Is VPM Worth the Upgrade?
What Is VPM?
Dive computers require a decompression model to determine No Deco Time and decompression schedules. The two primary models in use are the Varying Permeability Model (VPM) and the Bühlmann Model (ZHL-16C).
The VPM uses deeper stops to minimize bubble growth in the bloodstream and generally gets the diver out of the water sooner versus the Bühlmann model.
What is No Deco Time?
No Deco Time (or No Decompression Limit) is the total time a diver can remain at a specific depth without needing to decompress.
Why Deeper Stops? Bubble Prevention
The VPM asserts that deeper stops reduce bubble growth. Less bubbles equals less risk of decompression sickness (DCS).
Shearwater Dive Computers and VPM:
Shearwater computers come standard with the Bühlmann ZHL-16C profile, a gas concept model used for decades.
Due to diver requests, Shearwater developed a VPM software upgrade for the Perdix, Petrel, or Nerd dive computers. The VPM-B model and VPM GFS are both included in the upgrade.
The VPM-B upgrade adds a conservatism factor of 0 to +5 that can be user-adjusted to tweak the diver's profile.
Meanwhile, the VPM GFS option allows the adjustment of conservatism by altering the high gradient factor, offering the best of both worlds —VPM-B meets Bühlmann ZHL-16C. This will extend your shallower stops, giving you more time to off-gas.
Deep Stop Debate:
The technical diving debate of VPM VS ZHL-16C algorithms isn't going anywhere anytime soon. Human risk factors change from diver to diver, and nothing can keep us 100% safe while diving.
Dive conservatively, make small advances in your dives, keep your body in tip-top shape, and adjust your dive planning as necessary.
I recommend starting with the ZHL-16C algorithm, which is pre-loaded on Shearwater computers, and adjusting your gradient factor to offer the level of conservatism you feel is right.
We train our divers to utilize a gradient factor of 40/70 while using ZHL-16C when starting in environments here in the Great Lakes.
