Bühlmann and RGBM

[ChrisA]

How does Uwatec's "Bühlmann ZH-L8 ADT MB" algorithm compare with Suunto's "RGBM"
algorithm? I'm asking about both theory and practical result. I and NOT asking who makes the best computer. I'm trying to better understad the two algorithms.

 

[Charlie99]

Contrary to what you might think from the Suunto literature, their algorithm is a neo-Haldanian aka multicompartment dissolved gas aka Buehlmann algorithm.

"Suunto RGBM" emulates true RGBM by varying the limits of the dissolved gas model for future dives in response to things like excessive ascent rates, short SIs, and reverse profiles.

The overall setting of the Suunto RGBM algorithm is on the conservative (short bottom time) side.

Suunto has recently released a computer that uses a true RGBM model, but that 's not what is in the popular Cobra, Stinger, Mosquito, Vytec series.

In typical recreational diving, the Uwatec and Suunto computers will be similar, but IIRC, Suunto RGBM will have a bit shorter allowable bottom times than Uwatec. The "Suunto RGBM" algorithm will usually be much more conservative on repetitive dives.

 

[Dr Deco]

Hello chris:

I do not believe that the Buhlmann-style meters have any RGBM-like properties.

Models

There are two basic model concepts in use today. These have some common points and some differences.

One is the Haldane model that tracks dissolved nitrogen in multiple tissue compartments. Also nitrogen is assumed to be dissolved and remain dissolved throughout the dive and post-dive surface interval.

The other model is the Two-phase Model of which the RGBM is one example. Nitrogen (or helium) is considered to be in two states, a dissolved state or a gaseous state. The “dissolved” portion of the model functions in the same manner as a Haldane model. The “gaseous” portion however is considered to be in the form of micronuclei and surface tension plays a large role.

It is the surface tension effects that determine that slow ascents and deep stops are important factors in decompression. Haldane models would propose that the fastest route to the surface is through a quick ascent to the surface, thus minimizing the uptake of nitrogen (or helium).

Dr Deco :doctor: