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Hi
I know some of you are skilled progammers for deco softwares...
I am a trimix diver, rather new to the sport (but unless you have done it for 20+ years I guess you still are:) and I am wondering about the multiple dive plan softwares available:
I have run 10 dive plans for different scenarios (varying the depth, BT, configuration of deco gases, with/without helium) on several deco softwares/firmwares (ostc2, multideco, Ideco, ostc-planner, garmin mk2...) BUT all based on Buhlmann ZH L16C with the exact same parameters (GF, etc...).
For the same dive plan, differences of the overall deco time from one software to another are up 10 to 20%. And there is no apparent consistency/correlation between them, like we can't say for ex that one is just always a bit more pessimistic or optimistic etc... I know some programs account for the descent time as BT or not, but I doubt it could explain the differences.
It can entail +/-10 minutes on a 50minutes total deco time depending on which program you follow to plan your dive. Since we are taught to follow the plan by the minute, +/-10 minutes is a lot!
Does it mean we should all opt for "deco on the fly" estimates and use no computer...
I (naively) thought that because the algorithm (Buhlmann ZH L16C) and the parameters were the same for each device/software I ran, the programmed code would be the same or very close, thus yielding very little difference. But it does not seem to be the case ?
Tks
Jean-Luc

Hi Jean-Luc,

actually, Bühlmann did not provide an algorithm how to calculate a deco schedule. He provided a formular how to compute the on- and off-gassing of the tissues and another formular that gives the maximum pressure a tissue can tolerate. As mathematical equations, not as an algorithm. Besides, the GF factors are a "3rd party add-on" to the Bühlmann model which is specified more in prosa than in strict formulars. So it is quite a way to put all that into computer code which in the end provides a deco schedule.
Many aspects aren't specified at all, e.g. the 3 meter stepping may have it roots in traditions, but they are not in the Bühlmann equations. Also if a stop is timed in seconds resolution or only in full minutes is an individual decision of the programmer. The OTSC for example uses full minutes in the deco calculator and when calculating the schedule under water, but tracks the tissues during the dive in 2 seconds resolution and thus may release you to the next stop depth after less than a full minute. Also for some saftey reasons, the OSTC always assumes you are 1 meter below the indicated stop depths, which gives a little bit longer indicated schedules. Again, during the dive, the OSTC follows your actual depths and adopts the results.
Another aspect is ascent speeds. Bühlmann just said 'not faster than 10 meters per minute'. In may deco programs you can adjust the speed. The OSTC uses 10 m/min, but slows the final ascent to 1 m/min. So if your last stop is at 6 meters, the TTS will include 6 minutes for the final ascent instead of just some 30 seconds as other programs may do. The OSTC also adds 1 minute to all stops that contain a gas switch. You can configure this in other software, the OSTC always does it, last but not least to add some safety margins into the gas needs prediction.
So you see, there are many (and many more!) details that presumably are handled slightly different from deco software to deco software...

BR
Ralph

Thanks Ralph, your explanation is clear enough.

By the way, 10 to 20% discrepencies between different computations are insignificant, since total ascent time can be as different as 300% considering different models !

Regards.

Tks Ralph.
It's good to know more about the underlying assumptions and parameters of the OSTC.
Cheers
JL

Hi Didier
What kind of deco models do you have in mind when you mention differences of up to 300% with Buhlmann ZH L16c ?
Thankyou
Jean-Luc

Hi Jean-Luc,

I don't talk about only different implementations of Bühlmann ZH-L16, but about other tables (Doris, VPM, RGBM, MN73, Comex, ...)

Bühlmann ZH-L16 + GF seems to be in the middle of deco times given by all these models.

Regards.

merci Didier.
Yes that was my question : what other models/ tables could result in such differences compared to Bulhmann ZH L16c. I have already tried VPM and RGBM but don't know the other ones. Tks!
Jl

Hi Didier
would you happen to know a good dive shop in Paris ? ( I live in Singapore but will be visiting Paris soon).
thank you
JL

Hello,

Near Paris :
--> Boulogne Plongée, at Boulogne Billancourt
--> Aquatik, at Versailles
--> Fadis, at Versailles

I know the three ; depending on what you are waiting for as stuff. My own preference for Aquatik.

But a shop inside Paris, sorry, knowing no one.

Have a nice trip.

merci Didier

Hi JL,

I'm back to the forum after some days, and Motardplongeur already did answer you. I agree with him, no serious dive shop down town Paris (Le Vieux Campeur should be avoided, for several reasons). I also know the three mentioned shops, they are serious but take care of prices !

Regards.

Tks Didier and Motardplongeur.