Very clever marketing :-)

[Reg Braithwaite]

never mind.

 

[nadwidny]

... because Suunto uses what they call a "modified RGBM" algorithm, and I do not know the specifics of how it works.

Poorly at best. I have a Vyper that I put into computer mode sometimes just for fun and the results it gives are truly bizarre. I did a repetitive dive this week and on the 2nd dive I left the bottom (50ft) with 1 minute of deco showing, it added time on the way up (I was doing 2 minute stops) and didn't take away any time until I hit the 10 foot stop.

I was going to just ignore it and surface anyway but the boat I was on seemed to be the type that would've freaked out if I came up with a computer beeping with a deco violation so I waited it out.

 

[lamont]

Suunto's modified RGBM algorithm penalizes you HEAVILY for doing reverse profiles. Even a modest increase in depth on a subsequent dives can put your NDL in a ridiculous place quite quickly. In a case like that, I'm going to look at the computer, realize why it's giving me some numbers I KNOW I can't believe in, and follow my dive plan.

Bigger problem with Sunnto's punishment algorithm is what it does to you if you slip over into mandatory decompression. Even if its just a brief excursion of a minute or two, and even if you spend 15 minutes between 30 and the surface. It'll display the little '!' symbol and radically reduce your NDLs nearly 24 hours later.

So, if I nudge right up against the NDLs at 100 fsw and then head to the surface at 25 fpm and blow off my safety stops, that is better for me than a dive with an extra minute spent at depth but 15 minutes of shallow deco from 30->surface?

That is obviously wrong.

From the perspective of a computer manufacturer selling computers into the recreational market and worried about liability and dealing with a population of users who don't have the skills necessary to cope with even a few minutes of mandatory decompression, then this level of punishment probably makes sense. Recreational divers shouldn't be crossing the NDL line.

The guy who decided to sit out his dive also made the right choice for him. But if you've been doing NDL dives, I guarantee you that you can do another NDL dive and do 10-15 minutes of decompression on backgas and get out of the water cleaner than other recreational divers on the boat who can't hold a safety stop in open water without something to cling onto. But unless you're prepared to do that, mentally and with skill preparation, then you shouldn't do it (although, I'd argue that *anyone* can always assume their gas loading is a W or whatever on the PADI RDP and plug in your surface interval and then do the NDL the RDP says you can do).

 

[lamont]

That's a really interesting article, but there are definitely two ways to interpret what he is saying. One way is that since the computer told him he couldn't dive for 24 hours, he meekly sat the day out.

The other is that although he knew that the risk of getting bent if he used his brain instead of the computer was insignificantly low, he has decided that the benefit of following the standard rules every time outweighs the benefit of exercising his personal choice to violate them from time to time.

DIR divers violate all kinds of diving rules:

- crossing the NDL line in going into mandatory decompression
- going deeper than 130 feet
- going into overheads

None of those rules should be violated by the diver in that story. Religiously sticking to rules sounds like a great idea for that guy and will work for him.

It is a little refreshing to see DIR divers getting slammed for being too reckless and not following the rules. Usually we're getting crap for following too many rules and not thinking for ourselves...

 

[MXGratefulDiver]

Ahhem -

Doe Rey Me Fa So La Tea Dooooooe.

he he

Hey Bob (or anyone else of course),

Can you recommend a good first book on deco theory?

Thanks.

Hunter

I learned what I know by wading through one of Weinke's books (Physics, Physiology and Decompression Theory), taking workshops, and talking to a bunch of tech divers I know. I wouldn't recommend Weinke ... that stuff's so dry it'd put a geek to sleep.

This book seems to be getting a lot of good press ... it's new, and I'm not sure quite how available it is yet. Sounds like it's a lot more readable than Weinke, though ...

... Bob (Grateful Diver)

 

[tep]

...
RGBM is built around a few basic rules ... one of which is "no reverse profiles".
...
... Bob (Grateful Diver)

Interesting. I wasn't aware of that. Where can I find more information about the underlying assumptions of the various models? I'm contemplating this book:
Amazon.com: Deco for Divers: Decompression Theory and Physiology: Mark Powell: Books

Sorry if this doesn't belong in DIR...

 

[Dtaine]

This thread reminds me of something I've read recently:

"Remember that technology is no substitute for common sense, and a dive computer
only provides the person using it with data, not the knowledge to use it." From the Hollis DG02 manual.

Happy New Years,

Dave

 

[RTodd]

I learned what I know by wading through one of Weinke's books (Physics, Physiology and Decompression Theory), taking workshops, and talking to a bunch of tech divers I know. I wouldn't recommend Weinke ... that stuff's so dry it'd put a geek to sleep.

Let me second staying away from Weinke's book. It won't teach you much if you don't already understand this stuff well anyway. Buhlman's book, which isn't easy to read even if you can get your hands on a rare English version, is still infinately more comprehensible compared to Weinke's. I have used custom tables he cut for friends, and even discussed the topic with him briefly. I still couldn't make much out of his book and would have to devote an enormous amount of time to truly studing the thing rather than just reading it to get anything out of it. On the upside, I have a signed copy of his book in pristine condition since I gave up on trying to understand it.

 

[TSandM]

The Powell book is getting heavy recommendations from people whose opinion I respect, such as Gene Hobbs. It's on MY must-read list.

A book with a great deal of information about the various models and they assumptions they use, as well as their differences in output, is Lippman & Mitchell's Deeper Into Diving. I found it a very good read.

Another absolutely fascinating resource is GUE's "Mysterious Malady" DVD, which I highly recommend to anybody who is curious about this stuff. It's a series of interviews with people who are doing research in this field, and it's arranged into various topics, so you don't have to sit through the whole DVD at once. I found it riveting to listen to the guys who do this work talk about how little we actually KNOW about decompression sickness.

And hey, everybody, can we knock off the sniping? This thread has really crossed the line to where somebody ought to clean it up, but if we just get back to discussing the topics at hand, that would be nice.

 

[MXGratefulDiver]

A book with a great deal of information about the various models and they assumptions they use, as well as their differences in output, is Lippman & Mitchell's Deeper Into Diving. I found it a very good read.

Ah yes, that's a great book and quite easy to digest. Some of the material seems a bit dated, but it's got a ton of useful info in it. It's one of the sources I used when researching some of the material I wrote for my AOW class.

Definitely on my recommend list ...

... Bob (Grateful Diver)