Recreational Ascent Rate in the last 15 feet

[rsingler]

Yep. Or climb in the hot tub right after a dive and get skin bends. You and I agree, I think. There's something to the rate of change. I'm with you.

I'm just trying to get @dmaziuk to point out the flaws in my reasoning.
It's all a learning experience.

 

[FreeFlyFreak]

Yep. Or climb in the hot tub right after a dive and get skin bends. You and I agree, I think. There's something to the rate of change. I'm with you.

I'm just trying to get @dmaziuk to point out the flaws in my reasoning.
It's all a learning experience.

I edited and added the hot shower analogy later but I guess you were already replying.
But yes the body is just a larger and more complex gas container which gives up gas at different rates dependent on tissue types and blood perfusion.

An interesting subject for sure, I can see why the researchers who study it have done so for so long.

 

[Jay]

I'm just trying to get @dmaziuk to point out the flaws in my reasoning.

Don't hold your breath! The best so far is the "no data" argument. That can also be used as an attempt to argue against lots of things in life which have merit (and perhaps some physics) and commonsense behind them. Typically it’s used by politicians, and the like, or perhaps those with that mindset and are bored, to try to apparently justify something.

In this case, assuming we believe in the merit of a SS, we have a decision from there to make: fast, medium, or slow to the surface? Hmmmm ... let me think Yup - that's how complex it is!

 

[Fastmarc]

It does not vary the length, nor ascent rate. The way RGBM works, you get "reduced gradient" on subsequent dives, which really translates to shorter NDL.

What I meant was on shore dives we'd typically follow the sand all the way to waist-deep. Which effectively means having an extended safety stop with very slow ascent rate in the last 25-30 feet. On those profiles, w/ 60-ish minutes runtimes and max depths up to 30 metres, I never see single-digit NDLs after a week of 2-3-4 dives/day. We do 60 minutes SIs as well.

On squarish profiles with straight 3 minutes SS on the line and 40-45 minutes SI -- boat rules -- I had single-digit NDLs after a few days of only 2 tanks/day.

So from my very representative sample of 2 it looks like Cressi Leo likes slow ascent rates and longer stays in the top 15-20 feet and dislikes short surface intervals and "bare minimum" safety stops.

My last few dives really had me questioning my RGBM computer. The dives were with experienced divers from a boat, so we had longer bottom times. On the 1st dive we got down to the 96ft and gradually got shallower during the course of the dive and ended up with a dive time of 55 mins. Our SI was 1:16 mins. The 2nd dive was a similar profile with max depth of 91ft, but we were shallower sooner. Dive time 60 mins. On the 2nd dive I got down to 4 mins NDL even though I had gradually ascended to less than 40ft. I ascended about 8ft and it moved from 4mins to 99mins. HUH? I stayed there for a few mins and descended to see what would happen. Before I got to 40ft it was back at 4mins. I stayed a little and I got to 2mins. I went back up a few feet and it was back a 99mins. This was no where near the end of the dive BTW. Interestingly the other two divers I was with ran deeper profiles than I did on both dives. On the second dive while I was doing this, they were both at around 50ft doing their thing (photography, spearing lion fish) very comfortably within their NDLs. I had to just stay above and watch.
That 4mins to 99mins is too ambiguous for me.

 

[tursiops]

My last few dives really had me questioning my RGBM computer. The dives were with experienced divers from a boat, so we had longer bottom times. On the 1st dive we got down to the 96ft and gradually got shallower during the course of the dive and ended up with a dive time of 55 mins. Our SI was 1:16 mins. The 2nd dive was a similar profile with max depth of 91ft, but we were shallower sooner. Dive time 60 mins. On the 2nd dive I got down to 4 mins NDL even though I had gradually ascended to less than 40ft. I ascended about 8ft and it moved from 4mins to 99mins. HUH? I stayed there for a few mins and descended to see what would happen. Before I got to 40ft it was back at 4mins. I stayed a little and I got to 2mins. I went back up a few feet and it was back a 99mins. This was no where near the end of the dive BTW. Interestingly the other two divers I was with ran deeper profiles than I did on both dives. On the second dive while I was doing this, they were both at around 50ft doing their thing (photography, spearing lion fish) very comfortably within their NDLs. I had to just stay above and watch.
That 4mins to 99mins is too ambiguous for me.

There is this very old concept that you can stay as long as you want at 33 fsw or shallower...based on the pressure shallower than 33 ft being less than 2 atmospheres.....so when you go to the surface your pressure drop is always less than 50%. This assumes a LOT and totally ignores things like rates of decrease, gas mix, etc. Many computers/algorithms use the old concept....

 

[dmaziuk]

My last few dives really had me questioning my RGBM computer. ...

That does sound wrong. The jump from 4 to 99 is theoretically possible but you'd have to come way up and likely blow the ascent rate alarm, to trigger it. It wouldn't be a Suunto with a failing pressure sensor by any chance?

 

[dmaziuk]

...
Where going from a 3' to 5' safety stop drops 1% off your SurGF,
doing 5' and slowing your descent drops an additional 1-4% off that.

Check out SAUL dive planner -- try changing your times by a few minutes this way and that and see what that does to the probability of DCS.

I've no reason to doubt his base math is sound and the trends it's showing are right: appreciable difference happens when you change pDCS by an order of magnitude. 1-4% gives you changes in the 3rd or 4th decimal digit and I'd be surprised if the underlying numbers are accurate to 2.

 

[Fastmarc]

That does sound wrong. The jump from 4 to 99 is theoretically possible but you'd have to come way up and likely blow the ascent rate alarm, to trigger it. It wouldn't be a Suunto with a failing pressure sensor by any chance?

Nope. The ascent was very slow. Well below the ascent rate.
Mares Nemo Wide.

 

[scubadada]

I have enjoyed this thread very much. @rsingler's simulations were particularly interesting. You get a 0-1% decrease in your SurfGF for increasing your SS from 3 to 5 minutes. You get an additional 1-4% for the extra 4 1/2 minutes with the slow ascent. With both the 5 min SS and the slow ascent, you reduce your SurfGF by 2-5%. Overall, these changes seem relatively small.

@dmaziuk pointed out the SAUL probabilistic decompression algorithm. It has been brought up several times previously on SB, I have been following the development over the last 7-8 years. The following table shows the probability of DCS in % for diving up to the NDL using 3 different decompression algorithms. This is with 32% nitrox, depth is in feet, NDL in minutes.The SAUL algorithm includes a 3 min SS

So, for a clean, 1st dive, with a 3 min SS, the probability of DCS varies by dive time, or GF high, if you will. To put this into perspective, 0.5% is 1/200. 0.25% is 1/400, 0.1% is 1/1,000, and 0.01% is 1/10,000. Zero in the table is <0.01%

My impression is that we all have to choose a SurfGF range that we are comfortable with. We can continue to debate exactly how we get there.

 

[FreeFlyFreak]

I have enjoyed this thread very much. @rsingler's simulations were particularly interesting. You get a 0-1% decrease in your SurfGF for increasing your SS from 3 to 5 minutes. You get an additional 1-4% for the extra 4 1/2 minutes with the slow ascent. With both the 5 min SS and the slow ascent, you reduce your SurfGF by 2-5%. Overall, these changes seem relatively small.

@dmaziuk pointed out the SAUL probabilistic decompression algorithm. It has been brought up several times previously on SB, I have been following the development over the last 7-8 years. The following table shows the probability of DCS in % for diving up to the NDL using 3 different decompression algorithms. This is with 32% nitrox, depth is in feet, NDL in minutes.The SAUL algorithm includes a 3 min SS

View attachment 551923

So, for a clean, 1st dive, with a 3 min SS, the probability of DCS varies by dive time, or GF high, if you will. To put this into perspective, 0.5% is 1/200. 0.25% is 1/400, 0.1% is 1/1,000, and 0.01% is 1/10,000. Zero in the table is <0.01%

My impression is that we all have to choose a SurfGF range that we are comfortable with. We can continue to debate exactly how we get there.

Nice table, did you do one for Air too?