# Retrospective gradient factors on real dives

Robert Helling helling at atdotde.de
Fri Dec 9 14:43:31 PST 2016

```John,

> Am 09.12.2016 um 22:51 schrieb John Van Ostrand <john at vanostrand.com>:
>
> On Thu, Dec 8, 2016 at 9:36 AM, Willem Ferguson <willemferguson at zoology.up.ac.za <mailto:willemferguson at zoology.up.ac.za>> wrote:
>
> Would it be feasible to calculate retrospective gradient factors on real dive profiles? We sometimes use a VR3 dive computer which does give this information, but it would be very useful to to have it being calculated automatically for each dive.
>
>  Do you mean you want to see the decompression stops for a different set of GFs? Or do you want to automatically find the most liberal GFs that fit a dive profile?
>
> For the former questoin set up GFs in the Preferences and go back to view the dive. For the latter question I iterate through GFs manually to find the closest one.
>

I guess what Willem meant was to do something along the lines we do now for VPM-B planned dives: There we compute GFhigh/low such that you would get a similar decompression profile.

I have not yet answered to Willem’s original mail as I do not really know how one would implement this. Here are my thoughts: At each instant of time during the dive, we could compute a gradient factor such that the current depth would be the ceiling. That part is easy. One could then try to find  a line in the gradient factor vs depth plot that best approximates these points. The problem is that this makes only sense during the decompression phase of the dive, i.e. for the time where it makes sense to consider the current depth to be at least roughly the ceiling. During the bottom part of the dive as well as during the first part of the ascent this would give you far too low gradient factors.

So we would need to find out for which part of the dive to apply this. I don’t know what a good criterium would be: Something like the last 25% of max depth? Some part where the ascent is slower than the current ascent rate? We could take user input (maybe in form of the ruler) or we could find that last part of the dive where a linear approximation for the gradient factor vs depth plot is best (measured in chi squared or something similar).

Best
Robert
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.subsurface-divelog.org/pipermail/subsurface/attachments/20161209/c567537f/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 496 bytes
Desc: Message signed with OpenPGP using GPGMail
URL: <http://lists.subsurface-divelog.org/pipermail/subsurface/attachments/20161209/c567537f/attachment-0001.sig>
```