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View Full Version : To deep or not to deep... (stop)



Tassie Diver
09-03-2009, 20:57
Tech divers would be familiar with the concept of using deep stops as a method of reducing the severity of bubbles that occur during decompression. The theory is largely untested but is accepted dogma within the tech community.

In the latest edition of DAN SEAP's Alert Diver, Dr Andrew Fock from the Alfred Hospital in Melbourne, presents an interesting analysis of the reality of deep stops:

http://www.danasiapacific.org/main/_pdf/AD40-May_Aug09online.pdf (2MB)

The outcomes from the human trials he quotes are quite scary.

Deep stops anyone?

navyhmc
09-03-2009, 21:08
Interesting study, interesting read. Thanks!

comet24
09-03-2009, 21:31
There are deep stops and then there are deep stops. First stop of 42m for a 50m dive is a little to deep for me. The problem we now understand with deep stops is your still on-gassing at those deep stops. This on-gassing can be do more harm then any benefit gained by the stop.

I just ran the dive plan through V-planner but converted to ft. 164' for 25min on air w/ 36% and 100% for deco. With conservatism set to 5 my for stop is 105'. This guy planed first stop was 137' for a 164' dive.

Tassie Diver
09-03-2009, 23:16
Yeah, I was wondering about that myself. But specifics aside, it is a bit of an eye opener. It also makes me wonder about the merits of slow ascents until shallower than say 20m/60'.

navyhmc
09-04-2009, 00:53
After reading the article, it poses more questions than it answers:

What were the dive profiles-both actual and chamber? Were the deep stops in addition to or in exclusion to the planned "no deep stop" profiles?
What were the depths? Were rec dives included in the study profiles?

What is interesting is that DAN has an article from last year that concludes that in the Rec world, deep stops are of benefit along with the 30'/10m per min ascent. http://www.diversalertnetwork.org/news/download/marronifinal1888acapcoraut.pdf

Warning, a lot of statistical data to wade through, so let me sum up:


Ascent rates of 10, 33 or 60 fsw/min (3, 10, 18 m/min) were combined with no stops or a shallow stop at 20 fsw (6 m) or a deep stop at 50 fsw (15 m) and a shallow at 20 fsw (6 m). The highest bubbles scores (8.78/9.97), using the Spencer Scale (SS) and Extended Spencer Scale (ESS) respectively, were with the slowest ascent rate. This also showed the highest 5 min and 10 min tissue loads of 48% and 75%. The lowest bubble scores (1.79/2.50) were with an ascent rate of 33 fsw (10 m/min) and stops for 5 min at 50 fsw (15 m) and 20 fsw (6 m). This also showed the lowest 5 and 10 min tissue loads at 25% and 52% respectively. Thus, introduction of a deep stop significantly reduced Doppler detected bubbles together with tissue gas tensions in the 5 and 10 min tissues, which has implications for reducing the incidence of neurological DCS in divers.

navyhmc
09-04-2009, 02:57
Another interesting item: Using the Suunto Dive Manager, I made two Dive profiles: both to 100' for 16 minutes one with only a safety stop and the other with a deep stop at 70' and 45' each for 2 minutes. The nitrogen loading on the first profile was much higher: At 32' per the display, the first dive gave a No Deco Time of 40 minutes while the second deep stop dive showed a NDT of 64 minutes at 32'.

LiteHedded
09-04-2009, 07:18
I normally start pausing at 75% of my max depth on deep dives. no ill effects so far.
you're certainly ongassing slow tissues at these depths but the idea being that you're beginning to offgass the fast tissues. and by pausing you're controlling the size of microbubbles, keeping them small and the surface tension of the bubble high, so that off gassing from those bubbles is more efficient. that's how I understand it anyway.

in a typical dive I'm doing 30 second pauses and 30 second slides to the next stop depth

comet24
09-04-2009, 15:15
Accent rates should vary on deep dives(greater then rec limits) although there may be some benefit in varied accent rates on the deeper end of rec dives. When deep you want to get to your first stop as quickly as you can safely. Accent rates of 60' per. min. are better when deep. The rule of 30' per min is fine when you are shallow or hit your first stop. Slow accent rates from the bottom on deep dives are just increasing your on-gassing time.

LiteHedded
09-04-2009, 15:30
i cruise at a pretty good clip to my first stop

navyhmc
09-04-2009, 17:43
I stay with the 60'/min below 60' and 30'/min above 60'.

in_cavediver
09-04-2009, 17:51
I personally see some flaws to this. First - lets be realistic here. Deco theory is just that and I would NOT blindly go to 100m without a lot of work up to those depths. I also should be thrown to wolves for this sacrelidge of not exacting timing of stops or exacting ascents rates etc.

Its all mucketymuck and theoretical until you start applying it to your dives and your body. I personally like the beuhlmann algorithim with pyle stops or if I wing it - about 50% of max or average max for the deep stop then another at 25% or so then first stop - skipping the 25% if I have a stop sooner of course. These stops are somewhere between 30 seconds and 2 minutes depending on the conditions. Basically - pauses in the ascent for me to reflect on where I am and what I am doing next. I'll adjust this further if I use 50/50 and have a 70ft gas switch. I ascend more rapidily from deep to around 60ft or so, I then start slowing down. I also pad my last stop by several minutes on O2 if I have it. I like run-time tables and ensure I spend the requisite time at each stop and then mosey up to the next one. I almost always over stay stops and miss the run-time but that is OK for ME.

The result - I have a LOT of personal experience in the 100-150' range of deco dives with me feeling great after each one. I have some deeper dives to the 200' or so level but not as much personal knowledge. I won't go below 225' now without more knowledge in the 150-225 range. I know how I should feel during the dives I do and I know what I can get away with and how to fix it - IE, how much I can skip deeper with padding shallower.

My theory is you are seeing more divers progress much quicker to the deeper depths without understanding thier personal physiology. We also have more divers, diving deeper and treating deco models as gospel. (hint - they aren't and a table for a 300' dive is useless to you if you don't have personal expeirence with the model used at shallower depths). This used to be a long process and even I almost fell into the trap of rapid progression.

In the case in the article 'Bob' had no buisness where he was and frankly, he played Russian Roulette and lost. He made MANY mistakes, non of which was individually fatal but his lack of understanding killed him. It may be cold and callous to say it like this but its true. It wasn't the deco model he used, it was his lack of knowledge on how to 'bail out' that killed him. It was his ignorance that did it. Ignorance isn't always bliss.

texdiveguy
09-04-2009, 18:12
Interesting article.....very little data provided to make a case on any of the situations noted.

We are all different and our bodies respond to decompression in various manners. One model may work well for me time after time and then bam! That same model may create issues for a different diver the first run.

Nothing in decompression is law.......there are a few general guide lines which seems to apply well for most of the diving public, these you will study as you move thru technical classes and up the ladder.

Diving condition play SO much on the results of deco and the effects of deep/shallow stops.

Get the best training and education you can find, but be prepared for the unknow.

My deepest dive to date is 315fsw, but I much prefer shallower dives for many reasons.

Safe diving to all!

TommyB
09-04-2009, 19:34
Accent rates should vary on deep dives(greater then rec limits) although there may be some benefit in varied accent rates on the deeper end of rec dives. When deep you want to get to your first stop as quickly as you can safely. Accent rates of 60' per. min. are better when deep. The rule of 30' per min is fine when you are shallow or hit your first stop. Slow accent rates from the bottom on deep dives are just increasing your on-gassing time.

Might get buy with those rates for a bit with a non HE mix. Switch over to HE, and it's time to slow the ascent rate a bit :)
And yes, I have weekly experience in the 250-302 foot range.

HuntsDesk
11-19-2010, 00:27
Another poster already mentioned it but I think the biggest concern is someone who's not done the dive planning to know at what point they start off gassing, than you could potentially be doing more on gassing. I also dont think its good to deviate from your dive plan unless you have good reason (or a have super cool [I mean appropriate] computer capable of doing deco without keeping you down there till you run out of gas. :) I recommend the X1 or Shearwater).

I also agree with Tommy on the ascent rates. HE can get you bent if you push it from the deeper depths.

........something I had fun with was doing trimix rebreather training. One of the drills we did - Do a 200 ft dive, wait till the longest deepest part of your dive (where you'd have the most deco obligation), then bailout on to OC. Purpose to the drill is to show whether or not you'd planned bailout appropriately and (with relevance to this thread) see how quickly you can ascend to your deco stops. We were NOT exceeding 30 feet per minute but it was a good drill to show that most people werent ready to ascend that fast. It was also interesting to see how much EXTRA deco time was added if you werent ascending at a steady pace (slower than 30 feet per min). If anyone is curious we were diving with 10/50 dil.