I am Cavern certified with TDI. I did it because I wanted to dive the cenotes. I did that last year. Due to life, I am not able to go again for a year or so.
When I did it, I did it on a single tank, back mounted.
I want to go longer, and to the maximum depth and penetration. I also want to limit the time spent fumbling with gear on land. I also want some redundancy.
So, the next time I go cenote diving, I want to have 2 tanks.
So, which is better - doubles or sidemount?
Which is "idiot resistant"? (I don't use idiot proof, as things I thought were idiot proof, the idiots went against it.)
The problem with simple questions is they attract difficult answers
First, at word on backmount. Then sidemount. Then my view on what I choose to dive, and why.
I hope it might help stimulate consideration that helps you in your search for answers and a decision.
Backmount
I'll say the "standard" of backmount is the Hogarthian setup, with the isolatable manifold between the tanks. It's clever.
It's clever because for almost all of the critical failures, there is a redundancy.
The exception is the manifold. Roughly put, if you damage the manifold connection, you may lose your system and all your gas.
Most other cases, you swap to a backup and keep all of your gas.
The tanks are manifolded on the high-pressure side. This means that you won't be able to swap tanks around inside the system (there's no working up against a 200bar connection), but it doesn't matter - the tanks are bolted down on your back anyway, so they're staying put. The positive in this is, you don't have to manage opening and shutting valves or swapping regulators around. The gas will balance itself across the two tanks automatically. As I wrote above, it also means that you can reach all your gas from an alternate source, if the other fails (true for first- and second stages along with tank valves, not anything related to the manifold).
Now, you can also set up a single-tank to have the same touchpoints as the twinset in backmount. Your regs will be the same places, your inflators work the same way, the SPG is in the same place. Consistent.
This helps you in your development from one tank to two, and it helps you work together with other divers who have the other amount of tanks on their back.
This, however, is where the limitations begin to kick in. Of course, swapping from one tank to two means pretty much a whole new rig, but what's more, by the time you've got two tanks on your back, that system won't hold any more.
You can bring a third, fourth and tenth tank for that matter, but they'll be separate from the original system.
That's not necessarily a problem, but it does mean there are some limitations, and it does mean that as we progress deeper down or into an overhead, a decreasing portion of the dive is actually carried out on the clever base that we like. There are also some limitations to how much gas we can use from the individual tanks, but that's for another day.
Consider a 75m dive - we'd be on the backmounted rig only around 25% of the dive, and spend around 75% of the dive on individual tanks that are completely separate.
But, the general gist of it is, it's rather standardized - and it's rather simple.
Sidemount
It's complicated.
The reason I say it's complicated is because there isn't a "gold standard" in the dive industry, in the way that one might fairly say that the Hogarthian rig is in backmount. There are quite a few different takes on sidemount out there.
The original reason for going from backmount to sidemount, was to go through narrow constrictions.
To form a narrative, it'd be two divers going cave diving in their Hogarthian rig because it's clever, and they get to a narrowing. One says "oh well, end of cave" and the other says "oh well, end of backmount".
The bulk of sidemount divers use individual singles, which is to say completely separated tanks that have separate outputs (regulators, inflator hoses, SPGs).
The advantage to sidemount, regardless how you do it, is of course it's possible to penetrate narrow constrictions (sidemount or no-mount). But in almost all cases, the consistency from the traditional backmounted doubles, is lost - that is to say, you'll need to apply a different reaction pattern to diving in general, and specific scenarios (read: emergency protocol).
For many, this is the reason not to make the transition ("oh well, end of cave").
Others really don't like having to swap regulators to balance the tanks.
As for myself...
I dive mostly oceanic dives - what that mainly says, is
vertical. The implication is that for most of the longer dives, there's a correlation, in my case, to different gases needed, for decompression.
Personally, I like the double-backmount. It's consistent and well thought out. But it has limitations.
Sure, I don't like dragging it around, but most importantly, I don't like that it's really only [dare I say] "perfect" (in my subjective view) for one particular thing:
Recreational diving between 18m and 30m.
Anything shallower, one tank is enough. Pretty much anything deeper, I want or need at least one more tank.
That's me.
That's why I swapped to the Z-system, which - yes, yes - can do narrow constrictions (sidemount/no-mount), but is:
1) Consistent, and
2) Scalable
I can whack one tank into it, two tanks, three, four or a hundred for that matter, and it'll always work exactly the same way - and procedures will be interoperable with the Hogarthian rig so I can mix up the team or easily swap between sets if I need to (I only have my Z-system at this point, as I've sold everything else by now).
I can even add an additional layer by putting a rebreather on it. Same base.
So, the simple answer?
There is no simple answer, but I will advocate to assume and maintain an attitude that no equipment in the world is even a little bit fool-resistant. Good rig makes it easier for you to use your skills, but it willl never improve them.
So, the best investment you can make, is in
you. Your skills, your training and your diving.
If you're lucky enough to end up with a solution that takes you everywhere, from the start, you're one of few.
That said, if I could go back to my own Day 1 and choose, I'd pick Z-system, hands down.
That's my take on it, hope it's spurred on thought
Happy diving!
