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Why not just do the stops?

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that would take too long... hahaha

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Agree with Mike, Sunnto are nice looking computers but crap on time, day after day the first up the line are the Sunnto users while the rest carry on diving, Sunnto have to be the most conservative computer on the market and for this reason I never recommend them to divers, Sunnto if your listening that's 100's per year in sales you miss out on. What's worse is I have a Sunnto D9 but have it set to 27% Nitrox which is still a little behind my Oceanic on air!

 

Following on Drew's comment above, the RGBM algorithm used in newer Suunto (and other) computers is very different from the Buhlmann-based "Haldane" models used in the most other dive computers. Calling it "more conservative" may be correct (if misleading), but complaining that it "punishes" the diver doing repetitive dives misses the point. As for intentionally setting the computer to a nitrogen percentage less than the actual N2 content of the breathing gas in order to avoid doing deco stops you should do, thus overriding the computer's best efforts to keep you alive ... that strikes me as (avoiding the "S" word) ... a potential example of Darwinism at work in our species and the modern world.

 

Buhlmann models assume that all nitrogen in body tissues is in solution. The reality is that nitrogen in our bodies during a dive exists in solution and in free gaseous phase, namely bubbles. The modified version of the "Reduced Gradient Bubble Model" (RGBM) algorithm used in newer Suunto computers, developed by Dr. Bruce Weinke at Los Alamos National Laboratory, takes phase transitions between nitrogen in solution and gaseous form into consideration, which Haldane models cannot do.

 

It's not that RGBM "punishes" repetitive dives or short surface intervals. Rather, Buhlmann models fail to take into account the additional risks of repetitive multiple-day repetitive diving, inadequate surface intervals, and/or 'bad' profiles (saw-tooth, bounce, and reversed profile dives).

 

Setting a RGBM computer to 27% nitrox while diving on air to avoid having to make decompresion stops is asking for trouble, in my opinion, and may be riskier than just using an older and cruder Buhlmann-based computer. As someone in this thread has already asked [RD: it was John Bantin], what's wrong with just doing the stops? (Or to put it another way, what is it that you really like about decompression sickness?)

 

If you prefer the additional risk of doing repetitive dives with a Buhlmann computer (or if you're one of those divers who think driving your computer all the way to the no-dec stop time edge on every dive is smart diving), then I'd have to ask why you bought one of the very few RGBM computers that anyone makes in the first place? There are still plenty of aggressive dive-at-your-own-peril computers for sale in every dive shop, though Mares and other manufacturers have also released new RGBM-based computers or are about to do so. Something to do with those pesky lawsuits from the estates of dead divers.

 

None of this is meant to suggest that RGBM is perfect or that it guarantees you will not take a hit. I do think RGBM is superior (and safer) than Buhlmann models, but there are cases where divers using RGBM have taken DCS hits, though most if not all were technical divers doing very deep dives with rebreathers and/or exotic gas mixes.

 

I should also make it clear that even though I believe RGBM is safer, I use two DiveRite computers (Duo and a Nitek Plus), both Buhlmann-based - I carry both of them on every dive. They both still work and don't require replacement yet. I also have reliability issues with Suunto due to bad experiences with a Solution Nitrox that I owned years ago. But I am aware of the limitations of the Buhlmann algorithms and do what I can to compensate. For example, deep stops (starting at half the difference between the max depth and first deco-stop depth, or half the maximum depth on no-stop dives), avoiding bad profiles, and never driving the computers to the edge, particularly for multiple day repetitive diving (for example, on a liveaboard).

 

By not driving the computer to the edge, I don't mean that I limit my bottom times more than you. It means that I do my stops, and I take extra time on the safety stop on any dive that has pushed near or beyond the no-stop limits.

 

It would be interesting if someone here started a poll - who has ever taken a hit, or more than one, were they using a computer at the time, what compuer (and what decompression model), what gas, and was the dive part of a sequence of repetitive dives on multiple days.

 

Frogfish

 

P.S. RGBM isn't the only non-Haldane decompression model around, but it is the probably the best, and the only one (to my knowledge) that has been implemented on a commercial dive computer suitable for recreational diving. Other non-Haldane models include the Canadian Series model (implemented in the DCIEM Kidd-Stubbs dive tables), the EL model developed by the US Navy and implemented as a military computer for combat divers, the Slab model developed by Dr. Tom Hennessey for the British Sub-Aqua Club (implemented on tables), and the Varying Permeability Model (aka Tiny Bubble Model), and its successor, VPM, developed by Yount and Hoffmann at the University of Hawaii, implemented on tables and in software, which is generally viewed as the progenitor of Wienke's RGBM model.

 

Jolie Bookspan's excellent "Diving Physiology in Plain English" has a short discussion of the non-Haldane models.

 

Wienke has a book about RGBM out..

 

Wienke's book on Amazon

 

... and it is also covered in his Basic Diving Physics and his textbook on decompression theory and Technical Diving in Depth.

 

There's also a very interesting piece about Deep Stops by Richard Pyle, the University of Hawaii/Bishop Museum ichthyologist who independently discovered the value of deep stops about ten years before Wienke uncovered the theory behind them.

 

Richard Pyle on Deep Stops

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Not wishing to blow my own trumpet (wot, me?), Bruce Weinke has personally shaken me by the hand and said that my side-by-side comparison tests of computers are spot-on. John Lippmann has also found scientifically exactly what I discovered in my own way and has been in communication with me about these articles. One computer manufacturer looked for a way to suppress the information revealed but has had no luck! I am not an expert, I am merely a witness, but howmany divers regularly take ten or more computers side-by-side on a dive?

 

You can see them all archived on the www.divernet.com comparison tests pages. The latest is at: http://www.divernet.com/equipment/0306compsextra.shtml

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John,

 

I'm duly impressed (by what Wienke said when he shook your hand), but hardly surprised). I'm sure everybody here knows (or at least they should know) that the side-by-side equipment comparisons you've run as technical editor at Diver magazine are the best, and absolutely unique in the industry. How many diving publications, other than yours, have ever run a negative review or evaluation of any piece of dive equipment, particularly when it is an advertiser?

 

I was also interested to see that you cited Pyle and included an explanation of ad hoc deep stops at the end of your excellent comparison test of computers from the March 2006 Diver issue.

 

Since what I've just posted above could be read as advocating that users of Haldane computers practice ad hoc deep stops (which it is, and I do), it might be worthwhile printing Bruce Wienke's remarks in 2003 urging caution about the practice. This is from Another scuba-related website:

 

Folks,

-

The business of "P/1.6", "halving", and "gradient factor" rules for juxta-positioning deep stops (first and following afterward) are all ad hoc measures imposed on Haldane deco. Sometimes they work for very limited diving and ranges, but in general they are without real physical basis. They are attempts to get a dissolved gas (only) algorithm (Haldane M-values) to mock up bubble dynamics. And such rules are not self consistent for diving. They are also risky when they miss requisite deeper stops, but give shorter overall deco in the shallow zone.

 

They are a poor way to mock phase dynamics when dual phase tables, meters, and software are now available to do this self-consistently over a whole dive profile.

 

Diver, beware.

 

Bruce Wienke, Program Manager Computational Physics, C & C Dive Team

 

In another post, however, Wienke does make it clear that he endorses the idea of short ad hoc deep stops for all recreational no deco stop diving:

 

"Deep stops for recreational no-deco diving are safety stops, and the 1/2 rule holds up fine in this regime. This is a coupled rule for the new NAUI Rec Tables and it has been correlated in RGBM and by Bennett at DAN. Plus Marroni. Plus others. It extends to ALL recreational Tables, RGBM, Navy, etc that we use. The rule is "one minute at half the bottom depth" Remember, this is for recreational, no-deco diving.."

 

Those of us who mainly dive on coral reefs usually end up doing more than one minute at half the max depth as part of our normal multi-level ascent anyway.

 

Frogfish

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P.S.  RGBM isn't the only non-Haldane decompression model around, but it is the probably the best, and the only one (to my knowledge) that has been implemented on a commercial dive computer suitable for recreational diving. 

 

Excellent post, Robert. Very thorough and educative.

 

Just a small note to say that the DCIEM model is implemented in the Citizen Cyber Aqualand series of dive computers.

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Hey John

Bruce shook your hand for the review. He shook his fist at me when I told him that the record of RGBM having a clean record was not so clean because I took a hit diving within RGBM in PNG. Then a few others came to pour more rain on the RGBM party.

However, for dive guides and professionals, the computer quagmire is a very real dilemma. I mean, someone comes with a clean computer for 1 -2 weeks and leaves loaded, expecting the guide to stay with them or worse the company expects them to stay with the clients.

Now many dive ops switch DMs so they don't dive 4 dives a day 350 a year. But there are many who don't.

As for the EAN % switchers, I wish you all the best of luck and hope you stay out of the can. However, please let me know which trips you're on so I can avoid the same boat. I like having not to stop a trip for medical emergencies.

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I must admit, when I was diving in Malta we were diving as a threesome ;) two of us were on Aladins one on suunto. At first we took the piss out of our mate who had to stop up to 3 mins longer than us (macho diver banta) but one of us would stay with him for safety purposes, well that was me as I thought, in the long run its obviously better.

 

Anyway what do people think of the VR2-VR3 computers ???

 

Dive safe

 

DeanB

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Hey John

Bruce ... shook his fist at me when I told him that the record of RGBM having a clean record was not so clean because I took a hit diving within RGBM in PNG. Then a few others came to pour more rain on the RGBM party.

However, for dive guides and professionals, the computer quagmire is a very real dilemma. I mean, someone comes with a clean computer for 1 -2 weeks and leaves loaded, expecting the guide to stay with them or worse the company expects them to stay with the clients.

Now many dive ops switch DMs so they don't dive 4 dives a day 350 a year. But there are many who don't.

As for the EAN % switchers, I wish you all the best of luck and hope you stay out of the can. However, please let me know which trips you're on so I can avoid the same boat. I like having not to stop a trip for medical emergencies.

 

But Drew, didn't you hit the hot tub (or have a hot shower) that time?

 

By now I think we all know that isn't such a great idea, but it wasn't that long ago that most of the Aggressor fleet actually had hot tubs that people were jumping into right after coming in from diving.

 

You're absolutely right about dive guides. There are lots of boats that do same day turn-arounds, which doesn't allow the diving personnel much off-time for off-gassing. My friends on a boat here just had a customer (Russian nationality) who decided to take his newly certified 12-year old son on a bounce to 60 meters. Obviously without telling the dive master ahead of time, who sensibly did not follow them all the way down. Both of them were banned from diving the rest of the trip, but it was almost over. This was in Komodo, where there is no chamber - the closest is Bali, almost a two hour flight.

 

There is a line of thinking that the only reason more dive guides don't get hit is that beyond a certain point, repetitive diving can lead to a higher resistance to DCI, the result of a reduction in microbubble nuclei. Wienke has written about this.

 

Frogfish

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Following on Drew's comment above, the RGBM algorithm used in newer Suunto (and other) computers is very different from the Buhlmann-based  "Haldane" models used in the most other dive computers.  Calling it "more conservative" may be correct (if misleading), but complaining that it "punishes" the diver doing repetitive dives misses the point.  As for intentionally setting the computer to a nitrogen percentage less than the actual N2 content of the breathing gas in order to avoid doing deco stops you should do, thus overriding the computer's best efforts to keep you alive ... that strikes me as (avoiding the "S" word) ... a potential example of Darwinism at work in our species and the modern world.

 

Buhlmann models assume that all nitrogen in body tissues is in solution. The reality is that nitrogen in our bodies during a dive exists in solution and in free gaseous phase, namely bubbles. The modified version of the "Reduced Gradient Bubble Model" (RGBM) algorithm used in newer Suunto computers, developed by Dr. Bruce Weinke at Los Alamos National Laboratory,  takes phase transitions between nitrogen in solution and gaseous form into consideration, which Haldane models cannot do. 

 

It's not that RGBM "punishes" repetitive dives or short surface intervals.  Rather, Buhlmann models fail to take into account the additional risks of repetitive multiple-day repetitive diving, inadequate surface intervals, and/or 'bad' profiles (saw-tooth, bounce, and reversed profile dives). 

 

Setting a RGBM computer to 27% nitrox while diving on air to avoid having to make decompresion stops is asking for trouble, in my opinion, and may be riskier than just using an older and cruder Buhlmann-based computer.  As someone in this thread has already asked [RD: it was John Bantin], what's wrong with just doing the stops?  (Or to put it another way, what is it that you really like about decompression sickness?)

 

If you prefer the additional risk of doing repetitive dives with a Buhlmann computer (or if you're one of those divers who think driving your computer all the way to the no-dec stop time edge on every dive is smart diving), then I'd have to ask why you bought one of the very few RGBM computers that anyone makes in the first place?  There are still plenty of aggressive dive-at-your-own-peril computers for sale in every dive shop, though Mares and other manufacturers have also released new RGBM-based computers or are about to do so.  Something to do with those pesky lawsuits from the estates of dead divers.

 

None of this is meant to suggest that RGBM is perfect or that it guarantees you will not take a hit.  I do think RGBM is superior (and safer) than Buhlmann models, but there are cases where divers using RGBM have taken DCS hits, though most if not all were technical divers doing very deep dives with rebreathers and/or exotic gas mixes.

 

I should also make it clear that even though I believe RGBM is safer, I use two DiveRite computers (Duo and a Nitek Plus), both Buhlmann-based - I carry both of them on every dive.   They both still work and don't require replacement yet.  I also have reliability issues with Suunto due to bad experiences with a Solution Nitrox that I owned years ago.  But I am aware of the limitations of the Buhlmann algorithms and do what I can to compensate.  For example, deep stops (starting at half the difference between the max depth and first deco-stop depth, or half the maximum depth on no-stop dives), avoiding bad profiles, and never driving the computers to the edge, particularly for multiple day repetitive diving (for example, on a liveaboard). 

 

By not driving the computer to the edge, I don't mean that I limit my bottom times more than you.  It means that I do my stops, and I take extra time on the safety stop on any dive that has pushed near or beyond the no-stop limits.

 

It would be interesting if someone here started a poll - who has ever taken a hit, or more than one, were they using a computer at the time,  what compuer (and what decompression model), what gas, and was the dive part of a sequence of repetitive dives on multiple days.

 

Frogfish

 

P.S.  RGBM isn't the only non-Haldane decompression model around, but it is the probably the best, and the only one (to my knowledge) that has been implemented on a commercial dive computer suitable for recreational diving.  Other non-Haldane models include the Canadian Series model (implemented in the DCIEM Kidd-Stubbs dive tables), the EL model developed by the US Navy and implemented as a military computer for combat divers, the Slab model developed by Dr. Tom Hennessey for the British Sub-Aqua Club (implemented on tables), and the Varying Permeability Model (aka Tiny Bubble Model), and its successor, VPM,  developed by Yount and Hoffmann at the University of Hawaii, implemented on tables and in software, which is generally viewed as the progenitor of Wienke's RGBM model.

 

Jolie Bookspan's excellent "Diving Physiology in Plain English" has a short discussion of the non-Haldane models. 

 

Wienke has a book about RGBM out..

 

Wienke's book on Amazon

 

... and it is also covered in his  Basic Diving Physics and his textbook on decompression theory and Technical Diving in Depth.

 

There's also a very interesting piece about Deep Stops by Richard Pyle, the University of Hawaii/Bishop Museum ichthyologist who independently discovered the value of deep stops about ten years before Wienke uncovered the theory behind them.

 

Richard Pyle on Deep Stops

 

 

Frogfish I think you missed what I said, so I'll say it again slowly "What's worse is I have a Sunnto D9 but have it set to 27% Nitrox which is still a little behind my Oceanic on air!" I use an oceanic on air...... the Sunnto is on 27% so I can use it or it would permanently be bent, so yes do avoid the "s" word and read the post. ;)

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I have been using an old "blue" alladin nitrox as my primary computer since they were released.

It has had absolutley no care or attention other than a bi-annual serive and battery change.

 

For the last few years this old wristmount has been backed up by an air nitrox cable tied to my contents gauge.

 

Both have performed flawlessly.

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Frogfish I think you missed what I said, so I'll say it again slowly "What's worse is I have a Sunnto D9 but have it set to 27% Nitrox which is still a little behind my Oceanic on air!" I use an oceanic on air...... the Sunnto is on 27% so I can use it or it would permanently be bent, so yes do avoid the "s" word and read the post. ;)

 

If I misunderstood what you wrote, then my deepest apologies, but I don't think I have. You've said (twice) that you are have set your D9 to 27% Nitrox while your Oceanic computer is set on air. I assume that means you are diving on air (=EANX21, not EANX27) and that by "behind" you mean that the Suunto set on EANX27 still gives you less bottom time and/or requiresa longer decompression stops than the Oceanic set on air does.

 

I can only interpret your explanation that you dive with the Suunto set to EANX27 while diving on air "so I can use it or it would be permanently bent" to mean that the Suunto computer - not you - would be permanently "bent" (i.e., that it would lock you out) because you've chosen to follow the the least conservative of your two computers rather than adhering to the Suunto's decompression limits and instructions.

 

Most of the people I dive with would adhere to the principle that, when multiple computers are used, one should follow the most conservative computer regarding no-stop times, deco stops, etc. not the least.

 

If I understand you correctly, what you're doing here is the same as diving with two computers, following the least conservative profile of the two, and then putting the computer that is still "in deco" when you come up back in the water on 4 meters of line and hanging it over the side of the boat so it can "decompress" while you have a hot shower. If so, then I'm afraid the "s" word still does apply, as might the reference to Darwin. If I've misunderstood what you're saying, then don't be angry, just point out how I've misconstrued your meaning.

 

Frogfish

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Yes Robert I did take a hot shower. But after talking to a few doctors, they think I was probably already in non-clinical DCI and the hot shower pushed it to clinical.

 

Kriptap, I think what Frogfish was trying to say (and which I wholly agree with) is that using a precision instrument that is designed to keep you out of trouble in a dangerous way isn't something to be joked about.

I sure you know the consequences of your own actions, which at the end of the day is your own responsibility.

Frogfish is trying to emphatically deride that practice as unsafe and let people know exactly what the bad consequences are for messing with the EAN% settings to get bottom time.

 

But Drew, didn't you hit the hot tub (or have a hot shower) that time? 

 

By now I think we all know that isn't such a great idea, but it wasn't that long ago that most of the Aggressor fleet actually had hot tubs that people were jumping into right after coming in from diving.

 

You're absolutely right about dive guides.  There are lots of boats that do same day turn-arounds, which doesn't allow the diving personnel much off-time for off-gassing.  My friends on a boat here just had a customer (Russian nationality) who decided to take his newly certified 12-year old son on a bounce to 60 meters.  Obviously without telling the dive master ahead of time, who sensibly did not follow them all the way down.  Both of them were banned from diving the rest of the trip, but it was almost over.  This was in Komodo, where there is no chamber - the closest is Bali, almost a two hour flight.

 

There is a line of thinking that the only reason more dive guides don't get hit is that beyond a certain point, repetitive diving can lead to a higher resistance to DCI, the result of a reduction in microbubble nuclei.  Wienke has written about this.

 

Frogfish

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One thing I have noticed is the importance of surface intervals in determining the no-deco times given by the different models.

 

The Suunto algorithms are particularly cautious with short surface intervals. In cayman, where Kriptap dives it is routine to have short 45 minute surface intervals between dives on two tank trips. When I am diving in this the Suunto is very conservative.

 

When I dive in other places, such as the Red Sea, where it is routine to leave 1.5 hours or more between dives I find the Suunto can actually be more agressive than other brands.

 

Anyone else noticed similar patterns?

 

Alex

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One thing I have noticed is the importance of surface intervals in determining the no-deco times given by the different models.

 

The Suunto algorithms are particularly cautious with short surface intervals. In cayman, where Kriptap dives it is routine to have short 45 minute surface intervals between dives on two tank trips. When I am diving in this the Suunto is very conservative.

 

When I dive in other places, such as the Red Sea, where it is routine to leave 1.5 hours or more between dives I find the Suunto can actually be more agressive than other brands.

 

Anyone else noticed similar patterns?

 

Alex

 

This is due to the Suunto algorithm which - and Suunto says it themselves - is NOT a purebred RGBM implementation, but a fuzz factor hack on top of a Haldanean model(*). One way Suunto has simulated RGBM is to tamper with the surface interval off-gassing model to make it (much) more conservative.

 

There has been plenty of discussion on this at various tech. diving forums like Decostop etc, so I'll let you read it yourselves.

 

There is a paper by Suunto on the algorithm at http://www.dive-tech.co.uk/resources/suunto-rgbm.pdf

 

Also, a nice comparison of various models is at ftp://decompression.org/pub/Maiken/

 

timo

 

(*) I am not familiar with RGBM, but I am fairly familiar with VPM and as a rule, a real-time (=diving computer) implementation of a full bubble model would be prohibitively expensive in terms of processor and power consumption.

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After this discussion between Kriptap and Frogfish, I think the most interesting part is the wide variety of conservative to not so conservative models out there. My LDS sells a lot of Oceanics. They are a big shop, and thus many people here in San Jose have these computers. We are not seeing more people going into DCS, or at least nobody is reporting them.

 

At the end of the day, the question is which computer can we trust? This is after all part of life support.

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I use the UWtec SmartCOM (a console with quick disconnect and compass).

 

Likes: readable display (nice UI, show info you need while diving), long battery life, good ergonomics (relatively small size, compass mounted at top and angled for easy reading), built-in screen guard, easy download of dive data via IrDA (mac & PC), good algorithm (also see dislikes)

 

Dislikes: alarm beeping on ascent (very slow ascent rate for last 30 feet/10m; the alarm doesn't go off very often for me; but, can/will go off if you swing the console up to look at it), the software(JTrac) for my mac could use some user interface/features tweeking

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Maybe I should add this tale for those that want a less cautious computer.

 

I did a dive to the Shark Hole at Tikehau. It's around 40m deep. After less than an hour the dive guide wanted to do a second dive. I managed to delay things so that we got about an hour and ten mins surface interval.

On the way up, I found I had to hang for around 20 minutes. The dive guide took a lot of interest in what my computer said.

 

Afterwards, back in the boat, she said that she though that ten minutes deco-stop was quite enough and that 20 mins was totally un neccessary. I asked her what here computer said.

Her answer? "I don't have a computer!"

 

So there's your answer. Dive without one. She seems to be OK - or at least she was when I last saw her.

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I like the uwatecs a lot too, and all the new models (including the new aladins) have a microbubble level adjustment from L0 (normal) to L5 (most conservative). I found that with it adjusted to about L2 it is just as conservative as a suunto and it gets more conservative above that.

 

Now, as for getting bent, it depends a lot more on the physiology of the person than on what computer one uses. I've seen people get bent when following a suunto blindly, and I've seen people that never used a computer or tables and are still alive. I've done more than 1,000 dives with my uwatec on the least conservative setting (L0) and never had a problem, but it may happen some day, who knows.

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Hi Guys,

Thought I'd share some info here.

 

As part of my veiled past I spent two years in a very dubious corner of eastern Africa. Trying to decide what I would do stuck in the dust bowl of Djibouti for two years (horn of Africa north of Somalia) I turned to Diving. A friend I was working with a crazy diving dude by the name of Yves LeFevre (not the Club Raie Manta from Tahitit Yves) took me aside, I wasn't dive trained, and progressed to introduce me to the basics of SCUBA. He was a trained DM equivalent with the French FFESSM agency.

 

My first and only lesson was "OK, my Eeenglish friend, eer izz ze regulaator. You breeze wiz ziss. You must never ever 'old your bress and when yoo see zis needle (holding up the SPG) getting low down to zees red areaa zen you must come back to zee surface. D'accord".

 

A short lesson on equalizing and ascent speeds and I was trained. That was the first and only session I had for diver education in two years. We dived every day if we could, making use of the Red Sea which lapped at our doorstep. I would get tanks filled by the French Marines for free, and they never needed ID so that was great. They also made sure the reserves were open as we were using old J valve tanks.

 

I dived there for two years. Only a pair of shorts, tshirt, backpack, mask, fins and an air guage. No depth guage or computer. Never had any incident or cause for concern. There were a few muppets who got caught out whilst free diving down to scuba buddies, yanking their regs and fooling around for a couple of minutes then rocketing back to the surface....kaboooom.... But there was no training there, they had no idea of the mechanics of diving.

 

When I eventually returned to the "real world" I knew what I wanted to do. I embarked on dive training and was already starting to eye up the cameras currently available back then. Going through the education made me cringe. I started my Open Water course with about 600 dives under my belt but was cringeing everyday when the Rule Books were saying "now you can't do this" and You can't do that". We were doing those things with complete abandon in Djibouti. Never had an accident.

 

So I guess my point is that most dive computers are pretty darn conservative irrespective of their seemingly conservative or liberal allowances. Will the computer manufacturers ever make a computer that is completely on the button so far as true safety margins are concerned? I doubt it. By reading this thread we can see that some people choose to push those limits after having made their decisions based on their own faith and knowledge. With a computer that puts us close to the real safety margins how long would it be before people also try pushing those limits? It's innevitable, we're humans. We wanna push the envelope just a little, see what happens.

 

If any readers do decide to push their own personal limits and those limits set down by dive computer manufacturers then they do so based on their own assumption of risk. No one can say, don't do it. Its down to the individual. Hopefully they will live to tell the tale.

 

Cheers,

Mark.

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After this discussion between Kriptap and Frogfish, I think the most interesting part is the wide variety of conservative to not so conservative models out there.  My LDS sells a lot of Oceanics.  They are a big shop, and thus many people here in San Jose have these computers.  We are not seeing more people going into DCS, or at least nobody is reporting them.  ...  At the end of the day, the question is which computer can we trust?  This is after all part of life support.

 

Interesting point in an interesting discussion, one in which I hope nobody has been offended. I don't think that anyone here is saying that, say, Oceanic or Cochrane computers are intrinsically unsafe. Nobody intentionally designs unsafe decompression algorithms or manufactures unsafe dive computers, but there are clearly different ideas in the industry and among consumers about what constitutes "conservative enough".

 

The venerable US Navy Tables (ancestral to PADI's) were never designed to prevent DCS in all combat divers all of the time; the objective was to reduce the incidence of DCS in a population of young, fit, carefully screened and trained individuals to an "acceptable" level. An expected DCS incidence ratio of one hit in 500 dives might look reasonably safe to some people in certain situations, but not others. How comfortable should someone with more than 999 dives be with a more conservative algorithm expected to reduce the hit ratio to only 1 in 1000 dives? Or 1 in 2000?

 

Dive computer or decompression algorithm cannot directly measure dissolved and gaseous phase nitrogen in our tissues, nor can they take variations in susceptibility to DCS into consideration. These are mathematical models. Some of us are persuaded that RGBM can better model the actual physical processes involved than a Haldane-based algorithm, but that doesn't make it 100% safe. We all know things that are supposedly risk factors for individuals - age, lack of cardiovascular fitness, % body fat, alcohol consumption, dehydration, lack of rest, etc. And I suspect most of us also know someone like Drew, who is considerably younger, fitter, thinner than I am, and who doesn't drink, but who has taken an "undeserved" hit doing a dive with the same kind of dive profile that I have done many times.

 

We all have to decide what level of risk is appropriate for us, but also keep in mind others who rely on our judgment or may be tempted to follow our example. Kriptak made it clear in his 2nd post in this thread that he wasn't advocating that anyone else play with the EANX % setting on their computer to increase bottom time. But I did want to say loud and clear to others reading this that there are divers here who would consider this an unsafe practice. I have a friend who dives pure DIR rules. He believes that I'm crazy because I do things like dive beyond 30 meters on air instead of trimix, do more than two dives a day, and do dives that require mandatory deco stops without using twin tanks and deco gas, following the rule of thirds, etc.

 

LChan mentioned diving in the San Francisco Bay Area. I could be wrong, but it's my impression that recreational diving there is often limited to two dives a day, rarely extends over more than two or three sequential days, and that generous or at least adequate surface intervals are the norm, if only to get warm.

 

As the discussion here has highlighted, the difference between RGBM (modified or full-up) and Buhlmann-based algorithms in terms of bottom time and/or deco stop time is most noticeable on multiple day repetitive diving and when surface intervals are cut short.

 

This, of course, is the kind of diving we are most likely to be doing on trips on a live-aboard or at a full-out dive-oriented resort, especially in the tropics, and some of those liveaboards will be operating in remote areas of the world far from hyperbaric facilities.

 

Personally, I don't really think it matters much what computer is used for simple 2 or 3 tanks a day recreational diving over only one or two days. But I think it may matter a lot on a liveaboard when people are doing 3, 4, and even 5 dives a day for up to twelve days, or even longer. I'm afraid that these situations - when having a computer that is sensitive to the heightened DCS risks posed by repetitive multiple-day diving is most important - are also likely to be the situations where some divers will be tempted to cut corners with N2 % settings or else intentionally select a dive computer because it gives him or her more bottom time than other computers rather than for maximal safety. If this discussion only serves to make anyone in this situation think twice before buying a computer because it is generous with bottom time, then it will have been well worthwhile.

 

Frogfish

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Isn't it counterintuitive that the RGBM algoritm is more accurate yet more conservative?

 

If the computer modeled reality perfectly (including prior alcohol consumption, phase of the moon ...) then it could give us the absolute maximum safe bottom time for each and every dive. When factors are left out of the modeling equation then, with lifes and law-suites at stake, a worst-case scenario of a drunk, cold, dehydrated, elderly, obese diver must be assumed :lol:

Well perhaps not that extreme but algorithms must err on the side of caution and penalize bottom time more than typically needed.

 

 

So if older algorithms don't explicitly model the effects of microbubbles they should implicitly account for it by always adding an extra safety margin. In contrast, the RGBM model will know when microbubbles have interfered with off-gassing and only penalize the diver when it is needed.

 

So, why is RGBM more conservative???

 

One option is that older models are truly dangerous, but I find that a bit hard to believe.

 

Based on some of the earlier comments given in this discussion I think it is more likely that the current RGBM implementation is mostly the old model with microbubble-penalties added. Because the old model implicitly accounted for microbubbles by adding an extra safety margin you now account from microbubbles twice, resulting in a very conservative computer. Reality may be somewhere in between, where they have added the penalties but removed only part of the old model's safety margin. Alex's experience that RGBM is actally a bit more aggressive than the older models when you use long surface intervals could be the result of this.

 

If this is the case then we shouldn't blame the RGBM algorithm itself, just the way the early computers have implemented it. If RGBM is really a more realistic model of reality then, once implemented properly, it should start giving us more bottom time without increased risk. Untill such time I'll happily dive with my simple Genesis Resource computer and use its N2-loading bar graph to stay within my own safety comfort zone.

 

Bart

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Well,

 

I think it has been widely observed that the older models (USN, "raw" Buhlmann, etc.) are indeed if not outright dangerous, at least less reliable for a population of average divers (I sem to remember Mr. Buhlmann himself has adviced agaist the "raw" version of the algorithm). For this reason various corrections have been added. The most widely used have been "Pyle" stops and Gradient Factors for Buhlmann (For GF implementation over Buhlmann, see Eric Baker's excellent paper at http://www.gap-software.com/staticfiles/Un...ingMvalues.pdf).

 

Now, the bubble models (RGBM, VPM, etc.) DO model reality better than Haldanean models (see for example http://www.decompression.org/maiken/Bubble...Strategies.htm). I suppose there is no disagreement on this. But they are still models and the probability of an undeserved hit is still significant. There have been several studies showing that the susceptibility of an individual to DCS varies widely within short periods of time. Therefore, the only way to fully avoid DCS is not to dive. Building a better computer is not the answer since perfectly modelling the human physiology will for some time be outside the capabilities of computer science.

 

I personally plan all my tech dives using VPM-B (V-Planner SW) and in most cases it comes very close to what I did using Buhlmann and GF (or GAP RGBM with similar parameters). However, it is true that for long dives and especially repetitive long dives it is more conservative than my diving computer. I even add a factor or two of more conservatism if the water is very cold (<5C), if the dive will be stenuous, etc. For me, that extra conservatism is a good thing. Spending the extra 10 minutes doing deco is better than an airlift to a chamber any day.

 

For recreational stuff, I use a computer with a Buhlmann-based algorithm set to +1 altitude compensation, which brings it quite close to VPM-B for no-stop dives in the 30 meter range using air or nitrox. The major difference being that VPM (and RGBM) make you stop deeper as suggested already by Pyle based on empirical evidence and later by Baker based on theory.

 

If we look at what Suunto, Uwatec and other computer makers are doing, they are introducing "deep stop" algorithms more and more in their lineup. So, no matter what, bubble model concepts are filtering into the way we do decompression using mainstream computers no matter what algorithm we actually are using. If you do not like it, there are and will be computers out there that can do raw buhlmann and let you do max. bottom times and min. deco. For some, this is probably a wise move, for others not so.

 

I did a fair bit of studying on this when I started on tech. diving because having a family to support I wanted to know what I'm doing. My conclusions are probably apparent from my choices of computers/SW and I strongly encourage anyone to do a similar study so that you know what risk level you are at. Deco theory is no black art, neither is it rocket science.

 

timo

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Remember, the reality of diving is that you 'bubble' every time you dive, it's just a question of how much, and wheather it's your lucky day or not. :lol:

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