No-fly in Subsurface?

Rick Walsh rickmwalsh at
Mon Jul 25 02:11:42 PDT 2016

Apologies for the top-post.  The most informative resource on this I've
seen so far comes from DAN's 2002 workshop on flying after recreational


On 25 July 2016 at 13:10, Rick Walsh <rickmwalsh at> wrote:

> On 25 July 2016 at 10:33, Linus Torvalds <torvalds at>
> wrote:
>> On Sun, Jul 24, 2016 at 4:44 PM, Linus Torvalds
>> <torvalds at> wrote:
>> > On Sun, Jul 24, 2016 at 4:33 PM, John Van Ostrand <john at>
>> 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:
> 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
> 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.
> Cheers,
> Rick
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the subsurface mailing list