ABOUT 25 YEARS AGO, I wrote an article for DIVER extolling the virtues of the electronic dive computer. I pointed out that it could manage your dive, allowing you to make a multi-level ascent, stopping at deeper depths in what are now popularly called deep stops, so that much of the decompression requirement was shed before you got near the shallows.
I was castigated for my efforts. Why
I was not a member of the then all-powerful British Sub-Aqua Club, but joined soon afterwards.
Forced to start at the bottom as a Novice Diver, I quickly jumped through all the hoops but learned that I should really be doing my entire dive at a single depth and then make a direct ascent to the surface at a steady 18 metres per minute. These new-fangled computers were not allowed!
Peter Wingate, then London Regional Coach, told me recently that it was what BSAC believed safest at that time. He was my diving instructor. Times change.
Today, many divers gain only a perfunctory knowledge of decompression tables during training, and quickly abdicate responsibility for their well-being to the designers of the modern diving computer.
But both attitudes are wrong. All divers need a rudimentary knowledge of decompression theory, because how do we know that the information dispensed by our computers is correct We dont!
The base information still used by modern decompression experts continues to be that determined by Haldane for the Royal Navy around 100 years ago. There has been plenty of anecdotal information and observational research since, but as no diver will volunteer to allow his brain or spinal column to be sectioned in a controlled experiment, it is unlikely that we will
get anything better than is currently provided by the theorists and physicists.

Blue Seas scenario
hspace=5 If you strapped on a number of different makes of computer side by side and took them diving, the information they showed regarding the required deco would vary. Thats what we have just done for this test.
Its easy to be distracted by the peripheral features of these electronic marvels, features often used to gain a marketing advantage.
The specifications are easy enough to compare, but less easy to determine is their core function - the ability to bring us back to the surface safely and with our health undamaged.
The answers given by different computers may vary, but which is correct We will never know, because we took the cowardly route and always came up at the rate demanded by the most cautious. Yes, even here at divEr, we draw the line at risking our own health in pursuit of the truth!
We took a dozen computers diving, augmented by two on our wrists in case we should lose the rig for any reason.
We went on a series of dives during a one-week trip on a Red Sea liveaboard, Blue Seas. I have to thank the passengers for putting up with the inconvenience of having me do this, especially Stuart Wheelan, who more than once rode shotgun on computer-comparison dives.
The dozen computers compared here represent most of those now available. Only the Mares range was not represented (this includes the M1 RGBM [Reduced Gradient Bubble Model]) because the outgoing distributor had no stock at the time of the test.
Results varied slightly from dive to dive during the week, but the overall impression we give here can be taken as a reasonably accurate assessment.
All the computers were nitrox-compatible, but for fairness all were set to air. I dived with air and nitrox 32, which I breathed as soon as I was shallow enough to give myself some added safety, without altering any of those computers that could be adjusted during the dive.
All the manufacturers tend to deny that their products are intended for use by deco-stop divers, but we believe this has more to do with product-liability insurance than practical considerations.
We were not looking to reveal the extreme differences in these products. Instead we tried to adjust them with personal safety-level settings to get them more-or-less into line.
Otherwise, we might be adding nitrogen to fast-tissue models on some computers while we paused at depth to shed tissue-loading on the slower-tissue models of others.
We recommend that you use any of the computers tested here set with at least the same levels of caution described below, where these options are available.
Also, for comparison, on my wrist I had a Suunto D9 set to the standard RGBM100 algorithm with two-minute deep stops programmed in.
We took this as a target for what we thought was probably sensible. This is based on nothing more than prejudice and the fact that I have used this computer so set for several hundred dives similar to those we undertook
for the test. I dont think Im brain-damaged, but you can be the judge.

The Rosalie Moller comparison
The first dives of the week were used to get a feel for the computers and adjust them by means of in-built safety factor options where they were available to get them to perform in a similar way.
The computers were then left until they were fully clear before being taken on the actual comparison dives.
The displays were photographed on each dive so that we could analyse and compare the results later at leisure.
After a few days of deco-stop diving in the northern Red Sea, we went to the wreck of the Rosalie Moller, where we undertook a dive to 45m and stayed there until every computer was indicating the requirement to make deco-stops during the ascent.
We feel that this probably best reveals the similarities and differences between the computers.
We went down on to the seabed near the propeller and then up onto the deck, where we stayed for a while before ascending the superstructure at the aft, and finally making our ascent up the mooring line to our boat. It represented a typical leisure dive on this wreck.
Remember that this dive was part of a series of dives during a weeks cruise and may not compare with readings for a single dive carried out in isolation.
After 10 minutes at 43-45m, all the computers required a decompression stop during the ascent.

hspace=5 SUUNTO VYTEC DS £699
with transmitter
(RGBM50 with 2min deep-stops option)

Similarly performing computers include the Suunto D6 and Suunto D9

Like the watch-sized Suunto D9, the Vytec DS can
be gas-integrated by means of a radio transmitter fitted to the regulator first stage. It can also be gas-switched during a dive through three alternative nitrox mixes.
Deep-stop options can be one or two minutes long or switched off, in which case a 3min safety stop at 5m is displayed. There is a choice of two RGBM algorithms. Max ascent rate is constant at 10m/min.
With the deep-stop options set for two minutes, stop-times in the shallows were reduced to manageable levels. Deep stops are easily incorporated in multi-level reef dives. If disregarded, deco time in the shallows is increased to compensate.

hspace=5 SUUNTO GEKKO £199
(Suunto RGBM100 at factory setting)
Similarly performing computers include the Suunto Vyper, Suunto Stinger, Suunto Mosquito and Suunto Cobra

This computer is an entry-level version and uses the same standard RGBM100 algorithm as all the other Suunto computers such as the Vyper and Stinger, with an easy-to-understand menu-driven set-up.
However, unlike its siblings, it has no backlight to its display, no PC download function and can be set for nitrox up to 50% O2. Maximum ascent rate is constant at 10m/min. This program proved to be one of the more cautious and gave us lengthy stops in the shallows, including an optional 3min safety stop.

hspace=5 ALADIN PRIME £229
(Standard Buhlmann ZH-8L at factory setting)
Similarly performing computers include the Aladin Pro Ultra

An entry-level nitrox computer (for up to 50% O2) easily set up using the two buttons provided. It includes Smart Trak software for downloading to PC and has a backlit display. The maximum ascent-rate is variable from 20m/min at 50m to 7m/min at less than 6m. There is a manually activated 3min safety-stop timer.
This computer uses the original and well-proven Buhlmann ZH-8L ADT algorithm that might now be considered less than cautious by todays standards. This standard algorithm is an optional setting with all other Scubapro/Uwatec computers.

The instruction manual is almost unreadable because it is shared with the far more complex Aladin Tec.

hspace=5 ALADIN TEC £299
(with Reduced Gradient MicroBubble Level L1 option)
Similarly performing computers include the Smart Pro, Smart Com, Smart Tec and Smart Z

Similar in many ways to the Aladin Prime, the Tec also has Gauge mode and can be set to nitrox up to 100%. Its main feature is the ability to be set to one of four reduced-gradient micro-bubble settings as well as the standard Buhlmann ZH-8L ADT algorithm.
Maximum ascent rate is variable from 20m/min at 50m to 7m/ min at less than 6m. The manually activated safety-stop timer can be set for 1 to 5min. Includes Smart Trak software for downloading to PC and a backlit display.
With its reduced-gradient micro-bubble algorithm set at the minimum (L1), this computer seemed more cautious than any of the Suuntos as set up. In common with the Scubapro Smart oil-filled range, three other optional micro-bubble settings can add even more caution. The manual seemed overly complicated.

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(at Safety Factor 0)
Similarly performing computers include the Apeks Pulse and TUSA DC-Hunter

Seiko-made nitrox computers that can be switched between two mixes during the dive. They use a 12-tissue model Buhlmann ZH-L8 algorithm with a 30sec sampling rate that can be changed by the optional PC interface program. It is easily set up using the three buttons.
The difference between the two makes seems purely cosmetic, with the Cressi Archimede 2 afforded the better finish and choice of coloured surrounds. PO limits can be varied between 1.0 and 1.6 bar, sampling rates changed from 15 to 30sec and three alternative safety factors (SFO to SF2) can be set. Maximum ascent rates are variable (16m/min to 8m/min) according to depth. PC download kits are an extra-cost option.

hspace=5 TUSA DC-SAPIENCE £339
(at Safety Factor 0)
Similarly performing computers include the Scubapro Xtender

Employing the same algorithm as the two-nitrox-mix Archimede 2 and Quantum, this attractively slim Seiko-made computer is for single-nitrox-mix (per dive) use only. It has a novel and remarkably effective vibrating attention-grabbing device whenever new and crucial information is imparted under water, and a huge LCD with the most easily read information of any of the computers tested alongside it.
This model is very easy to understand how to set up. A PC download kit is available as an extra-cost option. All four Seiko-made computers tested were set to Safety Factor 0 option for this dive.
With safety factor SF1 set, this group was extremely cautious and introduced very long stops at 3m (that is14min more than any others on an earlier dive to 42m). With the SF0 used on the test they proved far more in line with the Aladin Tec (with L1 setting). All stop times are in addition to an optional 3min safety stop at 5-3m.

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OCEANIC VEO 250 £270
(at factory setting)
(at factory setting)
Similarly performing computers include the Oceanic Veo, Oceanic Versa, Oceanic VT Pro, Oceanic Atom

These are popular leisure-diving computers mainly intended for no-stop nitrox diving down to 30m. Of course, they can be used deeper. They have two-button operation for easy setting. Audible alarms can be acknowledged and turned off on the dive. Three alternative displays can be activated on the dive.

Variable maximum ascent rate is 8m/min, or 18m/min if deeper than 18m. Both computers use a Haldanian algorithm modified by Rogers & Powell (DSAT) with Spencers M values. Both can be set for nitrox values to 50%. A PC download kit is an extra-cost option.

These computers tended to be the least cautious of the group tested, with extended no-stop times and a long period of zero no-stop times displayed at the maximum depth before displaying the first 3m stop. However, on some dives in the series, extended periods of deco were required when compared to all the others set alongside them. With the deeper dives we were doing, it was difficult to anticipate which would apply. We deduced that if you stayed within the parameters for which the computers were designed they were OK, but if you went beyond you could be asked to make punitive stops in the shallows.

hspace=5BUDDY NEXUS £349
(Hard Conditions setting)

A Finnish-made computer intended mainly to address the closed-circuit rebreather nitrox market, but equally useful for open-circuit diving. It can be switched between two different preset nitrox mixes during a dive, either by means of a tap-switch or automatically.

Safety factors include two types of diving conditions as well as altitude settings. It has a variable ascent-rate according to depth. Good value for those requiring a fairly complex computer. A PC download kit is available as an extra-cost option.

Progressive stops meant time-to-surface decreased continuously on ascent. The very small display made all but remaining no-stop time of deco information quite hard to read. If you have less than 20/20 vision, this is not the computer for you. Slow maximum ascent rate with pauses at 24m and 12m (deep stops suggested for the Vytec DS) meant that stops got shed until 3m was finally reached.

hspace=5 DELTA P VR3 £1127
with VPM and colour screen
(VPM setting)

A 10-mix nitrox-switching computer that can be upgraded with software for trimix, heliox and closed-circuit
applications (including an independent O2 sensor). This
complex instrument can be used for almost any perceived diving requirement if you understand the manual!

It now has the option of different algorithms that can be switched between dives. The optional colour screen changes from green through yellow to red as deco status changes. It is not gas-integrated but almost every other option is available at a cost.

A new varying permeability model algorithm option smoothes out the dive profile. The VR3 so set gave multiple stops between 15m and 3m, making it more suitable for multi-level leisure diving than the conventional VR algorithm with its formal deep stops. An interesting graphic with colour changes helps the diver stay in the deco-stop zone. However, the manner in which the LCD figures are displayed makes them less than easy to read than most others here.

hspace=5 DIVE RITE NITEK HE £606
(at factory setting)
Similarly performing computers include the (single-nitrox-mix) Dive Rite Nitek

It is possible to set the Nitek HE, which is another Seiko-made computer, to match one of seven mixes of nitrox or trimix and change between them during the dive.
Two buttons make it uncomplicated to preset and the same applies to gas-switching during a dive. A PC download kit is available as an extra-cost option.
With an extended final minute at 3m but without the lengthy stops of other Seiko-made computers set on SF1, this computer is factory-set to the equivalent of SF0 but without any additional safety stop at 5-3m.
The display was rather small compared to the others tested alongside it, so good eyesight is needed.

(All the computers in this review have user-replaceable batteries)

Extreme deep-diving ichthyologist Dr Richard Pyle noted that if he made pauses in an ascent from a dive at a depth equal to around half the maximum pressure to which he had subjected himself, instead of ascending straight to the surface, and then repeatedly did the same thing again between there and the surface, he felt a lot better afterwards.
The idea is based on the theory that the human body can sustain a 50% reduced pressure change without suffering ill effects.
This is a technique that has been inadvertently used by reef divers over many years as they make their way gradually upwards, but it had never before been formalised in a written scientific paper. If you go to the deepest point of your dive first, then meander towards the surface, a lot of the required decompression time is shed before reaching that deco depth.
Always keen to gain a marketing advantage, it wasnt long before computer manufacturers hit on the idea of including a deep-stop option in some displays.
Although this is not based on any other real scientific evidence, giving slow tissues time to off-gas in this way, even at the expense of faster tissues on-gassing, makes sense!

Displays compared on a single 45m dive on the Rosalie Moller


The Final Time to Surface column shows how long the computers required the diver to hang around on the last stop. More caution or back on the boat quickly - its up to you!