Minimum gas calculation (was: Dive planner VPM-B problems)

[Willem Ferguson]

On 17/10/2017 22:37, Stefan Fuchs wrote:
>
> Now about the "5bar" result from your first plot. Yes, this makes no 
> real sense. Root cause is that my implementation currently always does 
> the calculation for the last manually entered data point and this in 
> your case is at 22m. This is so close to the possible gas change at 
> 21m that almost no gas volume is needed to go there. It would be 
> correct in this case to do the calculation for the second last entered 
> data point at 40m. But implementing this is not so easy because there 
> could be also the use case of a multi level dive with some bottom time 
> at 60m and then some more bottom time at 45m. Here we would really 
> need to check at which point of the dive (deeper depth vs. longer 
> bottom time!) the situation is most critical.
> For your simple example I have an easy proposal: Remove the last data 
> point and let the planner do the complete ascend starting from 40m.
> For your "plan2" the results look almost good and I even tend to 
> remember that I did use bottom SAC and deco SAC according to your 
> proposal:
> Ascend: 27bar*2 = 54bar
> Problem solving: 1min*20l/min*2*9=270l ~ 15bar
> Total would be 69bar, printed is 67bar... hmm, is this the 
> compressability thing once again or just comes from rounding - I have 
> to double check this.
> Ah, you did look at the green delta value again - I have to think 
> about the green value! ;-)
>
>
> Please feel free to provide more examples and comments! I will think 
> about possible improvements.
>
> Best regards
> Stefan

Hallo Stefan,

Let me just make sure, the green value is the minimum cylinder pressure 
when starting the ascent. This takes into account the minimum gas as 
well as another externally-specified minimum cylinder pressure. If it 
gives the green value as 159 bars and the minimum gas as 5 bars, where 
do the other 154 bars come from? There is a strong case for good 
documentation here.

The problem comes with cave dives where the geometry of the cave 
determines much of the ascent. In these cases it is necessary to specify 
at least part of the ascent to allow sufficient time to swim through the 
cave on the way up. In my typical case the cave entrance is at 15m where 
it opens into open water so this does not give the dive planner a lot of 
scope to do any useful calculation. In some perverse sense one may 
describe this as a multilevel dive. I attach a profile where these 
intermediate steps are pretty evident and where the ascent was specified 
to 15m in the dive plan. However, in this case the minimum gas 
calculation would be more meaningful if it were applied to the end of 
the bottom section and not at the point of exiting the cave at 15m. The 
problem then is, as you note, to distinguish between a multilevel dive 
and one that is not intended as multilevel. I wonder whether it would be 
possible to identify a "bottom gas", maybe the one that has the lowest 
fraction of oxygen (FO2) and do the minimum gas calculation for that gas 
mix. For an intended multilevel dive (say 10 min at 45m, 20 min at 30m), 
it would give the worst case minimum gas applicable to the deepest part 
of the dive.

As far as the utility of minimum gas calculations in multicylinder 
dives, our planning principle is always rule-of-thirds or 
rule-of-quarters. Based on this principle, one calculates the critical 
pressure when ascent has to start. If the minimum gas required is more 
than a third (or a quarter) of the start pressure of the cylinder, then 
it becomes almost meaningless. Well, maybe this would indicate that the 
plan is fundamentally flawed in terms of gas planning....

I am afraid I am not of much help in terms of finding a solution.
Kind regards,
willem


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