>The best solution is to wear two computers. To maintain a streamlined profile, many divers choose smaller, wristwatch computers or "puck" computers that can be mounted in a variety of unobtrusive ways, but the style of the backup is a secondary consideration to how it works.
>Different computers (even some from the same manufacturer) may use different algorithms to calculate nitrogen uptake and elimination, so it's vital that your backup computer provide the same level of conservatism as your primary computer. Some experts recommend that the primary and backup computers use the identical algorithm. When selecting a backup computer, consult the owner's manuals to verify that the algorithm is the same as — or at least compatible with — that of your primary computer. Before diving, verify that all default or user-adjustable settings are the same for both computers.
>Most important is that you wear your backup computer on each and every dive of a series so that it has the same history of your total nitrogen exposure as your primary computer. Don't make the mistake of switching to a new computer in the middle of a series unless your recent dive history can be downloaded to the new computer. Without this data, the new computer will underestimate your nitrogen load, putting you at increased risk of decompression sickness (DCS).
>If circumstances require that you switch to a "fresh" machine, says Eric Douglas, director of DAN® Education, a diver will need an extended surface interval to allow his body to offgas accumulated nitrogen. "I would follow the flying-after-diving guidelines before considering myself clear enough to start over," he says. Those recommendations call for a minimum 12-hour surface interval for a single no-decompression dive or a minimum 18-hour interval for multiple dives or multiple days of diving.
>For those who find it impractical to dive with a spare dive computer, the clear option is to carry some traditional backup equipment. A simple dive watch or bottom timer, a depth gauge and a traditional pressure gauge combined with submersible dive tables will provide the basic data needed to continue a dive and make a safe ascent. The important thing here is that you keep track of your decompression status or dive profiles so you can switch comfortably from computer to tables.
>Backup Gauges and Plans
>Having the right equipment and a backup plan is critical. "I also keep independent analog depth and pressure gauges on my regulator set," Douglas says. "If I'm doing a single dive with only one dive computer, and it goes out, I will still finish the dive, deferring back to my analog gauges. If I lose my dive computer in the middle of a series of dives, I may still finish the dive based on my dive plan and using my analog gauges. If I don't know my no-decompression status, I will abort the dive.
>"If your only depth and pressure gauges are in your dive computer, you have no choice but to end the dive. You cannot safely complete the dive if you don't know your depth, time and breathing gas pressure."
>There are a number of reasonably priced dive watches and bottom timers available in today's market. A wrist or console-mounted depth gauge is an inexpensive accessory (less than $100) that lends considerable peace of mind. Those who dive with air-integrated dive computers, particularly hoseless air-integrated computers, should also consider a simple, low-profile backup pressure gauge. Combination depth and pressure gauges can be found for less than $200.
>Monitoring ascent rate is another critical function that can disappear with a malfunctioning dive computer. A 30-foot-per-minute ascent rate means rising one foot every two seconds. By keeping a hand on the anchor line and your eyes on your backup watch and depth gauge, you can track your ascent rate manually. Just be sure these gauges are mounted where you can monitor both easily and still have your hands free to manage the ascent. It can be a cumbersome procedure if you're out of practice. This is another case where a buddy becomes invaluable. While it is unsafe to rely on a buddy's computer for decompression status, following a buddy's computer for ascent rate data is an acceptable and safe practice.
>Keeping your dive computer in peak working condition will go a long way to preventing failure underwater. Follow the manufacturer's recommendations for periodic professional maintenance. Before a dive excursion, check the function and battery level to make sure everything is in working order. If the battery is low or shows any signs of physical damage, replace it immediately.
>Before closing the case, inspect the gasket or O-ring seal for any nicks or other damage; replace it as necessary. Make certain both the O-ring and the groove in which it sits are clean and free of any sand, grit or other foreign particles. To ensure a proper seal, apply a light coat of lubrication (silicone grease) to the seal before closing the case, then check the seal by immersing the computer in water before your first dive. Also, check the wrist straps and buckles for signs of excessive wear. If they aren't in good shape, replace them before diving.
>If it's been a while since you've been diving, or if you have a new computer, review the functions of your dive computer to ensure familiarity. Know how to switch between various operating modes and display screens.
>Daily maintenance during a dive trip is also important. Rinse your computer in clean freshwater at the end of the dive day, if not after every dive. This prevents salt, corrosion and silt from interfering with the pressure sensors, buttons or obscuring the screen. Dry the computer, and store it in a safe location to prevent physical damage or loss.
>Bob Rossier is a longtime member of DAN and is a contributing editor to DiveTraining magazine. As a life-support systems engineer, he contributed to various NASA and U.S. Navy programs. He now specializes in training and safety for the marine and aerospace industries.
>© Alert Diver — Spring 2010