forum.heinrichsweikamp.com

Hi, after reading the attached blog on Helium halftimes, I was wondering if is possible to change the helium/nitrogen halftimes on my OSTC 2C?

https://www.shearwater.com/monthly-blog ... m-penalty/

I'm not planning on changing anything, it was more of an understanding if it is possible and what effects it has.

Thanks
Mark

These are the custom functions you can currently change -> file.php?2,file=599,filename=OSTC_Custo ... N_V294.pdf

Hi Mark,

you can't change the half-times nor the a and b values for both N2 and He - they are hard-coded into the OSTC Firmware. According to Buhlmann's books, you can set up a ZH-L model with different numbers of compartments. Over the time there have been implementations in dive computers with 8, 12 and 16 compartments. The lower numbers were mainly due to available compute power in the past. Anyhow, if you change the number of compartments, you also have to change the halftimes that these compartment have to get a good coverage of the model for different dive profiles. Today compute power isn't a problem any more and thus 16 compartments have become common because according to Buhlmann more than 16 do not bring any added value any more.
If you want to tweak the Buhlmann model about the way it handles N2 vs. He, you would need to fiddle with the a and b factors. Empirically, they are linked with the halftime of the respective compartment and by design to the max. tolarated over-pressures. The factors for N2 and He in turn have a relation with each other given by the different solubility of N2 and He in liquids.
All in all, the ZH-L16 model has 96 numbers that define what the model delivers. If you tune one of them, you probably have to tune others as well. Buhlmanns main work actually was spent on tuning these factors and have many volunteers to try them out - for various dive profiles. And that's the problem: whenever "new rules" are presented, have a carefull look for which gas mixes and for which dive profiles they are made and tested...

BR
Ralph