Questions about Nofly computation

[danou]

Hi all,

I notice that different computers give very different values for Nofly when carried by people diving together, so I wonder how the Nofly time is computed.

1) OSTC takes 60% (customisable) of the total desaturation time. OK, could anybody explain to me the rationale beyond this choice?

2) On the other hand, applying Bühlmann's model (with or without GF) to compute Nofly looks simple:

• the saturation of the 16 compartments at the end of the dive is known by the computer (even graphically shown by the OSTC);
• on commercial airlines AFAIK, the cabin pressure is set to 0,8 bar normally but could eventually fall to 0,2 bar (external pressure à 12 000 m.) in case of failure;
• Computing how much time is needed for the saturation of each compartment to drop below the M-value at 0,8 bar (or 0,2 bar) is straightforward, I mean for a computer of course , taking the maximum of this 16 times should give a good estimation of the Nofly time.

If this makes sense, I would really like the OSTC to compute and show both of these two Nofly times (in case of a cabin pressure at 0,8 bar or at 0,2 bar).

Thanks in advance for your comments and best wishes,
--
Daniel Flipo
OSTC 2N #6131

[scubatinoo]

It's a mysterium... So stick to the easy rule: Don't fly within 24 hours after your last dive. >:D<

[Lionel]

Have a look at for no fly computation
http://www.achim-und-kai.de/kai/tausim/ ... _e.html#k5

On the DR5, I reprogrammed the unit to show no deco time and max altitude. No Fly time is still visible somewhere but it is of no use for me. Assuming everything goes fine, air cabin pressure is likely not to be lower than the equivalent of a trip to 2400m (8000ft).

If things do not go fine and you get 0.2 bar, you'll probably have an "oxygen mask" on your mouth. Given the common name (including in safety message aboard airplane), I assumed it's 100% oxygen you'll be breathing. I am not sure calculating desat time for 0.2b is that meaningfull.

But in any case, it's better to wait to have a full desat before flying.

Lionel

[danou]

Lionel Wrote:
-------------------------------------------------------
> Have a look at for no fly computation
> http://www.achim-und-kai.de/kai/tausim/saett_faq_e
> .html#k5

Thanks Lionel for this interesting link. It answers quite well to my questions

> But in any case, it's better to wait to have a
> full desat before flying.

Of course… but it can take very long time!
--
Daniel

[Lionel]

[quote=Danou]
Of course… but it can take very long time!
[/quote]

Yes, with the rule that the longer you wait, the safer it is in mind and that all deco model are just real-time simulation of body saturation and certainly not the truth (ie your real desat level), I believe that the most "transparent" compromise for me is too know
- which altitude ceiling I can theoretically reach without trouble. Some dive computers do it and they also add some safety margin to the actual ceiling.
- and the total desat time.

Comparisons I've done between desat, 60% desat time and max altitude (for a air-cabin pressurized flight) from another brand of dive computer (UEMIS) suggest that you can theorically take your flight much earlier than the no fly time.

But while waiting a full desat might be overkill, you take more risks having an early flight whenever the altitude ceiling is ok than if you wait. From that angle, the 60% looks like a good compromise although the rational around that ratio could probably be discussed.

[sailor]

Lionel Wrote:
-------------------------------------------------------
> If things do not go fine and you get 0.2 bar,
> you'll probably have an "oxygen mask" on your
> mouth. Given the common name (including in safety
> message aboard airplane), I assumed it's 100%
> oxygen you'll be breathing.
-------------------------------------------------------

If an aircraft suffers a decompression and you have an O2 mask on your mouth you are not breathing 100% O2.
The regular aircraft O2 masks have a fixed relatively low flow of O2 which is mixed with ambient cabin air.
That is just enough to let a person without any N2 tissue-load survive at 0,2 bar.

Reiner

[scubatinoo]

In case of a cabin pressur drop the amount of O2 coming out of the mask will be your smallest problem if you have a N2 tissue load too... i would be more afraid of the low ambient pressure by itself...

[Lionel]

I don't know if cabin pressure loss are common. Luckily, I've never experienced such as thing despite a dozen transcontinental flights each year.

It's hard to tell what would happen between the effect of ambiant pressure and exact amount of O2 in your oxygen mask, also considering that the plane will probably quickly start to decrease its altitude to land somewhere.

Statistically, cabin pressure drop is a low occurence. Most divers have substantial intervals before rushing to the airport therefore the probability of having DCS from a pressure loss is probably extremely low. Cases of divers having DCS during or after a flight are also low but there are cases.

[danou]

Lionel Wrote:
-------------------------------------------------------

> ... I believe that the most "transparent"
> compromise for me is too know
> - which altitude ceiling I can theoretically reach
> without trouble. Some dive computers do it and
> they also add some safety margin to the actual
> ceiling.
> - and the total desat time.

We already have the total desat time, what about showing also the altitude ceiling on the OSTC?

It would be a useful information when you dive at sea level and live in altitude. I experienced this in La Réunion where nice BB are located at 800m and up. It sounds reasonable to wait after the dives before driving back to the BB, but how long? The answer would be ``wait until the altitude ceiling shown by your computer exceeds the altitude of the spot you go to''.

[scubatinoo]

danou Wrote:
-------------------------------------------------------
...
> It would be a useful information when you dive at
> sea level and live in altitude. I experienced this
> in La Réunion where nice BB are located at 800m
> and up. It sounds reasonable to wait after the
> dives before driving back to the BB, but how long?
> The answer would be ``wait until the altitude
> ceiling shown by your computer exceeds the
> altitude of the spot you go to''.

Luxury solution for a luxury problem. (:P)

[Lionel]

It's about calculating for each tissue the atmospheric pressure limit you can't go below considering the saturation of each tissue. Then you keep the highest calculated pressure from the previous calculation and apply an atmospheric pressure to altitude conversion formula. Atmospheric pressure might change as it is a weather condition so some tweaks (eg altitude references) might also be added.