Updated: Aug 7, 2020
"Should I bring my rebreather to Mexico?" This is probably the single most common question I get asked.
That is the shortest possible answer.
"Unless you want to," is, perhaps, the best possible answer.
Now buckle up for the long answer. Keep your head inside the ride at all times and do not monkey with the safety bars lest you slip out of the car, downward through a spiral of logical loop-de-loops, and into consequent madness.
Rebreathers are awesome, right? They automatically make you more awesome when you're wearing one. I'm actually not being tongue-in-cheek here. Rebreathers are, legit, awesome.
"I am a rebreather diver," has a certain cachet.
Not only do they take advanced training and practice in their safe use; they take a great deal more practice to maintain proficiency. The ability to master the increased complexity, failure points, and control required to dive a rebreather come only with time and experience.
So just to be wearing one is already a statement, of sorts, that you are no mere dabbler. The of time and money committed to the initial investment alone is something of a status symbol.
"Not only can I afford this thing," you proclaim simply by donning your rig, the full, proud declaration is, "I am no vacation diver - I dive as frequently as possible, because diving is life."
If that rebreather looks... shall we say... well-used. Well that adds another layer to the message, doesn't it?
This is not to say that the main reason everyone wears a rebreather is because they want to strut about like a horny peacock.
The quiet is enchanting. As is true neutral buoyancy. A nice, warm breathing loop is really comfortable (certainly in cold water or on long dives). The ability to make certain types of dives not only accessible, but arguably safer, makes it a potentially valuable tool. The reduced decompression stress and time on the body can be just as valuable. The peace of mind that comes with knowing you have about a dozen different ways to troubleshoot a problem and extricate yourself from potentially dicey situations allows a broader clarity of awareness in many ways.
And besides all that... they're just plain fun.
Because, when it comes right down to it, it really is just a really expensive toy. And taking your toys out of the box to play is fun.
But rebreathers also aren't just toys. They're also life-support equipment. The repercussions of a model train set malfunction do not include, "terrified suffocation to death," the same way they do on a rebreather.
No need to dwell on the obviousness of this point, but it is worth mention that there are still instructors out there that discuss rebreathers in the context, "This machine is trying to kill you; your job is to not let it."
Personally, I think that is a bit melodramatic, but there is a grain of truth.
I bring it up simply to remind that there is a good deal of complexity to using a rebreather. Significantly more than simply sticking a reg in your mouth and watching your depth, time, and gas supply.
The root of almost all the additional complexity of a rebreather has to do with PO2.
(Non-divers, I'm amazed you made it this far and have no idea why you're still reading, but now will probably be a good time to just tune out completely. Divers who have not taken, at least, a nitrox class... why? Stop reading, go enroll in a nitrox class. It can be done almost completely online and we both know you've been looking for some sort of diving thing to do.)
Oxygen is a stupid element for life to have evolved around. It's super caustic. It's hyper-reactive to everything. It makes everything be on fire really easily. And we're stuck with it. But we can only really survive a narrow band of exposure.
Between a PO2 of .16 to 1.6 -- outside of that we risk unconsciousness or death. Even with relatively short exposures outside that slim margin, which leaves little margin for error.
As a rebreather diver, what you are doing ALL THE TIME is monitoring to make sure you keep your PO2 exactly where you want it. The rebreather is not trying to kill you; the rebreather is just going to do its best to do what you told it to (which is, generally, maintain a specific PO2), assuming nothing breaks.
At depth, especially at a consistent depth, this is actually pretty easy. Assuming nothing does break, diving a rebreather is kind of a breeze.
As you start to get shallow, especially as you start to get shallower than just a few ATA and the volume/pressure differentials start to get more marked...
The volume difference ascending from 120 to 115 feet is almost nothing. The volume difference between 20 feet and 15 feet, however, are far, far greater.
Getting back to the original question (you thought I'd forgotten, didn't you):
Mexican caves are, mostly, shallow. Very shallow. Some caves your average depth will wind up being 15'.
When you ascend from, say, 20' to 15' in these caves a few things happen all at once:
- The volume in each airspace increases. Wing, drysuit, and CCR loop. Everything gets bigger all at once. Which, naturally, makes you more buoyant.
- So you've got to dump gas from all those airspaces to maintain neutral.
- Focusing on the rebreather loop, as the pressure on the loop is reduced and as the volume increases, the PO2 actually drops. Less ambient pressure = less partial pressures.
- The rebreather, now, sensing a drop in PO2, will react by adding oxygen.
- Which increases the loop volume even more making you more buoyant.
PSYCH! It was just a breakdown pile. You're now going to swim back down to 20'. Which means you need to add gas to all those airspaces again. And rebalance your PO2 to where you want it.
Now................ do that for a few hours.
Yeah, it all kinda becomes second nature. But only kinda. And this is also assuming a fully electronic rebreather - with a manual or a CMF rebreather some of those issues are lessened a bit. But even if it doesn't take all your focus to manage all that, you are spending your whole dive mostly manipulating airspaces. Burning through diluent, by the way, at a rare pace.
Oh, and by the way, when was the last time, as a rebreather diver, you practiced swimming on total open-circuit bailout with a pair of AL80s (as are the most common bailout tanks here)? You know they go super light when they start getting empty, right? Are you weighted properly to stay off the fragile ceiling without having to swim yourself down (and use up a ton of extra gas due to the effort)?
Even if you're not on bailout, if you're diving a configuration that uses offboard as diluent, and (perhaps) not used to all the up-and-down, you could be unbalancing your tanks and reducing your bailout radius without really noticing.
Remember what I said about an average of 15' around these parts?
What's your gas consumption rate? How long a penetration swim, at 1.5ATA do you figure you'd get out of 50cf/1400L?
If I'm in the zone, I'll turn a dive after about 90-120 minutes (that's still with another 90-120 minutes swim back out). If I'm on backmount I don't even need to switch regs. The whole dive is: Reg in mouth, swim and look around.
A little bit simpler, dontcha think?
I suppose, if we're doing something a little deeper, can add a smidge of complexity by strapping on a stage.
That said, there are places that require a second stage or maybe even some O2 as deco gas. Then we are actually getting into, "maybe the rebreather is the right tool for the dive" territory.
Or maybe you just want to dive it because it's your rebreather and you friggin love it. And that's OK, too.
I'm lucky that I get to switch configurations a lot and maintain proficiency on a lot. If the only thing you've been diving is your rebreather for several years... well, I'd suggest still doing a bailout drill from time to time, but yeah, you're probably going to be safer and a in a better position to enjoy your vacation if you're on your rebreather.
So, "Should I bring my rebreather to Mexico?"
No. You don't really need it. There is a LOT of cave where the additional complexity just isn't worth the bother.
Unless you want to. In that case, cool. Gimme the heads up to help you sort out sorb and O2 and then it'll give me the excuse to bring you to caves where the rebreather is better suited and those are awesome, too.