Can you do too much deco?

[L13]

remember that MultiDeco is only giving us two deeper stops of 40 seconds at 39 metres and 2 minutes at 36 minutes when compared to a GF 50/80 plan
This obviously gives a longer dive profile than say a GF 50/80 plan (adds in 27 more minutes) the water is 21degC / 70degF

The NEDU data definitively shows that GF 50/80 is SAFER than GF 30/80, and strongly suggests that GF 50/60 would similarly be safer than GF 30/60.

Lurker here among legends
While I too have keenly followed the 50/80 rationales, if you want a pee break at 39 metres and some more shallow hang time, then seems ok, no?

Will that extra 27 minutes mean the difference between a bend or not while climbing onto the boat before you sit down for another hour of relaxed surface deco? Idk. Probably depends a lot on the individual that day.

The END for 65% at 60m with 1.3 ppO2 (~18%) is like 5 metres? Nobody I know intentionally aims for a 5m END

Is there a conflict between:
a) trying to make the deco more conservative/safer, and
b) upping the fast-diffusing gas to levels higher than normal?

NEDU data says you are wrong, and that the extra deep time WILL lead to increased chance of getting bent(but still fairly small chance since we are far bellow GF 100). so ascend 6 more meters (less than a min) before your pee break to reduce the chance of bends.

 

[bubblemonkey2]

The NEDU data definitively shows that GF 50/80 is SAFER than GF 30/80, and strongly suggests that GF 50/60 would similarly be safer than GF 30/60.


NEDU data says you are wrong, and that the extra deep time WILL lead to increased chance of getting bent(but still fairly small chance since we are far bellow GF 100). so ascend 6 more meters (less than a min) before your pee break to reduce the chance of bends.

Would you mind re-posting said study here where they directly compared someone following actual GFs 30/80 vs. 50/80 (or 30/60 or 50/60 etc), for a dive similar to what the OP describes? I'm not sure that exists

If I remember correctly, the experiment involved forcing people to cut short their shallow deco if they did "deep stops," so that the profiles were "comparable" (i.e. same runtime). The result would be some very very whacky GFs--very far from any of those listed above, and obviously dubious.

This is however not what the OP is planning. The OP is planning a "tidy up the stages for a sec" brief pause at 39 metres, which is not what I would consider exceptionally "deep," and then doing another 27 cumulative minutes of shallow deco because they feel like they have the time. I personally wouldn't write in an actual stop there, but it doesn't sound that weird in this case. A lot of these mini "deep stops" will clear on the ascent before you even get there, in practice.

I don't think NEDU says much about the OP's dive plan, my recollection is it studied very different (and anomalous) profiles.

Now if someone were doing 5 minute stops at 50+ metres and then subsequently blowing past their ceiling later on, I too would not be surprised if more of them experienced problems. Or if they followed GFs of 10/60 or whatever, their runtimes could go beyond anything practical or safe for physiological or operational considerations.

My comps are on 50/80 most of the time... just sayin

 

[L13]

Would you mind re-posting said study here where they directly compared someone following actual GFs 30/80 vs. 50/80 (or 30/60 or 50/60 etc), for a dive similar to what the OP describes? I'm not sure that exists

I haven't kept links to most the details, and I'm sure you are right that it wasn't exactly those profiles (I seem to remember the GF high being ~85, but I'm not even sure about that), so I'm sure you can pretend that it didn't indicate that GF-low 50 was better than GF-low 30.

This is however not what the OP is planning. The OP is planning a "tidy up the stages for a sec" brief pause at 39 metres, which is not what I would consider exceptionally "deep," and then doing another 27 cumulative minutes of shallow deco because they feel like they have the time.

But at the end of that 27 extra minutes of time, you surface with GF-Surf = 60. If you did the GF 50/60 profile instead and then spent the same 27 minutes extending your shallower stops, you would surface with a GF-Surf significantly less than 60, and be even safer. You could extend the GF50/60 shallow stops by a good 12 min, surface with a lower GF-Surf less than 60, and be back on the boat a quarter hour sooner, and still be safer than both the GF 30/60 and GF 50/60 profiles.

Assuming the Buhlmann Model is Accurate Enough to be Useful
(otherwise, why are you using it with GF's to compute stops):
GF-low 30 (vice 50) always adds deco time, integrated deco stress, and gas consumed, for no change in GF-Surf (set by GF-high) and greater risk of getting bent. And you can always use any of that extra time/gas at shallow stops to reduce your GF-Surf and integrated deco stress further. This conclusion is implicit in the model.

The optimum spread across stop depths of any extra time is probably (though I'm not certain of this) found by reducing GF-high till your total planed runtime is hit, while keeping your GF-low ~= 0.85 x GF-high.

Any Haldane derived models (like Buhlmann) will produce a similar conclusion. Other models (like VPM) will produce different conclusions. Someday we may find a better model. But the NEDU data showed that the Haldane models were better predictors of DCS than the bubble models.

 

[L13]

Using Buhlmann with Gradient Factors to calculate your deco implies the following:

GF-High < 100 implies the Buhlman model is (close enough to) right
except he isn't conservative enough for my taste for all tissues.

GF-Low < GF-High implies the Buhlman model is (close enough to) right
except he underestimated the relative sensitivity of the faster tissues.

OR I guess you could go with:

The Buhlmann model isn't (close enough to) right,
but by putting 2 magic numbers on it we can make it look (close enough to) right
even though we can't explain why, but trust us it is more right with these magic numbers

 

[bubblemonkey2]

Sure, the 30/60 diver spends more cumulative time on-gassing, a bit more overall decompression is required.

The margin of safety is not simply about lowering the GF numbers, a good point.

But OP doesn't want to push a high GF early in the ascent. Is there merit to their "slow ascent" plan?

How fast is too fast? 80/80? 100/100? How much of it is specific to the person that day? (Fitness, physiology, hydration, repetitive diving etc)

Is repetitive diving with low GFs more risky because of the total cumulative gas absorption involved? Which is more likely to cause gas (dissolved or bubbles) to get stuck in the body somewhere: low GFs (more accumulation) or high GFs (more bubbling?)

 

[Nick_Radov]

different idea: since you have a small group of divers repeating the exact same profile many times, why not bring a Doppler bubble monitor and record their bubble grades? That could be interesting for them to see, how does it relate to exertion at depth, and also to adjust the decompression strategy if necessary.

Doppler bubble studies can be interesting but don't take them too seriously. There is only a loose correlation between bubble grades and clinical DCS symptoms. Some divers may have lots of bubbles but no symptoms (although there is a possibility of hidden damage that only shows up after repeated exposures). Other divers may be seriously bent with relatively low bubble grades. The test procedures are also somewhat subjective so two different technicians might record slightly different results.

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For the last 30 years, this book has been considered the

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[L13]

Sure, the 30/60 diver spends more cumulative time on-gassing, a bit more overall decompression is required.

The margin of safety is not simply about lowering the GF numbers, a good point.

But OP doesn't want to push a high GF early in the ascent. Is there merit to their "slow ascent" plan?

How fast is too fast? 80/80? 100/100? How much of it is specific to the person that day? (Fitness, physiology, hydration, repetitive diving etc)

Is repetitive diving with low GFs more risky because of the total cumulative gas absorption involved? Which is more likely to cause gas (dissolved or bubbles) to get stuck in the body somewhere: low GFs (more accumulation) or high GFs (more bubbling?)

Where is this 80/80 and 100/100 coming from? All anyone was advocating for was raising the GF-low to 50, making it 50/60 instead of 30/60. Not once has someone proposed that OP should raise his GF-high.

The ongassing through the slow ascent of the deeper stops results is slower tissues having to offgas more at close to the GF-high later in the deco profile. For each minute fast tissue offgassing you add at 30% GF, you add 10ish minutes of slow tissue offgassing at close to GF-high (60% in this case). It is a fundamentally bad trade off.

Low GF-Highs are better, all tissues are further from limits, but at a cost in time. Low GF-Lows are worse because they make the GF-High problem worse later in the deco and add a high cost in time.

 

[Celt]

Who is running the job?who is the dive supervisor? who are the archaeologists? where is the site? is there a chamber on site? what surface support? what are the conditions?and most importantly what kind of Mickley Mouse operating would look for decompression advise on an Internet forum.

 

[jale]

Hi
Why don't you try to approach other teams like Frank Goddio's which, I believe, did a lot of similar works at similar depths?

 

[leadduck]

You seem to be holding to the old deep stop theory that medium and slow tissue on-gassing is necessary to prevent fast tissue bubbling. What research do you have for that?

I don't hold to that theory and wouldn't suggest to move time from shallow to deep, like 20/80 or VPM does. I think that exertion at depth increases the decompression obligation in every tissue, fast and slow. That's an argument to prefer for example 40/70 over 50/80, but not a reason to pick 20/80. The other trick with calculating an virtually extended bottom time does just that; every tissue gets more gas, every stop becomes longer.