Buying the Octopus - does it need to be same model/similar?

[lowwall]

BTW, one small advantage of using a basic non-pneumatically balanced second stage is that it will most likely freeflow well before a balanced design. This means if you get 1st stage IP creep due to seat damage, mechanical blockage or freezing, your basic second will freeflow before the reg you are breathing from, and whilst it does that, you can keep breathing happily from your primary as the secondard effectively vents the freeflow.

And of course, if you just doing no-deco Rec diving, then you know using your secondary will be at most limited to some tens of minutes of use, so having it adjusted up a bit tighter (ie at a higher cracking pressure) means it's less likely to freeflow as you enter or leave the water.

Don't these two suggestions negate each other? Detuning the secondary will decrease the likelihood of it freeflowing before the primary. I also suspect that there is a greater likelihood a first stage issue will negatively affect a lower performing second stage before or more than a higher performing one. Therefore, if you are mismatching second stages for this purpose, you should be using the (optimally tuned) lower performing one as your primary.

But honestly this strategy appears to be suboptimal. It would require a first stage to fail in a very particular way that still delivers air, but either at a low enough pressure or somehow contaminated with particles of just the right size to block one type of second stage, but not another. I'm really struggling to see how such a scenario would evolve. Increasing the ability of your secondary to freeflow also seems to be a poor strategy compared to regular checking of the IP. Even if your reg does freeflow, you can still breathe it until you surface or reach a buddy.

In high reliability system that are part of a redundant backup design, it's very common to ensure your individual redundant systems are not designed, and these days, not coded, by the same team, for the very reason of avoiding systematic faults.

I'm not sure this is applicable here. First off, it's not software. Second there's a point at which systems are simple enough and well understood enough that duplicates are acceptable. For example, you wouldn't bother specifying two different brands of adjustable wrenches for your emergency toolkit. And regs are simple mechanical devices with each type having been subjected to millions of operating cycles to expose systemic flaws. NASA apparently agrees with this, I just checked and it appears they use identical mechanical primary and secondary oxygen regulators (POR and SOR - photo below) in their EVA space suits.

The SOV stores ~1.7lbm of usable oxygen which is then fed to the SOR, an identical regulator design to the POR, a two-stage motor-settable mechanical regulator capable of being remotely set to ~8000 set-points between 0 (shut-off) and 8.4 psid with a nominal set pressure of 3.7 psid. - https://ntrs.nasa.gov/api/citations/20170009505/downloads/20170009505.pdf [emphasis added]

 

[MaxTorque]

Er, i feel you may be rather over thinking this!

All things being equal, ie proper servicing, best replacement and repair strategy, best checking, best everything, faults can still occur. And using different regs means there is less likely to be systematic failures that affect both regulators simultaneously!

The case of IP creep or IP sudden rise after 1st stage failure is, after freezing, almost certainly the biggest cause of freeflows when actually underwater. Mis adjustments and poor servicing are as likely to show up prior to the dive, when you air-on your rig, or when you first jump in and shock everything.

Having a secondary that actually prevents your primary from freeflowing, in a REC single tank single 1st stage scenario is for me preferable. It means the over pressure is safely released, no chance of burst hose, it means i don't actually have to do anything but thumb the dive. And it means i actually get an early warning of things going south because it's much easier to notice a new stream of bubbles from your secondary as it hangs unused, than from your in-use reg.

The fact that the secondary is simpler, has less moving parts, also (probably) means it is more robust overall.


But by all means, use the same reg for both primary and secondary, i very much doubt you'd ever see a situation where that similarity mattered, but critically, i can't see any significant issue with using non-similar regs (other than spares commonality, which as i said, for a 2nd stage is a couple of O rings and a diaphram and perhaps a mouth piece, so not exactly costly or heavy/bulky)