Conservatism

[yvan]

Hello,

I've been reading throughout the forum topics and maybe I missed it. I've also had o look through the code and i'm afraid I'm to lazy to learn another more programming language.

Somehow, as everybody could be interested in, I decided to post my question. Could you explain what you understand under conservatism:


"The security level is fully customizable in all OSTC firmwares released. The major functions are CF11 and CF 12 .
A security level identical to the VR3 is possible by increasing the inert level of the gas at the gas setting.
Additionally CF16 modifies the conservatism of the deco predictions"


To be more precise, what are CF11 and CF12 doing in the calculation, are they in(de)creasing time, depth, compartiment constants, ... ?
I suppose decreasing CF16 is just making the integration more precise?

Greetings

Yvan

[sailor]

When increasing CF11 you can simulate a faster tissue on-gasing and when decreasing CF12 you can simulate slower tissue off-gasing.
So when you increase CF11 or decrease CF12 you increase the conservatism.
Be carefull not to do it the other way around, this might be dangerous.
When you see a dead men's skull displayed on the screen you did it the wrong way around, don't dive with this adjustment.
Decreasing CF16 gives you less conservatism, with increasing CF16 you will have more TTS.

[yvan]

Thanks for the answer.

But I have a mathematical mindset. So on the following basic equation for a compartiment tissue gas exchange

dPtis,gas/dt = 0.693/T*(Pa,gas-Ptis,gas)

on what factors are the CF acting or is the routine doing something else ?

It is just for me to know when I am thinking deco.
For the safe ascent criteria I know you are using Bulhmann's factors for He and N2.

[sailor]

I'm just a user and not a math professor so i don't know anything about the formula you mentioned.
What i understand from the WIKI online manual is that you increase the mathematical tissue on-gasing when increasing the CF11 value.
The Buehlmann standard is 100%, HeinrichsWeikamp is using a 10% safety margin ( 110% ) and the user can increase this safety margin furthermore.
So when you increase CF11 to 150% the computer is calculating a 50% higher tissue-load then the Buehlmann model.
Same applies to CF12 the other way around.
Have a look at the WIKI online manual here on the website.
Again, i can't answer questions regarding the mentioned formula and i'm only a user.

[heinrichsweikamp]

The formula (variables as used by Buehlmann) is

p_t.ig(t_e) = p_t.ig(t_0) + [p_I.ig - p_t.ig(t_0)] * [1-2^(-t_e / t_0.5)]

The part right of the "+" sign (the change) is multiplied by either the CF11 percentage or the CF12 percentage. CF11 or CF12 is chosen with respect to the algebraic sign of the change.

The code of the OSTC for N2 (similar for the He(lium) part) is

// N2
temp_tissue = (temp_atem - pres_tissue[ci]) * var_e2secs;
if (temp_tissue < 0.0)
temp_tissue = temp_tissue * float_desaturation_multiplier;
else
temp_tissue = temp_tissue * float_saturation_multiplier;
pres_tissue[ci] = pres_tissue[ci] + temp_tissue;

The change in tissue saturation is calculated and stored in temp_tissue.
The conditional branch checks if the tissue is saturated or desaturated and the corresponding multiplier is applied.
The modified change is added to the previous tissue saturation.

/Christian

[yvan]

Hello Christian,

thx for the open answer, makes it clear for me at least. It's like the algorythm is decreasing / increasing the compartiment periods.
I understand now why the algorythm is also working with 100% Buhlmann factors for determining the ascent.
Meanwhile it is quite complicated when working with the GF or if you install this feature on a next firmware,but a 100/100 setting can than re-equilibrate the situation.

Gr

Yvan