HOW TO BUY A DIVE COMPUTER
Computers truly revolutionized diving. Not only do they extend your bottom time by computing your dive on a multiple-level basis but, if you suffer a bends hit, a computer provides valuable information. A chamber director once commented that computers should be required diving equipment because computers donít lie and they donít forget. The two words most often heard at a chamber are "think" and "about". As in, "Well, I think I was at 85 feet for about 22 minutes . . ."
So here are some thoughts to ponder.
AIR or NITROX - If youíre already diving nitrox, you need a computer that allows you to program the percentage of oxygen in your mix. Nitrox-compatible computers are also good for air-diving because they can be set to show 21% oxygen (standard compressed air).
If youíre not currently using nitrox but think you might in the future, you could get a nitrox-compatible computer now and just use it on air settings. When you start diving nitrox, youíll save yourself the expense of buying another computer.
If you donít plan on using nitrox, a standard air computer will work fine. And itíll be a couple of hundred bucks cheaper than its nitrox-compatible brother.
AIR-INTEGRATED or STAND-ALONE - Both provide you with the same type of depth and time information. Air-integrated versions go one step further by giving you a digital readout of air pressure, and tying that in with the dive info. This yields an "air time remaining" display (in addition to "bottom time remaining") which can be useful in executing your dive plan. The downside is that if the computer fails during the dive (which is relatively rare), you lose the air pressure information as well.
A stand-alone computer only monitors depth and time, so requires a separate pressure gauge. Although you can buy stand-alones as a module if you already own a pressure gauge, theyíre commonly sold with a pressure gauge as a combo unit, and frequently come with a compass in a tri-gauge configuration. Stand-alones will also save you some money as theyíre generally a couple of hundred dollars less than air-integrated versions.
NUMBER SIZE & DISPLAY INFO - No matter your age, you need to be able to read the computer display easily. The size of the numbers on the computer is important. A computer that meets all of your needs but whose numbers are too small for you to read underwater will not be of great benefit. The key numbers are (1) depth and (2) available bottom time remaining. If you can read those easily, it might not be a big deal if itís hard to see maximum depth or total bottom time.
Also note what information is displayed on the screen during a dive. (And remember that the more information displayed on the computerís face, the smaller the numbers will have to be to squeeze everything in.) Some computers give you depth and remaining bottom time, a nitrogen graph, and an ascent rate meter. Others include maximum depth and total bottom time. Still others add temperature and dive number. Decide whatís "need to know" and whatís "nice to know" and find a computer that meets all your "need to knows" and as many "nice to knows" as possible.
ACTIVATION METHOD - For the first dive of the day, some computers activate automatically and some have to be manually activated, usually by pushing a button. Either method will work. (And if you do forget to push the button, surface, hold the computer out of the water, and hit the button to re-initialize it.) Once a computer starts recording dive info, it continues to run until it shuts off (usually 12-24 hours after the completion of the last dive), so computer activation is only an issue on your first dive of the day.
BATTERIES - Know your options when the battery dies. Many computers have user-changeable batteries. Some require a shop technician to do the battery change and others must be sent back to the manufacturer. Ask about the expected battery life and, if you canít change it yourself, what the expected turn-around time will be.
MEMORY - All computers have a dive log function so you can review the dives youíve completed. Find out how many dives the computer retains in its memory. Older computers only showed the last dive. Today, 10 dives seems to be the average. Ask if the dives stay in memory day-to-day (newer dives replacing older dives one-by-one) or if the entire memory gets wiped clean once the computer shuts down and you start a new day of diving.
DOWNLOADING - Some dive computers can be downloaded to a desktop computer. This requires you to purchase software with an interface, and gives you a graphical display of your diving. At the minimum, the graph shows your depth and time over the course of a dive. More sophisticated models show ascent rate violations, air consumption (for air-integrated models), and predicted nitrogen load in your tissues.
NUMBER OF TISSUE GROUPS - Youíll probably be touted on how many tissue groups the computer measures, generally ranging from six to twelve. This just means how many different rates of nitrogen absorption the computer algorithm (the theoretical mathematical model the calculations are based on) tracks. Thereís not necessarily an advantage to a lot of tissue groups nor a disadvantage to fewer groups.
CONSERVATIVE or AGGRESSIVE - These terms relate to how much bottom time a computer allows. An "aggressive" computer gives more bottom time than a "conservative" one does. Computers can be aggressive, conservative, conservative on deep dives but aggressive on shallow ones, or vice versa. And while there are no reliable statistics on what the "safer" way to go would be, knowing where a computer is aggressive and where itís conservative will allow you to choose one that fits your needs and comfort level.
CLOSING THOUGHTS - Whatever you choose, a dive computer is a valuable tool to use underwater. And since itís something youíll probably have for quite some time, take a few moments to analyze your needs and desires, and pick the one thatís right for you.