<div dir="auto"><div><br><div class="gmail_extra"><br><div class="gmail_quote">On 25 Jan. 2018 11:16 pm, "Willem Ferguson" <<a href="mailto:willemferguson@zoology.up.ac.za">willemferguson@zoology.up.ac.za</a>> wrote:<br type="attribution"><blockquote class="quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div class="m_-6597560273385316188moz-cite-prefix">On 25/01/2018 12:34, Robert Helling
wrote:<br>
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Willem,
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<div>On 25. Jan 2018, at 10:48, Willem Ferguson
<<a href="mailto:willemferguson@zoology.up.ac.za" target="_blank">willemferguson@zoology.up.ac.<wbr>za</a>>
wrote:</div>
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<div dir="auto">Hi Willem,</div>
<div dir="auto">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.</div>
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<br style="font-family:Menlo-Regular;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255)">
<span style="font-family:Menlo-Regular;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);float:none;display:inline!important">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.</span><br style="font-family:Menlo-Regular;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255)">
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how to you get the 16min from Subsurface? Because I seem to get
23min with GF 100/100:</div>
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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. </div></blockquote></div></div></div><div dir="auto">An easy mistake to make.</div><div dir="auto"><br></div><div dir="auto"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div text="#000000" bgcolor="#FFFFFF">Here is
my main point. I attach three images, all at gf=100/100 and cylinder
safety margin of 50 bar.<br>
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.<br>
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.<br></div></blockquote></div></div></div><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div text="#000000" bgcolor="#FFFFFF">
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.<br></div></blockquote></div></div></div><div dir="auto">In this example, and the 26min one below, the safety stop is in effecta decompression stop, so the dive exceeds the NDL.</div><div dir="auto"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div text="#000000" bgcolor="#FFFFFF">
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With a 15 litre cylinder we can do 26 min bottom time before hitting
the ceiling, with 100 bar remaining in the cylinder.<br>
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. <br>
<br>
I suspect that, if the erroneous error message about
<p style="margin-top:0px;margin-bottom:0px;margin-left:0px;margin-right:0px;text-indent:0px"> <span style="color:#ff0000">Warning: </span>this is more gas than available in the specified cylinder!</p>
<p style="margin-top:0px;margin-bottom:0px;margin-left:0px;margin-right:0px;text-indent:0px">is addressed, then the other problems are also likely to disappear.</p>
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<p style="margin-top:0px;margin-bottom:0px;margin-left:0px;margin-right:0px;text-indent:0px">Kind regards,</p>
<p style="margin-top:0px;margin-bottom:0px;margin-left:0px;margin-right:0px;text-indent:0px">willem</p>
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