No-fly in Subsurface?
John Van Ostrand
john at vanostrand.com
Thu Jul 28 06:12:25 PDT 2016
On Thu, Jul 28, 2016 at 2:52 AM, Rick Walsh <rickmwalsh at gmail.com> wrote:
> On 25 July 2016 at 09:33, John Van Ostrand <john at vanostrand.com> wrote:
>> Although my Cochran offers very generous amounts of no-deco time the one
>> conservative feature is its no-fly time. At the end of a week of
>> live-aboard diving it often displays no-fly times well over 24 hours.
>> I can only guess that it's offering desaturation time. The fact that in
>> practice a 20 tissue model no-fly time is longer than the 16 tissue model
>> also suggests it's a desautration level and the longer tissues of the 20
>> tissue model means longer desat times.
> In the DAN Flying After Recreational Diving workshop proceedings, Mike
> Cochran said that the Cochran dive computer calculates no-fly time as time
> it takes for all tissues to reach ambient pressure (i.e. desaturate) then
> add 12 hours. That sounds like a conservative approach and is no doubt why
> your dive computer gives long no-fly times.
> p82 (p83 of the pdf)
> He didn't say what they consider to constitute having reached ambient
> pressure (x half lives or a maximum difference between tissue and ambient
> partial pressure).
Thanks Rick. I don't know the half times of Cochran tissue compartments but
if the longest is 635 minutes (same as Bühlmann) so a 6x half time would be
63.5 hours. It would have to be a tissue tension threshold. I used 6x in a
comment because that duration is how long people consider a fully m-value
supersaturated tissue takes to de-saturate. That's somewhere around 1.5% of
the m-value and I presume that means that a one half m-value
super-saturated tissue would take only 5x to de-saturate.
John Van Ostrand
At large on sabbatical
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