Dive computers and DCS

[charlier]

Please, someone start a new thread - "Due to travel time to dive sites, land-resort sets 50 min dives with 50 mins SI - what dive computer should I purchase." Perhaps add to thread, many sites at Komodo require diving to ~25 m with moderate currents. Posted in a weaker moment and a bit snarky, sorry...

 

[scubadada]

that is because the Cochran algorithm is a "Navy Schedule" (IIRC, they are a contract vendor/supplier for the US Armed Forces and other countries too), and from there you can assign some conservation to it.... I dive one of their computers (EMC-20) as a "lost deco gas, get me home somehow" back-up to a Petrel.

My first backup computer was a Cochran EMC-14 (no longer available), It consistently had longer NDLs than my Oceanic ProPlus running DSAT

 

[rhwestfall]

My first computer in the early 90's was a Comander, about the only PDC at the time that no matter what you did, would not lock you out, constantly calculating a schedule to get you out. I used it until it "bricked" (my bad as I let the battery 100% drain, and that was a problem with that unit from long ago), and have an EMC-20 they gave me quite a very generous deal on as a replacement. I dial it back some as I am no longer a fit collegiate athlete, but rather an almost 50 year old "weekend warrior"... It can really run on the "ragged edge", so you need to understand it.

 

[uncfnp]

Discussing liberal vs conservative in tech/decompression diving is very different from the same discussion for recreational/NDL dives. So much of the controversy in the algorithms for technical diving do not apply to recreational, specfically deep stops.

My take regarding the terms and recreational diving, the algorithm that gives the longest NDL is inherently more "liberal" and the less NDL more "conservative" since they all (that I am familar with) end with slow ascents and safety stops between 15 and 20. As has been pointed out, RGBM computers will not be that much different on single dives but will start to deviate from Buhlmann/DSAT with repetitive dives. The difference will depend on many factors, some known and some guessed at, such as surface intervals, depth of dives, dive times, ascent rates, missed safety stops...

One thing I find interesting, and this speaks a little to stuartv's suggestion that bubble models may in certain situations have more risk due at least in part to deep stops and increased slow tissue loading, my Cressi does just the opposite. The Cressi's recreational application of a bubble model does not give credit as freely as others for moving shallow. It gives some but it is simply not comparable to Buhlmann or DSAT for "riding the NDL." And it will not count down the safety stop until you are shallower then 20. And like many recreational computers, once past NDL, it wants to be at the safety stop level period, at least at the deco times I have seen thus far.

So yes, for recreational diving and even "lite deco" my Cressi is
unquestionably more conservative then any of my other dive computers, even my Petrel on VPM +5.

Now to the OP's question of algorithm choice and risk. Like a lot of choices in life its a question of personal risk tolerance. Stuartv nailed it with the risk being exceedingly low for all recreational dive computers. If I was young and had no health risks I too would pick the dive computer that gave me the most BT. But not being young and with some special health risks I dive a very conservative profile. But even I refuse to dive the Cressi on its maximum SF setting.

 

[rhwestfall]

The fundamental "issue" is, do you feel "cheated' if your PDC says time to get out when others are still diving? When buddy pairs have different NDL results, it can get "ugly"...

 

[dmaziuk]

Please, someone start a new thread - "Due to travel time to dive sites, land-resort sets 50 min dives with 50 mins SI - what dive computer should I purchase."

I think the "so you want a new computer" sticky should instead read Two sets of Shearwater DSAT tables and then when you open it, it's "what computer should I buy".

 

[Ojai Diver]

I am reading these responses with interest.

It seems computers can vary in two different ways: 1. they might "pay the piper" with different algorithms requiring different stops, etc. but are perhaps equally conservative in effect,, or 2. they may simply accept more risk than other computers, however minute.

Is there any data available on DCS when used with particular algorithms? I had always heard, for instance, that the Navy tables, created by and for young healthy male divers, bent more than a few of them.

My understanding is that a somebody can climb out of bed on a diving day and based upon his/her health, amount of sleep, and many other factors will already have x amount of DCS risk for diving that particular day, before even suiting up.

I guess I am wondering if one can adhere to a computer's algorithm, and still get bent, and if so, how often does this happen?

And, if it happened, could it have been prevented on a more conservative computer?

Thanks.

 

[scubadada]

There is little data on episodes of DCS and specific dive profiles and/or decompression algorithms followed. DAN Project Dive Exploration may shed some light on this issue. Study information is said to be coming soon Diving Medical Research Projects — DAN | Divers Alert Network The topic is a different one when discussing no stop/no deco/recreational diving than when discussing deco/technical diving

Unexplained (previously, undeserved) episodes of DCS certainly occur. I don't know what proportion of all episodes these make up, perhaps someone else does. I expect it is reasonably high.

It makes intuitive sense that the less time spent underwater, the less the risk of DCS. The relative safety of all the available decompression algorithms might make teasing out very small differences nearly impossible. Individual risk factors may be more important in an episode of DCS than a difference in the algorithm followed

 

[dmaziuk]

Is there any data available on DCS when used with particular algorithms? I had always heard, for instance, that the Navy tables, created by and for young healthy male divers, bent more than a few of them.

Overall incidence of DCS is too low and poorly defined for any meaningful statistics. So the studies use the ballpark proxy metric informally known as "shadows of echoes of invisible bubbles" whose relationship to DCS is: 99% of those with DCS have a lot of them. Less than 50% of those who have them develop DCS.

So for starters there is a basic disconnect between the studies of "performance of algorithms" and "not getting bent".

Edit: the "too conservative" complaints, "sense of algorithm" write-ups, and the scubalabs so-called "study" are about the NDL numbers shown on the computer screen at various points of the dive. Their relationship to getting bent is nonexistent since it's the whole point of wearing a dive computer in the first place: it will compute the deco schedule for you and get you up safely. ("Safe" according to its algorithm.)

My understanding is that a somebody can climb out of bed on a diving day and based upon his/her health, amount of sleep, and many other factors will already have x amount of DCS risk for diving that particular day, before even suiting up.

I guess I am wondering if one can adhere to a computer's algorithm, and still get bent, and if so, how often does this happen?

Known risk factors are dehydration, high body fat, poor physical form. PFO is a definite risk factor.

There's people who set their computers to 36% nitrox and then dive air because it's "too conservative" otherwise. Would you call that "adhering to computer's algorithm"? -- I get the impression they still don't get bent.

And, if it happened, could it have been prevented on a more conservative computer?

Global warming could be a hoax, good guy with a gun could stop a bad guy with a gun, and an asteroid size of Texas could hit this planet tomorrow. Of those, I'd put my money on a more conservative dive computer... otherwise, who knows.

 

[uncfnp]

Diving=nitrogen loading=risk of dcs.
More diving=more nitrogen loading=more risk of dcs

Simplistic and given limitations for studies already mentioned somewhat theoretical but still appropriate.

I think that the general advise given by experts in dive medical for those at greater risk for dcs to "dive conservatively" validates the concept.

Like Scubadada I think the individual variables, all else equal, are the primary factors in recreational dives. Divers with low risk can probably dive pretty much any algorithm with little concern. Others can do everything right and still get hit. I have an acquaintance friend in Florida with similar circumstances as myself. She knows if she goes deeper then 80 for more then 30 minutes, even on nitrox, she will likely rash. For me, the single greatest factor is multidiving and longer NDL's. N=2

As to reporting, I have no doubt that Type 1 DCS is under reported.