Recreational planning with Subsurface?

[Willem Ferguson]

On 25/01/2018 11:16, Rick Walsh wrote:
>
>
> On 25 Jan. 2018 8:07 pm, "Willem Ferguson" 
> <willemferguson at zoology.up.ac.za 
> <mailto:willemferguson at zoology.up.ac.za>> wrote:
>
>     I am working on the user manual. Because of UI changes, this
>     includes replacing all the images in the section dealing with the
>     dive planner. I am trying to get the recreational planner to work
>     and I have a problem. See attached image.
>
>     If the initial cylinder pressure is not set, then the planner
>     provides a dive at the specified depth and duration is determined
>     by the NDL, limited by the present cylinder size and initial air
>     pressure. So I select a 12 litre 232 bar cylinder and do not set
>     the initial cylinder pressure.
>
>     1) The ceiling and profile do not approach the NDL limits based on
>     dive tables, even with GF=90/90.
>
>
> Hi Willem,
> In the example you provide, the air NDL at 30m is 13min with gf 35/75. 
> This looks about right to me. From memory the Buhlmann NDL (gf 
> 100/100) is ~17min, and the PADI and navy tables are 2-3 minutes 
> longer. Reducing the gf to 35/75 would reduce the NDL.

I suspect this is not the complete explanation. The NAUI, NOAA and US 
Navy tables all give the NDL with air at 30m between 22 and 25 min. With 
gf=100/100, Subsurface gives 16 min. In addition, the profile does not 
get close to the ceiling at NDL. I suspect that, with initial cyl 
pressure not specified, the whole calculation just does not work. 
Forcing the bottom time to 26 min (gf=100/100) brings the ceiling close 
to the profile which is more or less acceptable. So that part of the 
planner works. But if the bottom time is specified as much shorter than 
26 min, the NDL also is much shorter. There is a problem or bug there.

Then there is the question of the use of GF with recreational mode, 
since the Bühlmann option has been *de-selected*. Does this mean that 
VPM conservatism can also affect the profile? I thing there is a need 
for me to document the way thing work because the current text is 
clearly not sufficient. I have a feeling the only difference between a 
conventional ZH dive plan and a recreational dive plan is the latter's 
ability to automatically provide a NDL profile, which in this case does 
not work anyway.

>
>     2) Even more important, at the bottom it says that the dive
>     requires 1245 litres of gas, more than is in the cylinder. But
>     this is wrong because the 12 litre 232 bar cylinder takes 2784 litres.
>
>
> That looks like a bug. I'm guessing the calculation here assumes the 
> starting pressure was zero. If you don't enter a starting pressure, 
> the planner should assume cylinder working pressure.

I would agree. The gas volume calculation is accurate. The cylinder 
volume/pressure is lost somewhere.

Kind regards,

willem



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