Reduced Gradient Bubble Model
The RGBM developed by Dr Bruce Wienke at Los Alamos National Laboratory is a hybrid model which modifies a Haldanian model with factors to take some account of bubble mechanics to model gas phase production during decompression. The bubble factor modifies the M-values of the Haldanian model, making it more conservative.
Features of the modifying factor ξ include:
- ξ starts on the first dive of a repetitive series with the maximum value of one, so it will make the model more conservative or unchanged.
- ξ decreases for repetitive dives.
- ξ decreases as exposure time increases.
- ξ increases with increased surface interval.
- ξ modifies fast compartments more than slow compartments.
- ξ decreases with the depth of a dive segment
- ξ has more effect on repetitive dives which are deeper than previous dives in the series.
The effect is to reduce no-stop dive time or increase decompression requirements for repetitive dive in the following categories:
- Following a short surface interval.
- Following a long dive.
- Following a deep dive.
- Which are deeper than previous dives.
The model has been used to some extent in some Suunto dive computers, and in the HydroSpace Explorer computer, where it is a user selected option for computation formula, with a choice of additional conservatism factors.
The complete RGBM treats coupled perfusion-diffusion transport as a two stage process, with perfusion providing as a boundary condition for gas penetration of the tissues by diffusion. Either process can dominate the exchange depending on time and rate ceofficents.
Simplified implementations which require less computational power are available for use in personal decompression computers. These are dominated by perfusion. The inherent biological unsaturation of tissues is considered in the calculations.
The model assumes that bubble nuclei are always present in a specific size distribution, and that a certain number are induced to grow by compression and decompression. An iterative computation is used to model ascent to limit the combined volume of the gas phase. Gas mixtures of helium, nitrogen, and oxygen contain bubble distributions of different sizes, but the same phase volume limit is used.
The model postulates bubble nuclei with aqueous and/or lipid skin structure, in a number and size distribution quantified by an equation-of-state. Like the VPM, RGBM assumes the size distribution is exponentially decreasing in size. Unlike the varying permeability model, bubble seeds are assumed permeable to gas transfer across skin boundaries under all pressures.
The size of nuclei which will grow during decompression is inversely proportional to the supersaturation gradient.
At higher pressures, skin tension of the bubble nuclei reduces gas diffusion to a slower rate. The model assumes that bubble skins are stabilized by surfactants over calculable times scales, which results in variable persistence of the bubble nuclei in the tissues.
Read more about this topic: Decompression Algorithms, History of Decompression Research and Development, Mixed Phase Models (dissolved and Bubble Phases)
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