MOD, EAD etc calculations for CCR

[Willem Ferguson]

On 04/11/2014 11:10, Robert Helling wrote:
>
> On 04.11.2014, at 08:45, Willem Ferguson 
> <willemferguson at zoology.up.ac.za 
> <mailto:willemferguson at zoology.up.ac.za>> wrote:
>
> Willem,
>
>> The calculation of MOD, EAD and the like that happens in profile.c 
>> does not work for CCR because it is based on the gas mix info 
>> associated with individual cylinders. Function gas_mod in dive.h is a 
>> case in point. I was starting to mess around with these calculations 
>> to use dynamic fo2 values for each point on the dive profile (instead 
>> of fixed cylinder-specific fo2 values), but then I saw that this 
>> function is used in many places, including extensively in the planner.
>
> I am not sure exactly what would be the meaning of MOD for a CCR given 
> the dynamic O2 content. The only way I think this makes sense would be 
> the MOD of the dillutant (since that sets the minimum fO2 that the CCR 
> can deliver and that determines the depth).
>
I was thinking that it would be useful that a diver could review a dive 
and see at a particular moment, how far he/she was from the MOD, given 
the gas composition at that moment in time. Let's the fo2 was 35% at a 
particular moment and the diver was 30m deep. The diver should know that 
this was pretty close to the MOD for that particular gas combination. 
This might for instance affect decision making with respect to 
setpoints. Of course the dive computer should provide such facilities 
for decision making and/or automatic control during the dive, but for 
the review of a dive afterwards, this information might be useful.

> Indeed, gas_mod is used in various places but only the call in 
> profile.c is about the current gas, in all other cases the MOD of an 
> explicit cylinder is calculated (essentially to find the max depth to 
> switch to it). What I would probably do is to leave everything as it 
> is and only wrap the call in profile.c with an if(dc.dctype != CCR) { 
> and in the else clause make sure the dillutant is used.
>
> I guess the others EAD, END and EADD should be ok the way they are 
> since they use the info from fill_pressures which takes CCR-ness into 
> account.
>

At the moment it gives EAD of 0m throughout the dive and EADD of 
current_depth, but I have not looked at that code closely to explain 
that. However I note that fhe and fo2 are derived from cylinder-related 
data, using get_o2() and get_he().

> What do you think?
>
> Best
> Robert

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