Deep Stops Increases DCS

[victorzamora]

there's no obligation not to lengthen the deco on both the deep end (by a few minutes through deep stops on back gas) and then compensate on the shallow end with a modest GF high of 70 or even 60.

I believe that this issue has been addressed before, but I simply don't get why you would add deco time deep to then compensate for it shallow. It just seems unnecessary and counter to logic.

As for NEDU adding time at 70ft, that was on Air. I'm sure that a 70ft stop on EAN50, on a different profile than what the NEDU study ran, is a very different scenario and the depth of the stop isn't indicative of them being equivalent in concept.

 

[Redshift]

There is a lot to read and I still haven't covered all the available material.

A couple of questions come to mind...

Was the decompression schedule determined first for the shallow stops model and then the same time imposed on the bubble model? And, if so, despite the bubble model being free to distribute the stops, it had to use the 174 minutes, even though it might not be optimal (the paper says it may not be optimal for either of them). Here I think there is a problem in which one model may be forced to do something it doesn't want. If the bubble model was given just depth and bottom time what would have been the chosen deco schedule?

 

[kr2y5]

I just wonder... if, as someone mentioned here earlier, the goal was to demonstrate a general principle, rather than evaluate specific profiles/models actually used in practice, why not resume experiments on goats? One could have performed many more experiments, and on a wider range of parameters this way...

 

[LiteHedded]

I just wonder... if, as someone mentioned here earlier, the goal was to demonstrate a general principle, rather than evaluate specific profiles/models actually used in practice, why not resume experiments on goats? One could have performed many more experiments, and on a wider range of parameters this way...

because the navy doesn't want to know if it bends goats, they want to know if it bends sailors.

 

[kr2y5]

because the navy doesn't want to know if it bends goats, they want to know if it bends sailors.

Many folks in this thread raised the concern that the tested profiles were similar to, but not really quite like those people actually dive... the response was that the study was about testing a general principle, and not specific profiles. One could then argue that goats are also similar to humans... and one could also test a principle without using human subjects. AFAIK, Haldane's experiments on goats were the first proof that "deep stop causes DCS", which is why people departed from constant-speed linear ascent, in favor of profiles that allocate more time shallower. It would be fascinating, to me at least, to see similar experiments conducted over a hundred years later, with all the incredible modern technology that was developed since then... anyhow, I did not mean to insert a stick into an anthill, just randomly thinking aloud...

 

[boulderjohn]

One problem with goats is that it is really hard to teach them to use scuba gear and hold decompression stops.

That sounds like a silly answer, but it is actually serious. To use goats, you have to do your "dives" in a chamber. that has several advantages, but it has disadvantages as well. As was espiecially discovered when chamber experiments were done regarding oxygen toxicity, results in a chamber can be different from results under water.

 

[rjack321]

I believe that this issue has been addressed before, but I simply don't get why you would add deco time deep to then compensate for it shallow. It just seems unnecessary and counter to logic..

Because many people believe (based on decades of actually diving) that they feel better stopping or pausing or slowing down below the first required stop. This goes way back to pyle stops if not before. The idea that you need to protect fast tissues in one part of the ascent and slow tissues in another has biological plausibility too. Perhaps not so much for a very long air dive to only 170ft. But the situation might be entirely different for a short (20mins) bounce to 250ft on 15/55 trimix. All the fast tissues are obviously saturated and 5 mins worth of deep stops won't make a big difference on the slowest unsaturated slow tissues.

As for NEDU adding time at 70ft, that was on Air. I'm sure that a 70ft stop on EAN50, on a different profile than what the NEDU study ran, is a very different scenario and the depth of the stop isn't indicative of them being equivalent in concept.

For an 18/45 dive when you switch to EAN50 you are actually on-gassing N2 there at 70ft. fN2 goes from 37% to 50%. So adding time to a gas switch may be counterproductive (although many people do add 3 to 4 minutes to the 70ft stop, redistributing it from the 40 and 50ft stops). In VPM if you force the time after the switch the latter stops will shorten or reduce. But I don't know if these reductions are due to the dissolved portion of the model or the bubble phase. If these reductions are due to the bubble phase "clearing" and the validity of the bubble phase is dubious, perhaps people are actually increasing their risk by adding time at 70ft.

 

[John_ZAR]

For an 18/45 dive when you switch to EAN50 you are actually on-gassing N2 there at 70ft. fN2 goes from 37% to 50%.

Hopefully not, that can introduce isobaric counterdiffusion so you should have He in your deco gas.

 

[100days-a-year]

I wonder how many dives have been made without DCS or ICD using deep stops and nitrox based intermediate gasses?

How many cases of either in recreational Tech divers are incontrovertibly directly attributable to these two practices?

 

[UWSojourner]

I guess my question is how does the NEDU study inform practices like switching to EAN50 before the first mandatory low GF 40 stop? Or the shape of the deco once on a deco gas, which might be am 18/45 to EAN50 switch or a 12/65 to 21/35 switch. (these are just examples of current open circuit practices). The NEDU study's air backgas only and the Heat maps are hard to translate into open circuit diving with gas switches (not the O2 switch).

Thanks for your time here, I didn't think to ask this question while the RBW was active.

I don't think the heat maps really care if the dives are OC or CCR, have gas switches or don't. The heat maps are just looking at patterns of supersaturation and continued on gassing.

The heat map below is for OC 200ft 30min dive, 18/45, EAN50 at 70ft, 100 at 20ft. The profiles are generated by VPM-B+3 and GF 75/75. Each of those profiles then is shown with an extra 5 minutes of time spent at 70ft.

In addition the integral supersaturation for those profiles is shown. Integral supersaturation is simply a measure of time spent in a supersaturated state.

I would note the following:
1. The profiles surface at around 90 minutes.
2. The VPM-B+3 profile has deeper stops limiting early supersaturation. Of course this causes the higher supersaturations once the diver surfaces.
3. The VPM-B+3 profile has roughly 30% more supersaturation obligation once the diver surfaces (546.7 / 423.6).
4. The charts pretty much repeat the visual pattern of the NEDU study for A2/A1 profiles.
5. The additional 5 minutes at 70ft really doesn't impact the overall picture.