The Varying Permeability Model, Variable Permeability Model or VPM is an algorithm that is used to calculate the decompression stops needed for a particular dive profile. It was developed by D.E. Yount and others for use in professional diving and recreational diving. It was developed to model laboratory observations of bubble formation and growth in both inanimate and in vivo systems exposed to pressure. In 1986, this model was applied by researchers at the University of Hawaii to calculate diving decompression tables.
The VPM presumes that microscopic bubble nuclei always exist in water and tissues that contain water. Any nuclei larger than a specific "critical" size, which is related to the maximum dive depth (exposure pressure), will grow upon decompression (when the diver ascends again). The VPM aims to minimize the total volume of these growing bubbles by keeping the external pressure large, and the inspired inert gas partial pressures low during decompression.
VPM model description articles:
- VPM Explained by Corrado Bonuccelli. An illustrated, easy to follow description of VPM principles and operation.
- VPM for Dummies by Kevin Watts. This article, published in both English and French in the International Cave Diving Magazine Plongeesout, provides divers a good working understanding of VPM.
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