Appeared in DIVER February 2008

John John Bantin has been a full-time professional diving writer and underwater photographer since 1990. He makes around 300 dives each year testing diving equipment.


SOME YEARS AGO, I was a bit un-nerved when taking a pre-release version of the plastic Suunto Mosquito computer-watch diving. I was with Umberto Pelizzari in the Caribbean while he demonstrated his freediving skills. If you remember, for some time he was No Limits world champion.
What un-nerved me was the fact that the computer flooded. Some sort of early problem the manufacturer had encountered with the plastic moulding caused the flood. Obviously the problem was sorted before the Mosquito went on general release.
I had no such misgivings with the three-gas-integrated D9 or the two-gas D6 that followed, and I continue to wear both on my wrist on nearly every dive I do.
The D9 is a bit too chunky to wear as an everyday watch, but the D6 is not. The plastic Mosquito looked a bit cheap to qualify as everyday jewellery.
Now we have the Suunto D4, which slips neatly into the bottom end of the Suunto range of computer-watches. In a combination of composite material and metal, it looks as good as a watch. Its a full-function decompression nitrox computer with an RGBM algorithm and optional Deep Stops, and Umberto in particular will approve, because it has some important features for freedivers. In this respect, it also replaces the D3 digital dive-timer.
Besides accurate depth and time recordings, in freediving mode the Suunto D4 is programmed to capture the depth three times a second for an accurate maximum depth reading, while recording to the profile memory every second.
It also provides the dive time in minutes and seconds, and this is found in the centre of the display. The user-adjustable alarms can be set in advance to keep you informed of your progress throughout the dive.
It seems perfect for those who like to dive while holding their breath. Im too old and tired for all that athletic stuff. Early-morning exercise I can just about manage a buffet breakfast without breaking sweat.

Im more interested in the diving computer side of things, and the D4, adjustable for a single mix of nitrox between 21% and 50% and a maximum ppO2 set between 0.5 and 1.6 bar, slots in nicely alongside the D6 and D9 for most of the dives I do.
It uses Bruce Wienkes Reduced Gradient Bubble Model (RGBM), and you can choose to have the full version or a more aggressive 50% setting. The RGBM algorithm really takes effect, reducing no-stop times and increasing deco stops when doing a series of dives, as one might do on a liveaboard trip.
Some may criticise Suunto for being too cautious, but can you be too cautious when your long-term health is at risk

Display Legibility
I often hear people opine that the displays on Suunto computer-watches are hard to read. They are assuming it to be small because the watch face is small.
In fact, the information seems to be as clearly readable as on any other computer, thanks to the dot matrix figures. Its just that the information is given in layers, rather than trying to cram it all onto a single screen.

I used to scratch my head when it came to setting up a computer, but manufacturers have come to realise that we divers are not always particularly clever. I found the D4 intuitive when it came to setting time and date, nitrox mix, the depth and dive time alarms, my own personal caution level (or attitude setting), optional deep stop function and the sampling rate.
It was also easy to switch to Freediving mode, but I bypassed Air mode and used Nitrox 21 instead. I didnt want to get stuck in one mode, because I was going to use this computer for three dives a day for a week.
If you do a dive in Air mode, the computer has to wait until it has calculated that you have totally off-gassed before it will let you switch. It cant calculate otherwise.

During the dive, I could see the actual depth and remaining no stop time, or the total ascent time and the stop depth demanded.
By pressing one of the four buttons, I could alternate between the dive time and the current ppO2 with the actual nitrox mix set. This was displayed alongside as confirmation, too.
You can also see the time of day or water temperature by pressing another button, along with maximum depth achieved.
If you choose, the D4 will display a deep stop of one or two minutes duration as an alternative to a 5m safety stop that is timed down for three minutes. I guess most buyers will opt for the second pre-set option.
A simple graphic illustrates build-up of total ascent time, and another indicates the ascent rate as a fraction of the maximum allowed.
Overall, the D4 seemed very similar to the D6, but without the facility to switch nitrox mixes during the dive. The logbook, in common with the Suunto D4s more expensive siblings, gives data additional to the usual information.
It has extensive memory functions, including a nice little graphic profile of the dive, and dive-planning functionality. The logbook/profile memory can store up to 80 hours of diving, with a lifetime dive history also recorded.
It should be stressed that this is an entry-level model aimed at first-time dive computer buyers, or freedivers who will benefit from its quick sampling rate. As such Suunto has kept things elementary, but this computer doesnt look too bad as a watch, either!

Other computer-watches to consider:
Mares Nemo Excel £290
Scubapro Xtender £295
Apeks Pulse £318

Divernet Divernet
PRICE £295

WARNINGS Ascent rate, depth & time (optional)
NORMAL IN-WATER DISPLAYS Actual depth, no-stop time remaining, dive time, nitrox mix, actual ppO2, maximum depth, water temperature, time of day
DECO INFORMATION Stop depth & total ascent time
SAMPLING RATE Every 1, 10, 20, 30 or 60sec
DIVING MODES Air/Nitrox/Free
DEEP STOPS Iterative and optional (1 or 2min)
SAFETY STOP Optional and interchangeable with Deep Stops
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