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July 2002 Vol. 28, No. 7   RSS Feed for Undercurrent Issues
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The Backup Dive Computer

why leave home without it?

from the July, 2002 issue of Undercurrent   Subscribe Now

On one dive trip a few years ago, my Oceanic Prodigy computer flooded. On another trip my replacement’s battery failed. Both incidents happened at the end of my trips, but if they’d occurred midway through, I’d have been up a creek without a backup.

That’s because when I’m diving daily, my computer rarely clears to a de-saturated level, even overnight. So each dive must be measured as part of a series. Any break in the process and I no longer have accurate information to plan or monitor subsequent dives.

Most mainstream live-aboards and dive resorts rent computers, so replacements are generally available, but the rule of thumb is to wait a day to dive with a new computer. On a five-day dive package, that wait could cost me 20 percent of my bottom time.

Loss of dive time is just one issue. Let’s not forget that no matter what else it might do for you, a computer’s primary purpose is to keep you from getting bent. If you ever find yourself descending deeper than your dive plan, making more dives than you intended to, or even if you occasionally surface with your computer still in the “caution” zone, you’re flirting with decompression sickness. If you’re elderly, overweight, out of shape or don’t dive much, you’re even more susceptible to DCS. For all these reasons, you might want to consider the extra margin of safety a backup computer provides.

Who Uses a Backup?

We asked a few pros how they felt about backups. As in any discussion of dive gear, we got a wide range of opinions.

Tech diving maverick Bret Gilliam carries a spare computer in his luggage, but doesn’t dive with it, because, he says, “I haven’t had a computer failure since 1992.” In case of a malfunction, he says he’d just sit out 12 hours and use his backup. If you dive as often as Gilliam does, that might not seem like much of a sacrifice. But if you only make one or two dive trips a year, do you want to risk losing a full day of diving . . . especially on a liveaboard where there’s little else to do but listen to the other divers telling you what you’re missing? Carl Huggins, a pioneer of dive computer research who now runs the Catalina Hyperbaric Chamber, understandably takes a far more cautious approach than daredevil Gilliam. Huggins carries two computers plus an Aqualand watch. Yet, on repetitive dives he still uses tables.

Your backup should be as conservative as your
primary ... however, since manufacturers
consider their algorithms trade secrets,
comparing computers gets to be quite tricky

Many Undercurrent readers have reported incidents when a backup came in handy. Jesse Meyer’s (Pinole, CA) Citizen dive watch went into dive mode while his plane was descending into Cozumel, and didn’t revert to normal operation until his return flight. Nancy Balch’s (Louisville, CO) two-year-old Oceanic DataMax ProPlus also went haywire in Cozumel when the low battery warning started flashing — and kept flashing even after they replaced it!

But Jeffrey Reed’s (San Francisco, CA) sad tale of woe tops them both. In 1996, his one-yearold hoseless, air integrated Cochran computer died. The screen, says Reed, “would just go blank or refuse to leave surface mode at 20 or 30 feet.” Cochran replaced it with a Nemesis Pro that locked up on the third dive and wouldn’t unlock. After finally getting the Nemesis Pro repaired, Reed decided to use it as a backup and bought a ScubaPro hoseless, air integrated, nitrox-ready model. Reed recalls, “I took it to Bonaire and it also locked up without a way to unlock it (no, I didn’t violate its parameters, it just locked up). I returned it to the shop where I purchased it and they were unable, even with a PC, to unlock it. I traded for a hose-mounted, air- integrated Suunto. It works fine but I missed my wrist-mounted readouts. So when I went to Roatan last December, I used my old Nemesis Pro without a problem. Now I have an $800 Suunto as a backup to an old Cochran that has decided, late in its life, that it wants to be a reliable piece of equipment. Too weird!”

Unlike Reed, divers typically turn their older computers into backups when they buy replacements with more bells and whistles.

Which Backup to Buy?

Yet choosing a backup computer properly should involve more thought than that. For one thing, some computers are far more conservative than others when calculating decompression needs. That’s a function of the computer’s built-in algorithm, a mathematical model that uses a series of assumptions to approximate the absorption of nitrogen in various body tissues and the rate at which it comes out of solution. To play it safe, your backup should be at least as conservative as your primary. However, since most manufacturers consider their algorithms trade secrets, and way too complex to share with consumers, comparing them gets to be quite tricky.

Mike Ward, who operates Dive Lab in Panama City, FL, calls it “a big black science.” Usually, computers from a single manufacturer tend to use the same algorithm across the product range. But algorithms can be tweaked. Doug Krause, life support product manager for Oceanic, told Undercurrent that Oceanic’s algorithms have been modified over the years for new models and newer generations of older models. Krause concedes that “Oceanic’s owner’s guide is vague on algorithms.”

Jack Kuhn, proprietor of Harbor Dive Center in Sausalito, CA, recommends buying two identical computers, one as your primary and one for backup. Brett Gilliam agrees, adding, “That way you’ll be working with similar displays, too.”

Buying two new computers at once may be over your budget. Or, you may want different things from your primary (such as air integration) than from your backup. Your next best bet is to stay with the same brand, and test the computers against one another. Usually that means diving with both and comparing results, but Mike Ward points out that some dive shops have small recompression chambers for testing depth gauges, which can be used to compare 3-4 computers at once. He suggests that you note their readings every 10 feet down and up and chart any differences.

A couple of studies have grouped computers with similar performance characteristics. In the October 1999 Undercurrent (available online at Undercurrent) we ran a comparison chart from England’s Diver Magazine. You can also see a more recent “Freedom/Risk” chart on the Rodale’s website: But these charts are not precise. For instance, two computers in the same general group may provide different readings at different depths.

If your backup allows you to program certain parameters into your algorithm (such as altitude), you can probably adjust it to be no less aggressive than your primary. Meanwhile, there are other features you might want to look for (or avoid).

Size. On most dives, you won’t look at your backup, except perhaps to compare readings against your primary. (At last! Something constructive to do on a safety stop!) So you’ll probably want your backup safely tucked away in your BC pocket. Reader Clayton Fuller (Chula Vista, CA) straps his to the handle of his videocam housing, so he can check his depth while he’s taping. Others attach them to low pressure hoses, or wear them on their wrists. Wherever you keep yours, be sure it doesn’t add to snag problems underwater. Smaller watch-style models like Suunto’s Stinger and Mosquito keep a low profile and remain with you at all times.

Immersion switch. You don’t want to be fumbling for two manual activation buttons at the start of each dive, so be sure your backup is water-activated.

Readability. Displays should be big and bright enough so you can read them with a mask on, underwater, and in dim light.

User-Changeable Batteries. Reader Leo Dioguardi (Apollo, FL) changes batteries between his primary and backup at different times, so they’re less likely to go out at once. However, changing a battery mid-trip could wipe out the computer’s memory, as reader Mike Mullins (Woburn, MA) found out with his Oceanic DataMax ProPlus. So some divers change batteries before each trip.

Sonic Alarms: You don’t want this sort of intrusion from a backup. Some models allow you to turn sonic alarms off. Another nono is the lockout feature, when the computer stops providing decompression information once you’ve violated certain preset parameters. As reader Richard Jorgensen (Bradenton, FL) puts it, “this lockout thing is like having your car quit running after you exceed the speed limit.” In fact, one of the main uses readers cited for using backups was being locked out by their primary computers for correctable violations such as ascending too fast.

When traveling, you might want to pack your backup in a separate bag from your other dive gear — maybe in your carry-on. However airport security personnel may delay your boarding as they check it out or even insist that they check it for you and deliver it after your flight. That’s the only way, however, to protect yourself if your primary dive bag gets lost or delayed.

Table that Motion

If all this seems a bit too much, NAUI may have a low-tech solution for you. The venerable training agency has released a new, simplified, single table for no-decompression recreational diving. NAUI calls it a “No Calculation” table and claims it will make repetitive dive planning extremely easy and straightforward. According to Tim O’Leary, Director of NAUI Technical Operations, the new table “doesn’t rely on multiple tables involving variable surface intervals and residual nitrogen times. One table does it all.” No bells, no whistles, but it could get you back in the water. . . even if your computer crashes.

— Ben Davison

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