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hello
Both my OSTC2 and OSTCplus gave me some very long no fly and desaturation times (30 to 40 hours) after 90 to 120min shallow cave dives on air (average 10 meters).
I don’t understand why. Another diver with a Suunto got no fly times about half of mine.
thank you
jean-luc

Hi Jean-Luc,

well, remote diagnostics is not that easy in this case. I do not know how Suunto computers derive their no-fly time, but the OSTC hwOS does it as follows:

For the no-fly-time it actually computes a deco schedule targeting a surface pressure of 0.6 bar (instead of the usual +/-1 bar) and estimates the required time-to-surface, which is display as no-fly-time then.

For the desaturation time, it does more or less the same, but the target is that all tissues are down to a residual loading of max. 5% above equilibrium state.

So in both cases, times increase with a) higher ambient pressure (when staying at sealevel the tissues discharge slower than at altitude - same physics as if being submerged), b) higher residual loads on the slower tissues. The later is a quite dominating factor as the slower tissues discharge slower (as their name already suggests), plus they do tolerate much less supersaturation ("overpressure") compared to the faster ones.

So 90 to 120 minutes even at 10 meters (2 bar abs. pressure), maybe in series over several days, on air (which is the ppN2 richest gas divers can breath) may have exited your slow tissues enough to result in the figures you observed.
You can take a look at the tissue graphics that is shown in one of the surface custom views, there you can see tissue loading levels and loading distribution. Since firmware V2.9-something this graphic is in itself to scale. So if you see loadings there on the slow tissues, these will be your "troublemakers".

hope that helped,
best regards,
Ralph

Hi Ralph
Tks a lot for your detailed answer.
Usually I do deco dives with a very different profile (trimix or air, with nitrox50 and O2) and I had never noticed such a discrepancy in no fly time with other computers. These long shallow dives are new to me and I wondered if someone had experienced the same thing.
best regards
JL

Hi,

there is one more reason why no-fly-times of the OSTC with latest hwOS tech firmware will differ significantly from what other computers will show: as written before, it calculates true decompression times for being cleared for altitude, wheras most other computers (well, including the OSTC 4) just run static timers or replay the DAN recommendations in the one or other form.
The OSTC uses the Bühlmann model, and Prof. Bühlmann himself declared his model usable until down to an ambient pressure of about 0.5 bar. He always investigated the area of surface pressures < 1.0 bar, too, because being homed in Zurich, Switzerland, mountain diving was usual business. When calculating no-fly-times, the target surface pressure is 0.6 bar - much lower than typical cabin pressure on a jet airliner will be, so there's quite a safety margin in it that will cover problems with the pressurization system up to the point were the pilots are required to make an emergency descent.
Many other published models are mainly designed and validated for sealevel diving, because they were often developed or sponsered by the navies. For example in the Workman model, pressure is referenced to depth in seawater with the null point at water surface. Opposite to that, Bühlmann uses absolute pressures.

-R.

Hi Ralph
understood.
then before flying it would seem more useful and conservative enough to set the waiting time as for an ascent to 2000m, which is closer to the pressure of a cabin on a jet airliner than the 0.6 bar used for the "no fly time".
right?

JL

Well, the 0.6 bar originate from Bühlmann's book, it's the value he proposed. As the OSTC's philosophy is to stay as close as possible to Bühlmann, and not to add own twists here and there that are unknown/unanticipated to the user, no-fly targets these 0.6 bar.

When set to altitude modes, 1.000 meter selects 0.880 bar, 2.000 m select 0.782 bar and 3.000 m select 0.695 bar. Normal cabin altitude varies with aircraft type, usual values are 0.735 bar.

==> So 2.000 meter won't be enough, you'll need to select 3.000 meters!

Noted. Tks