Recreational planning with Subsurface?

[Rick Walsh]

On 25 Jan. 2018 11:16 pm, "Willem Ferguson" <willemferguson at zoology.up.ac.za>
wrote:

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>
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.

An easy mistake to make.

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.


In recreational diving, i.e. no-decompression diving (even though many of
us might do decompression diving as a recreational activity), the safety
stop is not considered in the NDL. It is extra padding for safety. If it
were required by the decompression algorithm then it would be a
decompression stop.
The recreational planner, like NDL tables, calculates the maximum bottom
time possible while still allowing a free ascent to the surface (and in the
case of the planner it also considers gas limits). In examples 1 and 2
above, than limit is calculated as 21min. At 22min, the ceiling would not
quite clear if the diver skipped the safety stop. The gas is not limiting
anything in either example.

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.

In this example, and the 26min one below, the safety stop is in effecta
decompression stop, so the dive exceeds 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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