<div dir="ltr"><div>Hi Robert,<br><br></div>I read and responded to your email when I
woke up at 4am on Friday morning thanks to a car alarm that wouldn't
stop, and my phone was blinking to say I had notifications. This time
I'm actually awake, looked at your screenshot, and have tested myself.<div class="gmail_extra"><br><div class="gmail_quote">On 11 September 2015 at 04:26, Rick Walsh <span dir="ltr"><<a href="mailto:rickmwalsh@gmail.com" target="_blank">rickmwalsh@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span class=""></span><p dir="ltr">I think we are following the model, and I agree it's silly. I believe the reason is:<br>
1. Crushing pressure is calculated at the greatest depth. The crushing pressure helps get rid of bubbles according to the model.<br>
2. The Boyle's law compensation takes the baseline value as the ceiling at the start of deco, taken as the end of the bottom phase (last user input point) of the dive. A deeper ceiling increases the calculated deco time.</p></blockquote><div><br></div><div>I was half correct. The greater crushing pressure from the 73 m bounce shortens the deco time. But the ceiling *is* greatest at the end of the bottom phase, so my second point doesn't apply.<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<p dir="ltr">The model assumes that deco starts at the last input point. This is ok for a square profile dive. But for a multi-level dive, especially with a deep bouncing, it doesn't make much sense. I think a better assumption would be that the Boyle's ceiling be taken is the deepest ceiling at any point in the dive. This gives very nearly the same result for a square profile dive, but in a multilevel dive such as your example, the decompression time would increase with the deep bounce, as one might expect.</p></blockquote><div><br></div><div>What I am referring to here is just another oddity in VPM-B, at least how we have implemented it, which could be addressed by taking the Boyle's ceiling as the deepest ceiling. Consider this 100 m dive as an example (for those playing at home, I've used 18 m/min descent, 9 m/min ascent, 12/70 back gas, 50% and 100% deco gas).<br><br>Subsurface dive plan<br>based on VPM-B at nominal conservatism<br><br>depth duration runtime gas<br>100m 6min 6min (12/70)<br>100m 14min 20min <br>66m 4min 24min <br>66m 1min 25min <br>60m 1min 26min <br>60m 1min 27min <br>---snip---<br>6m 17min 125min oxygen<br>3m 0min 125min <br>3m 29min 154min <br>0m 0min 154min <br><br></div><div>Say you remember that there's something interesting at 66 m, so you decide you'll pad out the first deco stop so you get to spend 5 min at that depth. Should be acceptable; you're not breaking the ceiling, but your deco time could be extended as a result. You manually add in waypoints at 66 m @ 24min and 66 m @ 29min. Now the plan is:<br></div><div><br>Subsurface dive plan <br>based on VPM-B at nominal conservatism <br> <br>depth duration runtime gas<br>100m 6min 6min (12/70)<br>100m 14min 20min <br>66m 4min 24min <br>66m 5min 29min <br>51m 2min 31min <br>51m 1min 32min <br>---snip---<br>6m 16min 118min oxygen<br>3m 0min 118min <br>3m 24min 142min <br>0m 0min 142min <br><br></div><div>The effect of the user deliberately extending the first deco stop is that they brush of 12 minutes of deco. In my opinion, that's silly and dangerous.<br><br></div><div>For comparison, I ran the same plan in the VPM-B Fortran program, and got total run times of 157 min (3 minutes longer than Subsurface) for 100 m constant depth, and 168 minutes with a manually entered 5 minute stop at 66 m. The Fortran program does increase decompression time for spending extra time at 66 m, which is what it and we should do.<br><br></div><div>I will send a patch or two to use the deepest ceiling as the Boyle's law compensation baseline. And maybe another to warn about the crushing pressure issue if I can think of a way to do it.<br></div><div><br></div><div>Cheers,<br><br></div><div>Rick<br></div></div></div></div>