Recreational Ascent Rate in the last 15 feet

[tursiops]

I see. Maybe I should have said, <=60 fpm post-safety stop as well. In the context of this thread's posts and poll results, that 60 fpm during final ascent seems pretty cork-ish, and is what I meant. Does not seem like hyperbole to me, but if you found it to be misleading, then maybe you have a point.

Welk. Now I'm confused. Is it 60 fmp, or 30, or what the computer say, or like a cork, or what? Cork is bad. 60 probably bad....15 probably good, 30 is what most computers say.

All I wanted to say was that I doubt you were taught to pop up like a cork from your SS.

 

[Wathdoc]

I usually take two minutes for the last six meters after the 5min stop even on shallow dives. It usually is a slow ascent to 3m and a minute or more there before coming to the surface. It obviously depends on surface conditions.

 

[chillyinCanada]

Among other things, I treat it like an exercise in bouyancy control.

 

[tursiops]

Among other things, I treat it like an exercise in bouyancy control.

Exactly!

 

[Wathdoc]

What people forget is what the pressure drop from 6m actually means before they shoot up. Its not far off being taken from sea level to the top of mount Everest( I know not quite). People would not be comfortable doing that.

 

[rsingler]

Here's why I started this thread.
Attached below is the graph from my Perdix for a dive to Little Tunnels in Grand Cayman last year. It was when I was attentive to extra time on my safety stops, but not so much on ascent rate in the last 15'.
The dive was to 108' max for 50 min; first dive of the day. Average depth 53'. EAN 32.
Min NDL during the entire dive was 11 minutes left. So, an average vacation dive, not pushing limits. Except maybe gas usage. Since I could see the boat above me the whole time, and hung out for 8 min total in safety stops, I surfaced with 375 psi.

Now that Shearwater Cloud has implemented graphing of the GF99 (GF "now"), it was fascinating to see what happened at the end of the dive:

The GF went from ongassing at depth to 25% upon reaching the Safety Stop at 15'. Perhaps because this was the first dive of the day, the GF declined rapidly while hovering at 15' for 5 min. But then upon ascending to 5' for a second stop, it jumped right up to 30% and then to 42% upon hitting the surface 3 min later. Had I had SurGF in the firmware at that time, it would have been interesting to see what my predicted surface GF would have been when I first left my final depth of ~55'. Putting the dive into SubSurface suggests that it was probably around 55% without safety stops.

In any case, a LOT is happening in the last 15 feet! Extra time at the safety stop clearly helps. But also, the abrupt change in GF suggests to me at least, that slowing that rate of final ascent means I'll be pulling the tab on the Coke can just a little more gingerly.

 

[dmaziuk]

The GF went from ongassing at depth to 22% upon reaching the Safety Stop at 15'. Perhaps because this was the first dive of the day, the GF declined rapidly while hovering at 15' for 5 min. But then upon ascending to 5' for a second stop, it jumped right up to 30% and then to 42% upon hitting the surface 3 min later.

I have a very strong suspicion that what you're looking at is an artifact of the model whose relationship to the actual diver on the actual dive is tangential at best. Progressively slowing down as you come higher up is common sense, but these glitzy infographics are nothing more than user interface eye candy.

 

[rsingler]

...nothing more than user interface eye candy.

Perhaps. But the mathematical model generating the graph revealed something that was not intuitive: a striking rate of change in GF in the last 15'.

For me at least, the whole point of this discussion is debating the relative value of a significant decrease in final rate of ascent. Buhlman and ZHL-16 and GF's have done well by us so far, no matter what GF you pick. And this bit of the math was a surprise.

 

[tursiops]

Here's why I started this thread.
Attached below is the graph from my Perdix for a dive to Little Tunnels in Grand Cayman last year. It was when I was attentive to extra time on my safety stops, but not so much on ascent rate in the last 15'.
The dive was to 108' max for 50 min; first dive of the day. Average depth 53'. EAN 32.
Min NDL during the entire dive was 11min. left. So, an average vacation dive, not pushing limits. Except maybe gas usage. Since I could see the boat above me the whole time, and hung out for 8 min total in safety stops, I surfaced with 375 psi.
View attachment 551584
Now that Shearwater Cloud has implemented graphing of the GF99 (GF "now"), it was fascinating to see what happened at the end of the dive:
View attachment 551585
The GF went from ongassing at depth to 22% upon reaching the Safety Stop at 15'. Perhaps because this was the first dive of the day, the GF declined rapidly while hovering at 15' for 5 min. But then upon ascending to 5' for a second stop, it jumped right up to 30% and then to 42% upon hitting the surface 3 min later. Had I had SurGF in the firmware at that time, it would have been interesting to see what my predicted surface GF would have been when I first left my final depth of ~55'. Putting the dive into SubSurface suggests that it was probably around 55%.

In any case, a LOT is happening in the last 15 feet! Extra time at the safety stop clearly helps. But also, the abrupt change in GF suggests to me at least, that slowing that rate of final ascent means I'll be pulling the tab on the Coke can just a little more gingerly.

How do you get the GF scale on the left? I seem to gt only a Depth scale.

 

[rsingler]

Added by hand for the graphic.