The reduced gradient bubble model (RGBM) is an
algorithm
In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...
developed by
Bruce Wienke for calculating
decompression stop
The practice of decompression by divers comprises the planning and monitoring of the profile indicated by the algorithms or tables of the chosen decompression model, to allow asymptomatic and harmless release of excess inert gases dissolved in ...
s needed for a particular
dive profile
A dive profile is a description of a diver's pressure exposure over time. It may be as simple as just a depth and time pair, as in: "sixty for twenty," (a bottom time of 20 minutes at a depth of 60 feet) or as complex as a second by second gra ...
. It is related to the
Varying Permeability Model
The Varying Permeability Model, Variable Permeability Model or VPM is an algorithm that is used to calculate the decompression stops needed for ambient pressure dive profiles using specified breathing gases. It was developed by D.E. Yount and other ...
.
but is conceptually different in that it rejects the gel-bubble model of the varying permeability model.
[ – Bruce Wienke describes the differences between RGBM and VPM]
It is used in several
dive computer
A dive computer, personal decompression computer or decompression meter is a device used by an underwater diver to measure the elapsed time and depth during a dive and use this data to calculate and display an ascent profile which according to th ...
s, particularly those made by
Suunto, Aqwary,
Mares
A mare is an adult female horse or other equine. In most cases, a mare is a female horse over the age of three, and a filly is a female horse three and younger. In Thoroughbred horse racing, a mare is defined as a female horse more than four ...
, HydroSpace Engineering,
and Underwater Technologies Center. It is characterised by the following assumptions: blood flow (
perfusion
Perfusion is the passage of fluid through the circulatory system or lymphatic system to an organ or a tissue, usually referring to the delivery of blood to a capillary bed in tissue. Perfusion is measured as the rate at which blood is deliver ...
) provides a limit for tissue gas penetration by
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
; an
exponential distribution
In probability theory and statistics, the exponential distribution is the probability distribution of the time between events in a Poisson point process, i.e., a process in which events occur continuously and independently at a constant average ...
of sizes of bubble seeds is always present, with many more small seeds than large ones; bubbles are permeable to gas transfer across surface boundaries under all pressures; the
haldanean tissue compartments range in
half time from 1 to 720 minutes, depending on
gas mixture.
Some manufacturers such as Suunto have devised approximations of Wienke's model. Suunto uses a modified haldanean nine-compartment model with the assumption of reduced off-gassing caused by bubbles. This implementation offers both a depth ceiling and a depth floor for the decompression stops. The former maximises tissue off-gassing and the latter minimises bubble growth.
The model has been correlated and validated in a number of published articles using collected dive profile data.
Description
The model is based on the assumption that phase separation during decompression is random, yet highly probable, in body tissue, and that a bubble will continue to grow by acquiring gas from adjacent saturated tissue, at a rate depending on the local free/dissolved concentration gradient. Gas exchange mechanisms are fairly well understood in comparison with nucleation and stabilization mechanisms, which are computationally uncertainly defined. Nevertheless there is an opinion among some decompression researchers that the existing practices and studies on bubbles and nuclei provide useful information on bubble growth and elimination processes and the time scales involved. Wienke considers that the consistency between these practices and the underlying physical principles suggest directions for decompression modelling for algorithms beyond parameter fitting and extrapolation. He considers that the RGBM implements the theoretical model in these aspects and also supports the efficacy of recently developed safe diving practice due to its dual phase mechanics. These include:
* reduced no-stop time limits;
* safety stops in the 10-20
fsw depth zone;
* ascent rates not exceeding 30 fsw per minute;
* restricted repetitive exposures, particularly beyond 100 fsw,
* restricted reverse profile and deep spike diving;
* restricted multi day activity;
* smooth coalescence of bounce and saturation limit points;
* consistent diving protocols for altitude;
* deep stops for decompression, extended range, and mixed gas diving with overall shorter decompression times, particularly in the shallow zone;
* use of helium rich mixtures for technical diving, with shallower isobaric switches to nitrox than suggested by Haldanian strategies;
* use of pure oxygen in the shallow zone to efficiently eliminate both dissolved and bubble phase inert gases.
References
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Decompression algorithms