Shearwater Perdix Computer Users running Open Circuit...question?

[Slow]

Medium. Always run out of air before NDL.

 

[admikar]

Tec mode on 40/80, so I'll call it on medium.

 

[DevonDiver]

What's interesting is that the tech mode default is 30/70, which is more conservative than the rec HIGH

That'd be an interesting insight to the mindset of technical divers... who don't care about NDL, but do care about decompression profiles.

 

[rb9228]

I've kept mine on medium setting and I've always been using the recreation mode.

 

[blac86]

30/70 for deeper coldwater dives (Great Lakes), 35/85 for shallower warm water (Mexico). If I left it at 30/70 in Cozumel, I'd drive the rest of the boat crazy with my deco obligations on many dives. Still more conservative than DSAT on my backup computer.

Can I ask why your low gf is so low? You're putting yourself in a situation where you are doing deep stops that cause slow tissue ongassing while your fast tissues offgas ineffectually. If you bumped that number up closer to 45-50 you would be able to do more deco where it is more effective, in 3ata or shallower. Not to mention most of your deco will be on your 6m and 3m stops where you can be on 100%. So if you ran like 45/70 you'd load most of your deco in the 6m and 3m stops and the rest would be in 21m or under so you could run 50% bottle and a 100% bottle. It's still on the conservative end of profiles and significantly more effective at decompression

 

[Boaty McBoatface]

What's interesting is that the tech mode default is 30/70, which is more conservative than the rec HIGH

Well, as I understand it, the tech default's GF-high of 70 is more conservative than the rec mode's GF-high of 75 on the conservative setting, but arguably tech's default GF-low of 30 is less conservative than rec's 35, despite the conservative intention behind using lower GF-lows. (It depends on what you think of deep stops.) On the other hand, the GF-low on a properly executed rec NDL dive is supposed to be irrelevant.

 

[Boaty McBoatface]

Some observed data points while diving my Perdix in groups with other computers, in case this helps anyone:

Perdix rec mode on medium conservatism seems to yield shorter, first dive (zero residual nitrogen) NDLs than many peers using other models (notably, Suuntos; on default), but NDL often isn't the limiting factor on first dives.

Perdix rec mode on medium then yields longer NDLs on subsequent dives.

Perdix rec mode on low conservatism seems to yield similar, first dive (zero residual nitrogen) NDLs to many peers using other models (notably, Suuntos; on default), so if you're diving deeper with people using other computers, and NDLs are becoming a limiting factor, you can match
up with them by setting Shearwater conservatism on low for the first dive and switch to medium conservatism thereafter.

 

[blac86]

Some observed data points while diving my Perdix in groups with other computers, in case this helps anyone:

Perdix rec mode on medium conservatism seems to yield shorter, first dive (zero residual nitrogen) NDLs than many peers using other models (notably, Suuntos; on default), but NDL often isn't the limiting factor on first dives.

Perdix rec mode on medium then yields longer NDLs on subsequent dives.

Perdix rec mode on low conservatism seems to yield similar, first dive (zero residual nitrogen) NDLs to many peers using other models (notably, Suuntos; on default), so if you're diving deeper with people using other computers, and NDLs are becoming a limiting factor, you can match
up with them by setting Shearwater conservatism on low for the first dive and switch to medium conservatism thereafter.

I would hesitate to recommend this without first achieving a working knowledge of decompression theory and what the differences between the three levels actually is

 

[Boaty McBoatface]

I would hesitate to recommend this without first achieving a working knowledge of decompression theory and what the differences between the three levels actually is

My gut shares that sentiment with you, and more knowledge is better, of course. On the other hand, let's think through that feeling:

On Dive 1, you're configuring one computer (Perdix; within its recreational presets, no less) from Medium conservatism to Low in order to emulate the default output of another computer (e.g. Suunto) which is widely used successfully without creating DCS problems.

Then, on Dive 2, you're changing your computer (Perdix) back to be *more conservative* than it was on Dive 1. The Perdix hasn't forgotten your first dive, so it knows what your tissue loading is, and on Dive 2 the Perdix doesn't care what your conservatism settings were before you went into the water for Dive 1, it only knows exactly what your body was exposed to, and it calculates your Dive 2 NDL based on your residual loading at that point in time and your then-current conservatism settings (GFs).

Gut feelings aside, it seems to me that in order for there to be a "problem" with doing this, either Suuntos (considered conservative) set to default must be bending people pushing NDLs on their first dives (not what we see happening) or Perdixes (Buhlmann 40/85) must be bending people doing repeat dives (also not widely observed).

The takeaway would seem to be that, relative to its peers, a Shearwater set to Low conservatism really isn't particularly Low on a first dive (only), but even set on Medium conservatism a Shearwater is *relatively* aggressive for repetitive dives. Or, you could say the same thing the other way around -- Suuntos on default (et al./RGBM) are *relatively* aggressive on first dives and relatively conservative on repeat dives. High/Medium/Low conservatism designations and default settings are somewhat arbitrary, and the models are just estimates, but we don't have widespread diver reports of problems for either, so there shouldn't be a problem with gaming a Shearwater in this way.

But hey, I'm green and certainly not a technical diver. Maybe I'm missing something (in which case I'm excited to learn).

Cheers

 

[RainPilot]

While I agree with everything you wrote above @Boaty McBoatface, I worry that people are missing the primary reason for conservatism and indeed GF themselves, which are to all intents and purposes a conservatism setting with a lot more granularity.

The idea is that, for a given diving day (cold, workload, fatigue, hydration etc) and a given person (BMI, vascularity, cardio fitness, PFO etc etc) there is a given level of conservatism required to maintain the same DCS risk that a pure Buhlmann /DSAT/RGBM etc would produce in a member of that testing group (young, very fit Navy types usually)

Thus, if on a given day you should be using Med, then using Low to match someone’s NDL is a bad idea. If you’re ok with Low, then why not use it throughout?

There’s a lot more to setting conservatism than just maximising available dive time.

Besides, if your buddy goes beep then it’s time to get back on the boat, regardless of who has what set.