Recreational planning with Subsurface?

[Willem Ferguson]

On 25/01/2018 12:34, Robert Helling wrote:
> Willem,
>
>> On 25. Jan 2018, at 10:48, Willem Ferguson 
>> <willemferguson at zoology.up.ac.za 
>> <mailto:willemferguson at zoology.up.ac.za>> wrote:
>>
>>> 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.
>
> how to you get the 16min from Subsurface? Because I seem to get 23min 
> with GF 100/100:

As far as the 16 min is concerned, I suspect Ferguson erroneously read 
the x-axis as 15+1 =16 instead of 20+1 which is 21 min. Here is my main 
point. I attach three images, all at gf=100/100 and cylinder safety 
margin of 50 bar.
1) a nominal dive to 30m using a 12l cylinder (10min-100-12.jpg). Bottom 
time 21 min with 75 bar remaining even though the ceiling is nowhere 
near the dive profile. So, there does not appear to be a deco limit, and 
the cylinder reserve gas specification has not been observed.
2) Same dive as above, but with 15l cylinder (10min-100-15.jpg). Exact 
same gas use as above. Reserve gas remaining = 106 bar, no ceiling 
anywhere near the dive profile. Here the important part of the ceiling 
appears to be the time it takes the ceiling to disappear, not the depth 
of the ceiling. Here the cylinder size had no effect whatsoever and 
neither deco load or available gas was the factor limiting the bottom time.
3) Dive to 30m with 24 min bottom time (24min-100-12.jpg). Again no 
obvious limitation of deco limits. Bottom time = 24 min. remaining air = 
54 bar. This is more or less what I would have expected with a 12 l 
cylinder. We are running out of air before hitting the NDL.

With a 15 litre cylinder we can do 26 min bottom time before hitting the 
ceiling, with 100 bar remaining in the cylinder.
It looks like the ZH algorithm calculates the NDL as 26 minutes. The ZH 
algorithm appears to work ok, I cannot see any *obvious* problem. But 
the planner has lost the cylinder size or the cylinder pressure or both 
and is not giving realistic NDL limits. Firstly one can lengthen the 
planner-provided no-NDL bottom time without encountering deco. This 
should not be. Secondly, if one takes a larger cylinder, the bottom time 
does not increase, even though one is far from deco. This should also 
not be. My feeling is that if the planner had the correct cylinder 
volume and pressure, everything will come right.

I suspect that, if the erroneous error message about

Warning: this is more gas than available in the specified cylinder!

is addressed, then the other problems are also likely to disappear.

Kind regards,

willem









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