Information is a valuable commodity, and knowing everything you need to know about your dive is a major aid to safety and enjoyment.
An instrument that tells you the time is a watch; if you can set it to start and stop it is a timer. If it tells you how deep you are or have been during a dive, it is a depth gauge. It is at the point when an instrument takes all this data and processes it, using a program contained in its memory, that it becomes a computer.
Models available on the market vary in their physical form and in the way in which they present information, but the standard package always includes basic data such as current depth, maximum depth, expired time, no-stop time, decompression-stop information, ascent-speed warnings and surface-interval time.
The information set then varies depending on whether the computer is intended for purely recreational diving, which tries to avoid decompression stops, or more adventurous diving.
If deco stops are catered for it will give us information on the depth and duration of the next stop and total time needed to surface. Recreational diving models will ask us to follow the passage of an arrow along a bar graph to determine our safe ascent.
The recreational diving models usually come from US manufacturers such as US Divers and Oceanic, as this meets the demand of the majority of their market. European manufacturers, principally Uwatec and Suunto, meet the demand of the more adventurous European diver, who is probably diving deeper.
The exceptions to this rule are found in the technical diving market, where the principal manufacturers are Cochran and Dive-Rite in the USA.
A computer suitable for nitrox diving differs from an air computer in that it allows you to enter the oxygen component of the gas you will be breathing, from which it deduces the nitrogen component.
This allows it to calculate your decompression requirements correctly, while also warning you of the safe depth limit for the nitrox mixture and your accumulating oxygen exposure time. The top-of-the-range models, such as Cochran's Nitrox Nemesis IIa, allow for up to two gas changes to be programmed in so that a more accurate decompression result can be calculated.
Some computers also read the pressure in the diver's cylinder and deduce from the rate at which it falls how long the gas supply will last. This consumption rate allows them to go further by calculating how much air will be needed for the ascent and for a pre-determined reserve, and then give users a precise estimate of how long they can remain at their present depth while breathing at their present rate.
They can then compare this with the decompression requirements and, if necessary, warn divers that they need to leave soon if they want to carry out all their decompression while air remains.
The air pressure can be read directly from the high-pressure hose to the instrument console, or transmitted to the dive computer from a sensor attached to the regulator first stage. Computers like the US Divers Scan 5, the Oceanic Data Trans, Cochran's Commander and Nemesis and the Aladin Air X use a radio transmitter to communicate between the regulator and the wrist unit.
The Aladin Air, Oceanic Datamax Pro, Suunto's Eon 350 and Favor Air Lux and the US Divers Scan 4 use the console's hp hose.
This year sees more models available with back-lit screens, making them distinctly easier to read in low-light conditions. This is a significant advantage so don't hesitate to choose this if it is an option.
The most compact instrument on our list is the Suunto ACW Spyder. The ACW stands for "Advanced Computer Watch", which describes it well as it is a chunky dive watch with all the features of a dive computer. But consider size carefully when choosing, as a computer with a larger screen allows the use of larger digits that are easier and quicker to read under water.
Most models offer the possibility of connecting your dive computer to your PC via a cable interface so that dive data can be downloaded. This data is typically entered in a dive log, which shows your dives in fine detail, allowing careful analysis.
All you have to do is tell the computer the things it can't know, like where you dived, your buddies and the size of set you were using.
If it is an air-integrated model it will have sufficient data on your breathing rate to be able to determine your air consumption rate during the dive.
Some models offer software that allows pre-planning of dives on the PC using the same decompression algorithm. Others allow this facility directly on the dive computer.