Diving 32% Nitrox with "Air" Algorithms

[arew+4]

There may be some misconception in this thread about what a NDL means, whether on a table or a computer. Yes, there is a lot of individual physiology and specific dive variability, but the way the NDL is chosen is NOT to put a line through the middle of all that variability, but rather use smaller NDLs to allow for most (not all) of that variability so that most (not all) people and dives will be "safe." Adding conservatism means to make that table/computer NDL even smaller, i.e. farther away from all that variability.

I get the impression some think the table/computer NDL is through the middle of the variability, so that adding conservatism means to make the NDL smaller...i.e. move it away from all the individual and dive-specific variability.

Here's a picture to try and explain:

View attachment 771924

I would think most of the folks on this board have at least a passing knowledge of statistics, I don't recall what the confidence interval is for the Navy tables or the various models used by computer manufacturers but would have to guess it's 95% or greater. The people I am referring to have demonstrated the model does not always work for them.

 

[tursiops]

The people I am referring to have demonstrated the model does not always work for them.

Any references for this? Just on the basis of likelihood, fast ascents in the top 30 ft are probably more responsible for minor DCS than using models that don't fit the individual (unless they have a PFO...).

Added: fast ascents violate the models, by the way...

 

[scubadada]

As per @tursiops. Available decompression algorithms are quite safe, conservative if you will. The rate below is 0.03%.

From Decompression Sickness - StatPearls - NCBI Bookshelf:

Epidemiology​

The incidence of decompression sickness, fortunately, is rare. Estimates for sports diving are three cases per 10,000 dives.

 

[dmaziuk]

... All that is known is that the model has failed them, therefore reducing the exposure while tabulating in the same way would reduce the risk of re-occurrence.

Is it known? Did it fail? The model comes with X% chance of clinical DCS (and contrary to the popular belief, the effect of lowering your High GirlFriend is not known to appreciably change that chance). Were they bent well within that X (in which case the model worked exactly as advertised)?

 

[L13]

What I am suggesting is that if a person's individual physiology does not fit the model, adjustments to the model may not adequately compensate for the these deviations as they are not well characterized. Do they uptake faster, do they off gas slower, is their deviation depth dependant? All that is known is that the model has failed them, therefore reducing the exposure while tabulating in the same way would reduce the risk of re-occurrence.

If they want to "reduce the exposure while tabulating in the same way" they should use lower GF's or more conservative settings. Using false O2 content is not "tabulating in the same way". If they don't trust the model as designed, why would the trust it with a false O2 content. Just by using it, they are implicitly saying it is generally correct, and just needs adjustment. Adjust it the way it was designed to be adjusted.

 

[Gandalf-the-Diver]

I have a dive buddy I sometimes dive with. He always uses Nitrox and his computer set to air. Out of all my dive buddies, he is also the only one I know, who actually has been "bent", and had the helocopter ride over to Vancouver, so he could go on a chamber ride. Ever since then, he uses Nitrox, but air setting, giving him a margin of safety. He does not want to go for another chamber ride.

 

[scubadada]

Is it known? Did it fail? The model comes with X% chance of clinical DCS (and contrary to the popular belief, the effect of lowering your High GirlFriend is not known to appreciably change that chance). Were they bent well within that X (in which case the model worked exactly as advertised)?

Perhaps from very very low to very very very low

 

[L13]

I have a dive buddy I sometimes dive with. He always uses Nitrox and his computer set to air. Out of all my dive buddies, he is also the only one I know, who actually has been "bent", and had the helocopter ride over to Vancouver, so he could go on a chamber ride. Ever since then, he uses Nitrox, but air setting, giving him a margin of safety. He does not want to go for another chamber ride.

He is wise to be more conservative. Not so wise in the the way he is accomplishing it.

 

[arew+4]

He is wise to be more conservative. Not so wise in the the way he is accomplishing it.

So where is the lack of wisdom in his approach, and do not say you shouldn't lie to your computer. Conservatism adjustments are arbitrary modifications to the calculations, either adjusting a variable or a bounding condition. Also lying to the computer.

 

[tursiops]

So where is the lack of wisdom in his approach, and do not say you shouldn't lie to your computer. Conservatism adjustments are arbitrary modifications to the calculations, either adjusting a variable or a bounding condition. Also lying to the computer.

Perhaps it doesn't matter for his diving, but the generic problem is that he is not tracking his oxygen use, and he is not displaying his O2-related MOD. But I suspect you already knew that, so why are you asking?