Padi Advanced OW - Deep stops??

[KenGordon]

BTW, you do realize, don't you, that your GF is always 0 until you start to ascend, so you will never reach your GF Lo ... until you begin your ascent.

If you put it like that it could be negative depending on how long you have been at that depth.

How about, "when do you have a deco obligation? When a normal ascent will lead to a GF exceeding GF lo."

 

[Skyflyer]

Just did my Advanced OW Cert and no deep stops on the deep dive specialty. Only a 3 minute safety stop at 15'.

 

[KenGordon]

That is my belief also and borne out by planning. I believe Ken has misspoken on this topic. It is difficult for many to discuss both no-stop diving and deco diving without confusing the two. I think we all agree about one thing, most recreational divers would not have a clue to what we are discussing.

The same dive to 30m (about 100ft) leaving the bottom at 15 minutes. No stop with GF of 95/95 but stops with a GF of 50/95

Depth Stop Run Mix pO2 EAD

Des 30 - 1 21 - -
Lvl 30 13 15 21 0.84 30
Sfc - - 18 21 - -

Dive # 1, ZHL-C+GF 95/95
Elevation = 0 m
CNS = 4%
OTU's = 11
Decozone start = 19 m
Gas 21 = 1167 ltr.


Depth Stop Run Mix pO2 EAD

Des 30 - 1 21 - -
Lvl 30 13 15 21 0.84 30
Asc 6 - 17 21 - -
Stp 6 0:36 18 21 0.34 6
Stp 3 1:00 19 21 0.27 3
Sfc - - 19 21 - -

Dive # 1, ZHL-C+GF 50/95
Elevation = 0 m
CNS = 4%
OTU's = 11
Decozone start = 19 m
Gas 21 = 1201 ltr.

It is a shame that recreational divers would have no idea what we are talking about. Press the buttons in the wrong (or maybe right) order and suddenly they are technical divers (apparently).

By way of comparison, a Suunto Vyper gives an NDL time of 17 minutes. I think that means you need to leave the bottom before minute 18 or get stops.

 

[scubadada]

You are wrong Ken, GF lo does not kick in unless you cannot make a no stop ascent to the surface at less than or equal to the GF hi. Go back and think about it. You should not be so condescending when it comes to no stop divers.

 

[stuartv]

The same dive to 30m (about 100ft) leaving the bottom at 15 minutes. No stop with GF of 95/95 but stops with a GF of 50/95

I ran this plan on 3 different planning tools. 2 of them varied based on the GF Lo and the 3rd did not.

I have always understood GF to be used as @scubadada said. I.e. If you can make a direct ascent to the surface without exceeding GF Hi, then GF Lo is not used. But, it is clear (now) that some implementations of GF use GF Lo differently (than I understood). However, it also seems to be the case that there is no ambiguity about how GF Hi is used.

And, I submit that using GF Lo, even when GF Hi will not be exceeded makes dubious sense. You will exceed GF Lo (when set to the normal 30 - 50 range) on almost any dive, other than the shortest, shallow dives. Using GF Lo that way would make almost every (okay, maybe only many, many) normal recreational dive into a deco dive. That does not make sense.

Example: I'm doing a dive that is intended to be an NDL dive, and I have my computer set to GF 30/70. I'm on the bottom and a direct ascent would have me at the surface with a GF of 65. In that case, clearly there is a point before I hit the surface where my GF will exceed 30. For my computer to give me a mandatory stop in that case makes no sense whatsoever.

Of course, this distinction is only relevant to people attempting to do NDL dives, which is not where GF are generally used. Once you are down long enough that you'll exceed GF Hi, it doesn't matter, right? Regardless, I think that defining a dive as a deco dive based on GF Lo does not make sense. It only makes sense (to me) to define it as a dive where you will exceed GF Hi on a direct ascent*.

*Obviously, the definition only applies to people using the Buhlmann algorithm with GF. Other algorithms implicitly have different ways of determining if there's a mandatory stop.

 

[KenGordon]

You are wrong Ken, GF lo does not kick in unless you cannot make a no stop ascent to the surface at less than or equal to the GF hi. Go back and think about it. You should not be so condescending when it comes to no stop divers.

Did you read the two plans? Same bottom time, same GF hi and different GF lo. The GF lo difference introduced the stops. Clearly the diver could get to the surface within the 95 GF used in the 95/95 plan, so if the 50/95 plan stops them then the cause is that the limit starts at 50.

I think it is page 145 of Deco For Divers which describes this, maybe page 147.

 

[KenGordon]

I ran this plan on 3 different planning tools. 2 of them varied based on the GF Lo and the 3rd did not.

I have always understood GF to be used as @scubadada said. I.e. If you can make a direct ascent to the surface without exceeding GF Hi, then GF Lo is not used. But, it is clear (now) that some implementations of GF use GF Lo differently (than I understood). However, it also seems to be the case that there is no ambiguity about how GF Hi is used.

And, I submit that using GF Lo, even when GF Hi will not be exceeded makes dubious sense. You will exceed GF Lo (when set to the normal 30 - 50 range) on almost any dive, other than the shortest, shallow dives. Using GF Lo that way would make almost every (okay, maybe only many, many) normal recreational dive into a deco dive. That does not make sense.

Example: I'm doing a dive that is intended to be an NDL dive, and I have my computer set to GF 30/70. I'm on the bottom and a direct ascent would have me at the surface with a GF of 65. In that case, clearly there is a point before I hit the surface where my GF will exceed 30. For my computer to give me a mandatory stop in that case makes no sense whatsoever.

Of course, this distinction is only relevant to people attempting to do NDL dives, which is not where GF are generally used. Once you are down long enough that you'll exceed GF Hi, it doesn't matter, right? Regardless, I think that defining a dive as a deco dive based on GF Lo does not make sense. It only makes sense (to me) to define it as a dive where you will exceed GF Hi on a direct ascent*.

*Obviously, the definition only applies to people using the Buhlmann algorithm with GF. Other algorithms implicitly have different ways of determining if there's a mandatory stop.

If you are doing a 30/70 plan and you would hit 65 on a direct ascent then of course you will get stops, that is the point, not to exceed 30 deep to protect the fast tissues.

People are often recommended GF based computers on this board. Many will be being used for no stop dives. I think those are poor recommendations for new divers. Clearly it is a bit complicated and there is scope for different implementations.

 

[stuartv]

If you are doing a 30/70 plan and you would hit 65 on a direct ascent then of course you will get stops, that is the point, not to exceed 30 deep to protect the fast tissues.

No. You will get stops with SOME implementations of GF. Not all. That was MY point.

As a side note, if you're fine with getting out of the water with a GF of 70, why would you feel like you're increasing your safety somehow by limiting your GF to 30 at an early part of your ascent? The whole basis of Buhlmann and its M-values is the notion that there is a specific pressure gradient where bubbles will form. Below that pressure gradient, they don't form. The value of that pressure gradient which produces bubbles varies based on things like which specific tissue you're talking about (modeled as compartments), dehydration level, and lots of other factors, some of which science may not even know yet.

So, if you're comfortable that you won't get bubbles at the surface with a GF of 70% of the M-value, what do you think makes it safer to keep your GF at 30% of the M-value during an earlier part of your ascent? Is there some evidence that suggests that a certain pressure gradient value produces bubbles when tissue tension and ambient pressure are both higher, but that same exact pressure gradient value doesn't produce bubbles when the tissue tension and ambient pressure are both lower (but still in the same mathematical relationship to each other), as in when you reach the surface and ambient pressure is 1 ATA?

BTW, have you watched Simon Mitchell's presentation on decompression controversies?


Towards the end (at 44:30), after debunking the "deeper" stops mandated by bubble models like VPM-B, he says that, for himself, he uses GF 50/70 or 75. And it's actually unclear (to me) why having a GF Lo that is ANY lower than the GF Hi is actually useful.

 

[KenGordon]

First of all, I am not championing GF. I am describing how it works.

Deco for Divers (first version) page 147

"The GF adaptions to the Buhlmann model work by inserting a stop during the ascent when tissue supersaturation reaches the value for gradient factor low."

Which implementation is different? One thing put forward to support the superiority of GF computers over proprietary ones is the supposedly open nature of the algorithm. If the planners do not produce consistent results then when you dive the plan how can you expect it to line up with your dive computer?

You should take care not to think that the 'current GF' properly represents the state of a diver. It simply represents the leading compartment. The rest of the compartments (themselves just a mathematical construct) are in whatever state they have got to. So a diver at a particular GF might have the rest understaturated or overstaturated as far as the GF. 'Current GF' values are not comparable with each other because of this. A diver limited a fast compartment at x% of its limit is not at all in the same position as one limited by a a slow compartment at the same percentage.

I am quite aware of the whole deep stops controversy leading on from the NEDU study. I think I have read all the threads and watched that video. I believe I understand Simon Mitchell's points.

As for why you might chose 50/70 rather than 70/70. These numbers are used to modify some other numbers. Those are per tissue compartment. They were derived by some theories, equations, then some experiments and some adjustment. They might be wrong, maybe the fast ones are too liberal. Deeper stops are more likely to limit on the fast tissues. For a square dive of any duration they will be saturated and likely first to limit an ascent. Adding deep stops to reduce that limit is a bit like reducing the fast tissues' limits. Of course that is just a made up explanation. Might be true, might be bogus, we really cannot tell. Maybe it is because deeper dive lead to more bends, or that fast tissue bends are more likely to be neurological.

My main point is that deco models are reasonably complicated and behave in ways hard to predict. Overly simple explanations being used beyond their useful scope leads to confusion. What might be true in a particular set of dives might not be in another. Blanket assertions about brand X vs brand Y because of the algorithm, especially NDL numbers, is not helpful.

 

[scubadada]

This thread started by asking about deep stops for recreational diving. Deep stops are not indicated. No stop diving do not require any stops, regardless of algorithm chosen. Deco diving is another topic