Dive computers, deco and tissue compartments
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    Question Dive computers, deco and tissue compartments

    Ok then. I admit it. Ive been reading a couple of websites and forums containing details on M values, Gradient factors and bent goats and I love it.

    Im even going to buy deco for divers for some bed time reading - Anyone selling a used one in good nick?

    But this has left me with a question relating to dive computers and their respective algorithms. but first let me explain a little more:

    Most ppl Ive dived with so far (non-tech, non deco) dive Suunto's. When I was kitting up a few years ago I didnt like the menu on the Suuntos and frankly think they are bloody awful to navigate. I had the foresight to have a good old google around and find a computer which would allow me to do gas switching in water as I had a fair idea that I would be bitten by the bug. Consequently I bought an Apeks Quantum with the theory that this cost a lot less (£150) than some of the cheaper suunto air computers did.

    On occasional while on dives with said buddies where we are getting close to NDLs my buddies are clear of safety stops a good 2 or 3 minutes before me.

    Now I believe the Suuntos are using a Buhlmann (Bend and Mend?) algorithm while my Quantum is using a "Modified Swiss Model by Randy Bohrer" based on 12 compartments.

    I am now wondering which "common algorithm" if such a thing exists is the most AND the least conservative (without setting additional conservatism modes within the computer)

    Is there a resource anywhere which provides a list of most commonly available algorithms and their aggressiveness

    I would be interested in seeing a comparison with a view to understanding if one type penalises a particular diving profile over another e.g. warm water based repeat diving algorithm / cold water deep algorithm if such a difference exists.


    Last edited by dustynet; 16-02-10 at 09:08 PM. Reason: cos I forgot the point of the post :-s

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    DIR - Resistance is Futile....... millssy's Avatar
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    OOO i like the way you think, i have been trying to find something similar for a while

    do you have any of the links for the DECO sites you've been looking at?

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    Guru HP's Avatar
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    Hi Owen

    Albert Buhlmann was Swiss - so that pretty much explains the origins of you algorithm on the Quantum Modified Swiss and Buhlmann are almost certainly versions of the same thing

    Buhlmann's algorithm was a continuation of the original algorithm / deco model developed by JS Haldane back in 1908. You often hear it being referred to as "neo- Haldanean"

    The trouble with all Haldanean models are that they are essentially just dealing with dissolved gas. As you'll no doubt be aware, we've worked out that those pesky little micro bubbles need to be accounted for too, because it's now widely believed that they're just as likely to cause us DCI problems.

    Because raw Buhlmann was considered to be relatively aggressive and was bending people, a guy called Erik Baker came up with something called Gradient Factors. Expressed as two percentages: EG 30/85, essentially what they do is fudge Buhlmann to alter its ascent profile. The first percentage (maximum 100%) is the how close we are prepared to go the original point where Buhlmann said you would bend fast tissues which control the deco in the early, deeper part of our ascent. The second percentage is how close we're prepared to come to bending, relatively, slower tissues as a percentage of their Maximum (M Value) in the shallower part of our ascent (I'm simplifying a little here)

    Using these gradient factors, you can essentially make Buhlmann a lot less aggressive in quite a big, but finely controlled way: both shallow and deep, separately and jointly. Put crudely - if Buhlmann is a Cod, you can use gradient factors to turn it into a Sardine or a Dolphin - IE you can change your ascent profile to such an extent that it's barely even recognisable as the original model.

    The point here is that most dive computers that are using their proprietary version of Buhlmann will have a number of similarities, and they're all trying to achieve the same goal - get you out unbent, but this can be varied by the manufacturer so much by tweaking, that knowing the computer uses Buhlmann, doesn't really tell you very much at all

    The example I quoted above - Gradient Factor 30/85 is one commonly used for Nitrox and air type mixes on shallower dives. You'll notice that the first figure: 30% means that you're being pretty conservative deeper - whilst the 85% figure means that you're coming relatively close to the maximum 100% when shallow, but using some degree of conservatism. As raw Buhlmann is a dissolved gas model, it tends not to be too far out in the shallows, but needs quite a lot of tweaking from the GF's when deeper to slow the ascent earlier and give our bodies time to catch up and get rid of bubbles before they become individually too big and get stuck or collectively too many as a whole. If you used 19/90, 50/50, 30/30 - the ascent and the stops called for would look totally different and you would think I was using a different algorithm.

    So where does Suunto fit into all this with it's RGBM that everyone says is bend and mend?

    RGBM is what's commonly known as a Bubble Model. VPM-B and RGBM are the two best known of the bubble models. More accurately they should be referred to as "Dual Phase Models", because unlike Buhlmann which has to be "fudged" with Gradient factors to deal with bubbles, they claim to handle both Bubble Phase gas and Dissolved Phase gas without modification or fudging. In a very simplified form, they do this by having tissue compartments, just like Buhlmann, but they then attach a theoretical bubble to each one of those tissue compartments. The model still controls dissolved gas, but when any one of the theoretical bubbles grows too large, or the overall volume of Bubble Phase gas in the body gets too great, then the algorithm will put in a stop as well. As earlier - this is a gross simplification for the sake of clarity.

    So being a Bubble / Dual phase model - why do Suunto computers not put more stops in deeper and then leave you shallow forever? Surely the claim to be a bubble model is just BS and they're using good old Buhlmann with a fancy sticker on the box?

    Lots of mixed gas divers are used to seeing profiles for Trimix dives looking, broadly, quite similar with Buhlmann GF and VPM-B. Normally they see the VPM-B profile putting more stops in deeper, starting deeper and Buhlmann less so but with longer stops shallow. IE Tech divers associate Buhlmann with bend you deep and then leave you shallow forever to fix it, compared to a good old Bubble model which produces a better "shape". You'll see lot's of Tech divers using Buhlmann based computers like Shearwaters, but jigging the model with some fairly heavy Gradient factors to get it to behave like a Bubble Model (something like 10/90)

    So when those experienced Tech divers sees a "Suuntoed up" Rec diver whizzing through where they would assume there should be deep stops, and then spending forever shallow - the normal reaction is - "B*ll*x is it RGBM - Suunto can say what they like - but it looks like old fashioned Buhlmann Bend N Mend to me"

    So who's right?

    The answer, I believe, lies in the fact that whilst Bubble models handle Trimix diving with deco gasses, broadly similarly to Buhlmann GF: Bubble / DP models handle Nitrox and Air (Recreational mixes) very differently to Buhlmann on Recreational style, no deco gas dives. IMO: they "believe" Nitrogen is a very slow gas (ie it doesn't need so many Deep Stops to control it coming out of faster tissues early in the ascent), but then "predicts" that it generates a lot of bubbles much shallower that take a lot longer to offgas

    As an example take a 20 minute dive on air at 30m ie: the standard PADI RDP No Deco Limit for a dive at the deeper end of the recreational range

    Except - lets use technical dive planning software to work out the ascent rather than the usual dive computer, RDP or Bendy 88's that Rec divers normally use

    The Buhlman ZHL16 with 30/85 Gradient factors ascent looks like this:

    20mins @ 30m then 9m/min to 12m and the following stops

    1 min @ 12m
    1 min @ 9m
    3 min @ 6m

    Looks pretty much like a standard RDP ascent with a few deeper stops and the normal safety stop at around 5/6m: All as expected

    The VPM-B (Level 2 Safety) ascent looks very different for the same dive:

    20mins @ 30m then 9m/min to 12m and the following stops

    1 min @ 9m
    12 min @ 6m

    Huge difference - a massive extra 9 minutes at 6m

    IMO RGBM on the Suuntos is doing a very similar thing to the VPM-B as we see it here. It's not failing to control bubbles - it just thinks they happen a lot shallower and it wants you to stop for a lot longer shallow to offgas them. IE in the recreational ranges - Suunto computers and the bubble model they use, hate Nitrogen and they punish it bigtime.

    Same dive on air, but lets do 30 minutes - Buhlmann wants 12 mins @ 6m, VPM-B wants a whopping 31mins !! You get the picture

    So which Recreational computer is right and what's the best way to get the most out of them safely with the maximum NDL time / minimum deco? Who knows, even with all the science - they're still just mathematical approximations of the way the body behaves. But I'll have a go anyway

    I think that Suunto computers are just fine and probably handle air and Nitrox mixes for recreational dives within minimum deco or small amounts of backgas deco pretty well.

    The whole concept of the latest Suunto models with a "Deep Stops" option is very odd? A computer that already claims to use a Bubble Control model is giving you the option of putting in Bubble Control Stops? Make up your mind chaps! They tend to put the deep stops in at about half your maximum depth, which in the Rec range is where conventional thinking says your faster tissues start offgasing and your tissues have stopped on-gassing in any meaningful way - so they're certainly not going to do any harm, but they seem to be hedging their bets somewhat

    Most people would be a lot better off using their brains and a bottom timer and developing more of a feel for deco IMO, but I realise I've got my finger in the dyke here, so if you're gonna use a recreational type computer, my recommendations for what they're worth would be:

    -Dive the richest possible Nitrox mix you can on any given dive - it makes an even bigger difference than you think, especially if you're believing what your Suunto tells you . That 30m dive on air with 40 minutes of backgas deco on VPM including 31 minutes at 6m? If you used Nitrox 36 for that dive - the model calls for ZERO stops - just a 9m/min ascent rate - no stops at all!!! Its a staggering difference. Bubble Models / Suuntos love O2 That doesn't mean dive silly PPO2's - it just means ramp the O2 up in the mix as much as you, remaining within a working PPO2 of 1.4.

    - Along the same lines, don't even bother to try and plan to do backgas deco using air with a Suunto computer - you'll run out of gas trying to do the 476 minutes at 6m

    -By all means use the Suunto Deep Stops setting, it can only help, whether it should be necessary with a Bubble Model based computer, is another debate

    -With Buhlmann based computers - if they look aggressive compared to your mates Suunto, consider padding the shallow stops to improve the quality and conservancy of your deco, and consider adding in your own Deep Stops by either doing 2 mins at half the max depth of your dive, or just slowing your ascent rate to 3m/min from half max depth

    - With all computers - if you rush to the surface after doing your 5-6m stop / safety stop / shallowest stop - you may as well not have bothered. The reduction in pressure between 5-6m and the surface is colossal - take your time: 3-6 mins will improve your deco a lot and is good practice for fine tuning your buoyancy

    - All of the is particularly true when you're do multiple dives on multiple days on liveaboard type holidays. You'd be unlucky to end up bent, but it's the difference between feeling sparky and feeling knackered all week

    - Get off the bottom pronto at the end of the dive. When most people reach their no deco limit at the end of a dive, they often faff around for ages like two deaf people fighting over a parking space, eventually get a DSMB up and then spend forever in the early part of their ascent. 9m/min is a good rate to use and its a lot quicker than many people realise. You need to get going !!!

    - In recreational diving - the non algorithm factors are at least as important if not more than the what the computer tells you to do. Are you warm with enough thermal protection (warmth makes a huge difference in answer to your earlier question - if you're not warm and your circulation isn't functioning well - how can you get rid of the inert gas in your tissues)? Are you properly hydrated? Are you holding your stops and controlling your ascent rates effectively? These are probably more likely to bend you than the little box of dreams on your wrist IMO

    Hope that helps, please make your own informed judgements, I'm not gonna push your wheelchair if it all goes horribly wrong, free advice is worth exactly what you paid for it, all the usual caveats apply
    Last edited by HP; 02-03-10 at 11:48 AM.
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  7. #4
    Notice my avatar. I am hard astern. BJ's Avatar
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    Just to put a spanner in the works, the latest research suggests that on dives with mandatory decompression stops, deep stops increase the risk of DCI. So it seems bend-and-mend is getting rehabilitated! I'll try to find the reference.
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    Nigel Hewitt nigelH's Avatar
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    And then some of us don't believe in micro-bubbles as much of the bumph I've read suggests you can detect lots of them going into the lungs but none coming out.
    The internal surface area of your lungs is so huge and the blood supply so good that the output gas tensions very accurately mirrors the gas partial pressures in the avioli.
    This, however, is why even a small PFO, bypassing the lungs, is so bad for you.

    John's point about deep stops is valid too. They move you to a far more complex part of the gas-flow graph so the tolerances on timings thereafter become more critical. That's why I like to dive computers and then overstay their time. They constantly are recalculating all the compartments so on the deep stops, where some compartment on the model are on-gassing and some are off-gassing are allowed for and my errors just adjust the rest of the profile. Mindlessly following a slate like most people believe is how to do deco is just looking for trouble. I view the computer as drawing a bent/not bent line and it is my plan to stay well away from that line. Of course if you are diving OC and you need a gas plan you're stuck with the slate but on the CCR I can stay as long as I want and OC is just bailout.
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    Aquanauts tea boy & GUE instructor James The Badger's Avatar
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    Quote Originally Posted by BJ View Post
    Just to put a spanner in the works, the latest research suggests that on dives with mandatory decompression stops, deep stops increase the risk of DCI. So it seems bend-and-mend is getting rehabilitated! I'll try to find the reference.

    As Howard pointed out, different models and strategies have differing benefits based on exactly what it is you trying to off gas For example there is a difference with He based mixes and air in terms of where and for how long you are stopping/pausing. Mixed based diving in particular demands attention to deep bubble control via ascent rate changes and pauses What may prove a useful strategy for one gas may not be so useful for another. DCI may be reliant on so many variables you just have to keep the big picture in mind when thinking about increasing or decreasing the potential risk and how you adapt your strategy to deal with it.

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    aka Chimp 1 or Mavis... GLOC's Avatar
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    Re: Dive computers, deco and tissue compartments

    Quote Originally Posted by nigelH
    This, however, is why even a small PFO, bypassing the lungs, is so bad for you.
    But then I have a hole 7-8mm x 12mm in my heart and I have done quite a few deep dives in the region of 45-70m and only had one minor hit (visual disturbances only, no other symptoms); I subscribe to the deep stop theories!

    The problem is we do not know enough about this, especially the deeper trimix dives...

    I know what I will continue to do though.

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    rEvolutionary HPDW'ing deepestandy's Avatar
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    Re: Dive computers, deco and tissue compartments

    Quote Originally Posted by GLOC
    The problem is we do not know enough about this, especially the deeper trimix dives...
    Which is exactly why I'm working on the next best thing... The underwater dice rolling machine! I think it will prove quite popular with the guys that like to use ratio

    On a more serious note to echo Gareth's comments, the only time I've ever been bent has been on a 15m no stop dive where I was way off deco (25 minutes away) and dived a very cautious profile. I think that for all the subtleties in deco planning you can't beat being properly hydrated, nice and warm and keeping post dive activity as low as possible for avoiding a hit.


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    TDI Instructor Trainer Mark Powell's Avatar
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    Quote Originally Posted by BJ View Post
    Just to put a spanner in the works, the latest research suggests that on dives with mandatory decompression stops, deep stops increase the risk of DCI. So it seems bend-and-mend is getting rehabilitated! I'll try to find the reference.
    Are you refering to the US Navy Study. If so I wouldn't draw quite the same conclusions.
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    GUE Instructor Garf's Avatar
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    the only hit I've ever had was after a ridiculously conservative dive (3 mins at the bottom at 21m on 32% followed by a 3 m per min ascent to 6 metres, then 1 m per min to surface) and I have no PFO. What this teaches me is that all deco models are just best guesses. Diving is a numbers game. Get in the water often enough and you'll take a hit. All you can do is minimise the probability and try to start getting a feel for what works for you and what doesn't.

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