Everything you need to know about the VPM-B algorithm: Reviews, safety, and settings.

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In the history of deep technical diving, theories about decompression have sometimes sparked heated debates. Divers exploring the depths using complex gas mixtures have long sought the perfect algorithm, capable of reducing post-dive fatigue and minimizing the risk of accidents. It was within this context of a quest for perfection that the VPM-B (Varying Permeability Model) algorithm emerged in the 2000s as the benchmark for technical diving, before being questioned by advances in contemporary hyperbaric medicine.

I remember my deep exploration dives using Trimix on the forgotten wrecks of the Bay of Biscay in the 2000s. We all used the VPM-B model on our planning software and dive computers, performing long, very deep decompression stops in the dark and cold water. Although we subjectively felt less fatigue immediately after surfacing, the interminable length of the overall decompression pushed us to the limits of our emergency gas supply. This heroic era marked the beginning of the transition to modern Haldanian models, which are simpler and scientifically validated.

The VPM model was originally developed by physicist David Yount and researcher Don Hoffman at the University of Hawaii, while studying the dynamics of bubbles in gelatin subjected to pressure variations. The practical implementation of this model for technical diving, as well as its coding in a computer language, was carried out by American engineer Erik Baker. Version "B" incorporates Boyle's Law to better manage the physical compression of bubbles during ascent.

Physically, this is a bubble (or two-phase) model. It starts from the premise that microscopic bubble nuclei always exist in our bodies, and that decompression will cause them to expand. Its philosophy involves imposing extremely deep decompression stops to crush these nuclei under hydrostatic pressure. This algorithm is open-source and publicly documented. It is mainly found on computers dedicated to technical diving, such as Shearwater (via an optional paid unlock key) or OSTC computers from Heinrichs Weikamp.

The adjustment of conservatism on the VPM-B is less intuitive than on a classic Haldanian model. Instead of adjusting saturation percentages, the diver modifies the physical value of the "initial critical bubble radius."

On computer interfaces like those found on Shearwater devices, this mathematical complexity is summarized by simplified presets ranging from "Nominal" (the standard algorithm profile) to "+5" (the most conservative setting).

Designed exclusively for technical diving with complex gas mixtures (Trimix, Heliox, Nitrox), the VPM-B manages multiple gas mixes with exemplary mathematical precision. Switching gases during a dive will instantly recalculate the entire decompression profile, perfectly adapting to the requirements of both open and closed circuit dives.

The VPM-B is a strict mathematical algorithm that does not take into account real-time biometric data such as the diver's heart rate or breathing effort to adjust calculations. On the other hand, it is extremely sensitive to sudden pressure changes. "Yo-yo" profiles or excessively rapid ascents cause an exponential increase in the calculated bubble radius within the algorithm, which immediately adds large decompression stops at the end of the dive.

Deep stops are the core of the VPM-B system. Where a classic Haldanian model would allow you to ascend from forty meters to fifteen meters without mandatory decompression stops, the VPM-B will require a very deep initial stop, sometimes as shallow as thirty meters, to compress the gas bubbles.

It was in the medical field that the VPM-B lost its status as an absolute reference. In 2011, the US Navy Experimental Diving Unit (NEDU) conducted a large-scale clinical study comparing deep-decompression models (VPM/RGBM) to classical Haldanian models (Bühlmann). The study had to be prematurely halted for ethical reasons because the protocol using deep decompression caused a significantly higher number of decompression accidents.

The contemporary scientific consensus today rejects the philosophy of VPM-B. Hyperbaric physicians have demonstrated that while a diver stops at great depth to compress bubbles, their slow tissues continue to absorb nitrogen and helium due to the high partial pressure. This increases the overall decompression stress during the final ascent near the surface, making the algorithm less effective than a Bühlmann algorithm coupled with Gradient Factors.

The main strength of the VPM-B is its historical significance. It facilitated the development of deep technical diving using Trimix and air in the 2000s, and it remains appreciated by veteran divers who are intimately familiar with how their own bodies react to this model.

The major weakness is the tissue over-saturation induced by the deep decompression stops. This unnecessarily prolongs the overall decompression time and increases the risk of desaturation accidents in a scientifically proven manner.

In the context of a dive with a group, the VPM-B's compatibility with other algorithms is very poor. If you are diving using the VPM-B and your buddy uses a standard Bühlmann ZHL-16C model, your decompression profiles will be completely misaligned, forcing you to stop much deeper than them. The entire team must absolutely use the same decompression model in order to dive together safely.

If you would like to analyze in detail the technical computers offering this option and compare them with other models, we invite you to consult our AquaExposure dive computer comparison tool to compare their performance and features.

The VPM-B algorithm is now considered a traditional technology in the world of diving. While it is not recommended for recreational divers or modern technical divers who prefer the transparency and validated safety of the Bühlmann algorithm combined with Gradient Factors, it retains its place in history for having paved the way for deep explorations. It evokes a sense of wonder and excitement.