult.


So I start thinking that while a similar safety buffer as outlined by the attached article would be damned useful, on sidemount a buffer as large as proposed might be heavy-handed. One is, after all, only reserving gas for one’s self, able to continue to move independently through the water, which takes far less effort than a requisite gas share on backmount. Maybe back off by 100psi or 10bar? Leaving a safety buffer of 300psi or 30 bar? Seems like that ought to be plenty to tolerate a bit of wiggle room and a bit of peace of mind during a long swim getting increasingly lopsided. I’d maintain that this is worth applying only in a team of two. The idea of two divers both suffering a regulator loss is improbable, but has been known to happen. The idea of three divers all losing a reg… we’re getting into the odds of being struck by lightning while scratching a winning lotto ticket.


A third diver with a donateable gas supply can offer a long hose to one diver for 5 or 10 minutes, then to the other diver for 5 or 10 minutes at some controlled point in some spacious length of cave to ensure that everyone has a comfy safety buffer. As opposed to waiting until both “injured” divers are down to the very dregs of their good tank in a tiny, crawly space - only a short distance from the exit - where there’s no room to even turn around, much less have a three-person meeting on a solution. There’s a zen koan that goes something like this: A man goes wandering up into the woods to find some master with whom he wants to study. Coming on an old man sitting by the side of a river the traveler asks the man if he knows where the master is. “Oh yes,” says the old man, “He lives on the other side of this river. If you carry me across I’ll show you the way.” Grumbling about the annoyance of the task, the would-be student hefts the old man onto his shoulders and starts across the river. Half-way across, suddenly, the old man forces the traveler underwater and holds him there with strength belied by his seemingly frail body. The man thrashes and fights and struggles to get free. But to no avail. The old man is simply too strong. As he yells in despair and is seconds from taking a lungful of water the old man lets him free and drags him to shore. “Just then,” asks the old man, “Just before you drowned, what was the single most valuable thing? The only thing in the entire world you truly wanted?” Realising that he was talking to the master he sought, and understanding washing over him, the student answered, “One more breath.”


Going forward when I’m one of two divers, the both of us on sidemount, I’ll be using more modest padding. But pad the gas supply I will. 100 psi works out to maybe 5 minutes of dive time. And knowing my buddy and I will definitely get home safe is well worth 5 minutes of penetration to me.


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