Deep stops, Pyle stops, Gradient Factor

[DA Aquamaster]

For deep dives I normally do a 1 minute stop at 1/2 max depth and then do a slow ascent (20-30fpm) to 20 ft for a 3 min safety stop. In cold water or on a hard working dive I will add a 1 minute stop at 30 ft and then add another stop for 1-3 minutes at 10 ft as well.

On a sloping bottom or wall, I will just plan to ascend to about 1/2 max depth and then step up in 20 ft or so increments during the remainder of the dive.

 

[Diver0001]

DA Aquamaster once bubbled...
For deep dives I normally do a 1 minute stop at 1/2 max depth and then do a slow ascent (20-30fpm) to 20 ft for a 3 min safety stop. In cold water or on a hard working dive I will add a 1 minute stop at 30 ft and then add another stop for 1-3 minutes at 10 ft as well.

On a sloping bottom or wall, I will just plan to ascend to about 1/2 max depth and then step up in 20 ft or so increments during the remainder of the dive.

This is pretty much what I do too. On deep dives along slopes or walls I'll ascend to 1/2 the deepest depth before we reach the NDL and then bimble my way up from there, ascending a few feet per minute. It's the only way to get an hour of bottom time out of a deep dive.

R..

 

[LightBlue]

Dr Deco once bubbled...
The means by which they are calculated are empirical. I am not aware of any laboratory studies that actually attempted to determine the best way of doing this.

Other responders have indicated how the stops can be calculated by various algorithms.

I see.
I understand GUE is teaching deep stops quite precisely, according to GUE tech 1 divers (I know a few).

It seems we all do these deep stops more or less the same way.

Tnx.

lb

 

[Kendall Raine]

You specifically asked about the purpose of deep stops. Put most simply, the deep stop is a mechanism for slowing the ascent rate early during decompression. The purpose of this is to allow inert gas which may present in bubble form to exit from bubble seeds before the bubbles can grow beyond a certain size. The size is described as the Laplace cutoff and marks the radius at which a bubble will tend to either grow or contract. If bubble seeds grow larger than the Laplace cutoff, they will tend to continue to grow as you ascend-bad. If they remain smaller than this critical radius, the surface tension of the bubble will tend to be sufficient to cause a sufficient pressure gradient forcing inert gas back into dissolved form for transport to the lungs for elimination-good.

Dual phase decompression models like RGBM and VPM recognize the pre-existence of bubble seeds alongside dissolved inert gas and so incorporate deep stops by nature. Haldanian models only recognize inert gas in dissolved form and seek to prevent the spontaneous formation of bubbles by controlling the critical tension (M Value) of inert gas on ascent. Put another way, Haldanian models don't really explain theoretically why someone who follows the table should have post dive venous gas bubbles. Nevertheless they do and most neo-Haldanian models have been tweaked (scientific term) based upon Doppler studies to reduce this bubbling.

Rich Pyle's 1995 article on deep stops recommends inserting a stop half way between the max depth and the first required (Haldanian) stop. This was a good start, but not very "computational." For example, dual phase models suggest the first dep stop occur between 4/5 and 2/3 of the max depth for helium based mixes, depending on the nature of the exposure and the gasses used. Helium mixes generally involve deeper stops than nitrogen only mixes. Helium and nitrogen involve different diffusion and solubility characteristics.

For recreational "No Decompression" exposures, the Pyle method is probably fine. In essence, this is what most people do anyway on a typical warm water reef dive-start deep and work your way into the shallows. The dive involving much more deco stress is the traditional square profile. You know, 60 feet for 60 minutes and head to the surface at 60 fpm. For an air dive to 60 feet for 60 minutes, I would do something like 40x1, 30x2, 20x3, 10x6 and take a full minute from 10 to the surface. While either ascent profile probably won't bend you, listening with a Doppler, particularly in the context of repet dives, would not take long to illustrate which schedule leaves you with fewer post dive bubbles.

 

[DeepTechScuba]

ALL SCUBA DIVING IS DECOMPRESSION DIVING.

That is an old quote from a very old IANTD decompression diving manual.

NDL diving (No Decompression Limit) works because your body tissues and fluids will tolerate a limited amount of pressure tension or gradience. But this only works to a point.

Look in the NOAA Diving Manual in the appendix with the depth charts and you will see that you may stay at a depth of 20 ft for an indefinite amount of time underwater.

The pressure at 20 ft is 1.6 ATAs derived as follows:

20/33 + 1 = 1.61 for seawater
20/34 + 1 = 1.59 for freshwater

This knowledge was determined emperically, in other words by experimentation. This tells us that our body tissues and fluids can tolerate a pressure tension change of 1.6 to 1. That is because the pressure at 20 ft is 1.6 ATAs and the pressure at the surface at sea level is 1.0 ATAs.

Armed with this knowledge, you can then determine what depth you should always perform your first deep stop, on any dive.

If your max depth for your dive was 20 ft, then you can ascend to the surface at any time, slowly of course, we teach 33 ft per minute, WITHOUT a stop.

Any max depth deeper than 20 ft should seriously consider a stop, rather than an ascent to the surface directly, albeit slow.

Why?

Because if we dive deeper than 20 ft, then we know that DCS is possible. We know it from the NOAA dive tables. The dive tables give us limitations of time at various depths for what we call NDL limits for dives deeper than 20 ft. But they are no guarantee against DCS. There is no guarantee against DCS, other than NEVER to dive deeper than 20 ft.

Since 1.6:1.0 is the change ratio in pressure that can be tolerated for an indefinite length of time underwater, we use this ratio to tell us at what depth to make our deep stop.

If you dove to 33 ft then your stop should be at 8 ft, computed as follows:

33/33 + 1 = 2 ATAs
2 ATAs / 1.6 = 1.25 ATAs
(1.25 ATAs - 1 ATA) x 33 ft = 8 ft

If you dove to 45 ft then your stop should be at 16 ft, computed as follows:

45/33 + 1 = 2.4 ATAs
2.4 ATAs / 1.6 = 1.5 ATAs
(1.5 ATAs - 1 ATA) x 33 = 16 ft

If you dove to 66 ft then your stop should be at 29 ft, computed as follows:

66/33 + 1 = 3 ATAs
3 ATAs / 1.6 = 1.875 ATAs
(1.875 ATAs - 1 ATA) x 33 ft = 29 ft

If you dove to 99 ft then your stop should be at 50 ft, computed as follows:

99/33 + 1 = 4 ATAs
4 ATAs / 1.6 = 2.5 ATAs
(2.5 ATAs - 1 ATA) x 33 ft = 50 ft

If you dove to 150 ft then your stop should be at 81 ft, computed as follows:

150/33 + 1 = 5.55 ATAs
5.55 ATAs / 1.6 = 3.5 ATAs
(3.5 ATAs - 1 ATA) x 33 ft = 81 ft

If you dove to 200 ft then your stop should be at 113 ft, computed as follows:

200/33 + 1 = 7.1 ATAs
7.1 ATAs / 1.6 = 4.44 ATAs
(4.44 ATAs - 1 ATA) x 33 = 113 ft

THAT IS HOW YOU COMPUTE A DEEP STOP.

That is also WHY you should do a deep stop. On shallower dives, we call the deep stop a "safety stop." But basically, it is the shallowest depth that you may safely ascend to, from your maximum depth, where you should stop, and pause for at least a minute, if not longer, to allow your body tissues and fluids to off-gass the excess pressure tension that you acquired during your scuba dive.

Hopefully that helps you understand the concept of safety stops and deep stops, and shows you how to properly compute them. I hope you are good with math, or else you will just be more confused now than you were before.

 

[BRW]

Greetings All,

Have been away in some nasty environs and just picked up
this leading thread (DeepTechScuba) on deep stops. Needs
a comment, or two. So,

DeepTechScuba

With all due respect to your thoughts on deep stops, they are

1) -- wrong on principles and gas dynamics,

2) -- wrong on application,

3) -- missing vital components of depth/time profile,

4) -- wrong using dissolved gas R-values (your 1.6)
which change at depth even within Haldane models,

5) -- not the way tech divers arrive at deep stops, using
Haldane and/or phase tables or software.

6) -- incorrect with respect to collected data used in Tables,
meters, and deco software.

Deep stops are a coupled bubble-dissolved gas, mini-max
staging procedure. They depend on mix, depth, time,
etc. Deep stops in a profile constrain free phases, while
shallow stops bleed free phases further, and eliminate
dissolved phases.

The method you describe is how early sat divers tried to
stage, and it doesn't work either for sat divers -- the limit
point.

Things are a way more complicated than you might think.

And it's easy to get "confused", even with any "math"

Regards, and good to be back,

Bruce Wienke
Program Manager Computational Physics
C&C Dive Team Leader

ut:

 

[DeepTechScuba]

I am honored to make your acquaintance, Bruce, and I have seen your name in many diving publications and have relished your articles.

I myself believe in a 1.6 to 1 pressure-to-pressure ratio for computing my first [deep] stop, since it is more precise than the unexplained "one-half of max depth" rule by Pyle mentioned in the earlier responses.

The empirical data certainly do support 1.6 in particular for air or nitrox diving. You are right in that helium and other gasses may very well be a totally different story.

I have played around with DecoPlanner and concluded that as you set the allowable gas tension tolerances more conservatively, you result in deeper first stops, as well as longer decompression times.

If 1.6 to 1 is indeed passe, and your own research and theories become mainstream, then we may very well be at the dawn of a new day. In the meantime, our mutual (yours and mine) certifying agency does endorse 1.6 to 1 without explanation, unfortunately, so I have tried to explain it.

I suppose if ordinary everyday nontechnical divers use a 1/2 of MOD procedure to determine their first deep stop, that is close enough to the truth, and better than assuming that 100 fsw for a deep deco dive or 15 ft for a deep NDL dive is always safe.

I prefer to use 1.6 to 1 and I have showed him how to calculate it and what empirical grounds it is based on.

Good luck with your latest theories. I look forward to your next publication(s).

 

[BRW]

DeepTechScuba,

Thanks for your comments. Your P/1.6 rule remains incorrect
as I detailed in my earlier post. Be careful if you employ it
within Haldane based deco software, as you may miss deeper
stops required by dual phase mechanics. Try some dual phase
software like GAP and ABYSS. Times are also very important
at deep stops, and arbitrary rules like "P/1.6" and "gradient
factors" miss the timing boat crucially. And with high risk.
No surprise since arbitrary times attend any arbitary deep stop
rule.

You are also wrong in stating that "both of our Agencies"
use a P/1.6 rule. Check the NAUI Tech tables for any mixture,
and any depth. What I stated in my earlier post IS amplified
therein. It's NOT ambient pressure divided by 1.6 to the
first deep stop in those Tables. Nor is it in dual phase deco
software you can find in the marketplace (GAP and ABYSS).
NAUI Tech/Rec tables are based on a full up, dual phase model
(RGBM), correlated with data in the field, and fitted to same.
Arbitrary rules do not enter anywhere here.

Nor in the sat regime if deep stops are juxtaposed safely.
Nor in very deep exposures of any length. Nor in the
field tested WKPP protocols. Nor here at LANL. Nor in
in some world record dual phase profiles employed by
cutting edge, extreme divers.

Using Haldane based software with deep stops imposed
by any prescription is NOT self-consistent. Nor are "gradient
factors" that are used to increase Haldane M, G, and
R-values. The problem is that you CANNOT mock up dissolved
gas staging algorithms to do bubble mechanics consistently
for all exposures and mixes. Same holds for the P/1.6 approach.

The "theories" you mention are not mine -- they are
college physics and chemistry 101. And have been known
since the late 1800s. They have been late coming on the real
diving scene because of medical inertia and requisite numerical
sophistication. Deep stops require dual phase dynamics plus
a computer to be done CORRECTLY across the board.


Regards,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Teal Leader

 

[BlueDevil]

I am a bit of a mathematical dyslexic so I am wondering if someone can give me an equation for claculating Pyle stops. I have seen two different methods and I am not sure which (if either) is correct. So I am sure there are some maths gurus out there who can give me a definitive answer.

Thanks, BD

PS I know that Pyle stops are regarded as outdated compared to VPM or RGBM but I just want to know for theoretical reasons.

 

[roakey]

Let me give you a couple answers but probably not the one you're looking for...

First off, Pyle Stops nowadays are known more commonly as deep stops. If you search the entire board for "deep stops" (including quotes), in titles only you get the following:

http://www.scubaboard.com/search.php?searchid=113184

(I don't know if this link will continue to work, which is why I told you how to do it) So I'd start there.

I guess I think of "Pyle Stops" as the stops that Richard Pyle stumbled upon while collecting fish -- it's an ad hoc protocol that he followed back when we knew even less about decompression than we do today. So though there were calculations to "compute" them, they were based on "what works, works" rather than any decompression model.

Nowadays we have a couple models, like VPM and RGBM, that actually calculate deep stops based on modern decompression models -- but these are not trivial calculations, so it's not a "bang it out on your calculator"-type formula.

So, at least in my mind, "deep stops", unlike "Pyle stops", are based upon a decompression model.

If you're doing real honest-to-gawd decompression diving, rather than using ad-hoc "Pyle stops" I would buy a decompression program that actually calculates deep stops based on its model.

If you're just doing recreational no-decompression (a misnomer) diving, there's a number of flavors of deeper stops you can perform, easiest being halve your depth and round up to the nearest 10 feet, do that again, etc., until you reach 10 feet and stop for 2 minutes, then surface and take it easy on the surface for a couple minutes before you start climbing out.

So, from 130 feet you'd ascend to 70, stop for 1, ascend to 40, stop for 1, ascend to 20, stop for 1, ascend to 10, stop for 2, then surface and hang out.

Links to check out, in no particular order:

VPM: http://www.decompression.org/maiken/VPM/VPM_Program_Site_Map.htm
RGBM: http://www.rgbmdiving.com/
GAP: http://www.gap-software.com/
Decoplanner: http://www.gue.com/decoplanner/

Wienke/NAUI on deep stops: http://www.gap-software.com/pdf/stopp.pdf

Roak