Bubbles vs dissolved gas

Posted 09 June 2026 · Field Note #2

VPM-B vs Bühlmann, two ways to think about deco

Every decompression algorithm is trying to answer the same question: how do I ascend without forming dangerous bubbles? They just start from different assumptions about where the trouble begins, and that single choice shapes the whole ascent.

First, the shared problem. At depth your tissues absorb inert gas under pressure. On the way up, if the gas in your tissues vastly exceeds the pressure around you, it can come out of solution and form bubbles. Bubbles in the wrong place are decompression sickness.

Bühlmann, the dissolved-gas approach, tracks how much inert gas is dissolved in each modelled tissue and limits how supersaturated any of them is allowed to get. Add gradient factors and you simply choose how far below that limit to stay. It is fundamentally a bookkeeping model of gas in solution.

VPM-B, the bubble model, makes a different bet. It assumes tiny bubble seeds, called micro-nuclei, already exist in your body before you even splash. Rather than limiting dissolved gas, it caps how much your tissue tension may exceed ambient pressure, since that excess is the gradient driving the seeds to grow. Because that limit bites the moment you leave the bottom, it naturally pushes your first stop deeper to hold the seeds in check.

Here's the practical truth that surprises people: a VPM-B profile and a deep-stop-biased Bühlmann profile often look remarkably similar. Conversely, raising your GF-low (the setting that governs how deep your first stop sits) shifts decompression time shallower, away from the VPM-B style. Neither model is "correct" in any provable sense. They are two reasonable hypotheses about where decompression stress actually lives, and the research community continues to debate it.

What should you do? Don't treat your algorithm as gospel. Run the same dive both ways, line up the two schedules side by side, and notice how runtime, first-stop depth, and total gas demand shift. Understanding why they differ makes you a far more deliberate diver than blindly trusting whatever your computer shipped with.

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