DCI in Thailand

[bradshsi]

Thanks to String and John for a very interesting discussion.

Ive plotted similar dives not on suunto dive manager and even that is distinctly unhappy. Assuming NO loading from the previous day you end up with 16 mins NDL - if there were similar dives the previous day that would be almost zero. Follow up dives get lower and lower until they run into mandatory stop territory.

I also felt the dives were a little "hot", certainly not allowed with tables and even with his computer, they were right on the NDL limit.



Feeling in a philosophical mood I recalled another comment I have seen on this board:

"All dives are decompression dives. The only question is whether you decompress at the surface or not..."

When you think of diving in these terms you conclude (or at least I did) that you need to set a safety margin (whether you are using tables or computers) that matches your comfort level and willingness to accept risk.

I design hydraulic pumps for a living. These pumps are built to high levels of precision using investment castings and excrutiatingly tight tolerances. Despite all this it's interesting to note that on test we encounter a noticeable scatter of performance results which we are generally at a loss to explain.

My point is that if we have this kind of scatter with a precision piece of machinery, how much larger scatter can we expect with that most interesting and complex machine - the human body ? You can take the same body on different days and on one day it will be DCS free and on another days you get bent.

Obviously as you increase your diving safety margin, the statistical chances (ceteris paribus) of getting bent decrease (but never to zero).

Thus it behooves each diver to look at his tolerance for risk and dive accordingly.

 

[John C. Ratliff]

Great post and discussion. Thanks to all who have participated.

I had previously read this DAN article

https://www.diversalertnetwork.org/news/article.asp?newsid=514​

and found it very informative and it certainly makes sense to me. I planned these mid stops into the few deeper profiles I have done.

Here are a few excerpts. I highly recommend the entire article.

... the introduction of an additional deep stop ... reduced the previously recorded 30.5 percent incidence of high-grade bubbles to zero.​

NAUI has now suggested that a deep stop might well be incorporated in recreational diving by taking a one-minute stop at half the depth and followed by a two-minute safety stop at the 15- to 20-foor level instead of the three minutes currently recommended.​

The secret of the deep stop rests in the paradigm shift of "beating the bubble" versus "treating the bubble." The former utilizes the deep stop to ensure that the fast tissue critical gas supersaturation is not exceeded and stops bubbles from forming in the first place. The long ascent to the 20-foot stop, as is currently done, involves "treating the bubble"; we know this produces 30 percent asymptomatic or so-called "silent bubbles" on the surface, which may be indicators of decompression stress or even potential DCS.​

This post from Wet-Willie was perhaps the most important piece of new information on this thread. If you'll look at the profiles that DAN represented as having benifited by the deep stop, you'll see that their profile was 25m for 25 minutes.

Contrast that with the first dive that Taipeidiver made, which was 28m for 61 minutes. While this was a "multi-level" dive, in my old way of calculating this would still be considered in these terms. That is why it would be very, very interesting to see the actual dive profile, and I encourage Taipeidiver to post that data if at all possible.

Thanks all for the information posted here. It is affecting how I will be diving with my new tool, the Cobra dive computer, in the future.

SeaRat

 

[Scuba]

This post from Wet-Willie was perhaps the most important piece of new information on this thread. If you'll look at the profiles that DAN represented as having benifited by the deep stop, you'll see that their profile was 25m for 25 minutes.

Contrast that with the first dive that Taipeidiver made, which was 28m for 61 minutes. While this was a "multi-level" dive, in my old way of calculating this would still be considered in these terms. That is why it would be very, very interesting to see the actual dive profile, and I encourage Taipeidiver to post that data if at all possible.

Thanks all for the information posted here. It is affecting how I will be diving with my new tool, the Cobra dive computer, in the future.

SeaRat

Nothing like an actual dive profile for evidence.

per Taipeidiver:

My average depths for the four dives were:

D1 - 14.6 metres
D2 - 15.8 metres
D3 - 16.4 metres
D4 - 14.0 metres

By using your maximum depth and total dive time to calculate subsequent dives no deco limits - irrespective of actual dive profile, you can introduce a great deal of conservatism, as long as limits are not exceeded. The less time at depth and longer time in the shallows, the greater the degree of conservatism in terms of accumulated gas loads, not withstanding possible exceptions.

A critical error in terms of computer and table use probably emanates from a lack of understanding about gas loads. As long as one stays in the green, or out of the red, or out of mandatory deco obligation, it is safe to surface according to the rec mantra. Result, many divers surface with high gas loads. Loads that can be higher than surfacing loads experienced by a diver doing a much deeper dive with mandatory decompression. Difference being using a better off gassing profile, and gases, employed before surfacing.

Ideally, I dive a profile where by the time I surface I'm not just in the green or out of the red, but well away from maximum acceptable loads in the green. This requires necessary time spent shallow to sufficiently off gas accumulated loads at depth. In conjunction with proper stops, ascent rate, and any gases used.

Additional relevant points from that article. Multiple dives to 20m for 20/25 min. with 3 hrs surface interval were used. Of the profiles compared: using 3,10,15m per/min ascent rate, using two stops, one at 15m other at 5m for 5 min. each, only one 5m stop for 5 min, or no stops to ascend, the lowest bubble scores (Good) resulted from utilizing two 5 min stops. One at 15m, the other at 5 m. regardless of ascent rate, with a 10m p/min ascent rate being best. Second was an 18m ascent rate with two stops, and third was the 5m ascent rate with two stops, which significantly increased time in water without producing a lower bubble score. Pointing to the issue of the additional on gassing that takes place at depth with increased time, and the need for a pressure drop needed for an optimum net off gassing.

Ascent rate with two stops--bubble score (lower is better)--and time to ascend.

10m(30 ft)/min (Profile 6)--15 + 6 m / 5 min---1.76--------12.5
18 m(60ft)/min (Profile 7)--15 + 6 m / 5 min--3.23-------11.5
3 m(10ft)/min (Profile 8)---15 + 6 m / 5 min---3.51--------18

Second best category was using one stop, except for the slower 3m per/min ascent rate which turned out to be worse than a 10m/min no stop ascent.

10 m/min (Profile 4)----6 m / 5 min----------5.23----------7.5
10 m/min (Profile 1)-----No Stop------------7.34---------2.5
18 m/min (Profile 3)-----6 m / 5 min----------7.38----------6.5

Slower ascent consistently yielded the worst bubble scores except in the case of its use with two stops, apparently accentuating the importance of a deep stop.
3 m/min (Profile 5)------6 m / 5 min---------8.07---------13
3 m/min (Profile 2)------No Stop------------8.79---------8

They said further research in this area would follow. Should be interesting.

 

[John C. Ratliff]

Well, it looks like I've found a way to edit and create a second entry. My apologies. I'll work on that. Anyway, my entry is below.

SeaRat

 

[John C. Ratliff]

Nothing like an actual dive profile for evidence.
per Taipeidiver:

By using your maximum depth and total dive time to calculate subsequent dives no deco limits - irrespective of actual dive profile, you can introduce a great deal of conservatism, as long as limits are not exceeded. The less time at depth and longer time in the shallows, the greater the degree of conservatism in terms of accumulated gas loads, not withstanding possible exceptions.
...They said further research in this area would follow. Should be interesting.

Scuba and others, my point was that this is the way we used to do the calculation, and yes, it can be quite conservative. But you cannot go by the "average" depth either, as the time at depth can vary depending upon time spent at various depths. For instance, diving at 28 meters for 61 minutes can be represented for two different dive profiles, with profoundly different nitrogen loads, as the same:

Depth, T1, T2
28.0, 2, 10
21.3, 3, 7
18.2, 19, 10
13.5, 5, 20
9.5, 7, 5
7.0, 8, 3
5.0, 17, 6
Ave: 14.6, 61, 61

Okay, something weird is happening here. I cannot get my entry to go into the forum. I'll try again later.

SeaRat

 

[Scuba]

I agree an average only tells a partial story, which can be very misleading in some cases. I should have been more clear, my point was not to argue against your position but to provide further insight about the actual profile beyond dive time and max depth with a safety stop and "controlled" ascent. Hopefully Taipeidiver will provide further details.

If nothing else this thread illustrates the importance of understanding some deco theory, since simply following a computer, or table guidelines for that matter, can lead to quite agressive dive profiles which many experienced divers find it best to avoid or to mitigate employing some of the methods discussed here.

Riding someone elses limits is a risky enterprise.

 

[String]

Agreed, the full downloaded profile would be very useful here. We've reached the limit of what we can scrape out of the incident without that.

 

[Batfish]

Wow. What a lot of discussion.

To me, as (in my opinion) an experienced diver, an instructor with plenty of experience in the area where this guy got DCS, his dive series just looks dodgy. Especially the 4th dive where he got bent. Too deep for a 4th dive. Not really a sensible depth for a 4th dive. I used to be a tour leader on a liveaboard in the Similan Islands and our 4th dive was always a shallow one, either a night dive or dusk dive. I tried to get people to stay 10 - 12m max. The last dive of the trip was also always a shallow one, 10m depth (there is a great shallow reef at Island No 1 in the Similan islands where we used to go).

Sorry to interrupt, please do carry on

 

[surlytart]

TD:

When I was spending some extra time at the Center for Hyperbaric Medicine (Virginia Mason, Seattle), the doctors there said something very similar. I seem to remember the figure they threw out for undeserved dci hits being around half.

Interesting. There are good reasons I ended up there. Equipment failure leading to a lot of extra physical work was one. The main thing that we did wrong though, is this: we'd switched our last three dives (out of five on the weekend) around in terms of profiles. There was little depth variation between the three, but they did go from shallower to deeper due to routine miscalculations. Differences of a few feet, like yours. And all three were pretty conservative in and of themselves.

Coincidence? I don't know. Definitely food for thought.