12 meter procedure - any good?

[tursiops]

PADI Deep Diver course 12-metre safety/decompression/"long" stop

I am soon to be doing my PADI Deep Diver certification to go down to 40-metres deep. The training manual says that a 12-metre safety/decompression/"long" stop for 1 to 3 minutes is sometimes done in addition to the 3 minute safety stop @ 5-metres. Up until reading the deep diver course book l...

scubaboard.com

 

[100days-a-year]

Put Another Deepstop In?

 

[rongoodman]

I‘m not sure I would categorize 36’ as a "deep stop", at least not in the sense as the term was once used. While hanging out there would reduce your surfacing GF, I don't know if it would be worth the bother, unless there was something interesting to look at.

 

[scubadada]

Boyle's Law supports this. If you're comfortable with the bubble expansion rate afforded by, say, 30 fpm at 30 ft (10 mpm at 10 m), you would need to be ascending at 8 fpm (2.5 mpm) or slower at the surface to not exceed that expansion rate.

For such rates, it's useful to think of the rate in terms of seconds per ft (or meter). An easy rule from the safety stop would be ascending at 10 seconds per ft or 30 seconds per meter.

This is a very simple no stop dive in Jupiter, FL, max depth 82 feet, avg depth 65 ft, 52 min. I always try to do my final ascent over at least a min, usually about 2 min. At the end of the safety stop, the GF was 17. The final ascent was over 2 min and the GF blossomed to 60. This pattern is the usual and reinforces why the final ascent should be relatively slow.

 

[lowwall]

From a purely theoretical view, a few minutes stop at 12m on an NDL dive makes a lot of sense. Assuming a max depth of 30m, this would prevent you from breaking the old Haldanean max tolerable pressure change no matter how quickly you ascended to that point. Actually I suspect that since you would be unable to fully saturate even the fastest tissues at 30m on a no deco dive, a 12m stop would ensure you couldn't go over a gradient factor of 100 no matter the ascent rate to that point.

Another way to look at it is that a midlevel stop will reduce the time spent at a high inert gas loading for fast tissues while having a negligible effect on slower tissues.

I'm pretty sure there was a paper that found a midlevel stop substantially reduced bubble formation. hold on ...

OK. https://www.researchgate.net/public..._bubbles_and_fast_tissue_gas_tensions_Article

The authors found adding a 15m stop for 5min to a 6m stop greatly reduced bubbles. They followed this up and varied the length of the stops and found 2.5 minutes at 15m was enough.

Europe PMC

Europe PMC is an archive of life sciences journal literature.

europepmc.org

The full text of the second study is available if you google the title.

 

[lowwall]

Diving with nitrox with the computer on air mode is safer because it keeps you farther away from NDL. There is no requirement in scuba that you dive to the NDL on your computer. It is permissible to ascend when you are 5, 10, 15 minutes from NDL--whatever you want. Why not put the correct gas in your computer and ascend before things get close?

This is what I do. I set the computer to the least conservative mode and actual gas mix and then monitor NDL. How close I get to it depends on the type of dive. If it's a squarish profile like a wreck, then I'll leave a lot more cushion than a multi-level dive.

Also, I don't think I've ever dove a truly square profile. There's always some way to get at least a couple of meters higher and get those NDL minutes farther away from 0.

 

[inquis]

I suspect that since you would be unable to fully saturate even the fastest tissues at 30m on a no deco dive

Fact Check: True
Typically 6 half-times is considered "saturated", and the fastest tissue in ZHL-16 is 4 mins. No-stop time for a 30m dive is 18 min (or less).

 

[L13]

Fact Check: True
Typically 6 half-times is considered "saturated", and the fastest tissue in ZHL-16 is 4 mins. No-stop time for a 30m dive is 18 min (or less).

4 min is the half-time for ZHL-16A, the ZHL-16C half-time is 5 min, but the result is essentially the same. Tissue 1 will be 92% saturated(vice 96% saturated) after 18 min.

For a direct ascent from 30m to the surface at the often recommended 9m/min, Tissue 1 will not exceed a GF of 75%(even if you far exceeded the 18 min NDL limit). The 3min 20sec ascent time is long enough for Tissue 1 to off gas to a safe level (75% of limit) even if saturated at 30m. However, a direct ascent from 30 m at 9m/min after 18 min does produce a GF of ~92% on Tissue 3 (the limiting tissue for that dive).

So do your safety stop for Tissue 3 , for NDL dives to 30m.

 

[Rollin]

Thank you @lowwall for the scientific background!

I'm wondering now if these are the articles are the ones @boulderjohn was talking about in his first post.

About the post of @L13 just above, if you go up after an NDL-dive and end up with a GF of 92%, I guess you chose to dive a very liberal DC or setting.

 

[L13]

About the post of @L13 just above, if you go up after an NDL-dive and end up with a GF of 92%, I guess you chose to dive a very liberal DC or setting.

In order to get the 18 min NDL quoted by others, you need to set the BHZ-16C algorithm to a GF Hi of 95%. I normally use 75%, but particular values of GF Hi/Lo are personal risk choices.

My point was primarily that for a 30m dive, Tissue 1 never exceeds 75% for standard ascent rates(slower tissues are limiting at 30m).

And that even with GF Hi set to 95%, you will surface with GF < 75% if you do your Safety Stop.