PADI Deco.....

[CamG]

Greetings ColinCB and you have asked a very thought provoking question that has spurred some good posts. In the recreational diving world every agency likes to teach conservatism which is meant to help reduce or manage the risk of DCS.
Decompression theory really is not as difficult as it sounds but many students in OW just are not prepared to learn the entire scope of it. In stead they are given parts of it to understand the logic behind safety stops and conservative dive planning.
I would suggest for you to research decompression theory and the methods that are used to calculate it. It cleared up a lot of my questions and helped me to better plan deeper longer dives once I understood the different algorithms and conservatism levels.
In the end you have to decide just what kind of dive you will plan and what conservatism to run on your computer or tables. I would rather know the information myself then trust my computer or any one else.
Take your time and research as you are now, you will be amazed at the information that is out there waiting to be digested.
If you feel so lead to enter the Tech realm then you will learn about doing deep stops correctly in a training class. Once you leave the Recreational realm you need to have a firm grasp of deep stops and advanced dive planning.
That all being said this is what I do when diving recreationally on dives over 60";
1. Plan the dive using RDP tables as max bottom time.
2. Dive redundant :manifold / independent doubles / pony bottle with single tank
3. Have specific objectives to meet on the dive, even if just a tour we discuss what if any issues we are trying to address in our diving technique.
4. Our descent rate is controlled but as quick as our buddies can go safely.
5. Our bottom time and depth not exceeded if anything it is cut shorter depending on team members and comfort levels.
6. Our ascent rates do not exceed 30' per minute and we do a stop @ half depth for 1 min. only. This a Recreational dive NOT A PLANNED DECO DIVE!
7. When we reach 18'-15' we do a 5 minute stop. All dives over 60'.
This is just our practice and not theory so take it as such.
Again I think that you are on the right track and desire to build conservatism into your dive plan. Continue to study all the while diving and gaining valuable experience mastering buoyancy and trim is paramount while pondering holding stops correctly in a horizontal to maximize off gassing.
Good luck and keep asking questions. That is what Scuba Board is about!
CamG Keep diving....keep training....keep learning!

 

[Divor]

Why do people talk about ongassing on ascent?
As long as you breathe air under pressure you can not offgas to begin with?

As far as I understand it, what you say is incorrect and you on- and off-gas the whole time. When you off-gas more than you on-gas, that's what's referred to as the off-gassing depth. So you are off-gassing the whole time. It is also very dependent on your dive profile, i.e. depth and tissue compartments involved (slow vs. fast)

 

[wedivebc]

Why do people talk about ongassing on ascent?
As long as you breathe air under pressure you can not offgas to begin with?

Not true As you ascend the pressure in some tissue will become higher than ambient pressure and you will begin off-gassing while other compartments that are further from ambient pressure continue to on-gas Depending on depth and time it can occur for quite some time before you reach the surface.

 

[SkimFisher]

Not the exact article from "Alert Diver", but close enough:

DAN News: Deep Stops

 

[TSandM]

Remember that decompression theory is just that . . . THEORY. Gas kinetics are modeled in simpler systems and extrapolated to behavior in human bodies. Gas absorption and elimination are driven by gradients, and as you absorb more gas at a given depth, that gradient decreases, so the rate at which you absorb MORE gas also decreases. This results in a half-life phenomenon. Different areas of the body are thought to have different half-lives, meaning that gas is both absorbed and eliminated from some regions more slowly than from others.

In addition, in deeper water, you have to ascend more FEET to generate the same fractional pressure change. (From 100 feet, you have to go to 30 to cut the pressure in half; from 30, you have to go only to the surface.)

The net result of the math is that decompression programs generate ascent profiles that are exponential . . . meaning one ascends more rapidly at depth, and slows approaching the surface. Marroni's divers used a linear ascent profile, so the people doing the 10 fpm ascent began that at the bottom and continued it to the surface. I don't think anybody is proposing that that is a good strategy.

The PADI 60 (or 30, as most instructors are actually teaching it) fpm ascent rate, combined with the 3 minute safety stop, is a way of creating a profile that ascends faster at depth and slower (stop) near the surface. It's better than no stop, but there may be better ways yet. NAUI, I believe, has adopted a stop at half maximal depth. GUE teaches a 30 fpm ascent to half maximal depth, and then 10 fpm from there onwards, achieved by what amounts to one minute stops at 10 foot intervals. Some of us add a very slow ascent over the last 10 feet to this (2 to 6 minutes to do the last bit).

As the incidence of DCS in properly executed recreational dives is extremely low, it will be difficult to gather data to show that any of these procedures is significantly better than another.

What IS definitely true, however, is that too slow an ascent rate from maximal depth is not useful, and in fact, may worsen bubbling. I suspect the reason the even slower rate in Marroni's study didn't show as much bubbling was that the very slow shallow portion probably deco'd out the slow deeper ascent part!

 

[halocline]

You're not being that harsh really, I know what you mean. I basically concerned with eliminating as much gas as possible so there will be little to no risk of DCI after the completion of the dive or subsequent dives. I've been reading about the use of half stops/deep stops for a few minutes as a safety precaution and I was wondering whether or not it had any merit within rec diving.

Since you have an interest in this topic, you should learn about decompression theory; you'll enjoy it and then you'll understand why a few people were 'scratching their heads' about your initial question.

The place to start is to get the concept of tissue compartments as a mathematical model. The idea is that N2 loading (and off-gassing) occurs at different rates simultaneously, and as such, there's a bit of complexity in predicting decompression requirements. At some depths/times during a typical dive, there can be simultaneous on-gassing and off-gassing predicted by these models.

There is a very simplified concept that you can apply to recreational diving, though, and that's that extending a shallow stop is widely considered to be the most efficient way to get rid of N2 in most typical recreational dive profiles. Deep stops are becoming much more closely looked at these days, but there is little consensus at the moment about their efficacy in recreational diving.

I'm not an expert, but my understanding of why an extended shallow stop is so beneficial to recreational diving is this: Because of the relatively short profiles in NDL recreational diving, it's often the faster compartments that control NDL, and those compartments can tolerate a higher pressure gradient than the slower compartments. Efficient off-gassing depends on getting to a depth where the controlling compartment has the highest pressure gradient it can safely tolerate, and for the faster compartments, that's shallow. If I remember correctly from the PADI encyclopedia (actually not a bad starting point for basic deco theory info) there is a study that showed that a 3 minute stop at 15 ft eliminated a very high percentage of excess N2 in recreational (non-deco) profiles.

Anecdotally, I find that I'm much less fatigued if I extend my safety stop to 5-6 minutes and then surface very slowly, often stopping at 10 ft and 5 ft. I believe that the lower fatigue level is probably associated with lower levels of N2.

I hope I'm reasonably on target with this, if not I'm sure there will be many experts happy to correct me!

 

[D_B]

I'll add that if you can hold a stop at 5 feet, that your going to have your buoyancy under pretty good control at that point

I think/thought it's reasonable to do a 1 minute half depth stop, if your dive is deeper than about 80ft?
... and following that, would a 3 minute stop at 20ft, then 3 at 10ft be a good practice/reasonable?

 

[Tigerman]

Not true As you ascend the pressure in some tissue will become higher than ambient pressure and you will begin off-gassing while other compartments that are further from ambient pressure continue to on-gas Depending on depth and time it can occur for quite some time before you reach the surface.

Which basically means youre not really offgassing, youre changing the "gas balance"?
As long as some tissue on-gas you cant be truelly off-gasing?

 

[ianr33]

Which basically means youre not really offgassing, youre changing the "gas balance"?
As long as some tissue on-gas you cant be truelly off-gasing?

The phrase "offgassing" in a deco context normally means that the LEADING compartment is offgassing. Other compartments may well be ongassing but as they are not controlling the deco they can be ignored.

Do a deep stop for 1 minute and you will be offgassing (good). Stay there for 30 minutes and you will be ongassing again (bad).

 

[Mr Carcharodon]

Which basically means youre not really offgassing, youre changing the "gas balance"?
As long as some tissue on-gas you cant be truelly off-gasing?

Some of both really. Your slower tissues will not be saturated for a recreational dive but your faster tissues will be. So as you ascend your fast tissues can be offgassing while your slower tissues are still loading. The ascent has to strike a balance between going slow enough but not too slow. Bubble models and disolved gas models come to different conclusions about the right balance.

The OP suggested stoping every few feet for a few minutes which is too slow unless you are within 30 fsw of the surface in which case it would be OK. Of course if you add signifcantly to your dive time is would be a good idea to know if your gas supply supports that plan.