Nitrox and computers

[boulderjohn]

I once did a deco dive in which my buddy brought along his new Suunto HelO2 to see how it would do. We did our dive by following a plan set by desktop decompression software, and he did his best to set the preferences to match. We followed our planned schedule, which did not match the HelO2. It locked up during our 30 foot stop.

 

[EFX]

So don't skip that 1 minute of deco. How hard is that??

Real hard if you're out of gas (see post #55).

 

[lynnpartridge]

I've send Deepblue a few emails saying that I don't think they have created a safe computer and have expressed my concerns about how what they think are "safety factors" are actually incredibly dangerous to the point that I would not be able to use one as a primary computer in a teaching environment and because of that, despite being an ambassador, won't promote it yet. Thankfully it is all programming things that they can update whenever they want, but I'm trying desperately to get them to do it.

I basically said this
Read Shearwater Recreational Nitrox mode
write code that makes your computer do what it can: 3 options for gradient factors, 3 gas mixes with decompression, and NEVER locks you out, for any reason. Continuously tracking your tissue loading regardless of what's going on. Any computer that doesn't do that is dangerous to me because if you HAVE to descent again for some reason, the computer just shuts down and that's a deal breaker for me.

regarding VPM, the reason you have to pay for the upgrade is because Shearwater has to pay royalties for the VPM algorithm, so it is there to cover their fees. Super annoying, but I don't have any faith in that algorithm anyway.

Just a quick clarification. We do not license VPM from anyone. We used Erik Baker's publicly available code as the basis for our own implementation.

 

[tbone1004]

Just a quick clarification. We do not license VPM from anyone. We used Erik Baker's publicly available code as the basis for our own implementation.

good to know, thanks Lynn!

 

[tursiops]

THIS TABLE IS WRONG FOR THE SHEARWATER CALCS....SEE MY LATER POST.

I couldn't resist comparing all my computers in the 60-110 foot range shown earlier.
Assumptions are AIR for all, first dive, no residual N2. Three tables are shown as well.

What did I learn?
SW Rec Mode Low conservatism is even worse than the USN tables.
The Dive Rites and the Zeagle (all made by Seiko) are nearly identical to the PZ+ in the OC1.
Nothing else jumps out at me.

 

[stuartv]

Real hard if you're out of gas (see post #55).

And, like I said in response to that, if that happens to you, should stay out of the water for 24 hours anyway. If your best bet is really in-water recompression, do that. If you are prompt, you should be able to swap tanks and do that before your computer locks you out. If you are out long enough for your computer to lock you out, then even if you do in-water recompression, you should know how much deco you had left to do, do it at least double that while using your computer in gauge mode, and then still stay out for 24 hours. If you skip even one minute of required deco and then want to dive again, I don't have any sympathy for you and your locked computer (assuming your are a Recreational only diver). If you are a certified tech diver and know what you're doing by choosing to skip a minute of deco, then you shouldn't be diving on a Rec computer anyway. But, if you were that, you probably wouldn't have run out of gas in the first place.

Seriously. A Recreational diver who goes into deco and then skips the last bit because they're out of gas? They SHOULD sit out for 24 hours. To desaturate AND to really think about what the heck they are doing. And a tech diver in the same situation (who is diving depending on a Rec computer)? Ditto. Maybe even moreso. A tech diver REALLY ought to know better.

Just my opinion, of course.

 

[tbone1004]

often times skipping decompression time is not in your control due to a myriad of variables. You may have bent the computer at say a gfhi of 70, but you are well within the times of say 85.

 

[stuartv]

I couldn't resist comparing all my computers in the 60-110 foot range shown earlier.
Assumptions are AIR for all, first dive, no residual N2. Three tables are shown as well.
View attachment 378311

What did I learn?
SW Rec Mode Low conservatism is even worse than the USN tables.
The Dive Rites and the Zeagle (all made by Seiko) are nearly identical to the PZ+ in the OC1.
Nothing else jumps out at me.

Interesting. I hope someone from SW will post an explanation of the difference between Rec and Tec with the same GFs. Maybe a difference in building in an expectation of a safety stop or not? If Rec mode assumes you'll do a 3 minute safety stop, I suppose it could allow you that extra minute or two on the bottom. I sure would have expected the NDLs to be the same for the same GFs.

 

[tursiops]

Interesting. I hope someone from SW will post an explanation of the difference between Rec and Tec with the same GFs. Maybe a difference in building in an expectation of a safety stop or not? If Rec mode assumes you'll do a 3 minute safety stop, I suppose it could allow you that extra minute or two on the bottom. I sure would have expected the NDLs to be the same for the same GFs.

Yeah, I meant to mention that. Glad you pointed it out. Surprised me, too.

 

[stuartv]

often times skipping decompression time is not in your control due to a myriad of variables. You may have bent the computer at say a gfhi of 70, but you are well within the times of say 85.

Exactly. That's the kind of thing I was referring to when I mentioned being a tech diver and knowing what you are doing by skipping the last minute. If I'm diving GFhi of 70, I might skip my last minute of deco, if circumstances called for it. Sure.

None of that applies to a Rec diver using a Rec computer. Or really even a Tech diver using a Rec computer (with its proprietary algorithm). We're talking about a computer locking you out for a deco violation. Doesn't that pretty much mean, by definition, it's a Recreational computer? And don't they all (meaning all the ones that could/would lock you out) have proprietary algorithms? I don't have any sympathy for a Rec diver who causes his computer to lock him out and then complains about not being able to dive again within 24 hours. And I really don't have any sympathy for a tech diver who does the same. If you're qualified to second guess and override a computer with a proprietary algorithm, then you should be okay to use that computer in gauge mode for your next dive.