Modern research/thoughts on Ascent Rates

[jadairiii]

Sorry, I was a WKPP member in the 90s. I assure you, the DCS rate was not zero.

You mean the guys in their trucks on oxygen and eating handfuls of Motrin were not doing it for fun?

 

[JonG1]

Wibs we sit on a 1.5 ppo2 on last stops on ccr quite happily, with cell occlusion through water vapour its potentially actually higher than that, so more like 1.6 (especially on your sweaty rEvo!)and cns is likely higher due to a constant then elevated ppo2 on deco.

Can't see the issue tbh.

Wasn't the 1m/min ascent designed to slow the last 6m because GUE/WKPP etc were pulling last stop at 6m so the reduction in pressure would be detrimental if a 3m/min rate is used.

I think I recall a thread on here about surfacing gf too that discussed a positive impact on SGF with a much slower last ascent rate.

It's also discussed on a TDF thread, which I'll see if I can find

 

[JonG1]

https://www.thediveforum.com/showthread.php?26694-6m-last-stop-vs-3m-last-stop

 

[Akimbo]

Aren't there some drawbacks to using pure oxygen for the 6m/20ft and above stops?

Such as:

• CNS and pulmonary oxygen loading is increased.
• Have to ensure you don't inadvertently drop below 1.6ATA (6m/20ft) as the PPO2 will be very high, rapidly increasing CNS
• Long deco hangs mean taking "air breaks" to give the lungs a rest - pulmonary oxygen toxicity

Given that a bottom phase PPO2 of 1.4 has been debated as being too high (have heard Dr Mitchell mention this), 1.6 seems almost excessive.

Reducing the oxygen content with a weaker mix (80%?) will mitigate this. As will ascending above 1.6ATA, but only when the ceiling's moved up.

Absolutely! There is no free lunch. Fortunately computers also track O2 exposure and anyone with advanced Nitrox training, which certifies for pure O2 use, "should" understand these constraints.

Unfortunately 80/20 does not have the advantage of zero diluent absorption during decompression.

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Other readers who are not fully familiar with these principles may find this thread interesting:

Info - Oxygen Toxicity Limits & Symptoms

Oxygen Toxicity Limits & Symptoms Oxygen toxicity limits can be very confusing, especially for PPO2 (Partial Pressure of Oxygen) levels above 1.6 ATA used in chamber-based hyperbaric treatment (recompression) and decompression tables. For example, here is a chart of one of the most common DCS...

scubaboard.com

 

[inquis]

Aren't there some drawbacks to using pure oxygen for the 6m/20ft and above stops?

I would call the items you listed "characteristics" rather than drawbacks. As with most things, the particular circumstances will dictate whether they might be driving considerations or not.

 

[Akimbo]

Because there are so many people with wildly different expertise reading this thread (and not participating) it might be worth providing a simplified perspective. Decompression algorithms are mathematical models used by decompression computers that "approximate" diluent absorption and release in divers in order to minimize bubble formation that can block blood flow.

Hundreds, maybe thousands, of different tissue absorption rates are grouped into manageable ranges for the convenience of calculations — typically 16 or 32 for optimum computer memory use. This is a massive simplification of the amazingly complex interactions at the cellular and atomic levels that constantly occur in our bodies. That said, it is pretty darn good after 120 years of research — but far from perfect.

 

[Wibble]

It’s done on thousands of dives every year with no ill effects.

Does this mean what I said was wrong?

 

[Superlyte27]

I

Does this mean what I said was wrong?Depe

Depends on perspective I guess. I don’t see any drawbacks. Constraints maybe. But it’s real world that works for 80% vs published data that works for 99.9% of divers. I’m part of the 80%

 

[Manatee Diver]

Context of the dive is important, I'm reasonably confident that this protocol is only used on dives with significant deco. A person incurring 10 minutes of deco at Ginnie shouldn't be spending an additional 20 minutes surfacing.

Certainly agree, I probably should've noted it for people that aren't familar with the super long deep dives that WKPP was doing when they developed the protocols. But I think that perhaps slowing down the last 20 feet is still valid. Maybe not the super slow 1fpm, maybe slow it to 10fpm? Heck I just did a no deco dive, the last few feet were super slow just to scan for boats so I don't get run over.

 

[grantctobin]

Certainly agree, I probably should've noted it for people that aren't familar with the super long deep dives that WKPP was doing when they developed the protocols. But I think that perhaps slowing down the last 20 feet is still valid. Maybe not the super slow 1fpm, maybe slow it to 10fpm? Heck I just did a no deco dive, the last few feet were super slow just to scan for boats so I don't get run over.

I think on any dive with a major deco obligation, mitigating the speed from 10’ to 1’ is prudent. E.g 1’/min for the last ten feet on a 6hr dive is relatively minor in the grand scheme of things. Add in some surface deco and general relaxation, and I’d assert that the margin of safety is improved. Caveat emptor, “major deco obligation” is a relative term.