Can You Believe It?
Every time you dive, you entrust your well-being to a mathematical formula that is intentionally being kept a secret from you. Yeah, we thought it was weird, too.
So we sent our Scuba Lab investigators backstage at the magic show to learn what dive computer manufacturers don't want you to know. The result is this first-ever practical guide to dive computer algorithms and what they mean to you, Joe and Josephine Diver.
What You Do Know
Let's start with the easy stuff--what you already know:
- Dive tables and dive computers are based on decompression algorithms, mathematical formulas that produce time and depth limits for you to follow.
- These calculations are based solely on theories, none of which have been proved or disproved. Given our lack of knowledge in this area, it's amazing these theories work as well as they do.
- Dive computers don't read tables, but use their programmed formula (algorithms) to constantly evaluate decompression needs based on current time and depth.
- Divers still get bent when they do everything "right."
What You Probably Don't Want to Know
- Identifying a dive computer's algorithm will not help you make a reasonable selection.
- In fact, the actual programming is proprietary to each manufacturer, which means it is a trade secret.
- More to the point, not one of the original algorithms is "pure." They all have been modified, adjusted or expanded by the manufacturer to cover situations not contemplated or calculated by the creators of the original algorithms.
- A manufacturer's changes usually take the form of restrictions, penalties, lockouts or other uses regarding every aspect of diving: multi-level diving, ascents, repetitive deep dives, flying after diving, diving at altitude, reverse profiles, decompression stops and more.
Before you despair, keep in mind that most of us could not give a lucid explanation of how a modern internal combustion engine works, but that doesn't keep us from driving to work. However, when we go to buy a car, it's nice to know which engine's actual performance meets our driving needs. And that's the approach we've taken here.
Scuba Lab to the Rescue
We are not directly comparing algorithms. We are comparing how particular computers are programmed and how these programs manifest themselves in actual use. These comparisons are based on Scuba Lab's work during the past eight years evaluating over 160 dive computers, collecting data about their performance on a variety of dive profiles in the ocean and in the hyperbaric chamber. To this end, we have grouped all the dive computers that have earned Tester's Choices by algorithm and their actual function.
The Good News
You don't have to know how to program a computer, identify slow and fast tissue groups or even spell "algorithm." In the end, your selection of a dive computer should be based on its actual performance characteristics and how they match or don't match your diving needs and style.
Group 1: Haldane/Spencer Algorithm
This model uses data from Rogers and Powell as programmed by Lewis and is based on tests by PADI's Diving Science and Technology group. It uses 12 tissue compartments. Computer displays in this group use color highlighting on two or more graphs and they are controlled by one or two push buttons. Older models are slightly more restrictive on deeper, repetitive dives.
Group 2: Modified Haldanian Algorithm
This model as used by Mares contains nine tissue compartments. Displays are controlled by two push buttons and electrical contacts.
Group 3: Suunto Reduced Gradient Bubble Algorithm
This model is based in part on work by Wienke and Hamilton and uses nine tissue compartments. Most computers in this group have color-highlighted graphic displays and use electrical contacts or a combination of electrical contacts and push buttons. Older models are somewhat more liberal.
Group 4: Uwatec Buehlmann ZH-L8 ADT Algorithm
This model uses eight tissue compartments. All these dive computers use electrical contacts and are self-adjusting to reduce risk.
Are You a Victim of The Conservative Trap?
As manufacturers re-program their algorithms, many computers are becoming more conservative, with settings or adjustments that actually make multilevel, repetitive diving impractical. Of course, the more conservative the dive computer, the lower the risk of decompression sickness (DCS).
However, there is only one way to avoid all risk of DCS: don't dive. So where do you draw the line between safety and impracticality? It is our contention that divers should be answering that question for themselves and, most of all, that they should be provided the knowledge and tools to do so.
The trend toward ever-increasing conservatism in dive computers is also a product of the American legal system, in which liability suits against manufacturers of everything from hot coffee to baby food have been forced to pay ruinous settlements. Not only have computer algorithms become more restrictive in response to this litigiousness, but the warnings in the dive computer instruction booklets have become excessive and, often, actually contrary to common and reasonable diving practices and the very functions built into the computer.
Example: All dive computer instruction books now tell you not to make decompression dives. Yet all modern dive computers not only allow for deco diving, they provide for decompression depths and times that are based on almost no data and are significantly beyond the recreational diver's ability. Rather than tell divers, "decompression is an emergency, don't do it--but here is an untested way to do it," dive manufacturers should come to grips with the fact that deco diving is common and then provide reasonable decompression information to their consumers.
Algorithms A to Z
Absorption--the uptake of a gas; also called on-gassing or in-gassing.
Body tissues--vary in how fast they absorb and release nitrogen, depending on fat content, blood flow, temperature, level of exercise, etc.
Elimination--off-gassing or out-gassing.
Saturated--tissues holding as much nitrogen as possible.
Supersaturated--greater partial pressure in the tissues than in the lungs, causing bubbles to form.
Tissue compartment or group--theoretical concept used to describe body tissues that absorb or release inert gas at different rates. Example: eyes (fast tissue group) transfer gases very fast and bone marrow (slow tissue group) transfers gases very slowly.
Tissue half-time--the time it takes a tissue to absorb or eliminate half a volume of gas; analogous to the half-life characteristics of various radioactive elements.
M-value--the maximum calculated gas load allowed when ascending to a particular depth.
- Decompression sickness (DCS) is caused by bubbles of inert gas, usually nitrogen.
- Bubbles can damage body tissues.
- Asymptomatic or "silent" bubbles may also cause damage.
- Most dive tables and dive computers use the same rate for absorption and elimination.
- Rapid ascents increase the likelihood of bubble formation.
- Susceptibility to DCS varies significantly between divers and for the same diver on different dives.
- Backing off to reduce risk is done by decreasing the no-decompression limits.
- Factors that affect susceptibility to DCS are thought to include temperature, age, hydration and exercise level, plus many others, but causative connections are not proven.
Time Line: Evolution of Decompression Algorithms
1880 -- French physiologist Paul Bert discovers nitrogen bubbles cause "bends."
1908 -- J.S. Haldane, English physiologist commissioned by the Royal Navy, develops first practical dive tables.
1915 -- U.S. Navy uses Haldane's methods to produce dive tables.
1937 -- U.S. Navy tables revised.
1957 -- U.S. Navy tables revised.
Late '60s, early '70s -- Dr. Merrill Spencer uses doppler bubble detection to provide convincing evidence that the Navy's NDLs should be reduced.
Late '70s -- Jeppesen modifies U.S. Navy tables with reduced NDLs; sets stage for more reduced NDLs throughout next decade.
1985 -- U.S. Navy develops new tables based on Maximum Likelihood Statistical Method, the mathematical probability of DCS, not on decompression tissue modeling.
Late '80s -- Significant contributions by Albert Buehlmann with Swiss tables, Buehlmann and Hann with German tables.
1988 -- BSAC tables introduced, not based on Haldanian methods ... PADI/DSAT Recreational Dive Planner and Wheel.
1990s -- Kidd/Stubs model completed by DCIEM, Defense and Civil Institute of Environmental Medicine of Canada ... Varying-Permeability Model by the Tiny Bubble group at the University of Hawaii ... The Reduced Gradient Bubble Model (RGBM) by Dr. Bruce Wienke.