Testing Dive Computers

When it comes to choosing and using a dive computer, how can you tell which models are the most cautious and which the most liberal in terms of the deco advice they give?

The truth is, no one computer fits the bill under all circumstances: the computer that seems most conservative using one dive profile can become one of the more liberal models when using another.

England’s Diver magazine conducted a series of tests to investigate the characteristics of full-function decompression-stop diving computers. Many American computer makers and American publications say little about decompression diving, telling divers to stay within the socalled “recreational limits” of 130 feet and not to make decompression dives. However, anyone who has been on a live-aboard knows that divers often go far beyond that arbitrary limit.

Furthermore, every dive you do is a decompression dive, because you breathe air under increasing pressure as you go down, then breathe it under decreasing pressure as you rise. You're able to do a no-stop dive only if you limit yourself to a given time for a particular depth and then come up at a prescribed rate of ascent.

Computers are an essential piece of diving equipment. Manufacturers add peripheral features to give them a perceived “added value,” but the core function remains the computer’s ability to measure pressure (depth) against time and relate this measurement to “tissue models” to represent what might be happening to your body during a dive. It does this with a mathematical model — an algorithm — that calculates the absorption of nitrogen and the rate at which the nitrogen comes out of solution. This is measured in “halftimes,” a “halftime” being the length of time it takes for a particular tissue to become halfsaturated with nitrogen at an exponential rate at a particular ambient pressure. Fast-absorbing tissues, like blood, have about a four-minute halftime, while heavy bones have much longer ones, in the neighborhood of 480 minutes.

Once a diver has gone beyond the limits of no-stop diving, fullfunction, decompression-stop diving computers give information about a diver’s nitrogen-absorption status, including the length and number of pauses necessary during ascent on a stage-decompression dive. When it comes to the core function, we can distinguish six distinct groups among the models (see chart below).

In any given group, each computer shares the same algorithm or mathematical model. So for the purposes of this test, Diver compared a representative from each group on a series of dives. Ascent rates varied. Group B computers were set at 30 ft/min., whereas others had variable ascent rates from 60 ft./min. at depth to 20 ft./min. in the shallows. They ascended at the maximum rate allowed by the most conservative computer at any given moment of the dive. They used the same surface interval for all computers.

The Performances

First Dive: 120 feet: After 9 min. to 120 feet, all the computers were within 2 min. of the same no-stop time. At 10 minutes, the Group E computer showed no-stop time left and the Group F showed 1 min. The others still showed two minutes of nostop diving.

After 17 min., the Group F computer had become the least cautious with a 2 min. stop at 10 feet, but Group D was asking for a 1 min. stop at 20 feet and a total ascent time of 6 min.

By 23 min. we had risen to 70 feet. Group A was the least cautious, closely followed by Groups E and B. By 28 min. and 16 min., Group C became the least cautious.

After 34 min. at 20 feet, Group C still wanted an 11 min. ascent time, whereas Group F required only 2 min. At 20 feet after 39 min., Group F reverted to no-stop diving, while the others still required between 4 min. (Group E) and 8 min. (Group C) of decompression before being free to ascend to the surface. They all eventually reverted to no-stop diving.

Dives Two and Three (180 feet): After 8 min. at 180 feet, the Group B computer went into deco-diving mode and showed a total ascent time of 6 min. A minute later all the other computers went into the deco-diving mode.

Two minutes later, the order changed. Group C became the most conservative followed by Groups E, D, B, A and F. During a slow ascent, after 13 min. at 130 feet, Group D showed the most caution and Groups E, B, and C tied for second most cautious, followed by Group A and Group F.

Group D stayed the most cautious throughout the rest of the dive and Group F the least. The others completely reversed their order between 100 ft. (16 min.) and 55 feet (23 min.).

Hesitating at 42 ft. meant that Group F added deco time, while Group E became less cautious. Group F first reverted to no-stop diving after 36 min. (7.6 m.).

Here the order of caution was Group D, followed by C, B, A, E, and finally Group F, although it was the Group B computer that reverted to no-stop diving soonest after E and F.

This time Group D proved consistently the most cautious because, unlike the others, it refused to clear a required 10-foot stop by doing extra time in the 10 to 20-foot range. This pattern was repeated on another dive to 180 feet for 63 min.

Dive Four: This involved a quick dash to 180 feet and a leisurely ascent through the 100 to 30 foot range, completing the dive after 35 min. Again, Group F was least cautious and Group D most conservative.

The Conclusions

All of the computers compared (and their siblings) are equally useful for decompression-stop diving. One cannot make a decision about which is most conservative simply by comparing them during a dive at one way point alone.

Because a computer gives you a mathematician’s idea of what might be happening to your body using model tissues rather than your own blood, skin, and bones, using one is an act of faith. You have to believe what the computer tells you. And you have to realize there is no way of knowing how close you come, each time you dive, to a decompression illness.

When using a computer, it’s your choice whether you make additional shallow water stops or surface when your computer clears the last stop from its display and reverts to no-stop diving mode.

However, we suggest that if you have a Group F computer, add optional conservatism (up to 50 percent is available) before undertaking this sort of diving. Those using Group E computers might do well to use them in “condition hard” mode rather than lesscautious “condition normal.”

If you use a Group B computer and dive with a buddy who uses a Group C or D model, you might adapt the algorithm by selecting an “Altitude 1” setting rather than “Altitude 0.”

Group C computer-users should note that their devices are not as cautious as others when used for a single dive.

Group A computer users should take heed of the additional and optional 3 min. safety stop offered between 16 and 30 feet (ST3 to ST1) on no-stop dives and perhaps add a similar safety stop to deco-stop dives.

Group D computer users should be sure to reserve enough air for the lengthy stops at 10 feet that might be required and have the necessary buoyancy control at that depth to be able to do the stop comfortably.

No one knows what is right for you. If you are older, overweight, or unfit, or if you simply have not dived very much, there’s reason to add an element of caution when you use any computer.

A version of this article originally appeared in the June issue of Diver Magazine. Undercurrent takes all responsibility for additions and edits. We have converted the original data from meters to feet. To subscribe, contact Diver Magazine by telephone (011) 0181 943 4288; fax (011) 0181 943 4312; e-mail 100737.2226@compuserve.com.