Deco without deco training

[Slamfire]

These dives were sketched out beforehand with V-planner so I knew the computer was giving me garbage. It was probably still wanting 40 more minutes in the water when I surfaced. Figure I was using it way beyond what it was designed for so the CYA mod kicked in!

Yesterday I did a 2 gas deco dive following previously cut tables from Buhlman gradient factors. I was also carrying a lost deco gas scenario on the slate. Because I like conservatism, I stayed longer at my last stop. When I surfaced I was only 6 mins away from fulfilling the lost deco gas overall run time, and yet the computer was saying I still needed 34 more minutes. CYA mode indeed.

The problem is that the Buhlmann-based algorithms in recreational computers are no longer used in technical diving because bubble-model/deep-stops models produce less DCS. So, a naive diver following a recreational computers recommendations for decompression stops will be at a higher rate of DCS than a technical diver blindly following a computer that implements VPM or RGBM.

My computer uses RGBM (not Suunto) and has "deep" stops. However, it can only handle one gas. So you're still much better off cutting V-Planner tables for dives where you really need the deep stops.

Trying to turn it into a human vs. computer argument is avoiding the larger problem that recreational divers that don't know anything about decompression while on the surface are only going to be asking for trouble by naively trusting their computer while underwater.

Couldn't have said it better myself.

Not knowing how the machine works is user error.

User error indeed. But even then, the manual only tells you gradient factors change, it does not tell you by how much or how that will affect your planning. The computer itself has a planning mode, but it is not very powerful. I did read the entire manual when I got the computer. However, at the time when I first got the computer I had no idea what gradient factors were.

I've also seen users make some interesting errors with their desk top deco software too. To me, anyone who uses a vanilla, neo Haldanian bend & mend algo for anything but the most minor oops deco infraction, is well into user error.

So what do you propose? By vanilla neo Haldanian are you referring to models without deep stops? Lot's of recreational computers these days come with "deep stops". And even then, implementing a deep stop ascent strategy is not that difficult. Just look at Bob's article and post. I may be wrong but I would have thought Buhlmann with gradient factors is neo haldanian. It's probably just semantics.

 

[Kern]

If you're using RGBM on any rec computer more than likely this is what you're using… "modified RGBM recreational algorithms (Haldane imbedded with bubble reduction factors limiting reverse profile, repetitive, and multiday diving), as coded into Suunto, Mares, Dacor, ABYSS, GAP, HydroSpace, Plexus decometers, lower an already low DCS incidence rate of approximately 1/10,000 or less. More RGBM decompression meters, including mixed gases, are in the works". (RGBM ALGORITHM OVERVIEW: CONCEPTS, BASES, VALIDATION, TESTING AND REFERENCES. p12). Lets not forget, If they had a fully iterative dual phase version of RGBM there would be no need to add deep stops. Deeper stops would be generated by the RGBM algo alone, to prevent bubbles from reaching a critical volume.

You're right, semantics, for me neo Haldanian means a single phase algo without any hacks to alter the output. However, I also think that Pyle deep stops, (& multiple gasses), on a rec computer are no more than a marketing ploy by manufacturers. After all, if I can blow off the safety stop, a deep stop is not required either. For rec diving I think straight Buhlmann is conservative enough without GF.

I have little interest in Pyle deep stops. For rec dives I'd rather concentrate on my ascent rate. For deco dives I think they're better than nothing, but far to arbitrary .

I'd be happy to use GF on deco dives, but I'd have to do a lot more research to satisfy myself that I was scaling the Lo/Hi correctly for the depth & time of the dive. Then I'd have to spend some time tweaking them for myself.

We've gone off on a bit of a tangent, so to address the OP a little. There are some who have done all the deco training available that I would not care to dive with. There are also those with no training, but who have the information, demeanour & processing power to have at it without formal training. Having said that, I don't think hard or soft overhead diving is a natural & obvious progression for all divers.

 

[FritzCat66]

However, I also think that Pyle deep stops, (& multiple gasses), on a rec computer are no more than a marketing ploy by manufacturers. After all, if I can blow off the safety stop, a deep stop is not required either...

I have little interest in Pyle deep stops. For rec dives I'd rather concentrate on my ascent rate.

Actually, there's at least one recent significant study contradicting what you just said, as far as rec diving is concerned. Not only does the deep stop make more of a difference than a slow ascent rate, but in fact the slowest ascent rates tested (3m/min) resulted in some of the highest observed Doppler precordial microbubble counts. I believe this is one of the studies that led NAUI, for example, to change their recommendations towards the deep stop for rec divers.

For the rec diver, it looks like one's best bet is, in fact, the deep stop along with the standard shallow stop, with a moderate ascent rate (10m/min) between them - not the fastest, but not the slowest either. Which means the addition of deep stops to a rec dive computer isn't gimmicky at all - in fact, I wish more of them would catch up.

As to the OP, I think the best thing to say is: a rec computer is designed for rec diving, which by definition is one in which there is no mandatory decompression. If, on a rec dive with a rec computer you happen to accidentally slip into deco, I would advise paying attention to the computer to get yourself out of it. But I would never plan on going into deco with a rec computer - the computer's just not designed for that. Planned deco diving is tech diving by definition, which is a whole 'nuther ballgame and requires different equipment and techniques.

 

[ianr33]

Actually, there's at least one recent significant study contradicting what you just said, as far as rec diving is concerned. Not only does the deep stop make more of a difference than a slow ascent rate, but in fact the slowest ascent rates tested (3m/min) resulted in some of the highest observed Doppler precordial microbubble counts. I believe this is one of the studies that led NAUI, for example, to change their recommendations towards the deep stop for rec divers.

Reading your link that study seems to have one enormous flaw to me. The slow ascents started from the bottom ! So even though they were acending those divers would still be ongassing.

Run a profile through V planner and it gives a depth where offgassing starts. For the first profile in the study (82 feet for 25 minutes) offgassing starts at 58 feet. So there is ABSOLUTELY NO ADVANTAGE in creeping up to 58 feet.

To me,the ideal ascent is:

1)A prompt ascent from depth at 30 feet/minute until 20 or 30 feet above the offgassing depth. (The offgassing depth is neutral,when you first reach it there is no net flow of N2. After a minute or 2 though you will be ongassing again)

2)Do a deep stop(s) for a minute or 2 if you want. No way can it hurt and could be very beneficial.

3)Once at 30 feet take as long as you want to get to the surface. Longer and slower the better.

The things to avoid are :
1)Slow ascents from depth (30 ft/minute actually feels pretty fast. On deco dives I often turn a minute or 2 early. That allows me to do a slower,more controlled ascent.

2)Doing a deep stop too deep. Stopping at 80 feet on a 100 foot dive is not a good idea.

 

[lamont]

Actually, there's at least one recent significant study contradicting what you just said, as far as rec diving is concerned. Not only does the deep stop make more of a difference than a slow ascent rate, but in fact the slowest ascent rates tested (3m/min) resulted in some of the highest observed Doppler precordial microbubble counts. I believe this is one of the studies that led NAUI, for example, to change their recommendations towards the deep stop for rec divers.

You need to be careful with what you are talking about.

The 3m/min no stop profile they did would best be characterized as "failure to get off the bottom", not just "slow ascent rate". Those profiles spent 3 minutes on each dive doing additional ongassing and then only had 5 minutes of what I would usefully call decompression -- compared to profiles that got off the bottom promptly and then did 10 minutes of deco, of course they did much poorer.

What would be interesting would be to see the 5@15m+5@6m profile compared to a fast (18m/min ascent) to 15m with a 1.5m/min ascent from there to the surface (2 minute stops).

The moral of that paper seems to be focusing on getting your ass off the bottom and not going into decompression. The paper should really break up the ascent time into time spent ongassing (faffing around deep) and time spent offgassing (decompression) -- I think that would make the results pretty obvious.

This is why when I'm scooter/shore diving and coming up from the deepest part of the dive to my first stop I just stay on the trigger and let my suunto ascent alarm scream about it. Slow ascent rates *before* you start driving a gradient are fairly pointless (other than for buoyancy control).

 

[Slamfire]

I don't think anybody's posted Erik Baker's paper on M-Values and Gradient Factors yet. When I first read it I found it extremely enlightening.

After understanding the theoretical the mechanics of tissue loading and off-gassing, you realize and understand why it is said that all dives are decompression dives. Then you start reading about shaping the ascent curve and it all starts making sense. Then you look at wrist computers with a different eye. Acquiring that knowledge, irrespective of using formal or informal channels, is key.

Nowadays, regardless of whether I'm doing multiple gas staged deco dives, or single gas fun recreational dives, I still follow more or less what Ian described:

1)A prompt ascent from depth at 30 feet/minute until 20 or 30 feet above the offgassing depth. (The offgassing depth is neutral,when you first reach it there is no net flow of N2. After a minute or 2 though you will be ongassing again)

2)Do a deep stop(s) for a minute or 2 if you want. No way can it hurt and could be very beneficial.

3)Once at 30 feet take as long as you want to get to the surface. Longer and slower the better.

The difference is that I am a little more "loose" on single gas shallow recreational dives (e.g. instead of spending 2 mins at exactly 40' I may take 3 mins to move from 40' to 30'). On staged deco dives I am much more disciplined, doing the time at the exact planned depths.

I always follow Ian's third point. I am no commercial diver, or dive professional. I am doing this for fun. I am on nobody else's clock but mine. There is no rush. Why should I go to the surface when I can stay down a little longer. There are plenty of things to see at 30' or higher in our Western Canada coasts; I don't get bored. I already paid for the gas on my tanks and I am not getting a credit if I bring them in for refill with a couple extra hundred psi.

 

[lamont]

I don't think anybody's posted Erik Baker's paper on M-Values and Gradient Factors yet. When I first read it I found it extremely enlightening.

One problem that I have with that paper is that it is blind about controlling gradient factors being in different compartments -- so comparing a 10/85 150 foot tech1 dive to a 10/85 tech2 100 foot dive, you'll wind up with that 85% GF in very different compartments. It is a start and it certainly gives a way to compare slightly similar profiles on the basis of what they do with GFs, but I'm unconvinced that you can compare very different profiles with the same GF usefully.

 

[Slamfire]

One problem that I have with that paper is that it is blind about controlling gradient factors being in different compartments -- so comparing a 10/85 150 foot tech1 dive to a 10/85 tech2 100 foot dive, you'll wind up with that 85% GF in very different compartments...

I always thought that Erik did that on purpose so that the concept could be easily explained. Once the concept is understood by the reader on one compartment, then the reader can realize that the M-Value relationship pressure graph (fig 3 pg 8) has to be created for each one of the 16 different Buhlman compartments. And then once that is clear, it then stands to reason that different compartments will be leading at different times. He doesn't spell it outright, but he does show tables comparing the different leading compartments at different points of a dive (Table 5 page 9).

 

[Kern]

Actually, there's at least one recent significant study contradicting what you just said, as far as rec diving is concerned. Not only does the deep stop make more of a difference than a slow ascent rate, but in fact the slowest ascent rates tested (3m/min) resulted in some of the highest observed Doppler precordial microbubble counts. I believe this is one of the studies that led NAUI, for example, to change their recommendations towards the deep stop for rec divers.

The main problem I have with this is that I didn't say anything about slow ascent rates. In fact I don't believe ultra slow ascent rates are necessary during any part of a dive. And although I don't do Pyle stops, I do actually have my own stop regime, it just starts at a shallower depth & is scaled for time & depth depending on the dive depth & time, proximity to NDL's (who's NDL's you may ask), whether I got up on the wrong side of the bed, house of the moon, etc.

What is the correlation between number of bubbles & DCS?

Also, if you look around a bit more you'll find studies that don't agree with the one you'v quoted. If one study in a peer reviewed journal made something a fact, we'd have cold fusion.

 

[Slamfire]

Yesterday I did a 2 gas deco dive following previously cut tables from Buhlman gradient factors. I was also carrying a lost deco gas scenario on the slate. Because I like conservatism, I stayed longer at my last stop. When I surfaced I was only 6 mins away from fulfilling the lost deco gas overall run time, and yet the computer was saying I still needed 34 more minutes. CYA mode indeed. My computer uses RGBM (not Suunto) and has "deep" stops...

I don't know why I bother trying to put in a good word for wrist computers and deco diving. A few mins ago I was trying to download the data for the 3 dives I did during the weekend. The staged deco dive I was referring to in the quote above spits out gibberish instead of the actual dive profile (see attachment).

When my computer locks up, it goes into bottom timer mode. I usually do staged deco dives in bottom timer mode to begin with. But on Saturday I had the "brilliant" idea to run it in regular mode so that I would have even one more lost deco gas scenario. As expected the computer locked up in bottom timer mode when I surfaced with a 34 mins of remaining deco. On Sunday I did 2 more dives (recreationals) in bottom timer mode using min deco rules. The profiles for those two dives come out fine.

I don't know if it just temporarily broke down for that particular dive or if the algorithm part of the computer is permanently FUBAR and now it'll only work in bottom timer mode. I guess I'll find out next weekend. Bottomline is: Don't trust gizmos -- they break down.