Modified Swiss model Algorithm

[maxguru]

Perhaps it's this kind of Swiss Model? Please, oh please!!!

http://http://www.tamparacing.com/ga...ish_bikini.jpg

m

[Art Greenberg]

Is this a "trust-me" Algorithm?

Aren't they all?

[gasdiver]

I believe it's similar to Buhlmunn with the difference being the Swiss model only uses 12 compartments. The Modified Swiss model Algorithm is the same algorithm in use by the Zeagle N2ition.

The N2ition is the same Sieko computer as the DR Nitek Duo as well. (If I'm not mistaken.)

[JS1scuba]

Reading advertising info for GP4000 dive computer and it advertises as "Modified Swiss model Algorithm ". I am not a deco algorithm expert,but know enough to bend myself really good,but this is a new one on me. What does it mimic- Buhlmun 16 with gradients,VPM,RGBM....? Anybody have any info?

Hi Kelly,

The Modified Swiss Algorithm in these computers was created by Randy Bohrer when Seiko got into the dive computer business back in the 1990s. They were/are a major player in air and nitrox dive computers world wide. Many of the Dive Rite units were made by them as were/are many other companies. I contacted Randy today and asked him about the algorithm and here is what he said:

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(from e-mail with Randy Bohrer)

The story of the algorithm development (for the Seiko computers) is fairly simple. There are two Buhlmann algorithms, the ZHL-12 and ZHL-16. Most believe that the -12 or -16 refers to the number of compartments. It actually refers to the number of a, b value (similar to M values) sets that the algorithm has. Both are 16 compartment algorithms. ZHL-16 is the newer algorithm. What is unique about it is that it is possible to calculate the a,b values based on the half-life time. Then you can develop your own algorithm with as many compartments as you like. Of course, selecting half-times that are far beyond what has been done with Haldanean type models (for example something in excess of 800 minutes for nitrogen) would probably not be valid.

So, the question becomes, which algorithm did we use for the Seiko-made units? The answer is ZHL-12 (the older model) with some custom modifications. The reason for that is that the ZHL-16 model produced NDLs that were very different from what dive computer users had come to expect. They were too short. We knew from the wide range of practical experience that the NDLs from popular dive computers were "safe", meaning they produced numbers that did not result in an unacceptable number of cases of DCS (DCI). We found that the older model, without modification, produced both NDLs and decompression stop times that were similar to other units already on the market.

We wanted to build in some extra conservatism in areas where there was traditionally an increased risk of DCS, so we looked at modifying the model. The two primary modifications were to freeze the gas loading calculations during the first 10 minutes of a surface interval, and to add a depth safety factor for repetitive dives. The gas loading "freeze" penalized the diver for short surface intervals, but as the surface interval became 2-3 hours or more, the safety factor had little effect -- just what we wanted. The depth safety factor penalized the diver for repetitive shallow dives, again affecting primarily the NDLs (the bottom time of 40-50 foot dives doesn't usually require decompression stops). The depth safety factor had little effect on deep dives (say 80 feet and deeper), where decompression stops would sometimes be required. The base ZHL-12 model has very good dive test results as well as practical field results, so we didn't feel a need to integrate safety factors that had much effect on decompression dives. The final modification was to select half times that would simplify the gas loading calculations. Because the particular half times we selected were not the same as the base model, we interpolated to determine the a, b values.
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An additonal comment he said in another email was:

I am familiar with the prototype version of that unit. It is a great computer and is rather “tried and true”.

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To summarize -- the Algorithm in these units is the Buhlmann ZHL-12 with modifications to "penalize" the diver for short surface intervals and to add a depth safety factor for repetitive dives.


Kelly, I also went back into my notes from Hamilton Research Ltd, and see that the oxygen tolerance calcs and CNS recovery incorporated into these units were from the NOAA oxygen tolerance values that Dr. Bill created. Those were also incorporated into all of the Seiko made computers (as well as most every other computer.)

I hope this helps you and others who are interested in what the "modified swiss model" is for these particular dive computers.

Regards,