"AREN'T YOU WORRIED ABOUT GETTING BENT?"
It's a question frequently asked of me by my non-diving friends. I couldn't say it's a question that never crosses my mind. However, so far as leisure diving goes, the days of poring over tables, checking and cross-checking the calculations, making a plan that is invariably the first casualty of the dive and then having to fathom out a new one while under the influence of high-pressure nitrogen seem to have gone forever.
I still recall, not so long ago, waiting religiously for three hours to pass, never a minute less, in order to have a second dive, and still being anxious about what the consequences might be of cutting it so fine.
When I went on my first liveaboard trip, I was horrified to see my highly qualified companion, a BSAC First Class diver no less, discard the hallowed BSAC table in favour of the PADI Recreational Dive Planner because, as he put it: "We'd only be able to make one dive each day with BSAC, but PADI lets us do three!"
Of course, the risks of getting bent during a dive today are as real as ever. It's just that instead of relying on a crude set of figures, and the foibles of watch and mechanical depth gauge, plus an ability to set both immediately before submerging, we have the rock-solid evidence of a pressure sensor connected to the precision of a time-counting electronic chip, combined with the calculating abilities of Einstein, all inside an instrument that is often as small as a wristwatch.
The risks have if anything become greater, because in the days of the one-dive-a-day tables we kept well away from the edge - if we were sensible. However, one could say that the worry has been removed because the calculations are now so precise.
Back in the early Ô80s, I joined some guys from Harrow BSAC to dive the wreck of the Maine in the English Channel. "We don't mind you using your computer," they told me, "but we'll be diving on tables and you must come up when we do."
We planned a dive to 30m for 20 minutes. It was to be a no-stop dive at the edge of what was allowable on the BSAC tables. During the ascent, they were intrigued to see that I insisted on making a stop. "We thought computers were supposed to give you more time," they said afterwards.
My computer had told me that we had actually gone to 33m for 22 minutes. It was the difference between our abilities to record the depth and time accurately, I with a computer and they with watch and depth-gauge. So that's the first thing a computer gives us - the ability to record the dive properly. That has removed one layer of worry.
Most tables assume that you spend all your dive at the greatest depth. Computers allow more complicated calculations, including the time spent at all the different depths to which you might go during a dive.
Another advantage of a computer is that it can monitor your ascent rate accurately. Not so long ago, divers were taught to ascend no faster than the smallest exhaled bubble. The problem was that you picked a bubble which grew larger as the pressure on it decreased, and so it displaced more water and accelerated as it went up - exactly what you should not be doing yourself.
So you constantly had to look for a smaller bubble to which to transfer your attentions. It was not very precise. Older readers might remember "Norman's Balls". We'll draw a veil over that subject; suffice to say that Norman rues the day that he let just about anyone play with them.
Today's computers warn you if you exceed a prescribed ascent-rate, and although they might vary from model to model, they all make us go up a great deal slower than those ball and bubble-followers of the past.
Then there is the question of the "repeat dive". The limitations of the tides and slack water around our coastline means that this was never really a problem for divers in British home waters. However, get abroad to places where you can dive as often as you like, and keeping track of your decompression status becomes a real problem.
Nowhere is this more apparent than on a typical liveaboard diving holiday on which there is hardly anything to do but dive, dive, dive.
You might have learnt to dive in the UK and got by without a computer, but embark on one of these trips without one and you'll either be left behind or, a far bigger problem, be diving by the seat of your pants. Without a computer, it is difficult to keep track of repetitive dives.
We are constantly advised to do our deepest dive first, but this is an idea that has recently been discredited in some quarters. When you start a series of dives on a liveaboard trip, inevitably the check-out dive is not very deep. From then on, your computer rarely clears to a desaturated level, so each dive becomes part of a series - even if you've been to bed during the surface interval!
It becomes inevitable that some dives will be deeper than the ones preceding them. Computers seem to be able to cope with this. It's all taken care of by the algorithm, the mathematical calculation used within the computer's program.
So is the information computers give regarding no-stop times and deco-stops required safe? It would seem so, for the great majority of people in most circumstances. This is indicated by the millions of man-dives made every year without apparent ill-effects. However computers do differ markedly in the information they provide. So although they can all be said to be "safe", some are obviously "safer" than others.
All computers from a single manufacturer tend to use the same algorithm across the product range. Rather than discuss the theoretical differences between them, we took an example of each make of computer available on the UK market and dived them side-by-side during a one-week liveaboard trip. We could then compare the information given. Each dive, at any given moment, was dictated by the most cautious example.
The computers were the DiveRite Nitek3, Mares Surveyor, Cochran Commander, Uwatec Aladin, Beuchat CX2000, Buddy Nexus, Suunto Vyper and the Suunto Solution (an obsolete model with an older algorithm, but very popular with readers). In each case we used them adjusted for air (nitrox 21).
We suspect that the DiveRite has the same algorithm as other computers made by Seiko. The Buddy Nexus, set for open-circuit, represented a wide range of computers made by Benemec under different brand-names, including the Orca Pilot.
Missing from the line-up were the new Cressi Archimedes computer and a new computer from Oceanic, the Versa, because they were not available at the time of the test.
It was too difficult to remember in detail how each computer performed, so we photographed them at critical moments during each dive and were able to examine the pictures of all the displays side-by-side at our leisure. We picked a representative selection to show here.
We must emphasise that the results are representative only of the dives we did during that one-week period. Under entirely different circumstances they might well have been different. What we offer you is the result of this experiment. I believe that what this information does do is provide me with an idea of which computer I would prefer to use!
I went with Chris Boardman, the former world-champion cyclist, on mv Salma, a new acquisition of Guido Sherif, owner of mv Coral Queen. During a typical week in the Fury Shoals and St John's Reef area of the southern Egyptian Red Sea, we undertook 21 dives, each of around an hour's duration.
Maximum depths varied between 53m and 10m. We did the deepest early in the morning, and progressively shallower dives during the day. All the dives of the week were part of a "series".
A typical day, the fifth day, began with a 6.30 start and a dive of 51min to 52.4m (Aladin) at Erg Abu Diab. After breakfast, we made our second dive to 28.9m with a total duration of 42min on the wreck of the Atlas, a WW2 tanker at Ras Banas.
An early lunch was followed by a dive of 63min and a maximum depth of 21.3m at the Fury Shoals Gardens.
Finally we made a 47min dive (no 18) at Sha'ab Matsur to a maximum depth of 21.9m. The dives were typical of the sort made by other passengers on the boat using a single tank, although we used independent twins so that our test was not compromised by any air-supply problems.
For example, on dive 19 (day 6) at Sha'ab Matsur, we headed down over the end of the plateau to the deep water where the sharks were. We spotted several grey reefs, a hammerhead and a thresher out in the blue.
After a few minutes, and once they had been scared off by the sudden presence of a boat-load of divers, we made our way back on top of the plateau and towards the reef. The quickening current caused by water squeezing over the tongue of the plateau gave us a free ride. We carried on in shallower water down the southern, sunny part of the wall that makes up the main reef.
Here we watched a large green turtle leisurely browsing for its breakfast and thought of our own. It was still only seven in the morning. It's when your stomach rumbles that deco-stops seem to take forever to clear!
A "no-stop" dive is just that, so comparing no-stop times is less than ideal. If you do a one-hour dive without a deco-stop, what's the difference between a minimum of one minute of no-stop diving remaining on a computer, and one and a half hours?
For this reason, we deliberately tried to go deep enough and long enough for the computers to switch into deco-stop diving. We then saw how quickly each one would switch back out of deco-mode and let us regain the surface. Of course, we were limited to the information displayed on the most cautious computer.
ALADIN (Scubapro/Uwatec) 012560812636
BEUCHAT (France) 0033 491 09 44 60
BUDDY (AP Valves) 01326 561040
COCHRAN (USA) 001 972 644 6284
CRESSI-SUB UK 01484 310130
DIVE RITE (Sea & Sea) 01803 663012
MARES (Blandford Sub-Aqua) 01923 801572
OCEANIC SW 01404 891819
SUUNTO DIVING UK 01420 587272
At first we used all the computers set as they had come from the manufacturer, in the belief that that is how most divers will come to use them.
However, there were some vast differences in what they told us during the dives. Some were indicating mandatory deco stops at times when others still indicated more than an hour of no-stop time remaining. At this time the DiveRite was clearly the most cautious, closely followed by the Suunto Vyper. There was a large gap between them, the Buddy, the Mares and the Uwatec Aladin.
The Beuchat, with an algorithm that added in caution tailored according to each individual dive, varied from dive to dive. The Suunto Solution sat resolutely in between, while the Cochran Commander seemed positively cavalier in the amount of no-stop time it offered.
We always ascended at the slowest rate indicated and this proved to vary between the Suunto Solution at depth and the Aladin in the shallows.
After the first couple of days we decided to reduce the disparity between the computers by making in-the-field adjustments to try to bring them more into line. We added user-adjustable degrees of caution to some where we could, and recommend this to anyone using these computers.
DiveRite Nitek3. This is a complex three-nitrox-mix computer but we used it at one mix and set that for nitrox 21. It almost always proved to be the most cautious instrument.
This was because it did not seem to shed the last minute of a mandatory 3m stop unless it was taken very close to that depth. Others would clear a stop at any depth shallower than 9m or 6m.
On one occasion we accidentally "bent" it and had to remove the battery (and replace it) to continue using it. I hadn't spotted that it had not cleared off the last stop despite waiting an extra five minutes at less than 5m for the Vyper, but that was obviously not shallow enough for the Nitek3.
It's interesting to note that had this been the only computer I was using, I might have felt very differently about missing this stop. I was consoled by the fact that the other seven computers on the rig (plus the two I was actually wearing) raised no such fear of impending DCI symptoms.
Back on board, we found that the instruction manual needed the intuition of a woman to allow us to understand how to reset the computer after replacing the battery. The logically thinking men among the other passengers got nowhere! Other DiveRite computers include the Nitek and Nitek C.
Mares Surveyor. This computer often went back to no-stop diving before the others, but then pulled itself back into line with an optional safety stop (indicated as "St3") once it got into the shallows. We would suggest that users treat this as mandatory.
Strangely, after one particular deep dive with mandatory stops, it decided not to display the optional safety stop. On this occasion it was one of the less cautious computers in our collection.
Its display, in common with that of the Mares Tutor and long-awaited Mares Air Lab, was clear and easy to read, with good illumination, and the two-button system made it easy to set functions between dives.
Cochran Commander. Designed by the man who is said to have invented the original pocket-calculator, this is a sophisticated machine that is set up by means of a combination of tap-switch, wet-finger contacts, and circuit-making using a metal object between its contacts.
It took two divers' brains working together to do it on board the boat, following rather complicated instructions. If you set it up previously at home with your PC and interface, many other options are available. We started with 25 per cent but ended up putting in a 50 per cent "caution factor" (the maximum setting) but it still seemed very incautious.
It would be one of the first to indicate a mandatory stop at, say, 50m, but this would always clear by the time we were back at, say, 30m and then an eternity of no-stop time was always indicated. It seemed very out-of-step with the others. Other Cochran computers include the Nemesis and the Navy.
Uwatec Aladin Pro Ultra. This computer used to be the one by which others were judged and represents a whole raft of siblings including the Aladin Air and Air X. It has a sophisticated algorithm which seems a little less cautious than it might otherwise be only in comparison with today's alternatives.
The manual suggests always making an additional safety stop in the 3-5m zone of 1-3min once the computer's mandatory stops have cleared.
The well-illuminated display we still consider to be the most easily read and least likely to be misunderstood. What we didn't like was the short amount of time-to-fly this computer suggested. We waited more than 24 hours before we took off.
Beuchat CX2000. With an algorithm developed by Comex, this seems very "French" in the way it is set up. We eventually decided to preset it on program No 2, for "harsh" conditions. Sometimes it was the most demanding computer with mandatory stops, and at others it seemed very incautious by comparison. It all depended on the individual dive, and the CX2000 seemed to have a will of its own in this matter. However, it was never very much out-of-step with the majority.
Buddy Nexus. This seemed incautious until we set it for "harder" diving conditions. Then it pulled very much in line with the Suunto Vyper. We felt the display was too small, with some of the figures too hard to read.
Other similar Benemec computers include the Dacor Sport, Orca Pilot and Zeagle. Of these, only the Nexus can be used also for constant ppO2 closed-circuit rebreather diving. With all the options available, it is rather tedious to have to set it up using a wet-finger sequence.
Suunto Vyper. Set for all the least cautious options, this computer was almost invariably still the most cautious of them all. This was because it added in a 3min safety stop to mandatory deco stops. It was only because the Nitek3 would not clear its last minute at 3m that it could claim to be "safer". The Vyper seemed to be the one we always had to wait for and this was sometimes in the order of four or five minutes extra at between 6m and 3m.
Other similar computers include the Cobra, Mosquito and Stinger. Menu-driven, with press buttons, it is easy to access all its features and has a well-illuminated display.
Suunto Solution. Many people know and trust the Suunto Solution but we must confide that it's beginning to show its age. We suggest that if you use one you do so at the first altitude setting when at sea-level. It was less cautious than the Uwatec Aladin and looked "positively dangerous" in comparison with the Vyper, equipped with Suunto's latest Reduced Gradient Bubble Model (RGBM) algorithm.
That said, many people have used Solutions without incident. Other similar computers include the Eon and Companion.
All the computers were very precise with timing, but not exact with depth measurement. For the sake of simplicity, we used the displayed depths of the Aladin in the tables.
Chart of Dive No 8 - Day 3 - Vasstu Wall
Chart of Dive No 12 - Day 4 - Valley of Kings
Chart of Dive No 15 - Day 5 - Erg Abu Diab
Chart of Dive No 19 - Day 6 - Sha'ab Matsur
This chart (Dive 19) shows a typical dive at the end of the week, with the read-outs from each of the computers tested. The columns A-H refer to the algorithm group represented by each model and in three cases on this dive the computer settings were deliberately adjusted in an attempt to bring them into line, as shown at the top of the table. The read-outs have been standardised for ease of comparison. The depth/time figures represent the deco-stop required, NST is no-stop time and TAT the total ascent time. The Suunto Vyper (Group G) and Mares (Group G) indicate an optional safety stop, denoted by "St" on the display.
|WHAT IS AN ALGORITHM?|
An algorithm is the mathematical calculation which sets depth against time and combines this with a set of theoretical tissue-models which go some way to representing the different tissue types in your body.
These model tissues have "half-times", which is to say the time it takes for them to become half-desaturated exponentially. These vary from short ones that represent the fast tissues like the blood, to long ones representing the slow-to-desaturate tissues such as those found in bones.
The calculation is also modified by other varying factors. Some computers use many different tissue models in their algorithms (the Beuchat uses 20 tissue groups with half-times from 5-480min), whereas others use far fewer. The Aladin uses eight tissue groups with a greater range of half-times (from 5-640min). Information regarding the algorithms of all computers is not always readily available.