Can you do too much deco?

[L13]

Deleted: statement was in error

 

[L13]

Pretty good and succinct voicing of the tradeoff idea, as long as we accept that it's bad to on-gas a bit more and spend more time in deco. Reasonable assumptions... but the deco obligation is completed in either case.

Which leads to... why not 80/80? How high is too high? We actually don't really know, and it probably varies by the day and diver. I'm sure we would all like to know.

Apparently some people think a GF_low of 50 is too high. Why do they think that? Is anyone reporting DCS at GF_low of 50? Is it is less at GF_low of 30? Or more? Is there any study that tested that without violating deco obligations?

More time in the water (exposure/operational) and more cumulative on-gassing/off-gassing sounds like two reasons to go for the higher GFs. But we're picking 50/80 arbitrarily instead of 100/100, or some other numbers.

What if someone were to tell you that your GF_low of 50 was too low and that you were wasting time on-gassing? Would you say that you are choosing GF_low of 50 to protect your "fast tissues?" Isn't this exactly what the GF_low 30 people are saying?

I feel like I am talking to a wall.

GF-High sets conservativism, pick what you are comfortable with. I'm not arguing for any particular value. Changing it has the expected effect.

But: GF-Low << GF-High is a bad idea no matter how conservative or not you are trying to be.

If you are comfortable with a GF-High of 80, good for you. GF-Low should be > 50 probably even higher. GF-Low = 80 is Buhlmann as designed with conservatism. GF-Low ~= 65 (per Doolette see below) might be better.

If you are comfortable with a GF-High of 60, good for you. GF-Low should be > 50. GF-Low = 60 is Buhlmann as designed with conservatism. GF-Low ~= 50 (per Doolette see below) might be better.

For those interested, here is the reference:

Gradient Factors in a Post-Deep Stops World

World-recognized decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

gue.com

Most of the people with whom I dive are in this ballpark.

 

[boulderjohn]

I do not like how the word "conservative" has evolved in regard to decompression diving. With recreational diving, the meaning of the word is clear, and it relates well to its common English usage. If you are aggressive, you are diving to the very edge of NDLs and creating a risk for decompression sickness. A more conservative approach stays away from NDLs and has less risk of decompression sickness. It thus coincides the common connotation of conservative = safer.

With decompression diving, things are a bit more complicated. A lower GF high is called conservative, and it is indeed safer. Things get complicated, though, when you use that word with GF low.

The word was specifically used in the VPM model. With VPM, the more you increased the settings (+2. +3, +4, etc.) to make them more conservative, the deeper your first stop was. That is consistent with the idea that you are staying farther and farther away from the m-value of the controlling compartment. For that moment in the dive, you can say it is safer, but is it safer for the dive as a whole? Carry it to an extreme, and you don't ascend--you just stay there ongassing. In fact, if you read the original WKPP deep stop theory, which used rounded off ATA's to determine first stop depths, at certain depths and roundings, you almost did your first stop where you were already diving. This nicely illustrates the point that to some degree, using very deep stops could be considered adding to bottom time.

The reason I don't like the word is that it creates confusion. Over the years I have seen people in threads like this making statements to the effect of, "I don't get it. Isn't it safer to be more conservative?" The answer is that is some cases, no, it is not safer to be more conservative.

 

[justinthedeeps]

I feel like I am talking to a wall.

I feel like I'm the only one listening... funny that!
Where you claim certainties here, I will keep posing questions.

As far as not having a big difference between GF_low and GF_high, I happen to agree that a smaller difference (or just a single GF) is probably good, but I'm not sure there is much empirical evidence to actually prove that, for divers who actually complete their deco obligations. It may well be that it doesn't really matter either way, as long as people can do the deco within reasonable runtimes.

For the OP's plan, having a lower GF_low has about as much effect as adding a couple minutes to your bottom time, or conversely, it's like ending a longer bottom time "early" to do a "deep stop" instead. So what? If it's in the plan and you do the deco, you will not explode.

If you believe in GF theory at all, then even a 10/90 plan should work out, as long as all of the deco is done. Suboptimal, maybe. You won't see me doing it, but this person should be safe from DCS if GF theory and current models are correct. If not, then you would have to accept that GF and deco models are wrong.

Longer and deeper dives than that? Then the GF strategy is going to have bigger effects to consider. Definitely go compare the plan and stops resulting from different GFs (just like the OP did)

Someone diving 90/90 is going to see your 50/80 dogma and say "wow buddy, what's with all the deep stops? Don't you know your deco isn't 'optimal'?" Let's admit that we're being fairly arbitrary here, and lighten up on people whose dive plans are totally fine.

 

[Wibble]

For those interested, here is the reference:

Gradient Factors in a Post-Deep Stops World

World-recognized decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

gue.com

Most of the people with whom I dive are in this ballpark.

I do wish they would have thrown in the graph of "Compartment Inert Gas Pressures" (Y-axis) to "Ambient Pressure" (X-axis). So much easier to understand.

Mark Powell explaining this in his lecture "Mark Powell: Intro to Deco Theory & Deep Stops". The diagram comes up around 20mins in.

 

[boulderjohn]

For the OP's plan, having a lower GF_low has about as much effect as adding a couple minutes to your bottom time, or conversely, it's like ending a longer bottom time "early" to do a "deep stop" instead. So what? If it's in the plan and you do the deco, you will not explode.

It's true. Deeper stops are the equivalent of longer bottom times, and you have to compensate for that during the shallower stops.

The problem with that is that many of the early proponents of deep stops believed that deep stops were so very, very good for you that you not only did not have to compensate for them, you could actually spend less time on the shallow stops. Doing deep stops, they believed, lowered your total decompression time.

These divers were not using computers that automatically add shallow time for the extra deep time. They were using pre-planned written schedules, and they were doing that planning in accordance with that theory.

 

[justinthedeeps]

Here are three diveplans in Subsurface, similar to what the OP describes.
Tissue saturation heatmaps are at the bottom of each.
GFs of 30/60, 50/60, and 80/80 are all shown (not necessarily in order)
Which plan is "best?"
The answer is, it depends--sometimes.
30/60 and 50/60 are nearly indistinguishable.
Should we really keep debating 30/60 vs. 50/60 in the context of the OP's dive plans?

80/80 gets you out faster... it should be obvious which one that is.

 

[EFX]

It's true. Deeper stops are the equivalent of longer bottom times, and you have to compensate for that during the shallower stops.

The problem with that is that many of the early proponents of deep stops believed that deep stops were so very, very good for you that you not only did not have to compensate for them, you could actually spend less time on the shallow stops. Doing deep stops, they believed, lowered your total decompression time.

These divers were not using computers that automatically add shallow time for the extra deep time. They were using pre-planned written schedules, and they were doing that planning in accordance with that theory.

The central idea in bubble models that incorporate deep stops is to keep bubbles below a certain number for a critical bubble radius. The surface tension of bubbles acts to inhibit their growth. As bubbles expand this surface tension goes down and bubbles will tend to grow in size. As they grow in size the surface tension decreases further which aids in their ability to grow producing a positive feedback effect. If bubbles are kept below a certain critical radius then we need less deco time at the shallower stops because there are less bubbles above a critical radius to act on. That's the theory.

When Erik Baker, P.E. introduced GF's the idea was to make the entire dive range more conservative and not just at surfacing (which is determined by GFHi). GFHi and GFLo set limits on Buhlmann's (100%) m-values. GFLo determined the depth of the first stop. If you look at a graph of one tissue compartment you can see the stair-step result of the GF's as the diver progresses from the first stop all the way to the surface. The GF of a particular stop increases linearly from GFLo to GFHi at the surface.

While the deeper first stop determined by GFLo added conservatism to the dive it also produced an undesirable side effect: longer shallower stops. This is because dissolved gas models don't track bubble size and quantity. (Hey, somebody forgot to tell the slower tissues to stop on gassing while we made those deep stops!) So, what was a conservative idea for bubble models became a liability for dissolved gas models.

 

[justinthedeeps]

Mark Powell explaining this in his lecture "Mark Powell: Intro to Deco Theory & Deep Stops". The diagram comes up around 20mins in.

This video is fantastic, thank you
We (and I mean not just me) should probably all watch it before returning to arguing about everyone's arbitrarily favorite GFs

 

[L13]

For the OP's plan, having a lower GF_low has about as much effect as adding a couple minutes to your bottom time, or conversely, it's like ending a longer bottom time "early" to do a "deep stop" instead. So what? If it's in the plan and you do the deco, you will not explode.

So, if you use deep stops, you have the less useful bottom time for the same total run time with a higher risk of DCS according to the NEDU data.

If you believe in GF theory at all, then even a 10/90 plan should work out, as long as all of the deco is done. Suboptimal, maybe. You won't see me doing it, but this person should be safe from DCS if GF theory and current models are correct. If not, then you would have to accept that GF and deco models are wrong.

Actually no. With 10/90 you spend more time at elevated GFs than with 90/90, and have a higher risk of DCS. Buhlmann GF theory has two components to risk Tissue Tension(GF) and Time. Extended times at elevated Tissue Tension is worse than short times at the same Tissue Tension.

Someone diving 90/90 is going to see your 50/80 dogma and say "wow buddy, what's with all the deep stops?

That isn't even remotely my position. I never once suggested 50/80 was optimum. I am saying that 50 is a MINIMUM (based on NEDU data), not that it is an ideal value, and higher most likely better, but exactly how much higher is how much better is harder to justify based on the experimental data. But there is good reason to believe that it should be closer to GF-High, with fairly good arguments for GF-Low = .83 x GF-High, and GF-Low = GF-High.

Someone diving 90/90 would get very little argument from me about his GF-Low choice as such. But I wouldn't be comfortable with GF-High over 85, and would prefer 70/85 to 85/85. And I would pad my shallowest stops to surface with GF as low as possible at the end.

Here are three diveplans in Subsurface, similar to what the OP describes.
Tissue saturation heatmaps are at the bottom of each.
GFs of 30/60, 50/60, and 80/80 are all shown (not necessarily in order)
Which plan is "best?"
The answer is, it depends--sometimes.
30/60 and 50/60 are nearly indistinguishable.

Actually, they are easily distinguishable.

While the deeper first stop determined by GFLo added conservatism ...

Actually, based on modern evidence, that statement is completely wrong. It reduces conservatism, while producing all the side effects you proceed to mention.

The Bubble models suggest that deeper stops add conservatism, but the Buhlmann model suggests that deeper stops are worse because of increased time at elevated Tissue Tensions. And the NEDU data suggests that the Buhlman model is a better predictor of DCS than the Bubble model, except when GF-Low is chosen to make it emulate a bubble model (GF-Low << GF-High).