Buhlmann deco question

[emmbee]

Hi all,

-- Posted on scubaboard, but nobody's answering, so I figured a better idea would be that site Stephen is always on

I picked up Deco for Divers (Mark Powell) last WE (which I loved), and was looking at the Buhlmann ZH-L16A tables.

Now, my understanding is that Buhlmann came up with the a/b values first (themselves derived from the compartment half-times), then converted that to the Workman-style M0/delta M so-called "A set", which was found to be overly agressive, and tightened as the B then C sets. This only affected the M0 values. The a/b values are correct to the 4th decimal as computed from the half times, and the closest M0 match (modulo some dodgy rounding) is the A set, so that makes sense. Is that correct?

I couldn't help but notice, however, that the delta M (which is the same for all 3 sets) was wrong for compartment 4. Try I as might, 1/.7725 is 1.294498381877023, not 1.2780. All other delta M match.

What's funny is that every set of tables I could find on the web have those same value for b and/or delta M. So either everybody just cut and pasted the values without checking, or I'm missing something  

-- Maybe I should get The Book, 30 quid on amazon, but my German *****  

Any idea?

Cheers,

emmbee

[Scubadec / Dec Hanniffy]

I don't know the answer to your question, but the book is excellent, well worth the money.
If you don't get an answer here you could try Yorkshire divers forum, I know Mark Powell is a member there.

[jonnyb]

    Hi Emmbee,

   You might get some answers here, but they may not be any use to you.... It was my understanding that Workman reworked Buhlmann tables to fit his M/DELTA theories on deco. Therefore Workman was copy and pasting as such. Also Buhlmann's sea level tables are only alterations to his original research at altitude, which he was more concerned with, and thus were over compensated, passive to say. Workman's M Values were much more aggressive, allowing the diver to push the boundries of there phsyiological condition.

  Anyhow, in summary, I am not goin to answer your question, simply because each answer will be different, none correct or wrong. Understanding Deco is merely understanding a Theory about Deco, it's all imprecise and dependant on varying factors which can not be put into any mathematical equation which exists at present. You are better understanding your Physiological condition and diving ability and trusting the most conservative set of tables.... Forget about it all, it will drive you mad.

   Just Dive Safe
       Jonny

[Stewart Andrews]

Sorry emmbee, you lost me on line three...

I simply strap on a VR3 and use the Vision software as a back-up (some deep tables also). For dives below, 70m I come up a tad slower and do the deep stops.

Suppose that is absolutesly no help to you!!

If you are on the Inspiration owners forum, there are some smart lads on that.
Best of luck ... dare I say it ...in your torment!

[Stephen McMullan]

Some correspondence from the Inspiration list:

Hi,

I'm just trying to track down the reason for an apparent "anomaly"
that a friend of mind writing a deco planner for kicks pointed out to
me.

In Buehlmann's model ZHL-16 the Nitrogen halftime for compartment 4 is
listed as 18.5 minutes

The formula for calculating the b co-efficient is:

b     = 1.005 - ( tht ^ - 1/2 )

So for compartment 4 (18.5min)

b = 0.7725


Now I've seen 0.7825 as the b value for comp. 4 being referred in two
places: Deeper into Diving 2nd Ed. Lippmann & Mitchell and also in the
implementation files of DDPlan

Alternatively in Erik Bakers article "Understanding M-values" he
refers to 0.7725 for b but when listing delta M (slope of the M value
line) which should be the reciprocal of b he puts down 1.2780 (which
is the reciprocal of  0.7825 and not 0.7725)


Sorry about all the terse detail - just looking for a resolution to
this "engineers argument".

Thanks,

Stephen


Gordon Henderson replied:


On Wed, 20 Jan 2010, Stephen McMullan wrote:

    Hi,

    I'm just trying to track down the reason for an apparent "anomaly"
    that a friend of mind writing a deco planner for kicks pointed out to
    me.

    In Buehlmann's model ZHL-16 the Nitrogen halftime for compartment 4 is
    listed as 18.5 minutes

    The formula for calculating the b co-efficient is:

    b     = 1.005 - ( tht ^ - 1/2 )

    So for compartment 4 (18.5min)

    b = 0.7725


    Now I've seen 0.7825 as the b value for comp. 4 being referred in two
    places: Deeper into Diving 2nd Ed. Lippmann & Mitchell and also in the
    implementation files of DDPlan


Someone, somewhere got it wrong and the rest copied... (And my 1st edition Deeper into Diving doesn't have the tables in it)

I wonder if it was deliberate so people could track the source

Although I do remember once extracting all the floating point numbers out of the Zplan binary to see what Will used as they were compiled into it, and I don't remember noticing any issues... Trouble is, that was 10 years ago and my memory isn't what it was... (Although I can do it again!)


    Alternatively in Erik Bakers article "Understanding M-values" he
    refers to 0.7725 for b but when listing delta M (slope of the M value
    line) which should be the reciprocal of b he puts down 1.2780 (which
    is the reciprocal of  0.7825 and not 0.7725)


My numbers came from Baker and I have 1.2780 in the OSTC version of DDPLAN for the 'b' table, but that came from my original DDPLAN data...

So, er, well spotted, that man ...

Gordon

[Stephen McMullan]

By the way the fella who discovered this is Mathieu who some of ye may have met at the TCD presentations - he hasn't even done his Nitrox course yet 

[Stewart Andrews]

That's it from me then... where's the nearest knitting night class?

My only saving grace was that I did suspect human error (initially), followed by copy and paste!

[Stephen McMullan]

That's it from me then...

Hold on there horse. You're not going anywhere. Matthieu only bought "Deco for Divers" last weekend so he could figure out what you were talking about in Trinity. I suggested he join Dalkey Scubadivers and get you to teach him Nitrox & Extended Range. He's got a lot of questions 

[emmbee]

... and for the same reason I had to read everything I could find on oxygen toxicity and CNS, too

Cheers

[Stewart Andrews]

Well, I'm an engineer and I don't remember getting into deco theory in that much detail. Perhaps he could explain 'stuff' to me (I promise to try to stay awake    )
I just like to spend as much time as possible underwater, preferably doing interesting dives!!

[RayWard]

I wonder does this copy and paste error affect many programs that are found on the n'interweb??

[Stephen McMullan]

There's was a bit of chat on the Inspo owners list about this but nothing hectic. Its a small "anomaly" possibly going all the way back to Prof. Buehlmann's publications themselves. As far as I can see there is an inconsistency in the numbers published in various places but nobody has said what the correct number is yet i.e. what Buehlmanns research intended. Regardless of whether your Buehlmann deco planner/computer has the "right" number or not I *think* its a small aberration and the situation is well and truly covered by the fact that we apply conservatism (i.e. gradient factors) and sure its all only theory anyway  . As to whether deco planners / computers are updated over time for the sake of mathematical accuracy to conform to the intended model I have no idea. Nothing has been mentioned in relation to Vision software or the APD Projection planner or anything else for that matter. I wouldn't think there would be an "emergency update", it might make it into a general bundle of updates at some stage at best.

[scuba steve]

well fair play to that man for noticing .

note to ones self never teach on a course he is attending .

[Stephen McMullan]

Just to finish this one off:

Stephen,

I researched this with Ernst Völlm, the co-author of Bühlmann for Tauchmedizin, because that is where the value of 0.7825 originates (at least in the latest (5th) edition).

His answer was:
You are correct, it is a printing error, the theoretical b value of compartment 4 with a halftime of 18.5 minutes for nitrogen is 0.7725 and not 0.7825.

So, it wasn't intentional and the value wasn't tweaked.

Rob

[Stewart Andrews]

Phew!
Can we all go diving now!!! - Oh yes, when the sea warms up a bit.