Shearwater PERDIX - VPM-B Upgrade

The VPM upgrade is supported only by the current firmware version of your computer and only when set in technical open-circuit or rebreather modes (i.e. recreational open-circuit mode is always Bühlmann.) An unlock code to enable the feature in the firmware is returned to you via E-mail within 72 hours.

The Varying Permeability Model (VPM) is a decompression algorithm that attempts to predict the behavior of bubbles within the human body during decompression. Compared to typical Bühlmann profiles, VPM dive profiles often have deeper initial stops that are believed to reduce bubble formation, along with reduced time at shallow depths. VPM-B (a version that includes adjustments for Boyles Law effects) is popular among the technical diving community for use in calculating dive tables with required stops. The algorithm was initially developed by David Yount based on laboratory observations of bubbles under pressure and coded into Fortran by Erik Baker. Shearwater Research has implemented VPM-B according to the original Erik Baker Fortran specification. VPM-B allows a conservatism factor to be adjusted over a range of 0 (least conservative) to +5 (most conservative), the default is +3.

In addition to implementing the Baker reference version, Shearwater Research has added an enhanced version because some divers believe the reduced shallow depth stop times may be too short for lengthy VPM-B profiles. The Gradient Factor Surfacing (GFS) option adds conservatism to the shallow stops on dives with significant decompression requirements (dives with total required deco exceeding an hour.) The GFS option is a hybrid that automatically chooses the decompression ceiling from the more conservative of the VPM-B profile and a Bühlmann ZHL-16C profile. For the Bühlmann profile a single gradient factor is used, adjustable over a range of 70% (most conservative) to 99% (least conservative), the default is 90%.