Deep Stops Recreational Divers

[boulderjohn]

I am sure many divers ascend more quickly than they should, but 200 FPM is smokin'.

 

[dmaziuk]

I am sure many divers ascend more quickly than they should, but 200 FPM is smokin'.

No it's not: from 3 m / 9 feet safety stop it's, rounding for simplicity, 10 feet at 200 fpm = 1/20 of a minute = 3 seconds. It's roughly one meter per second -- much faster than safe ascent rate from depth, but the $15 question is how unsafe is it over 3-5 seconds? At the end of a safety stop?

 

[dmaziuk]

Good question. This would be a good study, if it could ever be done. In my experience while working in the islands is more than half the people ascend rather quickly from the safety stop.

Well if they're "properly weighted" they'll be neutral at the safety stop with an empty tank and no air in the wing... so the moment they start ascending they turn buoyant and have to kick against the lift... Given the depth, they'll be topside before they finish turning upside-down. Unless they do their safety stop upside-down, of course.

 

[rongoodman]

Given the depth, they'll be topside before they finish turning upside-down. Unless they do their safety stop upside-down, of course.

A good reason to do them flat in the water, or even a little head-down.

 

[CandiveOz]

Suunto HelO2. It is an OC Trimix computer, same size as a Zoop.

Ken, thanks for responding. I downloaded the user manual for the HelO2 and Zoop to familiarlise myself with these computers. The original Zoop also didn't offer the ability to turn off the deep stop function but I noticed that the current Zoop Novo added this feature. I find it interesting that Suunto now offers 4 different versions of the RGBM algorithm. The Fused and Fused 2 generate quite different deco profiles past 33 metres compared to Tech and the Standard RGBM. The "deep stops" tend to drop off using Fused according to my initial testing done in DM5.

 

[NAM001]

Theoretically if you use a very fast tissue compartment, like RGBM's or ZH-L12's 2.5 minutes, it will on-gas very fast and reach NDL very quickly. That NDL will not be meaningful because the compartment will also off-gas very fast on ascent. i.e. with slow enough ascent rate it'll never actually hit its M-value. But if you blow that ascent rate it can.

Now whether there is an actual "2.5-minute tissue", or whether coming up 3 meters in 2 seconds really matters is another question.

(Note that dive computers tend to average ascent rates over longer-than-a-couple-of-seconds intervals before turning on ascent rate alarm. I expect such averaging has not been applied to the "200 fpm" number because, well, the surface is too close.)

I agree

 

[CandiveOz]

PDIS for rec diving is a pretty minor phenomenon in this regard, but if the theory is correct for tec diving, I believe it should be applied to no-stop diving as well, to lessen tissue decompression stress.

Thanks for the clarification on your position. I've look closely into PDIS and concluded that the developers at Uwatec, did approach this issue with a bit of science behind their implementation as compared to other developers who basically applied a half depth rule after exceeding a certain depth.

First, the deep stop doesn't come into play automatically when a certain depth is exceeded - this is why PD stands for "Profile Dependent." It requires time at depth for on-gassing to occur before a stop is recommended. And the initial recommendation is shallower than half the depth, starting at 8 meters; hence IS stands for "Intermediate Stop" rather than Deep Stop. The Stop then slowly increases the longer you stay at depth.

Also, the first 4 fast compartments out of the ZHL-16 (the first 2 in ZHL-8) are ignored and the 5-6 compartments (3 in ZHL-8) are taken into consideration when defining the stop based on the diver's profile. If graphed using the Scubapro SmartTrack software, you can actually see the 5-6 (3) compartments initiate the off-gassing phase at the intermediate stop. This off-gassing may not necessarily occur with just applying a rule of thumb method of half the depth or pressure regardless of time at depth. So there is a bit of logic behind Uwatec/Scubapro's PDIS.

 

[dewdropsonrosa]

https://repository.si.edu/bitstream...edings_safeascents.pdf?sequence=1&isAllowed=y

This is not exactly what I was reading, but does include discussion of 200fpm ascent rates. I continue the hunt waiting for the Potter Stewart moment - I will know it when I see it.

 

[rsingler]

Thanks for the clarification on your position. I've look closely into PDIS and concluded that the developers at Uwatec, did approach this issue with a bit of science behind their implementation as compared to other developers who basically applied a half depth rule after exceeding a certain depth.

First, the deep stop doesn't come into play automatically when a certain depth is exceeded - this is why PD stands for "Profile Dependent." It requires time at depth for on-gassing to occur before a stop is recommended. And the initial recommendation is shallower than half the depth, starting at 8 meters; hence IS stands for "Intermediate Stop" rather than Deep Stop. The Stop then slowly increases the longer you stay at depth.

Also, the first 4 fast compartments out of the ZHL-16 (the first 2 in ZHL-8) are ignored and the 5-6 compartments (3 in ZHL-8) are taken into consideration when defining the stop based on the diver's profile. If graphed using the Scubapro SmartTrack software, you can actually see the 5-6 (3) compartments initiate the off-gassing phase at the intermediate stop. This off-gassing may not necessarily occur with just applying a rule of thumb method of half the depth or pressure regardless of time at depth. So there is a bit of logic behind Uwatec/Scubapro's PDIS.

I really wanted to believe in the science, too! For me, bubble theory made such logical sense!
But based upon the developing evidence, I am coming to believe that the theory, while elegant, just doesn't correspond to the physiology of DCS. Therefore, "science" or not, I have concluded that Uwatec is not helping me, and I've turned it off.

For me (as an old hippie), it's a bit like Communism. "From each according to his ability; to each according to their need. Kum-bay-yah!" It's a fantastic way to live!
But as nice as it sounded, it just doesn't work in practice, does it? In fact, it's actually harmful.

 

[Steelyeyes]

I really wanted to believe in the science, too! For me, bubble theory made such logical sense!
But based upon the developing evidence, I am coming to believe that the theory, while elegant, just doesn't correspond to the physiology of DCS. Therefore, "science" or not, I have concluded that Uwatec is not helping me, and I've turned it off.

For me (as an old hippie), it's a bit like Communism. "From each according to his ability; to each according to their need. Kum-bay-yah!" It's a fantastic way to live!
But as nice as it sounded, it just doesn't work in practice, does it? In fact, it's actually harmful.

Harmful as in if I hang out for two minutes on my ascent like my PDIS recommends, even though I'm far from a NDL, that I could get symptoms of DCS? Is there evidence this has happened?