Deep stops, Pyle stops, Gradient Factor

[Charlie99]

newdiverAZ once bubbled...
k getting this thread back on topic. If you do a dive to 100' and you start deep stops at 50' aren't you still ongassing some of the slower compartments, and can't this cause a problem after certain amount of time.

The deep stops are to unload N2 out of the FAST comparments -- the ones most often associated with CNS problems. Yes, you are loading more N2 into the slower compartments, but that can't be avoided.

If you head straight from the bottom up to the first deco stop, or to 15' safety stop for an NDL dive, you will generate significant overpressure in the leading tissues, and probably create bubbles in the venous system, aka doppler bubbles. These often cause minor fatigue or subclinical DCS symptoms. I am convinced that the "less fatigue" of nitrox is related to this.

The deep stops give the fast tissues a bit of time to offgass and get a larger margin from the M value. Yes, the slow tissues will still be ongassing, but that is an acceptable tradeoff for few minutes that you spend at deep stops.

You can generate deep stops (for any profile, including NDL dives) with deco programs such as GAP and Decoplanner by setting the GF or gradient factors. I typically use lo = 0.2 or 20%, and Hi of 0.8 or 80%, or on some dives, 90%.

Erik Bakers article at www.gap-software.com explains deep stops in detail.

Charlie

 

[Charlie99]

newdiverAZ once bubbled...
k here's a profile question for some of the more experienced divers around. K 100' for 20 mins on 21%. (this is a hypothetical dive). by padi table is right at the ndl. by v-planner you have a 40 sec stop @ 30', a 3 min stop at 20', and a 6 min stop at 10'. Off gassing starts at 68.6 ft. Pretty big difference there?

GAP results for 100', 20 minutes; Descent 120fpm, ascent 40fpm to 60', 30fpm to 20', 20fpm to surface:

For GF 0.2/0.8
1 min stops at 50,40,30,20', 5 minutes at 10'.

GF 0.2/0.9 has same deep stops, and 2 minutes at 10'.

 

[Walter]

newdiverAZ,

"here's a profile question for some of the more experienced divers around. K 100' for 20 mins on 21%. (this is a hypothetical dive). by padi table is right at the ndl."

You are correct, but it exceeds the NDL of both the YMCA and the DCIEM tables.

I'm not familiar with the v-planner, please tell me a bit about this.

Charlie99,

What is GAP and why are you exceeding the max descent rate of 75 fpm?

 

[dc4bs]

Gas
Absorption
Program

It's a downlodable dive planner similar to vplanner, etc.

 

[Charlie99]

Walter once bubbled...
What is GAP and why are you exceeding the max descent rate of 75 fpm?

GAP is a freeware deco program. www.gap-software.com ZH-L16B or C algorithm. Gradient factors. Supports multiple gases, both nitrox and trimix. Option for 3%+3' depth conservatism. Nice graphical display of compartment loading at the end of each segment. You can set descent and ascent rates for different depth ranges. Some people don't like the profile entry method, but it works well for me.

He is now selling an RGBM version.

I use 120fpm descent rate as a safety factor --- it is faster than I will likely do on a direct descent, and therefore the program will caculate slightly more loading.

What is this max descent rate of 75fpm that you refer to? I'm not going deep enough to trigger HPNS. Some have reported narcosis can be intensified by fast descent -- it that your concern?

Charlie

 

[O-ring]

I would do a minute at 40, a minute at 30, a minute at 20 and get out.

But that's just me, I am not an instructor, and this isn't a recommendation and should not be followed.

 

[Walter]

75 fpm max descent rate comes from the US Navy Dive Manual. Narcosis is much more of a problem with a rapid descent.

 

[Charlie99]

Walter once bubbled...
75 fpm max descent rate comes from the US Navy Dive Manual. Narcosis is much more of a problem with a rapid descent.

I don't know much about the US Navy Table design, but their logic was probably the same as mine. If they design the tables assuming 75fpm descent and you go down faster, then you load more N2, particularly on short, deep, profiles. In my case, I run GAP at 120fpm, and descend slower than that.

 

[Braunbehrens]

O-ring once bubbled...
I would do a minute at 40, a minute at 30, a minute at 20 and get out.

But that's just me, I am not an instructor, and this isn't a recommendation and should not be followed.

That's exactly what I would do. If everything is cool I'll add a minute at 10 and then 1 minute every 2 ft from there on until I blow it. Great buoyancy practice and excellent for keeping those bubbles in check. On a long shallow dive I'd probably add that minute at 10 in any case.

 

[DandyDon]

I try to remind myself to Search The Forum before posing a question, as so many of questions have probably been answered in the respective forums. I tried searches on this here, though, and I don't think I've seen anything on this in DAN's periodical.

I read somewhere recently that someone had decided that it was better to do a one minute stop at half the maximum depth of the dive, then two minutes in the 10-20 foot range rather than a three minute stop around 15 feet. Trying to visualize little nitrogen bubbles attempting to work their way out of blood and tissue at reduced pressures - this would seem to make sense?!

Diving on Nitrox to depths of 70-110 feet on a Roatan wall, I normally work my way back up somewhat on the wall - basic multi level diving?, then return towards the boat over the reef - swimming around 30 feet, then finish with a three minute stop around 15 feet.

On a wreck dive, though, I might only work from 110 back up to 80 feet before hitting 1000 lbs, then work a slow ascent up the mooring line in a strong current. In the past, I opted for five minutes in the 10-20 foot range, often crowding with others, but if one minute at 55 feet (breathing air at 40-45 lbs of pressure) plus two minutes in the normal safety stop range would be better, perhaps I should take this approach instead?? Both of my compueters are Oceanic, and they seem to respond well to the above described wall dive; haven't tried them on the one minute at half depth idea yet?

So, could I get an opinion from an expert, or be referred to recent research?

Thanks, Don