Dive Computer No Deco Computations Question

[tursiops]

I recently ran into an older document (can't remember where) that expressed the opinion that you can't get toxed using a standard nitrox mix in an AL80 because you can't exceed MODs long enough to get a hit.

I think this is nonsense. I just played with MultiDeco a bit; 150 ft for 12 mins using 32%. You hit PPO2 of 1.77 for about 10 mins, get a slight deco obligation, CNS total 252%, and use less than 42 cuft (assuming RMV of 0.5 cuft/min). If you backed off to 140 ft but for 20 mins, you get more deco, only hit PPO2 of 1.67, CNS 111%, and still only used 63 cuft. Going the other way, to 160 ft on 32%, for 20 mins, you get 12 mins of deco, hit PPO2 of 1.97, CNS of 910%, and just barely can do it on an AL80.

 

[Belzelbub]

And when would that be the case?

Whenever you've messed up and check your dive computer for ascent guidance and find out that it's checked out.

I agree, it's a rare occurrence. My comment was to imply that while it's not a a frequent occurrence, it's important when it's needed.

 

[Nemrod]

I have never heard of a DM recommending this. In fact, in my experience, it is quite the opposite. I have seen DMs absolutely anal about the MODs--sometimes absurdly so. A DM in Bali panicked if we took 32% below 60 feet. A DM in Cozumel would not let me use 32% on a dive planned to 80 feet because he was afraid I might momentarily lose buoyancy control and plummet below the 111 foot MOD.

As for getting O2 toxicity on a NDL dive using an AL 80, I recently ran into an older document (can't remember where) that expressed the opinion that you can't get toxed using a standard nitrox mix in an AL80 because you can't exceed MODs long enough to get a hit. You don't tox just by straying below the MOD--it takes a while. All the cases I know of are on technical dives, and the violations were significant.

I can only say what I know and what I have seen more than once, in fact, routinely in some places. It makes my OCD meter peg.

 

[Bento.Box]

Messed up in what way?

I agree, it's a rare occurrence. My comment was to imply that while it's not a a frequent occurrence, it's important when it's needed.

But that's almost never the case.

if it happens, how do I know as a diver that the computer switches over to an algorithm than is designed / proven to provide the best ascend guidance in terms of not getting DCS after a severe deco violation? (example: you're already surfaced and there's medical treatment including a chamber nearby - should you then follow your "superior" diving computer's guidance back to depth instead of leaving the water and starting oxygen treatment while informing medical services?)
Does it also take all the other risks into account? As I probably have a fatal equipment malfunction and / or I'm severely injured if I cannot control my depth at all, which resulted in both bottom and ceiling violations for extended periods of time.
But if it doesn't take those risks into account, then I have to fall back to my emergency strategy either way.

Don't get me wrong, there's plenty of diving computers superior to Suunto in some ways (that's why I also bought my Ratios), but not coming with an arbitrary emergency algorithm that you probably won't be able to follow anyway is not really a big concern for the majority of recreational divers in my experience. In contrast, the abysmal handling of multiple tanks, is something that people care about.

 

[Belzelbub]

Messed up in what way?

There are examples in this thread. Starting a deco stop, then going down to retrieve an item. The Suunto didn’t like it. My Shearwater and Garmin would recalculate without some mythical emergency algorithm that you are talking about.

I honestly couldn’t connect the rest of your post to what I was talking about.

Again, only Suunto and AWU seem to think it’s a good idea for an in-water lockout. The rest will continue to provide ascent information without an emergency algorithm.

 

[Bento.Box]

There are examples in this thread. Starting a deco stop, then going down to retrieve an item. The Suunto didn’t like it. My Shearwater and Garmin would recalculate without some mythical emergency algorithm that you are talking about.

That will not lock you out.
It will continue to calculate and probably your deco stop time will extend.
No problem with the Suunto.

I honestly couldn’t connect the rest of your post to what I was talking about.

Why?

Again, only Suunto and AWU seem to think it’s a good idea for an in-water lockout. The rest will continue to provide ascent information without an emergency algorithm.

Again, not a concern for a diver.

Providing ascend information without switching to a medically proven emergency algorithm would concern me on the other hand, as the information could be false. E.g. Bühlmann only calculates tissue saturation.

 

[LI-er]

As for getting O2 toxicity on a NDL dive using an AL 80, I recently ran into an older document (can't remember where) that expressed the opinion that you can't get toxed using a standard nitrox mix in an AL80 because you can't exceed MODs long enough to get a hit. You don't tox just by straying below the MOD--it takes a while. All the cases I know of are on technical dives, and the violations were significant.

^^This. There is not a single reported incident of CNS 02 toxicity in a diver diving a single tank with a nitrox mix of less than 50% within recreational depths.

The whole CNS 02 toxicity thing is so overblown it's not much more than a myth.

 

[Alekseolsen]

That will not lock you out.
It will continue to calculate and probably your deco stop time will extend.
No problem with the Suunto.



Why?



Again, not a concern for a diver.

Providing ascend information without switching to a medically proven emergency algorithm would concern me on the other hand, as the information could be false. E.g. Bühlmann only calculates tissue saturation.

Can you please explain, in normal words, how the correct response to beeing 1 feet shallower than your deepest stop ceiling (which on the SUUNTO is dangerously deep) would be unsafe? Simply continuing calculating to reach target GFhigh is the correct approach, the Suunto recommendation of ascending to safety stop depth is in certain conditions attempted manslaughter

 

[Belzelbub]

Why?

It had very little to do with the conversation.

You were talking about going back down after surfacing, gear malfunctions, etc. None of which is what was the subject of conversation. I've repeatedly stated that, while I prefer a dive computer that won't lock out, I do understand the reasoning for a computer that will lockout for certain violations AFTER the dive.

The discussion was around in-water lockouts. These are during the dive, not while the diver is on the surface.

And, I really don't know what you were trying to say with the gear malfunctions. Yes, of course, sometimes there are gear malfunctions or other hazards that may necessitate a deviation from the ascent profile. Only one computer will know all that, and that's the one between the diver's ears. I'm simply asking that my dive computer continue to provide information. Clearly that's not too much to ask as mine do. I have 5 dive computers in the family at the moment. 3 Oceanic, a Shearwater, and a Garmin. All of them continue to work during the dive. The Oceanics will lock out post dive due to violations.

 

[dmaziuk]

Providing ascend information without switching to a medically proven emergency algorithm would concern me on the other hand, as the information could be false. E.g. Bühlmann only calculates tissue saturation.

You know a "medically proven emergency algorithm"? Pray tell.

The purpose of the dive computer is to get the diver out of the water not bent. It follows that

a) if two different models get the diver out equally "not bent", all the extra sophistication in one of them is indistinguishable from snake oil, and

b) if you push yourself outside of the computer's "not bent" range, the computer failed to fulfill its purpose. It's not a "medically proven emergency", it's a FAIL.