No-fly in Subsurface?

[Robert Helling]

Willem,

> Am 26.07.2016 um 16:42 schrieb Willem Ferguson <willemferguson at zoology.up.ac.za>:
> 
> Be fair, it's a hugely multidimensional problem that is not easily tractable. The DAN project at experimental decompression to 8000 ft had pretty low sample sizes but there was (if I recall correctly) one serious case of DCS in the order of 12 h after repetitive diving. Was it just a single individual response? Really part of a pattern? Difficult to say. I would say a first-level approach is to use the standard models we have and predict saturation levels when flying after various surface times. Question is, what cutoff level of PN2 (or bubble size) indicates safe flying condition? Given this lack of understanding, if a recreational diver, diving 6 dives a year, with no understanding of PN2 and how to manage it, asks me this question, I would probably use the guess of 24h before flying. This diver does not understand the complexity of the problem and wants an answer. For DAN there are potentially real costs related to the consequences of their advice, so I understand their point of view.
> 
> In the past we have made extensive use of the NOAA table for surface-time-before-flying. But I have no idea what the basis was for that table and we often forget we have to take it with a pinch of salt.
> 
> Alternatively, we sometimes plan the last dive(s) to do deco as if we are doing the dive at the maximum altitude we will experience. I do this when I need to cross mountains of 2000m altitude to get home after waiting and additional 2 h. We have not had incidents yet, but we hardly have a large sample size upon which to base such a practice.

I agree that (as many problems in decompression) this is a highly dimensional problem that makes empirical assessment hard. And all the studies point out that variations are dominated by personal factors.

I agree that it sounds reasonable to „use the standard models“ (this effectively means doing an altitude dive to 8000ft or treating the time between dive and flight as a very long deco stop at 1bar ambient pressure before surfacing to the altitude of 8000ft) but as Linus pointed out, this leads much much shorter no fly times, nothing like 12h. And indeed the DAN text mentions that some consider a 2h interval sufficient (I would say it’s actually quite hard to get from the water to a plane that takes off within 120min unless you are a military diver and the plane is waiting for you)

They also mention that DCS symptoms (without flying) can take as long as 6h to appear, so with shorter times it could be that it wasn’t the flight that triggered the symptoms, they only came late, which on a plane is of course more severe as help is not available).

So we are actually discussing how much extra safety factor we are adding to the „standard model“ at that question it seems is dominated by guesswork.

Best
Robert
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