Diving computers offer the user a wide range of peripheral functions these days. Thats the magic of silicone-chip technology and hi-tech battery power.
However, the core-function of any diving computer remains the same - to monitor your time against depth, and calculate the level of absorption of gas that might be going on within your body while you are under water.
This allows you to control your ascent rate, your remaining no-stop time, and any decompression stops you might need to make during an ascent, to avoid the onset of the symptoms of decompression sickness. These can vary from merely irritating, through serious, to fatal - so its a serious subject. The problem is that all decompression theory is exactly that - theory!
So how do you choose a diving computer Do you get one the same as your friends Do you trust the judgment of the dive-shop assistant Do you get the one that looks nice Or do you choose it because you like the price
A straw poll among passengers aboard Emperor Divers liveaboard mv Pegasus in the Red Sea this October revealed all of these motivations. However, few of those passengers could understand why we wanted to go diving with 11 different computers ganged together on one rig. Surely all the computers would give the same information under water, wouldnt they?
Sadly, this proved not to be the case.
I was diving courtesy of Emperor Divers, aided and abetted by George Buxton, a veteran of Diver in-water comparison tests. We took the 11 different computers plus two personal computers each strapped to our arms, and we did some serious dives during the course of a week, many in the order of 50m deep.
The idea was that the 11 computers would represent most, if not all, of the computers currently available.
In taking them beyond the limit of no-stop diving, we would be able to detect the differences in the algorithms each one uses, thereby revealing the differences in approach of the various physiologists and mathematicians who write the software.
Its the mathematical calculation that attempts to keep us safe from the ill-effects of breathing nitrogen while under pressure.
The amount of nitrogen varies within the range of gas mixes available for us to breathe while under water so, for this exercise, we standardised on one mix. It was the mix which is most often available, nitrox 21, commonly known as air.
Most of todays computers can be adjusted for a range of nitrox mixes and this was true of all the units we used. Air contains a lot of nitrogen so is, in fact, one of the least safe gas mixes to breathe when it comes to the possibility of DCS.
All the computers were set for air or nitrox 21. However, George and I chose to switch to a second supply of gas, nitrox 32, as soon as we had ascended within the range of its maximum operating depth, but we decompressed assuming that we had continued breathing air for the whole dive. In this way we built in a safety margin.
We carried the nitrox 32 supply on our backs alongside a single tank of air. Regulators used were the Apeks TX40, Mares Planet Ti, Mares Abyss MR22 and Poseidon Dive.
Because we did more than one dive per day, each computer also computed residual nitrogen levels so that no-stop times and required deco-stops became adjusted to take the previous dives and surface intervals into account.
Passengers on board mv Pegasus are normally limited to a maximum depth of 30m but we had made special arrangements with Emperor Divers to exceed this.
Resident dive-guides Jim and Tish were helpful in dropping us from the boats inflatable into deep water further from the reef than normal. This normally worked well, but it did involve us in a couple of swims from hell when we found ourselves at the required depth for the test but without sight of the bottom.
On one occasion we swam at depth in the blue for more than 23 minutes before we could detect the bottom, such is the dramatic way in which the Red Sea drops away into deep water beyond its reef edges.
The nitrox membrane-system aboard mv Pegasus delivered reliable supplies of nitrox 32 from its two compressors for use as the decompression gas.
Previous Diver comparison tests of computers found us using each instrument directly from the box, with factory default settings in place. This time, experience gained from those previous tests allowed us to pre-set each computer, where it was possible to do so, so that it could be brought closer into line with the decompression information it displayed. This dispensed with most of the disparities in performance found previously.
We recommend that buyers of any of the computers tested here start by using them set with the adjustments we made in place, unless they have personal reasons for setting even more cautious limits to their diving.
Confronted with a mass of information on a variety of differently designed and often unfamiliar LCDs, it was impossible to recollect after a dive all but the most outstandingly different results that were offered.
To make our interpretation of the results accurate, we photographed the computers from time to time during each dive, taking a snapshot of the information displayed, at the same times and depths, with 36 different way-points in each dive.
In this way, we retained positive evidence of what each computer displayed and could later study it at leisure.
In most cases we were then also confident that the mandatory deco requirements displayed were sensible, and we always ascended at a rate that triggered no fast-ascent warnings on any of the computers.
That said, no-one can tell you how close you come to getting decompression sickness or, even more likely, sub-clinical DCS.
Those computers that seem less cautious might in fact be telling the truth about your decompression status, while the others might just be keeping you in the water for longer than necessary. Or some might simply be more cavalier with your health. We have no real way of knowing.
Some computers now take into consideration deepwater stops in their supposed ability to reduce the chances of micro-bubbles forming. The theory, put simply, is that by reducing the chances of the build-up of symptom-free micro-bubbles during an ascent less time is required decompressing in the shallows.
Should a bubble form at depth, it will not go away in the shallows no matter how long you stay there, so the latest computer-algorithm writers are now more inclined to follow the deepwater-stops route.
With the computers ganged-up side by side, we were naturally bound to do both the deepwater stops required by some and the long hangs in the shallows required by others, to avoid bending any of them.
Deepwater stops are a relatively new procedure for square-profile divers, such as those who wreck-dive in Britain. However, multi-level-profile divers have been using this approach for years, in the form of a natural progression towards the shallows up a coral reef, for example.
The latter will find that this new generation of computers probably does not affect their established diving routines.
All our test dives lasted around an hour, with sufficient time spent at a depth to see all the computers go into deco-stop mode before we started our slow ascent.
This report is not intended to be a comparison of features offered by the individual computers. That can be obtained by studying the product brochures and manufacturers websites.
Instead it is a comparison of the decompression information that the computers display during a dive. This is something that cannot be discovered while trying to make up your mind what to buy in a shop.
The computers ranged from the popular-in-America Oceanic Veo 250 to the popular-with-tekkies Delta P VR3. We advise you to choose a computer that matches your style of diving. The units named in brackets are products similar to the tested model and known to use the same algorithms.