No-fly in Subsurface?

[Miika Turkia]

On Mon, Jul 25, 2016 at 6:10 AM, Rick Walsh <rickmwalsh at gmail.com> wrote:
>
>
> On 25 July 2016 at 10:33, Linus Torvalds <torvalds at linux-foundation.org>
> wrote:
>>
>> On Sun, Jul 24, 2016 at 4:44 PM, Linus Torvalds
>> <torvalds at linux-foundation.org> wrote:
>> > On Sun, Jul 24, 2016 at 4:33 PM, John Van Ostrand <john at vanostrand.com>
>> > wrote:
>> >>
>> >> So rather than calculate the deco stop at 0 ft for a safe ascent to
>> >> 8000 ft,
>> >> maybe it should do a desaturation to an arbitrary percentage, like 1.5%
>> >> above ambient at 0 ft. Isn't 6 half-times considered desaturated?
>> >
>> > That might be a useful thing to do regardless of no-fly times. And
>> > yes, many dive computers seem to consider the two things the same
>> > (sometimes with a "no-fly is desat time with a minimum 24-hour")
>>
>> Side note: I'm not going to have time to look at it, since the merge
>> window for 4.8 just started, but I'm assuming that it should be fairly
>> easy to just hook into calculate_deco_information() at the end, where
>> we have the tissue saturation data set up for the after-the-dive
>> situation.
>>
>> I *think* you could just solve it analytically by looking at each
>> compartment, calculating how long to desat for that compartment
>> (exponential decay towards surface pressure with the factors for that
>> compartment), and just taking the max time.
>>
>> Sounds like something Robert could do in his sleep in five minutes.
>> Robert?
>>
>> Or perhaps Rick knows what the VPM-B model considers desaturated.
>>
>>
> The VPM-B model uses the same method (ok there's a different assumption on
> effective water vapour partial pressure but it's nearly negligible) and same
> theoretical tissues as Buhlmann to track gas saturation, so the desaturation
> time will be the same.  The difference between the models is in how the
> allowable gradient (difference between current ambient pressure and
> tolerable ambient pressure) is calculated.
>
> Something similar to how Linus calculated the no-fly time could be done with
> VPM-B, but I'm pretty certain it would also end up with a very low no-fly
> time compared to dive computer calculations and guidelines by DAN and
> others.  I don't think we should implement a feature that produces less than
> commonly accepted no-fly times, lest someone relies on it and ends up bent.
>
> I wouldn't be surprised if it could be shown that the Buhlmann method could
> be applied to determining no-fly times, but with a different theoretical
> 'governing tissue' for lower than atmospheric pressure (e.g. flying), which
> is slower than the slowest of the 16 Buhlmann tissues.  I have no idea what
> half-life the governing tissue would have, or what the corresponding 'a' and
> 'b' factors would be.
>
> Being proprietary, I don't think we will ever know how Suunto calculates
> no-fly times.  Maybe we could look at data from the dive computer and
> back-analyse something.
>
> The OSTC method is apparently just a portion (default is 60%) of the
> desaturation time:
> http://forum.heinrichsweikamp.com/read.php?2,9870,9878

I guess this is the only exact information we have gathered so far.
And of course the DAN recommendations are based on real life testing
with clear basis of statistical data, but no algorithms behind them.
Other than that, no info on what the no-fly is based on.

Anyway, my OSTC gave about 1 hour less nofly than my Suunto did (IIRC
50% RGBM and Suunto DC without that option gave similar no-fly). We
are talking about 26-27 hours of it after multiple days of 3 dives /
day on air. Profiles were hugging the NDL limit with the OSTC often
going over if the surface interval was more than 2 hours. (This was a
surprise to me as the general rumor is that Suunto's are more
conservative, but his apparently is only true with short surface
interval..at least when comparing with the OSTC Sport, GF enabled.)

> A no-fly feature could be implemented, but unless an analytical method is
> published and accepted, I don't know what the calculations should be.  I
> think options for discussion could be:
> 1) Implement DAN's guidelines without complicated calculations: 12hr for
> single no-deco dive, 18hr for repetitive no-deco dive, >18hr (maybe 24hr?)
> for deco dives
> 2) Use the method Linus did previously, along with a really conservative
> factor (ignore gradient factor preference) to come up with something similar
> to DAN or dive computer calculations
> 3) Use the OSTC method: 60% of desaturation time (we have to determine how
> many half-lives there are in effective desaturation)
> 4) Use a Buhlmann based method, but with a really slow "tissue", fudged
> until we get the answer we expect
> 5) Something completely different - are there guidelines from someone's
> NAVY? NASA?
>
> The problem with all these methods is that they need to be calibrated to get
> the expected value (or risk being more aggressive than any other
> recommendation).  Unless it's backed by published research, I think I'll
> personally stick to DAN's advice.

How about using the OSTC method of 60% of desaturation but falling
back to DAN recommendation if that value is higher. The DAN
recommendations are easy to use even without support from Subsurface,
but I like to try to keep the DC happy as well when flying. Right now
it is just guess work, and didn't succeed on my latest attempt :D

miika