Have you ever wondered how modern dive computers make your underwater adventures not only safer but also more enjoyable? In this article, we’ll take a look into dive computer algorithms and unravel why they matter to scuba divers.
Dive computers are one of the most important pieces of scuba gear, and I firmly believe every diver should own one. They have become a substitute for dive tables and are widely used by technical and recreational divers alike. When it comes to buying their first dive computer, divers care about price, esthetics, and functionalities, comfort (wrist mounted, dive watch style vs. console dive computer) and ease of use. At best, they rely on the advice of their trusted scuba shop or dive instructor.
One of the most overlooked aspects while deciding on a given dive computer model is the type algorithm that is being used by each computer to calibrate your dive profile (no decompression limits, ascent to the surface, maximum bottom time, dive time, surface interval) to avoid decompression sickness.
In this post we will take a closer look at what are dive computer algorithms and how they work, the different types of algorithms and why they matter when you are deciding which dive computer to buy. Different algorithms are suitable for different dive profiles. Therefore, before deciding on buying a specific dive computer, you should have an idea on the type of diving you are going to do.
Whether you are buying your first or simply a new dive computer, keep reading if you would like to make an informed choice.
What Are Dive Computer Algorithms?
Dive computer algorithms are mathematical models that compute the optimal ascent of a dive in order to avoid decompression sickness. They are also known as decompression algorithms and are used both in technical and recreational diving to compute your no decompression limit, eventually your decompression stops, your safety stop, and minimum surface interval between two or more dives.
Decompression models are also the backbone of dive tables, which are still used by (some) technical divers to plan their dives. The main difference between dive computer algorithms and decompression tables is that dive computers run the decompression model in real time, whereas if you are using dive tables you will need to recompute your dive parameters every time you breach some of the limits (e.g. no decompression limit).
As you might expect, there are several decompression models currently in use by scuba divers. Consequently, not all dive computers use the same decompression algorithm. And choosing which algorithm works best for you requires some basic understanding of the functioning together with the advantages and disadvantages of each decompression model.
How do Dive Computer Algorithms (Decompression Models) Work?
In general terms, dive computer algorithms take certain parameters as input and use them to determine your no decompression limits, ascent rates, decompression stops, maximum depth, and surface intervals. Input parameters may include depth, time, water temperature (cold or warm water), gas mixture, and other variables.
The ultimate goal of each algorithm is to get you to the surface with the least amount of residual nitrogen in your body while also ensuring that harmful nitrogen bubbles do not form.
The way in which input parameters are processed is determined, respectively, by the decompression model underlying the dive computer algorithm. There are two main conceptual frameworks when it comes to decompression modeling. The first one, also known as dissolved phase decompression models, focuses solely on managing dissolved gasses in the body during asymptomatic decompression. The second one, known as bubble phase models, assumes the formation of gas bubbles in our body during asymptomatic decompression and considers both dissolved gasses and the formation and behavior of gas bubbles.
At this stage, it is important to note that each decompression model has its own advantages and disadvantages. To date, no single decompression model has been proven to be an optimal representation of actual physiological processes. All decompression models and computer algorithms provide us with approximations (some of which, not all, have been calibrated with the use of experimental data).
How Do Dive Computer Algorithms Differ?
We just saw that there are two main families of decompression models, and consequently of dive computer algorithms. Dissolved phase decompression models and bubble phase decompression models. Let’s take a quick look at how each of these differ.
Dissolved Phase Decompression Models
| – Grounded on the work of John Scott Haldane, developed further by the Swiss physician Albert A. Bühlmann – Based on mathematical hypotheses and hyperbaric chamber experiments – Assumes that nitrogen is absorbed by body tissues during descent and released during descent – Does not attempt to model complete bubble formation dynamics (no micro-bubbles) – Uses the so called M-Values to compute the bubbles formation limit as a function of the differential between ambient pressure and dissolved nitrogen in your body |
Bubble Phase Decompression Models
| – Based on the work of Bruce Wienke – Based on mathematical hypotheses and currently tested in off the shelf dive computers – Attempts to model complete bubbles formation dynamics by assuming that micro bubbles form in body tissues during ascent and that the early elimination of tiny bubbles reduces the risk of larger bubbles at a later stage – Focusing on predicting the rate of bubble formation and eliminating them during the ascent by means of deep stops |
At first sight, bubble decompression models or algorithms seem more realistic to the extent that they try to model nitrogen bubble formation and behavior in a diver’s body. However, they are mostly based on mathematical hypotheses that have not been subject to hyperbaric chamber trials. Dissolved phase algorithms are instead well tested, even if they don’t model exactly the physiological off-gassing process that takes place in a diver’s body while surfacing of a dive.
This doesn’t mean that dissolved phase (also known as dissolved gas or Haldanean) algorithms are worse than bubble algorithms. To the contrary, the differences between those models make them better suited for different dive profiles.
Let’s take a look at the most widely used computer algorithms in modern diving applications.
What Are The Most Widely Used Dive Computer Algorithms?
At the top of our list, we have the Reduced Gradient Bubble Model (RGBM) and the Bühlmann ZHL-16C Algorithm. They are hands down the most widely used and tested decompression algorithms, with some diving computer manufacturers using some slight variations. For example, some algorithms might be calibrated to support longer dives or to have a shorter decompression time, depending on the dive modes you select.
I have reviewed 59 dive computers produced by Suunto, Garmin, Aqualung, Scubapro, Apeks, Mares, Cressi, Tusa, Shearwater, Oceanic (including the Apple Watch Ultra) and 38 computers use the Bühlmann algorithm and 21 use the the RGBM algorithm (or some variations thereof).
Let’s take a quick look at the basics of each single algorithm.
Bühlmann ZHL-16C Algorithm
The Bühlmann ZHL-16C Algorithm is the most widely tested dissolved gas algorithm on the market. Its wide use both in recreational and technical diving makes it also one of the most tested in all types of diving applications. Technical divers use a modified version of the Bühlmann ZHL-16C Algorithm, allowing them to set the gradient factors to adjust the algorithm’s (and dive computer) level of conservatism.
In a nutshell, gradient factors (high and low) allow you to adjust the overall ascent profile of a dive. Gradient Factor High (GF High) controls the overall length of your decompression time, whereas GF Low determines the depth of your first decompression stop.
Advantages of the Bühlmann ZHL-16C Algorithm
| Adaptability: The Bühlmann ZHL-16C algorithm can be tailored to suit individual divers’ profiles, allowing for personalized decompression calculations. Multiple Tissue Compartments: It uses 16 distinct tissue compartments to model gas absorption and release, providing more precise decompression calculations than simpler models. Real-time Monitoring: Many modern dive computers use variations of the Bühlmann ZHL-16C, providing real-time data during dives, allowing for immediate adjustments and safer ascents. Widely Accepted: It’s a well-established algorithm, widely accepted in the diving community, and supported by numerous dive computer manufacturers, ensuring its reliability and availability for divers. |
Disadvantages of the Bühlmann ZHL-16C Algorithm
| Complexity: The Bühlmann ZHL-16C Algorithm is complex and may require specialized training to fully understand and use effectively. Conservative decompression: It tends to be conservative, potentially resulting in longer decompression times and reduced bottom time for recreational divers, particularly in the case of multiple dives or multi-day diving. Lack of Personalization: The algorithm may not account for individual differences in nitrogen uptake and elimination rates, leading to overly cautious recommendations for some divers. This is particularly the case for unexperienced users. Limited Precision: Some critics argue that the ZHL-16C lacks the precision of more modern algorithms, potentially impacting its accuracy in predicting decompression requirements and resulting in higher post-dive fatigue. |
Reduced Gradient Bubble Model (RGBM Algorithm)
The Reduced Gradient Bubble Model also known as RGBM Algorithm was developed by Bruce Wienke in 1990, and first implemented in dive computers by Suunto in 2002. Throughout time, and thanks to wide coverage on the internet, adoption by NAUI, as well as commercialization in metric and imperial units of measure, the RGBM model has gained wide popularity and is still used by many dive computer manufacturers.
The RGBM model calculates dive profiles by considering the formation and growth of microbubbles in addition to dissolved gas. It is based on the assumption that eliminating the formation of micro-bubbles during deep stops reduces the risk of decompression sickness. Accordingly, it adjusts ascent rates based on bubble dynamics, aiming to minimize the risk of DCS.
Advantages of the RGBM Algorithm
| Enhanced Safety: RGBM dynamically adjusts decompression calculations, reducing the risk of decompression sickness by considering both dissolved gases and bubble formation. The dynamic adjustment is particularly useful in case of uneven dive profiles (frequent changes of depth, fast ascent rate, skipped surface interval, missed safety stop). Reduced Ascent Times: It often allows for shorter ascent times compared to traditional models, enabling longer bottom times. Reduced Fatigue: By minimizing unnecessary decompression stops, RGBM helps divers conserve energy and enjoy longer, more efficient dives. |
Disadvantages of the RGBM Algorithm
| Conservative Profile: RGBM tends to provide more conservative dive profiles, which may lead to shorter bottom times compared to other algorithms, potentially reducing dive enjoyment. Particularly during repetitive and multi-day diving. Variation Among Brands: Different dive computer manufacturers may implement RGBM with variations, leading to inconsistency in decompression calculations between devices. Bubble Formation Assumptions: RGBM makes certain assumptions about bubble formation, which might not accurately represent individual divers’ physiology, potentially resulting in unnecessary decompression stops. Overly Cautious Ascent: RGBM can trigger deep safety stops even when they are not always necessary, extending dive times and air consumption. Lack of Personalization: The RGBM algorithm may not fully adapt to a diver’s specific physiological characteristics, potentially limiting the algorithm’s efficiency for some divers. |
VPM-B Algorithm
The VPM-B decompression model, developed by Dr. Erik Baker, combines the VPM (Variable Permeability Model) with bubble dynamics. It calculates decompression stops based on dissolved gas levels and bubble growth and dissolution, providing a more precise and conservative approach to avoiding decompression sickness in scuba diving.
Advantages of the VPM-B
| Enhanced Safety: VPM-B takes into account the formation and behavior of gas bubbles, reducing the risk of decompression sickness compared to traditional models. Reduced Decompression Time: It often allows for shorter decompression times due to its adaptive approach, making dives more efficient. Individualized Decompression: VPM-B can be customized for individual divers, considering their specific physiology and dive profiles. Reduced Gas Consumption: By optimizing decompression schedules, it can help divers conserve breathing gas. Improved Dive Planning: VPM-B provides a more precise and conservative decompression model, enhancing overall dive planning and safety. |
Disadvantages of the VPM-B
| Complexity: The VPM-B algorithm is relatively complex and requires a deep understanding of its principles for effective use, which can be challenging for some divers. Conservative Nature: Some divers may find VPM-B to be overly conservative, particularly for no-stop dives. In case of decompression dives, it leads to longer decompression times compared to other algorithms, which can be seen as a disadvantage when time is a limiting factor. Potential Learning Curve: Mastering the VPM-B algorithm and its software tools can require a significant investment of time and effort, which may not be appealing to all divers, especially beginners. |
DSAT Algorithm
The DSAT (Diving Science and Technology) decompression model calculates safe ascent profiles for divers. It considers nitrogen tissue saturation and estimates off-gassing rates during ascent, helping divers avoid decompression sickness. DSAT utilizes a modified Haldanean approach, combining deep stops with no-stop limits, optimizing safety for recreational diving. The DSAT was the basis to develop PADI’s rdp tables.
Advantages of the DSAT Algorithm
| Conservative Approach: DSAT is known for its conservative dive profiles, prioritizing diver safety by reducing the risk of decompression sickness. Adaptive Algorithm: It adapts to real-time dive conditions, adjusting decompression calculations based on the actual dive, making it more personalized. Easy to Understand: DSAT algorithms are user-friendly, aiding in dive planning and execution. Proven Reliability: DSAT algorithms have a long-standing history of reliability and have been widely tested in various diving scenarios. |
Disadvantages of the DSAT Algorithm
| Conservative Decompression: It can sometimes result in unnecessarily long decompression times, reducing bottom time and limiting the enjoyment of recreational dives. Limited Adaptability: The DSAT Algorithm may not account for individual differences in divers, potentially leading to over-conservative or under-conservative profiles. Not Suitable for Technical Diving: DSAT is designed for recreational diving and may not provide adequate decompression profiles for more advanced technical dives. |
Which Algorithm is Best Suited for Which Dive Profile?
Based on the advantages and disadvantages of the most widely used dive computer algorithms, you can already see that each type of algorithm is most suited for different dive profiles. Consequently, when choosing a dive computer, one of the main considerations you should make is which kind of diving are you going to do most of the time.
| Dive profile | Best Suited Algorithm |
| Single recreational dive per day | RGBM or DSAT |
| Multiple (3+) recreational dives per day or multi-day diving (e.g. liveaboard, dive trip) | Bühlmann ZH-L16C |
| Technical dives (with deco) | Bühlmann ZH-L16C |
| Recreational dives | RGBM or VPM-B |
| Irregular dive profile typical of new divers (e.g. missed safety stop) | RGBM |
| Dive with frequent depth changes (e.g. underwater photograph) | RGBM |
Which Dive Computer Algorithms Are Used By Different Scuba Brands?
In the following sections, you will see the algorithms used by the most popular dive computers/brands. As you can see, computers developed for technical diving mostly use the Bühlmann ZH-L16C algorithm, whereas computers developed for recreational diving mostly use either the RGBM or the DSAT algorithm.
Shearwater Dive Computers
Shearwater dive computers employ the Bühlmann ZHL-16C algorithm with Gradient Factors, offering divers extensive customization options. You have significant control over adjusting the algorithm’s conservatism or liberality to suit your preferences. Moreover, a VPM software upgrade is available, allowing a switch to the VPM algorithm in technical open-circuit or CC (rebreather) modes while retaining Bühlmann for recreational open-circuit mode. VPM-B profiles generally feature deeper initial stops and reduced shallow-depth time compared to standard Bühlmann profiles. In the VPM-B/GFS mode, the Gradient Factor Surfacing option adds extra conservatism for extensive decompression dives exceeding an hour, dynamically selecting the more conservative ceiling between VPM-B and Bühlmann ZHL-16C profiles. In the recreational mode, Shearwater’s standard gradient factors are 40/85 (medium conservatism gradient factor). You have the option to select either a lower level of conservatism, corresponding to a gradient factor of 45/95 or a higher one, corresponding to a gradient factor of 35/75.
| Dive Computer | Algorithm |
| Shearwater Peregrine | Bühlmann ZH-L16C |
| Shearwater Perdix 2 Ti | Bühlmann ZH-L16C |
| Shearwater Teric | Bühlmann ZH-L16C |
| Shearwater Petrel OC/CC | Bühlmann ZH-L16C |
| Shearwater 3 Fisher | Bühlmann ZH-L16C |
| Shearwater Nerd 2 | Bühlmann ZH-L16C |
Suunto Dive Computers
Suunto offers various iterations of the RGBM algorithm across their dive computer models by employing the RGBM algorithm across the board. You can still adjust the level of conservatism, but all in all Suunto dive computers remain quite conservative. Suunto uses a standard RGBM model in its recreational dive computers, and a technical version of the RGBM in its technical dive computers. The Suunto Fused RGBM is integrated into models like the Suunto EON Core, EON Steel, DX, and D5. This innovative algorithm automatically transitions between the Suunto Technical RGBM and the full RGBM, effectively managing decompression sickness risks during deep dives. Furthermore, it boasts rebreather compatibility and supports dives down to depths of 150 meters (492 feet).
| Dive Computer | Algorithm |
| Suunto Zoop Novo | Suunto RGBM |
| Suunto Viper Novo | Suunto RGBM |
| Suunto D6i Novo | Suunto RGBM |
| Suunto Cobra 3 | Suunto RGBM |
| Suunto D4I Novo | Suunto RGBM |
| Suunto DX | Suunto Fused™ RGBM |
| Suunto EON Core | Suunto Fused™ RGBM |
| Suunto D 5 | Suunto Fused™RGBM |
| Suunto EON Steel Black | Suunto Fused™ RGBM |
Aqualung Dive Computers
Aqualung utilizes the proprietary Pelagic Z+ algorithm, conceived by Dr. John E. Lewis. This algorithm draws inspiration from the widely acknowledged Bühlmann ZHL-16C model, aiming to optimize dive durations at greater depths without penalizing divers engaged in consecutive deeper dives. Divers have the flexibility to tailor their experience by incorporating a Conservative Factor (Aqualung’s variation of gradient factors), which adjusts the available dive and No Decompression/O2 times based on an altitude 915 meters (3,000 feet) above the activation point. Additionally, Deep and Safety Stops can be integrated for No-Decompression dives, further enhancing user customization.
| Dive Computer | Algorithm |
| Aqualung i770R | Z+ Algorithm |
| Aqualung i770R TX | Z+ Algorithm |
| Aqualung i300C | Z+ Algorithm |
| Aqualung i330R | Bühlmann ZH-L16C |
| Aqualung i470TC | Z+ Algorithm |
| Aqualung i200 | Z+ Algorithm |
| Aqualung i100 | Z+ Algorithm |
| Aqualung i550C | Z+ Algorithm |
Scubapro Dive Computers
ScubaPro employs an advanced Predictive Multi-Gas ZHL8 ADT MB algorithm, offering extensive customization options. Divers can fine-tune the algorithm’s conservatism to align with their experience, age, and physical fitness by selecting various microbubble levels. Additionally, the algorithm factors in a diver’s breathing rate, skin temperature, and heart rate (based on the model, which may feature either a built-in HRM or utilize the ScubaPro HRM belt). It intelligently detects the diver’s exertion level, integrates it into the workload assessment, and dynamically adjusts the decompression plan accordingly.
All algorithms used in Scubapro computers are based on Bühlmann ZH-L16C original algorithm however you will notice two slight variations:
- ZH-L16 (16 Tissue Compartments): This model considers 16 different tissue compartments within the human body to calculate nitrogen uptake and release during a dive. It is more advanced and provides a more accurate representation of a diver’s physiological processes. The ZH-L16 model is generally considered more conservative and safer, as it takes into account a greater number of variables.
- ZH-L8 (8 Tissue Compartments): In contrast, the ZH-L8 model only considers 8 tissue compartments. While still a reliable decompression model, it is less complex than the ZH-L16. This model is often considered less conservative and may allow for longer bottom times and shorter surface intervals compared to the ZH-L16.
Divers often choose between these models based on their experience level, risk tolerance, and the specific dive profile they plan to undertake. The ZH-L16 is typically favored for technical diving and conservative recreational diving, while the ZH-L8 may be suitable for more liberal recreational diving profiles.
| Dive Computer | Algorithm |
| Scubapro G2 | ZH-L16 ADT MB PMG |
| Scubapro Galileo Luna | ZH-L8 ADT MB |
| Scubapro Galileo Sol | ZH-L8 ADT MB PMG |
| Scubapro A1 | ZH-L16 |
| Scubapro Z1 | Bühlmann ZH-L16C |
| Scubapro M2 | ZH-L8 ADT MB |
| Scubapro Mantis 1 (M1) | ZHL8 ADT MB |
| Scubapro Mantis Black | ZHL-8 ADT MB PMG |
| Scubapro Chromis | ZH-L8 ADT MB |
| Scubapro Aladin Sports Matrix | ZH-L16 ADT |
| Scubapro Aladin One Matrix | ZH-L16 ADT |
Mares Dive Computers
Mares, like Cressi and Suunto, predominantly employs the RGBM algorithm in its range of dive computers, known for its cautious approach to decompression planning. The extent of user customization varies across different Mares models. However, it’s worth noting that the Mares Genius and Sirius computers are now equipped with the Bühlmann ZHL-16C algorithm, offering divers the flexibility to fine-tune their conservatism levels. Users can select from four predefined settings or manually configure their preferred parameters within the “Custom” settings option.
| Dive Computer | Algorithm |
| Mares Quad | RGBM Mares-Wienke |
| Mares Smart | RGBM Mares-Wienke |
| Mares Puck Pro + | RGBM Mares-Wienke |
| Mares Sirius | Bühlmann ZH-L16C |
| Mares Genius | Bühlmann ZH-L16C |
| Mares Quad Air | RGBM Mares-Wienke |
| Mares Smart Air | RGBM Mares-Wienke |
Cressi Dive Computers
Cressi dive computers are equipped with the Wienke RGBM algorithm, sharing similarities with Suunto’s approach, recognized for its cautious approach to decompression. This algorithm facilitates secure decompression planning for a series of dives across consecutive days. It also offers users the flexibility to fine-tune the level of conservatism to align with their personal preferences, along with the option to include or exclude deep and safety stops as desired.
| Dive Computer | Algorithm |
| Cressi Raffaello | CRESSI RGBM |
| Cressi Cartesio | CRESSI RGBM |
| Cressi Neon | CRESSI RGBM |
| Cressi Goa | CRESSI RGBM |
| Cressi Michelangelo | CRESSI RGBM |
| Cressi Donatello | CRESSI RGBM |
| Cressi Leonardo | CRESSI RGBM |
Tusa Dive Computers
Tusa dive computers are equipped with the classic Bühlmann ZH-L16C algorithm and have the possibility to switch among two pre-set levels of conservatism (SF1 and SF2, respectively normal and conservative modes).
| Dive Computer | Algorithm |
| Tusa TC1 | Bühlmann ZH-L16C |
| Tusa DC Solar Link | Bühlmann ZH-L16C |
Apeks Dive Computers
The Apeks DSX comes with a Bühlmann ZH-L16C algorithm, with user configurable gradient factors. In recreational (Sport) mode, you have the option to choose 3 pre set levels of conservatism: Low (90/90), Medium (35/80), and High (30/70).
Oceanic Dive Computers
Oceanic dive computers come with dual algorithms (older models don’t have this option). They offer the flexibility of selecting from two distinct algorithms, granting you the autonomy to tailor your device settings. This feature proves invaluable, particularly when diving in a group, as it permits synchronization with your fellow divers’ equipment. Additionally, you have the freedom to switch between these algorithms as per the specific diving requirements. The Pelagic DSAT has a more liberal approach during recreational dives, effectively extending dive time for repetitive, multi-level dives. Alternatively, Pelagic Z+ is a prudent choice for either conservative recreational diving or adventurous deep and decompression dives, thanks to its adaptation of the Buhlmann ZHL-16C algorithm, optimizing bottom time without compromising safety.
| Dive Computer | Algorithm |
| Oceanic Pro Plus X | DSAT or Z+ |
| Oceanic Proplus 4.0 | DSAT or Z+ |
| Oceanic Geo Air | DSAT or Z+ |
| Oceanic VEO 4.0 | DSAT or Z+ |
| Oceanic Oci | DSAT or Z+ |
| Oceanic Geo 4.0 | DSAT or Z+ |
| Apple Watch Ultra | Bühlmann ZH-L16C |
| Apple Watch Ultra 2 | Bühlmann ZH-L16C |
Garmin Dive Computers
Garmin’s only dive computer, the descent MK1 uses the Bühlmann ZHL-16C algorithm. Just like the Apeks DSX, it has user configurable gradient factors with three pre set levels of conservatism in the recreational diving mode.
Conclusions
Dive computers are not made equal. Besides comfort, size, brand, easy-to-read screen, and popularity, there is one essential component that should be taken into account when choosing a dive computer. Decompression algorithms used in each single dive computer determine their level of conservatism given your dive profile.
If you are a newly certified diver still figuring out your buoyancy, you should be looking at a dive computer with an algorithm that takes into account for possible irregularities of a dive such as frequent depth changes or skipped surface intervals. On the other hand, if you are an experienced diver, you might want to have a computer that allows you to fully customize the level of conservatism based on your own dive profile.
Of course, although dive computer algorithms play a crucial role in helping you avoiding DCS, you should always bear in mind that no algorithm accurately predicts the level of nitrogen absorbed and released in your tissued during a dive. And that there are several other risk factors contributing to DCS which no algorithm takes into account: jet lag, percentage of body fat, alcohol consumption, cigarette smoking, your body’s reaction to water temperatures, previous DCS incident, level of body (de)hydration, and health status.