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Shearwater Research Upgrade to VPM-B Deco Algorithm

Shearwater Research


The Shearwater Research VPM-B software feature upgrade allows your Shearwater Research Predator, Petrel, Petrel 2, Perdix, Perdix AI, NERD, or NERD 2 model dive computer (but NOT the Teric model) to be switched between a choice of VPM-B, VPM-B/GFS or Buhlmann ZHL-16C with Gradient Factors decompression modelling algorithms.

The VPM-B upgrade is supported only by the current firmware version of your Shearwater Predator, Petrel, Petrel 2, Perdix, Perdix AI, NERD, or NERD 2 dive computer, and only when set in technical open-circuit or rebreather modes (i.e. recreational open-circuit mode is always Bühlmann.) However, the recently introduced Shearwater Teric dive computer does NOT support the VPM decompression algorithm. An unlock PIN code to enable the feature in the firmware is sent to you via E-mail.

Shearwater Research Upgrade to VPM-B Deco Algorithm Features

  • Allows choice of VPM or Buhlmann algorithm
  • Includes both VPM-B and VPM-B/GFS
  • VPM dive profiles often have deeper initial stops
  • We email an unlock code to you after purchase

The Varying Permeability Model (VPM) is a decompression algorithm that attempts to predict the behaviour of bubbles within the human body during decompression. Compared to typical Buhlmann 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 Buhlmann ZHL-16C profile. For the Buhlmann profile a single gradient factor is used, adjustable over a range of 70% (most conservative) to 99% (least conservative), the default is 90%.

Please Note: As of 2018, recent decompression research has called in to question the benefits of deep stops and Shearwater Research does not recommend the use of the VPM-B algorithm for dives with planned decompression.

How To Order

If you are purchasing this upgrade with no other physical items... please choose In-Store pickup for your shipping method to save on shipping costs. We will email you the software unlock code.

If you're purchasing for use with an existing dive computer you own, please supply the Serial Number of your unit in the comments section on checkout or via a separate email to us. The unit serial number (eight hexadecimal digits) and firmware version number are shown in the information displays (aka status screens) of your Shearwater dive computer. Repeatedly pressing the Select Button (Right), from the main display, will cycle through the various information screens. See the user manual for more information. Note that the serial number is NOT the CE certification EN13319 found on the outside of the case.

IMPORTANT – Once your order is received with your serial number, the unlock PIN code will be emailed to you as quickly as we receive it. This often happens overnight, but can take 1 to 2 days given the time delay between Australia and the time zone of Shearwater in Canada.

Please Note: you will also need to update your firmware to the latest firmware before using the PIN code.

Buhlmann versus VPM Decompression Schedules

The Varying Permeability Model (VPM) is a bubble model based decompression theory alternative to the Buhlmann gas content model widely used by many dive computers to compute no-decompression limits and decompression schedules for dives. Most technical dive computer implementations of Buhlmann 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 the 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-emphasised, 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 Buhlmann with a 30/70 gradient factor. If you don't understand the differences and why you would choose one algorithm or the other, we recommend using the factory defaults.


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