Every dive computer runs a decompression algorithm — a mathematical model that estimates how much inert gas (nitrogen, or nitrogen plus helium in tech diving) your body tissues absorb and release during a dive. The algorithm determines your no-decompression limit, required safety stops, and decompression obligations. Different algorithms produce different answers for the same dive profile, which is why two divers at the same depth can have different NDLs on their wrists.
Bühlmann ZHL-16C
The Bühlmann model, developed by Albert Bühlmann at the University of Zurich, is the most widely used and well-understood decompression algorithm. It models the body as 16 theoretical tissue compartments, each absorbing and releasing gas at different rates (half-times ranging from about 4 minutes to over 600 minutes).
The key advantage of Bühlmann is transparency. The math is published, peer-reviewed, and used by thousands of divers and researchers worldwide. When paired with gradient factors (GF), divers can fine-tune conservatism precisely. A setting of GF 30/70, for example, means you begin ascending at 30% of the maximum tolerated pressure gradient and surface at 70% — adding a significant safety margin over the raw algorithm.
Shearwater dive computers (Peregrine, Perdix, Teric) use Bühlmann ZHL-16C with user-adjustable gradient factors. This gives divers full control over conservatism and makes their dive plans reproducible and verifiable against published tables.
RGBM (Reduced Gradient Bubble Model)
RGBM, developed by Bruce Wienke, takes a different approach. Instead of only tracking dissolved gas in tissues, RGBM also models microbubble formation — tiny gas bubbles that form in blood and tissues even within no-decompression limits. The theory suggests these microbubbles contribute to decompression stress, and RGBM adds penalties to account for them.
RGBM is used by Suunto and Mares dive computers. In practice, RGBM tends to produce shorter no-decompression times than Bühlmann at comparable depths, especially on repetitive dives and multi-day diving. The algorithm also penalizes rapid ascents and deep-to-shallow profiles more aggressively.
The criticism of RGBM is that it's proprietary — the full implementation details aren't publicly available, so divers can't independently verify the calculations or understand exactly why their computer is giving a specific NDL on a given dive.
VPM-B (Varying Permeability Model)
VPM-B, originally developed by David Yount, models bubble nuclei — microscopic gas pockets that exist in body tissues. The theory proposes that these nuclei have a "permeability" that varies with pressure and time, and that controlling the growth of these nuclei is the key to safe decompression.
VPM-B is primarily used in technical diving software (MultiDeco, Subsurface) and some Shearwater computers. It typically produces deeper decompression stops than Bühlmann, pushing divers to spend more time at mid-depth and less time in the shallow zone. Whether this produces safer outcomes is debated — some research suggests deeper stops may increase overall inert gas loading.
What This Means for Your Diving
For recreational diving within no-decompression limits, the practical differences between algorithms are small — typically a few minutes of NDL at recreational depths. A Suunto running RGBM and a Shearwater running Bühlmann GF 40/85 will give you roughly similar limits on a single 80-foot dive.
The differences become significant on repetitive dives, multi-day diving, and decompression diving. RGBM applies cumulative penalties across multiple dives that Bühlmann doesn't. VPM-B prescribes different stop depths than Bühlmann for decompression dives.