DCS Within No-Stop Limits

[leadweight]

A recent issue of the Canadian magazine, Diver, discussed the problem of divers suffering DCS when their computers were within the no-stop limits. In addition to the usual possible causes such as dehydration, poor physical condition or a fast ascent the author notes that while one diver is within the no-stop limits of his computer he may be well beyond the no-stop limits of another brand of computer.

The author actually goes so far as to say that he has identified a brand of computer that he can link to several diving accidents, although he will not disclose its name for fear of a lawsuit.

Think about it for a minute. In a series of repetitive dives, computer A shows 40 minutes of no-stop dive time 25 minutes into a dive a 50 feet. Computer B shows 20 minutes right beside computer A. Twenty five minutes later Computer A would show 15 minutes of no-stop time remaining. Computer B would have passed its no-stop time 5 minutes ago, and might be asking for a 5 minute deco stop. As compared to the diver with computer B, the diver with computer A missed a deco stop when he ascends in the no-stop mode.

Computers are only models of what is going on, but if there is a brand of dive computer out there that is causing accidents solely because its model is too generous with bottom time, that is a scary thought.

 

[Scuba_Steve]

I'd give the guy a little more credit IF he divulged this supposed bad computer, and maybe gave us some examples of these tests/actual dives that have been done to prove his theory.

It's nothing but spin until then. So what's this hero's name?

 

[leadweight]

I'd give the guy a little more credit IF he divulged this supposed bad computer, and maybe gave us some examples of these tests/actual dives that have been done to prove his theory.

It's nothing but spin until then. So what's this hero's name?

I don't have a copy of the magazine, and I do not remember the author's name. As it is Canadian, it is not available around here. I saw it while traveling. You should try to get it as you are located in Canada. After reading the article and thinking about some of the tests that have appeared in other publications, I am quite convinced that I know where to point the finger. Its not just spin. Do you suspect your computer?

 

[Charlie99]

A good place to start with analysis on the possible connection of different dive computers with DCS rates would be to find out what data is available on the relative DCS rates associated with the various tables.

The DCIEM table, for example, is much more conservative than either USN table or PADI tables. Bruce Wienke has some relative risk info in his Technical Diving in Depth book. The risk-of-DCS vs. time-at-depth curve is suprisingly soft. In other words, the fuzzy grey zone where risk goes up slowly is huge.

One of the most liberal family of computers out there (excluding ancient ones based upon USN tables) is based upon the PADI/DSAT model. Many others have large amounts of padding.

 

[leadweight]

Charlie:

I just looked at four different sets of tables that I have around here. At 60 feet, the difference is minimum 50 minutes and maximum 55 minutes no-stop time. The 55 minutes is from PADI and also the pre RGBM NAUI tables.

When I look at the graphs published in the May 2005 Scuba Diving magazine, the differences in remaining bottom time at 60 feet are much greater than 5 minutes. No doubt there is a big grey zone. But my concern is that someone might be way close to dangerous side of the curve. In other words, they used up most of the grey zone.

I don't have a lot of accident data available to me. Divers that I know who got bent tended to either be constantly pushing the envelope, photographers who went down to get some prize shot at the end of a dive, or divers without computers who were not following the tables because they were diving along with divers that had computers, but did not realize their profile was significantly different.

I can't say that I have seen any kind of a trend myself. But the article makes me wonder about just how much padding is enough, and if you take some out, is that not enough.

 

[String]

Its perfectly possible to get bend staying within tables/profile - after all these are just averaged statistical model and not everyone falls within the distribution curve.

IIRC last year someone over here found incidents of divers getting bent with computers and found a few potential causes.

Case 1 is the " Push the NDLs to within seconds on every dive" - this sort of diver goes within seconds of his NDLs on every dive thereby testing the model to its extreme

Case 2 is the up/down fast ascent diver - People need to realise that decompression models have maximum and minimum ascent rates, if you violate those you are outside the models scope.

Case 3 - the undeserved hit diver - some people are just unlucky. It happens

Combine case 1 and case 2 divers and its perfectly possible to stretch the limits of the computers.

Some computers have different algorithms which give a different calculation (ie guesstimate) model and some computers with the same algorithm have more safety buffer fudged in than others. Its worth noting this isnt a "computers are evil" thing - computers just implement a statistical model just like tables and these affects are just as possible on those.

 

[redacted]

If he could make that case, he could be a rich man. I think he just has a suspicion with no credible evidence. I like my liberal Oceanic computer. I can add all the additional conservativness I want, when I want. I like a computer that helps me, not one that limits me unnecessarily.

 

[DA Aquamaster]

A computer needs to be used with caution as all it knows in most cases is time and depth (and in a few cases water temp and air consumption) so it remains up to the diver to add in and consider all the other factors that may contribute to DCS.

When I started diving US Navy tables were the standard and it was routine to add in lots of fudge factors. We avoided square profiles, rounded up to the next greatest time and depth, and in some case to the next greatest time after that as well for cold water dives and/or hard working dives.

With computers, people seem to forget that every dive is already essentially a square profile and that the lower multilevel NDL's used by most computers are not always adequate to make up for extra risk posed by a cold water dive and or a hard working dive. Other risk factors such as a diver who may have had too much to drink the night before and who may also be on day 5 of a 5 day dive trip with repetitive dives over multiple days are not taken into account. Many computers specifically state in the manual that they are not designed for repetitive diving over multiple days, but many divers do exactly that with them and then claim they are victims of an unwarranted hit as the computer said they were still ok.

So my thought is that the conservatism of the computer is far less important than the conservatism of the user. For recreational diving I use a Sherwood Wisdom which is a very liberal computer - one of the most liberal around. However it also requires a 30 fpm ascent rate and "recomends" a 3 minute "safety" stop at 20 ft for every dive deeper than 30 ft. The slow ascent rate and essentially mandatory 3 minute stop go a long way toward offsetting the hazards of an otherwise liberal computer. A user could theoretically blow it off, but they would be stupid to do so, just like a diver in the past would have been considered stiupid to dive the US navy tables to their limits as well.

Most computers provide a graphic display of N2 loading and offer a green and yellow zones in addition to a red decompression zone. A prudent computer diver is well advised to extend a safety stop, gas permitting until he is well back in the yellow or even back in the green.

 

[wunat]

Do dive computers and dive tables guarantee divers no DCS?

I have been told since I have started diving, they are just a tool and model which has been tested and prevented divers from getting DCS statistically. If this understanding is right, no matter how conservative the computers are, the risk of DCS is there. The only way to avoid it is probably just not dive at all.

In any case, do correct me if I am wrong.

 

[String]

I have been told since I have started diving, they are just a tool and model which has been tested and prevented divers from getting DCS statistically. If this understanding is right, no matter how conservative the computers are, the risk of DCS is there. The only way to avoid it is probably just not dive at all.

In any case, do correct me if I am wrong.

Correct. No decompression algorithm whether its on a computer or table guarantees no DCS, they merely demonstrate you are statistically unlikely to get DCS by sticking within their limits.