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Advice by different makers of computer varies considerably. For our comparison test, John Bantin went diving with 17 computers. Then, to make sure, he subjected them to simulated dives in a pressure chamber.

Diving computers have now entered the fourth phase of their development since the original and often suspect SOS 'bendy-meter' of 25 years ago. This was a device which attempted to simulate by mechanical means what happens to body tissues under pressure.
Electronics and the silicone chip enabled the complex mathematical algorithms of scientists like Buehlmann to be used in the Decobrain, and across the Atlantic, the Edge used software based on Spencer's no-decomression limits.
Both these computers gave readings not dissimilar to those of today. However, power supply was a problem, and we had to wait for the revolutionary lithium battery before computers became conveniently small and reliably watertight.
The next development was the obvious one of combining air consumption analysis, based on information from the cylinder's pressure reading, with the calculations for no-stop or decompression stop diving.
Another major development was the Buehlmann calculation moel ZH-L8 ADT. Instead of adding safety by simply shortening no-stop times, Buehlmann added in extra parameters such as work-rate, water temperature, and the effects of provocative diving practices such as saw tooth profiles and gross disregard of ascent rates.
The fact that perfusion by nitrogen into the organs is not constant is also accounted for, and the model considers inert gas not only in its dissolved state, but also when in the form of micro bubbles within the body.
We got hold of all of the computers available at the time of writing and took them diving alongside each other to get a good impression of their in-water differences.
We also took most of them (some arrived too late) on a controlled dive, with the help of Terry Knight, in the pot at South East Diving Supplies Ltd, in Gillingham, Kent.
These pot dives were to 39m for 16min with a water temperature of 19°C. The no-stop time with the BSAC 88 tables is 13min including the ascent back to 6m (bottom time 10min). A 16min dive to 39m requires a 1min stop at 6m.
This simulated a typical British square profile dive, ignoring the advantages conferred by a computer for multi-level diving. It should be said that almost without exception the manuals of every computer tested warned in some way that it was dangerous to make dives that needed decompression stops.
We took two minutes to reach the final depth. At this point, we noted what each indicated as remaining no-stop time, including what was required after 15min and at 16min. We ascended at the maximum rate each computer allowed and stopped short of each required decompression stop until it was time to move on up. In some cases we ignored a stop to see what the computer asked us to do next.

The Aladin range of computers is made and marketed in the UK by Uwatec. These have all recently been updated with the ZH-L8 ADT program, together with graphic displays considerably bigger than their popular predecessors.
The new Aladin Pro is a full function decompression-stop computer. In either console or wrist-mounted form, it gives all the information a diver needs about decompression in a clear and unmistakable manner. This includes depth, dive time, maximum depth achieved, remaining no-stop time or decompression-stop time and depth together with the total time needed to safely make it to the surface.
The speed of ascent is displayed as a percentage of the allowable ascent rate from a particular depth, and this is variable according to that depth, with the slower rates being closer to the surface. This varies from 7-20m/min. It also logs 19 dives. A simpler version, the Aladin Sport, distinguishable by its black rather than grey case, uses the same software but displays less information. It's really for those who intend to stick to no-decompression-stop diving. It shows decompression-stop depths but not the time needed to wait there. One just stays until the stop depth shown clears from the display before moving upwards. It doesn't allow for forward dive planning. It's considerably cheaper though.
Doyen of the Uwatec range is the Aladin Air X. At round £600 it's also the most expensive. This takes into account the diver's air consumption by measuring the pressure of air remaining in the cylinder and includeds this in the decompression model. A heavy air user will need to make more or longer pauses on the way up. This also allows for a cylinder pressure display (eliminating the cylinder pressure gauge) and a prediction of remaining air time, with all the added safety that gives.
Remaining air time should not be allowed to fall below time needed to surface, even though air time will increase with reducing depth since the remaining air will go further as the diver ascends.
The Air X does away with a high-pressure hose connection by using a small radio transmitter which plugs into the high pressure port of the regulator 1st-stage. It's very neat. This is paired with the wrist unit. The chances of meeting another diver with the same frequency of transmission and is said to be one in 50,000.
The wrist unit (there is no need for a console) is rather snazzily designed in an avante garde case which is exceedingly slim and wraps around the wrist with a strap long enough for a bulky drysuit sleeve. The air information is displayed on a screen separate from the decompression information screen.
During the controlled dives to 39m for 16min, with one of the Aladins, we overshot a required stop at 3m. (It can easily happen in real life.) The Buhlman ZH-L8 ADT program sent us back down to 6m where we had to wait before being allowed back to 3m. This extended the dive time considerably (by 8min), but interestingly enough, the planned no-stop time for a dive to 25m two hours later was 8min longer than if we'd come to the surface in a perfectly executed ascent as we did with the other Uwatec computers. With a dive carried out in a textbook manner, the first 3m stop came up after 11min, at 15min a stop of 3min at 3m was indicated and at 16min the first 6m stop requirement appeared. Total time to the surface at this point was 8min, and the resulting total dive time was 27min.
For a second dive two hours later, the dive plan mode (not the Sport) suggested 20min at 24m.
The Italians market the Mares Genius. It is also made by Uwatec and performs in exactly the same way as the Air X. It's in a rather more chunky case and displays the air information on the same screen as the everything else. I found this easier to use since I could see all the info at a glance and it was not necessary to rotate my wrist. It is very handy to be able to instantly compare remaining air-time with the time needed to reach the surface.
The Spiro Monitor 3 Air had my favourite display that used some easy to understand icons to tell you which mode it was currently in. It would have been my favourite computer all round if it had not been marred by a strap that proved too short to go round my dry-suited forearm adequately.
It's also available cheaper if you choose not to have the transmitter, and hence do without the air calculations. In this case it's simply known as the Monitor 3.
All the above mentioned Uwatec-made computers automatically take into account atmospheric pressure, (for diving at altitude or in very depressed weather conditions). They are fired up automatically on entering the water, and dive log-book and planning modes (no planning with the Aladin Sport) are accessed by wet finger contacting a combination of two of four contacts.
They use the same decompression model and display identical information regarding the crucial decompression requirements. They each interface with an IBM-type PC, with the right hardware, to allow you to print out a full graphic of a recorded dive profile. Suunto is another brand favoured by British divers. They are made in Finland, and use a slightly different decompression philosophy in that they demand a very slow ascent right from the deepest point. If you exceed the limit, a large 'SLOW' appears and stays on the display should you not slow down. Even in the pot, we found the maximum ascent rate (10m/min) so slow we were always in danger of exceeding it, and the 'SLOW' motif appeared time and time again.
The Suunto Solution and the Suunto Eon gave exactly the same results, but the Eon is air-integrated by means of a conventional high-pressure hose, and gives remaining air time as well as cylinder pressure read-outs. It's very tidy, being only about the same size as a conventional pressure gauge.
The 39m 16min dive took a total dive time of 24min to complete. After the 2min it took to reach the ultimate depth there was only 7min of no-stop time left. After 10min the first 3m stop was actually indicated and by the time 16min was up, a 9m stop showed. However, the very slow ascent demanded by the Suunto meant that the deeper stops began to clear long before we arrived at them, and the total dive time turned out to be reletively brief. These stops were not actually shown in the form of depths and times, but as depths and total ascent times, which included the stops. It means the same. Stop depths are shown on a rather hard-to-distinguish graphic display.
The Suunto Companion is an economy model with slightly fewer features, but ironically, what we found to be a slightly easier-to-read display, since it gave the stop depths in figures rather than as a gap in a depth bar graph, as with the Solution and Eon.
It appeared to give a very similar result to its more expensive siblings, but in a slightly different way. Total ascent times were not shown, but the depth of each required stop was. However, this really limits the Companion to no-stop diving. It does give, however, planning and logbook modes.
The Suuntos are activated by being immersed, and the surface functions called up by three wet finger contacts. You need to preset the altitude adjustment, but this does mean that you can 'dial in' additional safety by opting for an altitude setting while at sea level.
All the Suuntos suggested a no-stop time of 21min for a second 24m dive two hours after surfacing.
The Companion remembers five dives in its log mode.
Both the Eon and Solution remember the last 25 hours of diving in ordinary logbook mode and can be interfaced with a PC to give graphic read-outs of logged dives.
Two computers which are aimed very much at the American leisure diver are the US Divers Matrix and the Oceanic Prodigy. They come in neat, round, rubber wrist pods. Although their displays are arranged in a slightly different way, for all intents and purposes one can consider them to be identical items, and both use a data base from DSAT (Rogers/Powell).
Americans are very good at making things simple and easy for even the least technical minded to understand, and both these computers work on the 'traffic light' principle in that they both show a build-up of a bar graph which represents nitrogen loading. This is aligned to a green, yellow and red zone. By keeping the bar graph in the green and ascending as it nears yellow, the diver stays out of decompression-stop requirements. Remaining no-stop time is predominently displayed.
One can use the computers for deco-stop diving by allowing them to go into the red sections which are marked off with the required stops. The ascent rate indicator is composed of a number of segments which show as one accelerates upwards. These are also aligned against a three-colour margin. Staying out of the red keeps you safely away from 19m/min plus. The maximum in the green zone is 10-12 m/min.
These computers have an alternating display which clearly shows additional information such as maximum depth achieved and elapsed dive time, but you have to wait for the information to come round again before you can be sure of what you've seen. The great joy of these computers is their utter simplicity. That said, they do have to be activated by pressing a button before diving, and I have to admit to having to return, embarrassed, to the surface on more than one occasion because I forgot this.
On our pot dive to 39m for 16min both displayed a no-stop time remaining of 9min after a 2min descent. The first stop showed after 11min of diving. After 15min we needed to stop at 3m for 2min and by the time we left the bottom after 16min the first 6m stop was indicated. Staying in the green section during the ascent (10-12 m/min) and stopping at slightly deeper than 3m on the way up (the 6m stop had cleared by the time we got there), our total dive time was 24min before we made the surface.
Dive planning mode suggested a second dive of 26min to 24m, two hours later.
The Sherwood Encore looks superficially similar, but this could be misleading. It's made in Japan and uses algorithms developed by Bohrer, based on research by Buehlmann. It's fired up by touching two contacts with either wet fingers or the water. We found no-stop timeswere quite short, and by the time we'd reached the bottom on our 39m pot dive we had only 6min left. The first stop (3m) however did not actually display until we'd been under water for 10min and the 6m stop came up after only 14min. Ascent time showed as 9min by the time we had completed our bottom time and the whole dive with stops took 26min surface-to-surface. The repeat dive to 24m two hours later was planned at 23min.
The variable ascent rate is controlled by the depth digits flashing if you exceed 16m/min in 30m or more, 12m/min in 10m-30m, or 8m/min in less than 10m.
We found the display neat and easy to understand but the the legends 'mins', 'metres', and 'ascent time' were so small I couldn't figure them out without my specs.
The Sea & Sea Profile is also made in Japan. It distinguished itself in that it gave the shortest no-stop time once we were down at 39m. (5min). Staying down for the full 16min meant punitive decompression-stops (we waited 9min at 6m). Thirty seconds later it required a stop at 9m but this was shed on the way up. No total ascent time was displayed and the whole display flashed, together with the legend 'caution', when in deco mode. The stop depths, but no actual stop time is displayed.
The whole dive, including the ascent, took 41min, and a second dive, two hours later, to 24m was planned with a no-stop time of only 18min.
It really is very cautious and intended either for no-stop diving or reef diving where there's plenty to look at while you're hanging about in the shallows.
We liked the positive user-friendly buttons, which neatly did away with any frantic finger licking when we needed log or planning modes. We also liked the way the computer neatly locked on to either its wrist mount or console, and especially liked the way it was possible to pull up a mini graphic display of your dive profile, there and then on its own screen immediately after the dive. Trust the Japanese to come up with a product with so much consumer appeal.
The Mares Divemate is a one-off design from Italy, made in Finland. It's a talking dive computer!
Now this may sound a little silly, but try one first. If you're busy doing something under water other than just loafing around - teaching or photographing, for instance - it can prove to be very useful to have a voice in your ear telling you of your changing depth, your dive time, your remaining no-stop time, or indeed, telling you to slow down if needed during an ascent. It attaches to your mask strap and unclips at a moment's notice.
It should prove useful to anyone diving in poor viz, because it does away with squinting at your computer with the aid of a torch. It has a visual display too.
The Divemate uses an older modified Buehlmann algorithm than its state-of-the-art brother, the Genius. The user has a choice of programs for either normal or harsh conditions, but even in the former it still gve decompression-stops that seemed familiar to us.
After the 2-min descent to 39m we were left with 8min of no-stop diving and the first stop was actually displayed after 10min. A stop at 4m was required by 15min and at 16min the total ascent time, including stops, was shown to be 8min. The total dive time was 23min, with a second no-stop dive to 24m, two hours later, planned at 29min. No deco information is relayed vocally and we relied on the exceptionally clear and easily-read display to show depth, dive duration, stop times and depths.
There were also some red lights that come on if you beat the variable 20-10m/min ascent rate. These red lights are bright enough to see in the poorest conditions, so even if you don't have it next to your ear, the message is still unmistakable.
The Divemate will remember 10 dives in its log. Having used it, we found it to be a very useful instrument, limited only by the possibility that the battery might not last as long as those of some of its rivals since it has to power a small speaker system.
There is a choice of five altitude selections which must be entered manually by the user.
The new Scubapro Trac air integrated diving computer was almost impossible to get hold of. Scubapro in this country seemed unwilling or unable to supply us with one so we bought an example from a shop the other side of the Channel. Replacing the normal pressure gauge on its high pressure hose, it comes in a rather chunky console which has space for a compass too.
The display is very large and very easily understood, with two massive command arrows, (up, down, and stay where you are) and graphic bar representations of nitrogen build-up and air time remaining, together with traditional numerals.
If you ascended too quickly the downward arrow appeared, and the allowed ascent rate was variable between 27m/min at more than 30m to 15m/min at less than 15m.
We didn't receive it in time for the pot test dives but I've dived extensively since with the Trac alongside other computers. The Hann P-6 algorithm is called into play for the program, which seemed to give very short no-stop times and lengthy waits in the shallows if you went into deco-stop diving. During a dive to 30m, the Trac demanded an 8min stop at 3m when other computers with the Buehlmann ZH-L8 ADT software said that only 3min were needed.
However, once I got used to these stop times, it didn't seem so much of an imposition, and we tended to keep the Trac out of deco mode by ascending gradually. This, naturally, was no problem on coral reefs, since one just worked up into shallower water. It was a little boring during a typical UK wreck dive.
Strangely enough, on one dive, when I chose to trust my life to an alternative computer, and ignore the 4min/3m stop demanded by the Trac, I arrived at the surface with no trace of my omitted deco stop on the display. It was a detail of my dive I had the option to keep secret from the dive marshall. This is not necessarily a good thing.
The Cochran Captain is an economic diving computer made by an American manufacturer new to the British diving scene. Unfortunately, the importer was unable to supply it in any sort of mount so we borrowed the pod from the Sherwood Encore and took it diving. We found it reletively difficult to read under water because the display layout was so unfamiliar.
During our pot dive, the remaining no-stop time after the descent was 9min and after 16min a 1min stop at 9m was indicated. The total dive took 25min and the planned no-stop dive to 24m, two hours later, was 26 min.
Cochran is an important manufacturer of technical diving equipment in the States, and their high-profile offering is the Cochran Nemesis II, a fully air-integrated variable gas mix computer, with radio connection between the regulator 1st-stage and the wrist unit.
Wading through the bound manual deserved at least an honours degree. It was so complex, as the computer offers so many options. However, for the dedicated technical diver this is the ultimate in electronic wizardry, and no self-respecting 'tekkie' will want to be seen dead without one.
In order to get both wrist and transmitter units to spring into life one has to do a quick impression of Ringo Starr on them.
Unfortunately, we were unable to get the rather large transmitter unit together with the wrist unit into the pot to see how it performed, and we were unable to successfully change the calibrations from feet to metres and PSI to Bar. I made several successful dives with it, but I felt we were unable to get a good enough comparison with other units. We will be reviewing both Cochran computers, in depth, in a forthcoming edition of In Gear.
The DiveRite Bridge II is a more simple nitrox computer that's been recently improved. It's a very neat and nicely made (in Japan) unit with an exceptionally long strap which will pass round the thickest suit sleeve. It feels expensive.
This new Bridge is now easier to use than its predecessor. You enter in your O2 percentage by means of two of the three command contacts. The Bridge also registers the build-up of Oxygen Toxicity Units (OTU) with an oxygen limit indicator.
We set it for Nitrox 21 (air) and took it on our pot dive along side the other air computers. 14min had passed at 39m before the first stop was indicated and after 16min the ascent time was seen to be 6min. The ascent rate was variable from 20m/min beyond 18m to 10m/min in the final six.
If the ascent was too fast the display flashed and beeped a warning. Most of the other computers reviewed here beeped audible warnings too.
With the Bridge II, the total dive time worked out at 22min and the second dive to 24m two hours later was planned at 23min. This computer uses a modified Buehlmann algorithm for nitrogen uptake calculations and the Hamilton/Boher algorithm for OTUs. It can also double as a rather bulky calender/watch and can be interfaced with a PC for dive profile readouts. It's another computer which we will revew in depth at a later date.
Decompression theory is an act of faith. You never know how close you come to disaster. Diving computers have undoubtedly added a large degree of safety to scuba diving, but they deserve intelligent use. Stay well within the limitations of any computer that you finally decide on owning and using.