Varying Permeability Model


The Varying Permeability Model, Variable Permeability Model or VPM is an algorithm that is used to calculate the decompression needed for ambient pressure dive profiles using specified breathing gases. It was developed by D.E. Yount and others for use in professional 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 Hawaiʻi to calculate diving decompression tables.
Several variations of the algorithm have been used in mobile and desktop dive planning software and in dive computers.

Theoretical basis

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, will grow during decompression when the diver ascends. The VPM aims to minimize the total volume of these growing bubbles by keeping the external pressure sufficiently large and the inspired inert gas partial pressures relatively low during decompression. The model depends on the assumptions that different sizes of bubbles exist within the body, that the larger bubbles require less reduction in pressure to begin to grow than smaller ones, and that fewer large bubbles exist than smaller ones. These assumptions can be used to construct an algorithm that provides decompression schedules, designed to eliminate the larger, growing bubbles before they cause problems.
Varying permeability refers to the layer of molecules surrounding the bubbles, which may vary in permeability to gas molecules in the bubble and the surrounding medium, and which affect the diffusion of gases between the surroundings and the bubble, and the variation of compressibility of the bubble under changes of pressure.

VPM Dive Planning Software

  • V-Planner: VPM-B & VPM-B/E, VPM-B/FBO.
  • MultiDeco: VPM-B & VPM-B/E, VPM-B/FBO, ZHL-B, ZHL-C, GF, and GFS.
  • Ultimate Planner: VPM-B, VPM-B/U, VPM-B, VPM-B/U, ZHL-B, ZHL-C, ZHL-D, GF and GF/U.
  • DecoPlanner: VPM-B.
  • HLPlanner: VPM-B.
  • JDeco: VPM-B.
  • PalmVPM: VPM.
  • DivePlan: VPM.
  • Baltic Deco Planner: VPM-B.
  • Subsurface: VPM-B.

VPM Dive computers

  • V-Planner Live: VPM-B & VPM-B/E.
  • MultiDeco-X1: VPM-B & VPM-B/E, VPM-B/FBO, ZHL-C, GF, and GFS.
  • MultiDeco-DR5: VPM-B & VPM-B/E, VPM-B/FBO, ZHL-C, GF, and GFS.
  • Shearwater Research Predator, Petrel, Perdix and NERD models: GF, VPM-B plus GFS.
  • RATIO Computers: iX3M series and iDive series VPM-B and ZHL16-B.
  • TDC-3 with MultiDeco-TDC: VPM-B & VPM-B/E, VPM-B/FBO, ZHL-C, GF, and GFS.
  • HeinrichsWeikamp OSTC4: VPM-B