Going into deco

[TSandM]

I think one of the head-shaking things about this particular set of dives is that we had five divers in the water, doing similar or identical profiles, using several different computers or other methods of dive planning/monitoring, and four of us came out quite happy about the decompression aspects of the dive. One did not. And not only was that computer unhappy, it was TWENTY TWO MINUTES unhappy.

Now, I know decompression is not a science, and I've read extensively enough to understand that various algorithms will generate significantly different answers about no-decompression time, or required deco. But this seems to me to be an egregious example of the possible variation.

I really like the way I have learned -- I do deco on EVERY dive, and plan for it. It becomes habit, and I'm sure it increases my safety.

 

[Blackwood]

It also dislikes you violating what it thinks is 55 mins of deco if you forget to put it into gauge mode before an accelerated deco dive. Operator error on both accounts there though.

Lesson learned: always bring some cave line so you can throw your Sunnto overboard and let it do an extended stop during your SI

 

[JeffG]

Lesson learned: always bring some cave line so you can throw your Sunnto overboard and let it do an extended stop during your SI

What a waste. just pull out the battery.

 

[DandyDon]

Anyone heard of this? Who out there who does "light" deco regularly (not necessarily full-on tech diving with deco mixes) is using deep stops? Where do you generally like to do your 3 or 6m deco obligation - @ 3/6m level or deeper?

Oh yeah - as long as one works out of it well, not just satisfying the deco obligation but extending that. The more off gassing I can do on deep and shallow stops, the more energy I'll have at dinnertime, but overdrawing your computer a handful of minutes is not biggie if you have work-out plans and resources for it.

TDI teaches a 6 hour SI after any Deco, I think. I've dived with North Carolina divers who don't see anything more than 90 minutes as worthwhile.

 

[Charlie99]

I think even the liberal computers have enough safety built into them that you can do that kind of bail-out to the surface without getting DCS. That's what they claim, and they know they'll get sued if their algorithm is wrong. The grey fuzzy area is when you get into light deco, where you might or might not get bent if you bail out.

What is grey and fuzzy is that the DCS risk increases only slowly with increased depth and time. It isn't that the liberal computers claim that you can punch out and get to the surface without getting DCS, it is just that they are saying that the statistical risk is within the range that manufacturer has chosen as "normal and acceptable".

Bruce Wienke, the developer of the RGBM algorithm, has an interesting chapter on Decompression Risks and Statistics in his book Technical Diving in Depth. The relative risks of different profiles, as inferred from the results of many reported and analyzed dives, is much much fuzzier than I had thought.

As an example, increasing bottom time on an 80' dive from 15 minutes to 60 minutes increases the risk of DCS only by a factor of 5. Put another way, if the acceptable risk chosen by a table developer or computer algorithm designer were to vary by only a factor of 5, then the results would be a no-stop or NDL time of only 15 minutes at 80' for the conservative table or computer, and a whopping 60 minutes for the liberal one. That far exceeds the smaller variations between the various computers and tables commonly used. For example, the DCS risk estimate for the USN 40@80' vs the PADI 30 minute at 80' is that there is about a 60% increase in DCS risk. OTOH, after 40 minutes at 80' a computer based upon the PADI/DSAT model will be showing 9 minutes at 10' as a required deco stop. But the DCS risk of a direct ascent is less than doubled compared to a "within NDL" ascent after 30 minutes at 80'. Obviously, with a conservative computer there would be even more decompression obligation showing after 40 minutes at 80'.

You may or may not get bent doing a fast ascent when well within NDL limits. You may or may not get bent doing a fast ascent when a ways beyond NDL limits. The odds change much more slowly than most people realize.

 

[diverdowndh]

I posted but it was supposed to go into another thread... not sure how it got here but I just removed it... sorry!

 

[in_cavediver]

You may or may not get bent doing a fast ascent when well within NDL limits. You may or may not get bent doing a fast ascent when a ways beyond NDL limits. The odds change much more slowly than most people realize.

This is true but other factors not accounted for in the tables also play a role. Think dehydration, exertion or injury. Some of these change day to day and person to person. I personally know a diver who got bent after following his computer exactly as it asked. His dive buddy didn't get bent.

I personally think that the issue of deco, missed deco, the bends and NDL's is a lot like PP gas blending. Do your dive with excellent technique, you can get away with a lot over the accepted limits. Do it with poor technique, you may have a problem even if you're well within accepted limits.

 

[Sharky1948]

TDI teaches a 6 hour SI after any Deco, I think.

Don,
I don't recall this in any of my TDI training, nor can I find it in the manuals. Can you elaborate?

Thanks!

 

[DandyDon]

Don,
I don't recall this in any of my TDI training, nor can I find it in the manuals. Can you elaborate?

Thanks!

You know, I may have just got that from my Inst - I'll have to ask him where it came from. Sorry...

I think the primary fear from many US divers about overdrawing your computer a few minutes is that the charter may not let you did again that day. :silly:

 

[Sharky1948]

Thanks, Don! It doesn't sound like a very practical guideline. I can't imagine asking the captain (or the other divers) to hang around an extra 4-5 hours on a wreck site!

What he may have been referring to was the amount of off-gassing that would take place. Assuming a 90 minute tissue half-life, in 6 hours you would off-gas 94%...pretty close to where you started the day.