Meaning of GF settings (was: deco calculations)

[Jan Schubert]

On 01/05/13 09:13, Dirk Hohndel wrote:
> Miika Turkia <miika.turkia at gmail.com> writes:
>> 55/96 seems to get quite close to what my computers thought during
>> these dives, but I believe Suuntos (stinger and Vyper) are more
>> conservative than that (not that I would know what these GF values
>> really mean).
> Neither do I. But as I mentioned in my commit message - the shape of
> that ceiling seems awfully strange to me at times. Yes, I can easily
> hide that (switching to 3m increments usually does the trick), but it
> still makes me nervous.

Please note that GF is related to Bühlmann and Suunto is not using
Bühlmann at all but RGBM (Reduced Gradient Bubble Model). AFAIK RGBM is
not publicaly documented but might be based or similar to VPM, both
bubble based models. In difference to Bühlmann these models try to avoid
the formation of bubbles in tissues right from the the beginning while
Bühlmann assumes there would be bubbles anyway and tries to keep them
small (smaller than a critical size). In real life this results in
deeper deco stops at VPM and and shallower but much longer ones for
Bühlmann. Nowadays most dive Computers use Bühlmann meaning this is more
tested (funny side note: Definitions for the maximum tissue saturations
in the Bühlmann model are based on tests done by Haldane using goats in
pressure chambers).

Anyway, back to the Meaning of GF: As said, Bühlmann defines maximum
overpressure values for all 16 tissue compartments (there are also
easified models using "just" 8 compartments). Bühlmann itself does not
know of GFhigh and low. If you set GFlow=GFhigh=100% then you use plain
Bühlmann (mostly as most dive computers and calculations ad some more
safety beside this). To add more safety to deco calculations and to
adapt to individual preferences (like fitness, illness, coldness, ...)
the GF limitations has been introduced meaning we just use 75% of the
maximum overpressure defined by Bühlmann assuming you use a GFhigh=75%.
Sidenote: The GFhigh defines the max. overpressure when surfacing, NOT
when being/reaching/leaving the last deco stop (this is a point many
people get wrong). To make it more complicated^Wadjustable there is an
additional GFlow setting which defines a (usually) lower/reduced maximum
overpressure for the beginning of the decompression phase. This also
tries to adapt to a (disputed) deep stop based theory and heads towards
the bubble based models in avoiding the formation of bubbles. In other
words: GFlow and GFHigh are "just" a linear reduction of the max.
overpressure values for all the 16 tissue compartments. The higher the
GF settings are the shorter and shallower the deco stops but the more
risk in getting bend. Note: GFhigh and also GFlow can be defined with
values more than 100% if one like to use such settings.
And to make it even more complicated/easified there are also these 3m
based stops introduced (10ft for imperial based people), meaning there
is no floating deco ceiling ("smooth" using definitions by subsurface)
but fixed stops based on depth in multiples of 3m.

Personally I dive GFlow=30% and GFhigh=95% (which most people would
consider quite aggressive), meaning in the beginning of the ascend I
will not break a gradient factor of 30% (but will get a deco stop
before) which gets linearly raised to 95% when surfacing.

Please come up with any more questions to decompression theorie (or
medicine) if there is something not clear to you.

HTH,
Jan