THE first item sold as a diving computer, more than 20 years ago, looked like of a bit of pottery and two condoms in a tin box. It was dubbed the "Bendymeter" and it was as dangerous as it sounded. We have come a long way since then.
Modern electronic diving computers have changed the way we dive. Old-style decompression tables were complex, inaccurate and liable to user error. Even greatly improved versions such as the BSAC 88 tables and the PADI Recreational Dive Planner and Wheel restricted the user to a pre-determined dive plan that did not necessarily take into account changing conditions during the dive.
Today a high degree of accuracy in measuring depth and time, combined with the mathematical wizardry of those who write computer programs, means a diver can keep a close watch on his decompression status throughout the dive.
It seems only yesterday that I strapped my first electronic marvel to my wrist. It was called the Decobrain. It was not so different in function to modern computers, but because of contemporary battery technology and the fact that microchips were still in their infancy, it was the size of the instrument binnacle from a Ford Cortina.
In those days, about 12 years ago, my computer was regarded with deep suspicion by those who did not understand what it could do. Now it seems that every diver eventually ends up using one, if not two.
What does a diving computer do? It gives information that helps you avoid the onset of decompression illness in case you go too deep, for too long, or if you ascend too quickly, or a combination of the three. Most computers still use data based on the empirical test results achieved by Haldane at the beginning of this century.
A computer's main function is to measure depth and time at set periods, and calculate how much nitrogen a diver's body is absorbing and later releasing. It does this by using a mathematical algorithm with a number of factors built into it representing token body tissues.
These "model" tissues have half-times that vary from short (2min) to long (480min). Because body tissues soak up nitrogen exponentially, these theoretical figures represent half the time a tissue takes to become half fully saturated.
Electronics technology has advanced and the complex mathematical algorithms of physicists like Buelhmann in Switzerland, Rogers and Powell in the USA, and Spencer with his "M" values for tissue half-times, can now be incorporated into instruments that offer more peripheral functions, or have been reduced to watch-like dimensions - or both!
Prices can vary from less than £200 for a simple no-stop computer to nearly £1000 for a complex gas-integrated machine, intended for complex technical dives.
Because a computer takes samples of depth and time every few moments, it can also measure, and therefore allow the diver to control, ascent rate. All modern computers provide this facility, including the entry-level models like the Aladin Sport, Suunto Companion, Oceanic Data 100, Cochran Captain and Benemec Data/Orca Pilot.
Mainstream full-function diving computers like the Suunto Solution, Cochran Commander and Aladin Pro can interface with home PCs to reconstruct a moment-to-moment log of every dive. This is usually presented in the form of a depth/time graph with additional bar graphs representing model tissue nitrogen saturation status.
Computers with such a comprehensive spec also tend to give the user a total "time-to-the-surface" at the safe ascent rate, including any required decompression stops. As the diver ascends, the reducing pressure allows his body to off-gas. The time-to-the-surface information also helps a diver with his air-management.
Some computers are "air-integrated". The Suunto Eon, Suunto Favor Air, Scubapro Trac and Aladin Air are connected to the diver's TPR (as cylinders are now called) by a high-pressure hose attached to the regulator's first stage, and they add air-management calculations to the displays. They measure the breathing rate previously monitored, equate it to the ambient depth, and incorporate a figure for "remaining-airtime". Putting the computer at the end of a hose can make it inconvenient to look at. State-of-the-art computers such as the Oceanic Datatrans, Aladin Air X, Monitor 3, Mares Genius and Cochran Nemesis II use a small radio transmitter plugged into the hp port of the regulator. This measures tank pressure and transmits it to the computer mounted in a wrist unit.
Some computers offer something unique. Benemec in Finland makes a talking computer that is sold both as the Mares Divemate and the Orca Audio Pilot. It can be attached to the mask strap and is clearly heard, even through a thick hood. It may seem like something of a gimmick, but any diver who finds his or her eyes concentrating on something other than the instruments - including photographers, cameramen and even diving instructors - will find it invaluable. Technical divers also find it useful to have a voice reminding them of their depth and time when narcosis might tempt them to forget to look at their gauges. Computers like the Sea & Sea Profile actually display a mini-equivalent of the interfaced PC display. The Mares Guardian probably has the easiest display to read and is brightly illuminated for night use or poor visibility. Both units use large push-buttons rather than wet-finger contacts.
Many of these computers are available for nitrox divers. They can be adjusted to match the percentage of oxygen (and therefore nitrogen) in the breathing mix, with decompression requirements modified to suit. In addition they monitor oxygen toxicity levels - an O2 clock counts exposure levels - and warn if the maximum level of ppO2 is being reached in case the diver is going too deep.
You can get nitrox versions of more simple computers such as the Oceanic Data Plus, the Data/Orca Nitrox and the Dive Rite Bridge II; established favourites like the Aladin Pro Nitrox and Suunto Nitrox, and unique-feature models like the Orca Audio Pilot Nitrox.
For nitrox divers who want something state-of-the-art, there is the radio-controlled, air-integrated Aladin Air X Nitrox. Those who want to go one better can buy the hugely complicated Cochran Nemesis Nitrox IIa. This can distinguish up to two gas switches during the dive and amend its display according to the gas being breathed.