# Graph documentation [was: Re: Meaning of GF settings]

Dirk Hohndel dirk at hohndel.org
Fri Jan 11 06:25:54 PST 2013

```"Robert C. Helling" <helling at atdotde.de> writes:
> On Jan 10, 2013, at 7:29 PM, Jan Schubert wrote:
>>> Thus I would not worry at all if two different models (or
>>> implementations) vary in their prediction for your stops by one or
>>> even two meters.
>>
>> I'm not worrying about the differences in depth (as it is quite obvious
>> where this comes from) but the much longer deco in the shallow part.
>
> My point is, this is effectively the same. If you for example change
> the rule for "end of deco" from "the ceiling is at the surface" to
> "the ceiling is at 0.5m" for an arbitrary number where the change is
> likely not very significant as the pressure difference it implies is
> quite small. And using a different rule how you round your depth to
> meters has exactly this effect. But the resulting change in stop time
> at the last stop seems quite significant. This is because the ceiling
> graph at the end is almost flat, the slope is so small. What I am
> saying is that even though several minutes difference seem quite
> important at the end of deco they are equivalent to very small
> variations in the (allowed) ambient pressure and thus are not relevant
> for the quality of your decompression.

Here is an interesting piece of code from the dive computer
implementation of Buehlmann's algorithm:

#define  DIST_FROM_3_MTR         0.28

if (mydata.pressure > mydata.surface)
{
pressure = 3 * (unsigned int)((mydata.pressure - mydata.surface + DIST_FROM_3_MTR) / 0.3);
mydata.pressure = ((float)pressure)/10 + mydata.surface;
}

This clearly chops off the last bit of 'tail' in the shallow part. It's
not a lot (0.02 bar - about 20cm), but my guess is this helps to shorten
especially the last stop.

/D
```