Features, Functions & Algorithms
The following is a brief overview of some of the questions and mysteries surrounding dive computers. As dive computers become more and more popular, new manufacturers are quickly bringing products to the market to satisfy the growing need.
Dive Computers – a Brief History & Purpose
Ever since the first diver put their head underwater, man has worked on means to allow that experience to continue as long as safely possible. Equipment and methods have evolved with the express purpose of providing greater duration for the underwater experience. In addition to the external components, one cannot overlook the critical physiological limitations that our bodies bring into the equation, and the risks associated with them.
Drowning aside, the greatest risk divers face are barotrauma injuries. Some, such as ear and sinus issues, are generally obvious and dealt with on an intuitive basis. However, the more serious barotrauma risks lie within our bodies and the tissues that make it up. These can generally be grouped into two major categories – lung over-expansion injuries and decompression sickness. Both essentially involve the expansion of gases within and around our tissues as we move from an area of great pressure to one of lesser pressure, such as when ascending on a dive.
Lung over-expansion injuries occur when gases are trapped and cannot be expelled through normal respiration to avoid the tensioning of lung tissues. This cannot occur via a breath-hold, obstructions within the lug passages, or simply due to an excessive rate of free gas expansion as a result of ascending rapidly. With the myriad of complex passages, and the density of the minute lung alveoli, gases may simply not be able to push themselves out into the airway fast enough to allow the diver to exhale them. Once trapped, should the diver continue ascending, the pressure of the gases may exceed the mechanical ability of the surrounding tissue to contain them, and as a result, a rupture or tear of the tissue occurs. Depending on the extend of this breech, the gases may now directly enter the circulatory system, or relocate themselves elsewhere in the chest cavity, in general, wrecking havoc on our bodies and greatly increasing the likelihood of serious injuries or death resulting from this dive. It is good to avoid this on each and every dive.
Decompression sickness involves gases that generally are in solution within our blood or other tissues, and that due to the lowering of the ambient pressure, such as during an ascent, now begin to expand to the point where they actually come out of solution and form actual bubbles within our body. Depending on the size, quantity and location of the bubbles, thus may present a very serious risk. Once bubbles begin to form, the gases they contain cannot be removed from the body through the normal exhalation process. The trapped bubbles may flow freely or lodge themselves within us, causing great risk of permanent injury or death. Once again, it is best to avoid this during our dives.
So, with so many risks to deal with, how do we manage them to reduce the likelihood of an injury? As noted above, the portion of the dive that poses the greatest risk is the ascent, where the ambient pressure is lower than during the deeper part of the dive. This includes the surface, where the pressure is the lowest, or, heaven forbid, any visit to a greater altitude after diving, such as traveling in the mountains or flying after diving. The key is to understand the process, and once that knowledge is there, to utilize tools that help us best plan and execute our dives in a manner that provides the lowest risk of an injury.
Dive tables were invented right at the onset of diving, as soon as the practitioners of the sport recognized that injuries often resulted from diving. Coincidently, it was not diving at all, but deep construction techniques, working in pressured ‘caissons’ for early bridge construction, that helped illustrate the hazards associated with returning to the surface after being at depth for extended periods. Many deaths and permanent injuries were observed in the construction industry before the medical community linked them to the fact that they were directly correlated to the time and pressure the workers experienced during their shifts. From these observations research grew, and before long the first tables began to appear, providing guidelines and methods that greatly reduced the risk of decompression illness.
Tables had limitations, and in order to avoid them from being overly complex, the developers used rounding techniques for depths and times. In reality few of us dive exactly like the tables are designed, at exactly the depth chosen for exactly the time specified, but until recently there was little that could be done to improve on this.
Welcome to the 70’s and the explosion of the electronics industry. Suddenly, with on-board pressure transducers and math processors, the algorithms used to develop the dive tables could be “live”, and dives could be calculated dynamically, by slicing the dive in small segments and calculating tissue gas loading and unloading throughout the dive, taking advantage of the actual depth at any given point in the dive, and not rounding up like the written tables did.
Today’s dive computers offer features such as integrated tank pressure gauges, programs that can be adjusted for the use of Nitrox of Trimix blends, multiple gases, and more. Dive planning allows you to see where you are before any given dive, and the dynamic algorithms also help alert you to remaining bottom time for any given depth, letting you avoid or manage decompression obligations.
In addition to the basic algorithms, possibly the most important feature in any dive computer today is the ascent rate indicator and alarm functions, allowing divers to accurately control their ascent, not by intuition, or watching your smallest bubbles, but by observing actual physical data and indicators that help you monitor your actual rate of ascent at any point in the dive. Even on free ascents, divers can now use good buoyancy control techniques to maintain safe, healthy ascent rates, and minimize the risk of barotrauma incidents.
So, What Exactly are Dive Computer Algorithms?
With the dive computer now used to calculate your maximum dive time, you have the tools in hand allowing for longer and safer dives, and greater control of surface interval times between dives. The dive computer has taken over from the dive table as the diving tool to calculate all your necessary parameters to enjoy scuba diving.
Dive computers use mathematical algorithms to calculate these safe diving limits. They will make adjustments for both ‘fast’ tissue groups and ‘slow’ tissue groups. It uses this information to estimate the nitrogen in your body. Based on the amount of time you’ve been submerged and your depth, from this the computer then calculates how much longer you can safely remain underwater. Various parts of our bodies take on gas and give off gas at differing rates, so the creators of the algorithms use various tissue group ‘models’ to help make adjustments to the calculations not only during a given dive, but also throughout a day or more of diving, with the sole purpose to help manage and reduce risk of injury.
Different Type of Algorithms
Algorithms vary by model from each manufacturer and only a few algorithms are actually used. Some algorithm used may also be a modification of an existing algorithm. When you want to consider buying your next dive computer, the main feature to look at is the algorithm that the air decompression limit monitor uses.
Algorithms with different manufactures use some varying factors in their algorithms such as mentioned with fast and slow tissue groups. The solubility, permeability, and blood contact for a given tissue, say for example a kidney, is called a “tissue compartment or tissue group.” Dive computers use these “tissue compartments” to simulate the effect of a gas on your body at depth. The more “tissue compartments” a computer measures, the more accurate (theoretically) the computer can gauge the effect of a dive on your body. 9 to 12 compartments is fairly standard, but a few consumer models have 16 or more.
Here are four of the main algorithms used for dive computers:
Group 1: Haldane/Spencer Algorithm
The Haldane/Spencer model uses test results from PADI’s Diving Science and Technology and was developed by Rogers and Powell. Within their algorithm they use 12 separate tissue compartments. Oceanic and Sherwood use this algorithm model for their dive computers.
Group 2: Modified Haldanian Algorithm
This algorithm model was used by Mares and is based on nine tissue compartments. Now the latest Mares dive computers use the Reduced Gradient Bubble Model.
Group 3: Suunto Reduced Gradient Bubble Algorithm
This model is based in part on work by Wienke and Hamilton and uses nine tissue compartments. Suunto were the first to use the RGDM and now other manufactures are starting to use it too. The big difference of this algorithm is they consider micro bubbles that are in the blood stream as a result of nitrogen build up. The theory was they consider that these micro bubbles are a precondition of larger bubbles which can lead to DCS.
Group 4: Uwatec Buehlmann ZH-L8 ADT Algorithm and the ZH-L8 ADT MB.
This algorithm model uses eight tissue compartments and has been recently updated with two additional letters, the Buehlmann adaptive model has been expanded to be called the ZH-L8 ADT MB which stands for micro bubble. This algorithm is used by Uwatec and Scubapro.
Shortfall of Algorithms
Algorithms may not be able to account for age, previous injury, ambient temperature, body type, alcohol consumption/dehydration, and patent foramen ovule (hole in the heart).
Which Algorithm is the best?
Removing of factors like cost, looks, other features etc then the more “tissue compartments” a computer measures, the more accurate (theoretically) the computer can gauge the effect of a dive on your body. 9 to 12 compartments are fairly standard nowadays, but a few dive computers are now using 16 compartments.
Even if two algorithms have the same results, manufactures will add there own factor of safety so they can set there dive computers to aggressive or conservative. An aggressive computer gives more bottom time than a conservative dive computer, and it’s wise to understand