The Varying Permeability Model (VPM) is a bubble model based decompression theory alternative to the Bühlmann gas content model widely used by most dive computers (including the models we sell) to compute no-decompression limits and decompression schedules for dives. Most technical dive computer implementations of Bühlmann also incorporate Eric Bakers' user selectable Gradient Factors modification.
Among many long time technical divers a trend toward significantly deeper initial stops developed, based mostly on anecdotal reports within the community that some divers 'feel better' after such dives. This trend was a factor leading to an increased popularity of the VPM algorithm because those dive profiles typically have deeper initial stops, along with reduced time at shallow depths resulting in a 'smoother' profile. However more recently the whole concept of deep stops, and the VPM algorithm, have come under increasing criticism from the community of decompression experts. Some believe the dubious benefits of deep stops have been over-emphasized, especially for deep diving on trimix. Some dive computers such as the Shearwater models, offer VPM as an alternative for those divers who still prefer the VPM algorithm, although Shearwater recommends their default of Bühlmann with a 30/70 gradient factor. If you don't understand the differences and why you would choose one algorithm or the other, Dive Gear Express recommends using the factory defaults.