EXTRA June 1999 DOES YOUR COMPUTER RUN AT YOUR SPEED? COMPARISON TABLE

We did it before, now we've done it again - but even better! John Bantin carries out comprehensive tests on diving computers representing every deco-stop model on the market. His mission: to find out if they are angels, devils or simply doing their job

GROUP A Mares Air Lab Mares Guardian Mares Surveyor Mares Tutor

GROUP B Suunto Eon Suunto Favor Suunto Solution Alpha Suunto Solution Nitrox Suunto Spyder ACW Suunto Viper

GROUP C Aladin Air Aladin Air X Aladin Air X Nitrox Aladin Pro Aladin Pro Nitrox Mares Genius Monitor 3

GROUP D Dive Rite B'Air Dive Rite Bridge II Dive Rite Nitec Dive Rite Nitec 3

GROUP E Dacor Sportster Hydrotech Data Ocean Reef Plus O1 Orca Pilot & Pilot Audio Orca Pilot Audio Nitrox Orca Pilot Nitrox UBS Chameleon Zeagle Status II

GROUP F Cochran Aquanox Cochran Commander + Cochran Commander Nitrox Cochran Nemesis + Cochran Nemesis Nitrox IIa

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 user?
The answer is: "With great difficulty." So Diver decided to find out the facts.
Equipped with an armful of diving computers, we carried out a series of carefully planned and monitored dives to see how various models performed. The tests described here rate the characteristics of each type of full-function decompression-stop diving computer against the others. Just about every model available in Britain is represented.

The Computers
Compared to a modern desktop PC, the diving computer is a simple affair, though it is not long since it was regarded as something of a marvel. Certainly it is impressive compared with the crude tables which it has largely replaced as a means of controlling decompression on dives. The low cost of some models rivals that of the old-technology watch-and-depth-gauge combination.
Manufacturers might have added peripheral features to their products to give them a perceived "added value", but the core function always remains the computer's ability to measure pressure (depth) against time, and relate this to a number of "tissue models" to represent what might be happening to your body during a dive. It does this with a mathematical model - an algorithm.
We absorb the inert part of the gas we breathe, the nitrogen, so we need to calculate that absorption, and the rate at which the nitrogen comes back out of solution.
But your body is complex, so the tissue models vary. A "half-time" indicates how long a particular tissue takes to become half-saturated with nitrogen at an exponential rate at a particular ambient pressure. A four-minute half-time represents fast-to-absorb tissues like the blood; much longer ones, such as 480 minutes, represent the heavy bones.
Every dive you do is a decompression dive, because you breathe gas (usually air) under increasing pressure as you go down and breathe it under decreasing pressure as you come up. Only if you limit yourself to a given time for a particular depth, and come up at a prescribed rate of ascent, are you able to do a no-stop dive.
Most leisure divers in warm conditions limit themselves to no-stop conditions and a wide range of computers are suitable for this type of diving. Some are purpose-designed (notably many popular US products), while others have been detuned by their manufacturers to provide an entry-level product at the lower end of the price scale.
Full-function decompression-stop diving computers can give information about a diver's nitrogen-absorption status, including what stage-decompressions (pauses during an ascent) are necessary once he has gone beyond the limits of no-stop diving. Northern sea conditions tend to limit shallow-water diving, so most European divers want full-function decom-pression-stop machines.
These computers are available in many different forms, including those able to integrate with air-supplies, to match to a breathing mix other than air (nitrox), or even to double as an everyday watch. However, when it comes to the core function, among the plethora of models available we can distinguish six distinct groups.
Within a given group each computer shares the same algorithm or mathematical model. So for the purposes of this test we were able to compare a representative from each group on a series of dives. These occurred during a dive trip on which repetitive diving would have rendered many tables impractical or just too limiting - that is, a typical trip.
The different computers are referred to as Groups A to F. If yours is not listed under any of these groups, it is probably either a very old model or one designed for no-stop diving only.
One UK distributor of a US brand wrote to us to say that its computer was "not sold as a deco-stop diving computer" but was "suitable for dives to 99.5m", implying that it was possible to do a no-stop dive to this depth! Putting aside other more relevant dangers such as oxygen toxicity encountered at such depths, we challenge the practicality of making a descent and safe ascent within the required time limits.
Be careful of claims made in advertising material. The computer might do the dive OK, but you might not! That particular brand is omitted from our listing.

The Tests
We made the dives at the Giftun Islands, outside Hurghada in Egypt. I was accompanied on each test dive by Jan Ellingsen, former skipper of mv Poseidon's Quest, mv Colona IV and my Colona 2 when they were operated on Red Sea safaris. Jan is the very experienced pioneering diver who discovered and named Pfeiffer's Reef in the Sudan.
We did two dives a day for four days. In each case the surface interval was more than two hours and less than three between the first and second dives of the day. The first dive was a very deep one (37m on the first day followed by first dives of 50m-plus thereafter). Each second dive was limited to a depth that allowed no-stop diving for at least 45 minutes with all the computers - usually 25m or less.
Naturally there was a lot of information to assimilate from all six displays, and this changed from moment to moment. To make accurate notes we photographed all the computer displays together during each dive, every time one of them reached a critical waypoint. These photographs were available for close analysis later.
By the second day, each morning dive was commenced with a surface interval of more than 19 hours since the previous dive. Each computer had to make its calculations allowing for "repeat" diving. The computers were each set for air (where there was a choice) and we followed a dive-plan for air, but used twin 12-litre cylinders and breathed nitrox 32 (consistently supplied at 31.9 per cent oxygen by Emperor Divers) once we were shallower than 36m. This added extra safety to our dives.
All the computers were impeccably synchronised by time but there were slight, if inconsequential, differences in the depth displays. Some computers automatically recognise the salinity of the water, whereas others are fixed for sea or fresh water. For continuity in the commentary, we always used the same computer to describe the depth.
Many of the computers had the facility to add an element of personal caution to the decompression required. We used them all at the factory setting, although our results might persuade you to do otherwise with any computer from a particular group (see chart, page 26-27).
Ascent-rates varied. Group B computers were set at 10m/min, whereas others had variable ascent-rates from 18m/min at depth to 6m/min in the shallows. We always ascended at the maximum rate allowed by the most conservative computer at any given moment of the dive.
The first dive of the series saw the computers in effect jostling for position, and by the second day we were noticing a pattern in the way some of them performed. But at first it seemed that no clear picture would emerge as to which would prove the most cautious.

The Performances
After 9min on the very first dive to 37m (see graph below), all the computers were within 2min of the same no-stop time. Without moving from that depth, after 10min the most cautious was the Group E computer with zero no-stop time left, followed by Group F with 1min. The others still showed two minutes of no-stop diving.
We stayed at 37m for 17min, by which time the Group F computer had become the least cautious with a 2min stop at 3m, but Group D was asking for a 1min stop at 6m and a total ascent time of 6min.
By 23min into the dive we were up at 22m and Group A looked least cautious, closely followed by Groups E and B. By 28min and 16m it was Group C that looked to demand the most in the way of decompression stops.
With 34min gone and at 9m, Group C still wanted an 11min ascent time, whereas Group F required only 2min. After 39min and at 6m Group F reverted to no-stop diving, while the others still required between 4min (Group E) and 8min (Group C) of decompression before we would have been free to ascend to the surface.
Staying at 6m the other computers reverted to no-stop diving, so would have allowed us to finish the dive, in the following order: Group E - 46min; Group B - 48min; Group A - 51min; Group D - 52min; Group C - 54min.
The deep dive on Day Two saw us momentarily descend to 57m (see above), although we quickly regained the previously agreed maximum depth point of 55m.
This time, after staying there for 8min, it was the Group B computer that first went into deco-diving mode and showed a total ascent time of 6min. This was closely followed a minute later by all the other computers.
However, the running order was completely changed by ten minutes of diving, with Group C being most conservative followed by Groups E, D, B, A and lastly F in that order. During a slow ascent, after 13min and at 40m, it was Group D which showed most caution, followed by Groups E, B and C together, A and lastly Group F.
Group D stayed consistently the most cautious throughout the rest of the dive and Group F the least. The others completely reversed their running order between 30m (16min) and 17m (23min).
Hesitating at 13m meant that Group F added deco time, while Group E became less cautious instead. It was still Group F that first reverted to no-stop diving after 36min (7.6m).
At this moment 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 it was the computer from Group D that proved consistently the most cautious because, unlike the others, it seemed to refuse to clear a required 3m stop by doing extra time in the 9-6m range. We finally broke the surface after 68min under water.
This pattern was repeated on the similar profile deep dive (54.3m/63min) on Day Three (bottom left). The deep dive on Day Four (below) involved a quick dash to the maximum depth and a leisurely ascent through the 30m to 10m depth range. This allowed us to complete the dive after only 35min duration. Again Group F was least cautious and Group D most conservative.

The Test Dives

Day	Depth (m)	Time (min)	Surface Interval
1(am)	37	58	-
1(pm)	27.7	62	2hr 10min
2(am)	57.6	68	19hr 30min
2(pm)	22.2	45	2hr 40min
3(am)	54.3	63	19hr 26min
3(pm)	23	54	2hr 20min
4(am)	53.7	35	19hr 20min
4(pm)	18.9	45	2hr 50min

The Conclusions
What did we learn from analysis of the photographs of the displays from all the computers on all the dives?
First, all of the computers we compared (and their siblings) are equally useful for decompression-stop diving and one cannot make a decision as to which is most conservative simply by comparing them during a dive at one waypoint alone.
Remember, a diving computer is not calculating exactly what is happening to your body during a dive. It has no way of knowing your physiology. What it does, is give you a mathematician's idea of what might be happening using "model" tissues, rather than your own!
So using a computer requires an act of faith on your part. You have to believe in what the computer tells you and there is no way of knowing how close you come, each time you dive, to a decompression illness.
We added extra safety by switching to nitrox 32 during the final 36m of our ascent. You can make additional shallow water stops or surface as soon as the computer you are using clears off the last stop from its display and reverts to no-stop diving mode. The choice is yours.
However, we suggest those of you with Group F computers add some optional conservatism (up to 50 per cent is available) before undertaking this sort of diving. Similarly, those using Group E computers might do well to use them in "condition hard" mode rather than less cautious "condition normal".
If you use a Group B computer and are diving with a buddy who uses a Group C or D model, you might choose to adapt the algorithm by selecting an "Altitude 1" setting rather than "Altitude 0". Group C computer-users should bear in mind that their machines are not as cautious as some others when used for a single dive as opposed to one in a series.
Group A computer users should take heed of the additional and optional 3min safety stop offered between 5m and 3m (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 3m which might be required, and have the necessary buoyancy control at that depth to be able to do the stop comfortably.
These are our recommendations based on the experience of this and other tests and "gut" feeling. No one knows what is right for you. If you are elderly, overweight or unfit, or simply have not dived very much, these are all reasons to add an element of caution when you use any computer.
No computer guarantees to keep you free from decompression illness and not one of their manufacturers recommends dives that require decompression stops. So, dive within your limitations and always err on the side of caution. Safe diving!

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FLYING AFTER DIVING
Certain diving computers display a shortened interval for when it is "safe to fly", compared with the total desaturation time. This is based on certain assumptions by the algorithm writer regarding the aircraft cabin pressure, which might not be correct at the time the aircraft takes off.
Other computers simply count down from 24 hours after the last dive. This often confuses people, because the "time to fly" might be more than the displayed total desaturation time. Some computers simply display expired surface interval and recommend that users wait at least 24 hours before flying.
Current medical thinking suggests waiting 24 hours after diving before flying or waiting 48 hours after a decompression-stop dive.

COMPARISON TABLE

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Appeared in DIVER - June 1999

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