Dive Computer Algorithms For Dummies

What Is a Dive Computer Algorithm?

A dive computer algorithm is, in essence, a set of mathematical formulas used to calculate safe dive limits, factoring in a variety of real-time measurements, such as depth, time at that depth, water temperature, gas mix, cylinder pressure, ascent rate, etc. All dive computer algorithms are intended to keep the risk of decompression sickness (DCS) to an acceptable level and work using theoretical models, based on research and data from actual dives.

Although the source data for most algorithms is the same, the resulting calculations often differ. Why does this happen? One of the primary reasons lies in differing methods of interpreting the data and adjusting large numbers of initial best estimates. Another obvious source of difference is in determining the maximum acceptable risk. Finally, thanks to highly advanced microscopic physics experimentation and the use of mixed gases in diving today, the scientists are learning more about how different gases act in our bodies under pressure and update their algorithms accordingly.

The Theory Behind the Algorithm

There are lots of algorithm variations and proprietary computations used in modern dive computers. Let’s look at a few major theoretical models that serve as a basis for most of them:

Haldane Decompression Theory was proposed in 1908 by a Scottish physiologist John Scott Haldane, who is now considered to be the father of modern dive decompression theory. Haldane was the first researcher to apply a scientific approach to predicting and preventing decompression in a systemized way. Thanks to his experiments on goats, Haldane found that the body could tolerate a certain amount of excess gas with no apparent ill effects. Goats saturated to 50 msw (165 feet) did not develop DCS if decompressed to half the ambient pressure. He also noticed that divers could surface from a depth of 10 meters (33 feet), without developing DCS. In order to explain his observations, Haldane suggested in his dive decompression theory that we consider the body as a group of tissues, and created a mathematical model to describe how each of the tissues absorbs and releases gases. He further put limits on the amount of over pressurization that the tissues could tolerate and introduced the concept of half time, which is the time required for a particular tissue to become half saturated (or desaturated) with a gas. He suggested 5 tissue compartments (note, these theoretical tissues do not directly correspond to any particular body tissue) with half times of 5, 10, 20, 40 and 75 minutes. Based on his research Haldane developed practical dive tables that included slower ascent rates as the diver approached the surface. Although some of the Haldane’s findings have since been proven wrong, a lot of what he discovered was used for the computation of the US Navy Air Tables (the industry standard up until the 1980s) and is still at least partially used in all other recognized air decompression tables and most dive computer algorithms.

Bühlmann Decompression Algorithm was created back in the 1960ies by a Swiss physician Dr. Albert A. Bühlmann, and, at the time of publication (1983), was regarded as the most complete public reference on decompression calculations. Building on the previous work of John Scott Haldane, the researcher modeled the human body as a number of theoretical tissue compartments, which absorb (on-gas) and exude (off-gas) inert gases at different rates. Bühlmann also used the concept of half time, however, unlike that of Haldane, Bühlmann’s algorithm considered 16 tissues with half-times up to 635 minutes and introduced factors that attempted to model the variation of supersaturation limit with depth. Several versions of the Bühlmann algorithm have been developed over the years and adopted by many dive computer manufacturers. The naming convention used to identify the algorithms is a code starting ZH-L, from Zürich (ZH), limits (L) followed by the number of tissue compartments, and other unique identifiers.

The Varying Permeability Model (VPM), originally developed by researchers at the University of Hawaii, is based on laboratory observations of bubble formation and growth in both inanimate (such as gelatin) and in vivo systems exposed to pressure. The VPM presumes that microscopic bubble nuclei (also called seeds) always exist in water-containing tissues. Any nuclei larger than a specific “critical” size, which is related to the maximum dive depth (exposure pressure), will grow upon decompression (when the diver ascends again). The VPM aims to minimize the total volume of these growing bubbles by keeping the external pressure large, and the inspired inert gas partial pressures low during decompression. The model depends on a few key assumptions:

different sizes of bubbles exist within the body;
larger bubbles require less reduction in pressure to begin to grow than smaller ones;
fewer large bubbles exist than smaller ones.

These are used to construct an algorithm that provides decompression schedules designed to allow the larger, growing bubbles to be eliminated before they can cause problems.

The Reduced Gradient Bubble Model (RGBM) was developed by Dr. Bruce Wienke and based in part on the Buhlmann algorithm and the classical VPM bubble theory. It is, however, conceptually different from the latter in that it rejects the gel-bubble parametrizations. The RGBM is characterized by the following assumptions:

blood flow (perfusion) provides a limit for tissue gas penetration by diffusion;
an exponential distribution of sizes of bubble seeds is always present, with many more small seeds than large ones;
bubbles are permeable to gas transfer across surface boundaries under all pressures;
the Haldanean tissue compartments range in half time from 1 to 720 minutes, depending on the gas mixture.

The first dive computer manufacturer to incorporate this model was Suunto, nowadays many other manufacturers use variations of RGBM.

The Diving Science And Technology (DSAT) Model is based on the studies that were used to develop the PADI Recreational Dive Planner (RDP) which is still used and relied upon by thousands of divers around the world. As it was mentioned before, until the mid-1980s, the US Navy tables were the industry standard in dive decompression theory. Based upon their research and empirical observations the Navy made revisions to the tables previously developed by Haldane and tested the new tables with US Navy divers. Subjects were all male in their 20’s and 30’s and reasonably fit. The test criteria were bends/no bends. As a result, six tissue compartments were used with the slowest halftime of 120 minutes. When scuba diving began to emerge as a popular activity, however, it became apparent that certain modifications needed to be made. In 1983, Dr. Raymond Rogers began analyzing the US Navy tables and compared them to the needs of recreational diving and the latest findings by decompression physiologists. He found that the US Navy tables had several disadvantages for recreational divers. First, more recent data showed that the US Navy no decompression limits were perhaps a bit too generous for civilians diving for fun since the test group of the USN didn’t reflect recreational divers who include females and people of all ages. Secondly, the 120 minute half time used for surface interval credit, while appropriate for decompression diving, seemed excessively conservative for recreational divers making only no-decompression dives. Following this dive decompression theory research, Rogers, with the help of DSAT, went onto developing the PADI Recreational Dive Planner. The most significant decompression theory change was the choice of a 60-minute halftime compartment as the basis of repetitive diving. As a result of this, the RDP gives about twice as much surface interval credit. In addition to that, tests done using Doppler ultrasound flowmeters showed that silent bubbles often formed at USN table limits, so Dr. Rogers concluded that tables for recreational divers would have somewhat shorter no-decompression limits for single dives.

One thing to be aware of is that different dive computers can produce different results even if they use the same underlying model. This is based on the fact that many manufacturers add their own modifications and conservative settings to the results. Overall, even though some algorithms are more liberal or lenient, while others are more conservative, all dive computers available on the market today should keep you safe if you use them within their limits and follow their guidelines.

Algorithms by Dive Computer Manufacturers

Aqualung - Pelagic Z+

The Pelagic Z+ used in Aqualung computers is a proprietary algorithm developed by Dr. John E. Lewis, who has been with Pelagic for over 34 years. It is based on the popular Bühlmann ZHL-16C algorithm and was developed to safely maximize dive times at depth without penalizing the diver for performing repetitive deeper dives. The user can customize the algorithm by adding a Conservative Factor (the dive time remaining and No Decompression/O2 time will be reduced to the values available at the altitude level that is 915 m (3,000 ft) higher than the actual altitude at activation) as well as Deep and Safety Stops for No-Decompression dives.

Atomic - Recreational RGBM

Atomic uses a recreational RGBM algorithm that is based on the Dr. Bruce Wienke model. It’s not an overly conservative algorithm but it is also not a very liberal one. Realistically it is somewhere in the middle of the pack. The age you enter, the risk level, and the exertion level selected for a dive will all influence the conservatism of the algorithm.

Cressi - RGBM

Cressi computers come with the Haldane and Wienke RGBM algorithm, which is similar to the one Suunto uses and is known to be rather conservative. The algorithm allows for safe decompression calculations for multiple dives spread out over multiple days. The users can adjust the level of conservatism based on their personal preference, as well as add or remove deep and safety stops.

Garmin - Bühlmann ZHL-16C

The Descent Mk1 (currently the only dive computer by Garmin) utilizes the Bühlmann ZHL-16C algorithm and has highly configurable conservatism settings. You can choose from three pre-set conservatism settings, or enter custom Gradient Factors. It is also possible to change the duration of the safety stops.

Mares - RGBM or Bühlmann ZHL-16C

For most of its dive computers, Mares, similar to Cressi and Suunto, uses the RGBM algorithm, which is pretty conservative. Depending on the model, the user will have more or fewer adjustment capabilities. However, Mares Genius dive computer currently comes with the Bühlmann ZHL-16C algorithm. It has the option of personalized conservatism settings, where you can choose from four pre-sets or set the values directly under the “Custom” settings option.

Oceanic - Dual Algorithm - Pelagic Z+ (ZHL-16C) and Pelagic DSAT

Oceanic dive computers provide two different algorithms built-in, and it is up to you to choose which one to use. This option comes in handy when diving with a group since you can adjust your device to closely match the computers of your buddies. You can also switch between the algorithms depending on the diving you will be doing. Use Pelagic DSAT for liberal recreational diving, as this algorithm safely maximizes dive time for repetitive, multi-level recreational dives. Go with Pelagic Z+ for more conservative recreational diving (when applied to standard recreational diving, the algorithm increases the conservative factor by 15-20%) or liberal repetitive deep and decompression diving. The Pelagic Z+ uses the Buhlmann ZHL-16C database and was designed to maximize dive times at depth without penalties.

ScubaPro - ZHL8 ADT MB

ScubaPro uses a Predictive Multi-Gas ZHL8 ADT MB algorithm that comes with lots of personalization capabilities. You can choose different microbubble levels to adjust the conservatism of the algorithm to match your experience level, age, and physical conditioning. The algorithm also includes a diver's breathing rate, skin temperature and heart rate (depending on the model, with either built-in HRM or with ScubaPro HRM belt), senses the diver’s effort, incorporates it into the workload calculation, and then adapts the decompression plan accordingly.

Shearwater - Bühlmann ZHL-16C with optional VPM-B and VPM-B/GFS

The basic decompression algorithm used by Shearwater computers is Bühlmann ZHL-16C with Gradient Factors. This allows for a ton of adjustment capabilities. You can pretty much influence the underlying algorithm to the n-th degree, making it as conservative or liberal as you wish. Additionally, there is the option to get a VPM software upgrade. The upgrade allows switching to the VPM algorithm when the computer is set in technical open-circuit or rebreather modes (i.e. recreational open-circuit mode is always Bühlmann). Compared to typical Bühlmann profiles, VPM-B dive profiles often have deeper initial stops along with the reduced time at shallow depths. In VPM-B/GFS, the Gradient Factor Surfacing 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 Bühlmann ZHL-16C profile.

Suunto - RGBM

It’s pretty much common knowledge that Suunto uses a very conservative algorithm. You can somewhat manipulate it through the conservatism settings but it overall tends to be one of the most conservative algorithms you can find. There are a few different modifications of the Suunto RGBM algorithm used in different computer models. Suunto Vyper Air, Vyper, Cobra, Cobra3, Zoop, D4i, and D6i, come with the standard RGBM adaptable algorithm. Suunto HelO2 and D9tx computers have the Suunto Technical RGBM - an advanced algorithm that provides flexibility and safety during ascent through continuous decompression. Finally, Suunto Fused RGBM is used in Suunto EON Core and EON Steel, as well as Suunto DX and D5. This algorithm automatically switches between the Suunto Technical RGBM and the full RGBM to effectively manage the risks of decompression sickness on deep dives. It has rebreather capability and supports dives down to 150 m (492 feet).

Can You Influence the Algorithm?

As described in the previous section of our article, most, if not all scuba computers have the feature to set conservatism settings. As a rule of thumb, lower end dive computers, targeted towards novice divers, will allow very simple manipulations, while technical dive computers will have much more complex adjustments available.

Regardless of your skill level and experience, it is strongly recommended that you read the manual and find out exactly how the manipulations and adjustments work. It would be common sense not to try to read about it when you’re on the dive boat and getting ready for your dive. Take a few moments to read through the details and try them out before you go on a trip to understand how to set conservatism settings or other customizable options on your specific model.

Which Algorithm Is Best?

There is no definitive answer to this question. In fact, a 100% accurate decompression table or dive computer algorithm will most likely never be obtained. The complex nature of human physiology means that a certain amount of conservatism is required. Currently, it is up to you to decide what you are comfortable with and what suits you best. A younger, physically fit, somewhat-aggressive diver might prefer a more liberal algorithm that maximizes the bottom time, while an older, perhaps not-quite-as-fit diver might want to go more conservative to stay on the safe side.

Hopefully, as the knowledge of decompression physiology improves and technology develops, we will see more dive computer algorithms tailored to some extent for the individual, based on personal biometrical data and the planned dive profile.

References

Azzopardi, Elaine & Sayer, Martin. (2010). A review of the technical specifications of 47 models of diving decompression computer. Underwater Technology: The International Journal of The Society for Underwater. 29. 63-72. 10.3723/ut.29.063.
Hamilton, R. W. & Rogers, Raymond. (1994) Development and Validation of No-Stop Decompression Procedures for Recreational Diving: The DSAT Recreational Dive Planner: Diving Science and Technology Corp.
Nico A.M. Schellart. Neo-Haldanian and bubble models, Bühlmann, bubble grow and Bubble dynamics; Available from: URL: http://www.diveresearch.org/download/Publicaties/Haldane%20en%20bellen%202006.pdf
Wienke, B. Modern Decompression Algorithms: Models, Comparisons, And Statistics; Available from: URL: http://www.tecvault.t101.ro/ModernDecompression_Wienke.pdf
Wienke, B. (2019). RGBM Algorithm Overview: Concepts, Bases, Validation, Testing And References.