Meaning of GF settings

[Jan Schubert]

On 01/05/13 12:17, Jan Schubert wrote:
> 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.

Oh forgot to put some notes to Dirks question about the "odd shape" of
the deco ceiling curve: When calculationg the deco ceiling all the 16
tissue compartments has to be taken into account. The deco ceiling is
defined by the compartment/tissue having the highest overpressure,
called the leading tissue. This leading tissue can (and will!) switch
during a dive resulting is such mentionable jumps (ups  and downs) when
looking at the smooth deco ceiling curve not based on 3m steps. In my
opinion nothing strange but really nice to see as all the dive planning
software (not saying that subsurface is supposed to be such a software!)
I know is just printing run time tables based on 3m steps. Personally I
really like this profile based visualization and being able to switch 3m
stops on and off - helps me understand decompression theory better.

Thx a lot,
Jan