Is Your Old Computer Still Safe?

The algorithm, the mathematical calculation a dive computer uses to make real-time data measurements on time, depth and gas mix, cannot be seen behind the glass counter in the store, but it's the most important thing to understand about your computer. Just as the launch of a spacecraft is a spectacular thing to see, it's the navigation algorithm that sees the crew safely to its destination and back again to Earth. The same can be said of your dive computer's algorithm -- it's designed to keep you from getting hit with decompression sickness (DCS).

As we age, our bodies change, so it could be the dive computer you bought at age 55 may not have the more conservative algorithm suitable for you at age 70. So, how do you know if it's time to retire that old computer? And how do you decide which is the right one to purchase next?

The First Questions to Ask

To evaluate your current dive computer, ask these questions: Can it be set for nitrox? Does it allow you to set degrees of caution? Does it tell you how long your air will last? Can you read and understand the display easily?

If the answer to any of these questions is no, dump that anachronism and get a modern, more suitable computer. Your tired eyes will also enjoy your new computer's clearer display; many computers now have super-sharp dot matrix systems. If you choose one with an organic light-emitting diode (OLED) colored display, its main figures will change from green to amber if caution is required, and then to red if you've really outstayed your welcome. At night, and in poor visibility, such an illuminated display can be a godsend.

Aside from that feature, don't be misled by the shiny perfunctory knobs and buttons offered by manufacturers as sales bait. You need to ask about the algorithm and understand its ability to return you safely to the surface. After all, what good is a clearly visible display if you don't understand what it's telling you? Buying a diving computer without having a perfunctory understanding of what it does is buying blind.

So it's worth understanding a little bit about the development of the algorithm and diving decompression tables.

A Short, Important History of How Your Computer Works

Way back in 1908, a Scottish physiologist named John Scott Haldane was commissioned by the British Royal Navy to prepare the first proper decompression table. He based this on extensive experimentation with DCS in goats, saturating them with nitrogen to depths of 165 feet. Since then, other physiologists have modified Haldane's discoveries to try to theoretically improve things, but, by and large, they're still using the same basic information.

In the 1960s, Albert Bühlmann, a professor at the University of Zurich, came up with a decompression algorithm for use in an early Uwatec computer, and since it's now freely available in the public domain, most computer manufacturers use a modified version of that. (Look for the nomenclature "Bühlmann ZH-16" with varying suffixes in computer specs.)

Here in the U.S., Drs. Ray Rogers and Michael Powell designed the PADI recreational dive planner, and their work was then turned into the DSAT algorithm for no-deco-stop dives to a depth of less than 100 feet. This proved unsuitable for European divers, who habitually go deeper, so later Americanmade computers came with dual-algorithms -- DSAT and Pelagic+, a derivation of the ZH-16. It is important to know which of these algorithms your computer is set to.

There was a time when divers made a single dive or maybe two in a day, but the modern traveling diver now may make up to six dives in a day. With short surface intervals, an allowance should be made for residual nitrogen levels from previous dives.

The American physiologist Bruce Wienke came up with an algorithm taking into account asymptomatic microbubbles that may be present in a diver but will most likely be added to, making them larger and symptomatic, on a second dive. Many modern computer manufacturers have bought into Wienke's work, most notably, Suunto, Cressi, Atomic and Mares. Some divers complain about the punitive deco stops mandated on subsequent dives, but that is the algorithm writer attempting to keep you safe from harm. Older divers, with less efficient hearts, lungs and circulatory systems, are more vulnerable to an otherwise unwarranted attack of DCS on repeat dives. Driven by the competition, other manufacturers, particularly Scubapro, have added adaptations to the ZH algorithm to account for these microbubbles.

The problem with all of this is that no algorithm writer can write one specifically tailored for you. It's all based on hypothesis and Haldane's original research from more than 100 years ago. Not enough divers get bent to provide sufficient data, so computer manufacturers tend to err on the side of safety, while insisting at the same time that none of their products can protect you from getting injured by DCS.

Then Dick Rutowski, formerly the deputy diving coordinator for the National Oceanic and Atmospheric Administration, proposed that leisure divers could be safer if they reduced the amount of nitrogen they breathed by increasing the percentage of oxygen -- i.e., breathing nitrox. At first, Rutkowski was pilloried for his suggestion, but now nitrox use is commonplace among recreational divers. It still amazes me that so many older divers cling to the use of air, but remember that it's actually nitrox 21 because it contains 21 percent oxygen.

Setting Your Computer Straight

Modern computers can all be set for nitrox, and if you are getting longer in the tooth, you should certainly breathe that. However, setting the computer to match the nitrox mix does not add safety, it merely increases your no-deco-stop diving time. If you want to add safety, either set a less oxygen-rich mix or add a level of caution, which most modern computers allow you to do.

You might set your computer to air (nitrox 21) when using nitrox 32 -- always bearing in mind the maximum operating depth of the mix you are actually breathing. In this way, it calculates for a higher level of nitrogen absorption. Many divers just use the computer straight out of the box at its factory settings. Wrong!

It's always worth reading the manual. For example, if you buy a Scubapro computer for its microbubble settings but leave it set at MB0, you are not allowing for any micro bubbles whatsoever. Set it at a micro-bubble setting (MB1 To MB3). Other dive computers allow you to set Safety Factors (SF) or Gradient Factors that, provided you follow your computer's advice on the way up, decrease the calculated rate at which you off-gas the nitrogen that you've absorbed during the dive, making your ascent slower.

Many computers can be operated in conjunction with a transmitter that plugs into your regulator first-stage. If you think this sounds too complicated, let me offer you an appropriate analogy: There was a time when we only had a gas gauge to go by in our cars. Nowadays when you drive, which do you refer to first on the dashboard, the gas gauge or the "remaining miles left" indicator? It's the same with gas-integrated computers. They tell you not only the pressure of gas in your tank, but how long it will last you at the depth you're at considering the rate at which you've been breathing. Like the miles-left indicator, you'll soon get used to watching the remaining air time. Keep that longer than the remaining no-stop time and you shouldn't get into trouble.

Modern diving computers can be adjusted to accommodate the fact that we are not as fit as we were. They now give better information regarding nitrogen absorption, especially considering that we now typically make repetitive dives, and that info is displayed in a much-improved way.

The mantra still applies: There are old divers and bold divers, but few old, bold divers. Be one of the latter by evaluating your current computer, and if it doesn't meet the criteria I listed above, then be smart enough to know what facets are required in the new one you'll buy -- and understand how it works.

-- John Bantin