Unlock your Shearwater Perdix, Petrel, Perdix, Perdix AI or Predator for the optional VPM-B algorithm with this unlock code. The Shearwater computers come standard with the Buhlman model installed. After this upgrade your Shearwater dive computer will have the option to switch between VPM-B, VPM-B/GFS and Buhlman ZHL-16C with gradient factors decompression modelling algorithms. When ordering, please supply your serial number (eight hexadecimal digits) during the check out process in the notes box and the firmware version number that are shown in the information displays of your computer. You can find this info in the the information screen of your computer by pressing the right button to cycle through the information screens.