DECOMPRESSION TABLES WERE DESIGNED FOR MILITARY DIVERS. They assume that a diver descends to one level, does a job of work, then ascends as safely but as quickly as possible. This is called "diving with a square profile".
    Leisure diving is subject to different rules. Divers want to swim about, enjoying the view. Not confined to one depth, this is called "multi-level diving". However, to use any decompression table safely, the leisure diver must assume that the whole of the dive is spent at one (the maximum) depth.
    A long time ago, an enterprising Italian came up with an idea to help leisure divers judge what they might need in the way of decompression. It was called the SOS Decompression Meter, often affectionately known as the "Bend-O-Matic".
It had a smart stainless-steel case and a dial with a needle which moved, during the dive, progressively from green to red. It seemed like a good idea, but when I opened mine up, I was dismayed to find there was little more than a pair of condoms and a bit of porcelain inside. That was back when sex was safe and diving dangerous.
    Then came pressure sensors, micro-chips and, finally, lithium batteries. The modern, compact diving computer was born. Computers can calculate decompression status during multi-level dives.
    There are fewer than a dozen brands of diving computer available in the UK. Under each brand name there are many models, but within each brand those models do much the same so far as a diver's decompression status is concerned.
    There are differences between how various brands gauge that status, but nothing you can detect at the time of making that important purchasing decision.
    You the customer must assume that, of all the different theories and mathematical calculations used by the different computer designers, any one will keep you safe from decompression illness.
    Diver has covered the relative characteristics of different brands in this respect in past computer comparisons (Pick Your Algorithm, September 2001). You can make value judgements on the core "decompression management" function of a diving computer only with the help of such an article or by your own past experience.
    However, the micro-chip designer has other ways of making his computer more attractive to you at the point of sale. He can build in lots of value-added features.
    First to arrive was a water-temperature display, followed by a calendar/watch function and a cascade of other features. Dive-planning mode, adjustable O2 percentage for nitrox divers, gas-integration, downloading to PC, dive-simulation and so on are all now possible.
    Of course, it's always nice to know what time of day it is, and it is especially important to be able to read the display in low light conditions, which is why more and more manufacturers include as a feature a self-illuminating display (no diving lamp needed).
Nearly every computer also now has an audible alarm that will draw your attention to any visual warning - particularly useful for the inattentive or distracted diver.
    Dive-planning allows the diver to work out, and see displayed before diving, a no-stop time for a given depth, taking into account previous dive profiles. Some computers allow more sophisticated dive-planning for deco-stop dives.
    Gas-integration keeps an eye on your breathing-gas management, and displays an estimate of the amount of time left with the remaining supply, taking into account the ambient depth and your breathing-rate during the previous part of the dive.
    Knowing both your gas-supply time and the required total ascent-time can be very useful, and it is an excellent idea never to let the second exceed the first!
    The ability to download all your dives from your diving computer to a PC now makes dive-log-keeping very simple. It usually provides a digestible depth/time graph too, a quick and easily assimilated view of how you carried out your dive.
    When I was once involved in the rescue of another diver, it was interesting to relive the moments and to be able to marvel at the way we both seemed to survive a rapid ascent, recorded faithfully on the graph, without any damage to our health!
    Dive simulation is a feature that enables divers to get to know their computers, and fully understand the information displayed, before actually getting in the water. With so much information now available, and the need to understand the computer's various functions paramount, this dry-run feature can be very important.
    The opportunity to add caution to a computer's calculations can be used by those who think that they might be more susceptible to DCI than others. Or it could be used simply to draw your computer into line with that of your buddy, which might be significantly more cautious than your own.
    The ability to replace an exhausted battery while away diving could be important; the ability to replace a broken strap easily is offered by only one maker but could be convenient. Perspex screen-guards prevent damage to the screen itself and can be replaced when they get too scratched.
    Nitrox users can take advantage of extended dive times by using a nitrox-compatible computer. The facility to choose a particular maximum PO2 setting gives them more choice when it comes to a maximum operating depth for a given nitrox mix. Those who switch to richer nitrox mixes during an ascent to accelerate off-gassing will need a computer that can switch gases too.
    Recently, some computer manufacturers have developed special ways of making their products safer for diving.
    Repeat-diving scenarios with short surface intervals are being accommodated by some through specific attention to the circumstances that might give rise to the formation of micro-bubbles.
    At least two manufacturers now offer computers with a choice of the mathematical algorithms which simulate what happens to your body on a dive.
    Are certain computers over-cautious, others not cautious enough? We at Diver can't tell you where the line should be drawn, because we have no way of knowing what's happening to you, as an individual, when you dive. Nor does any computer designer.
    What all computers do is give you specific control of your rate of ascent, a very important aspect of decompression management.
    I believe it is often the rate of ascent that should be questioned when divers suffer decompression illness even when following a well-recognised dive table.
    But again, manufacturers differ in philosophy on this point. Some, such as Suunto, believe that a fixed maximum rate of ascent is best. Others, such as Scubapro/Uwatec, believe in a variable rate, faster at greater depths where pressure differences are less.
    Nearly all computer manufacturers recommend that you wait for 3-5 minutes in the shallows (between 5 and 3m), even after a no-stop dive or after all your mandatory deco-stops have cleared from the display.
    Many manufacturers now add this safety-stop alongside other displayed information so that the user can judge it accurately. Slow ascents and safety stops are thought to be a major contribution to safe diving practice.
    I was recently told that a study of regular divers and non-divers revealed that those who dive will all, without exception, suffer some changes or damage to the blood vessels in their retinas.
    I have been diving regularly from the time that the DecoBrain, one of the first decompression computers, arrived on the market. I dive as much as anyone. I had my retinas checked recently and was told that no evidence of damage could be found.
    How can that be? Perhaps because I have always used a computer for my diving, coming up slowly and stopping in the shallows for a good period. Who knows?
    I certainly would opt to dive with a computer rather than without. In fact, I never dive without a back-up computer, too.
    However, safety has never been a strong selling-point on any item, from cars to computers. Rightly or wrongly, people want perceived performance. So what value-added features should you look for when choosing a diving computer? Here are 20 questions to ask:
1: Does it give details for full deco-stop diving or is it a no-stop-only diving computer?
2: Can you read the display easily - not only the main figures but all those little icons as well?
3: Can you read it in the dark?
4: Is the surface-operation user-friendly?
5: Could you follow the shop-assistant's demonstration easily?
6: Is it intuitive to use?
7: Manufacturers are increasingly replacing wet-contacts with buttons. Which type do you prefer?
8: Is the instrument automatically activated once in contact with water or do you have to remember to switch it on?
9: Is it breathing-gas integrated?
10: Is it nitrox-compatible?
11: Is it console- or wrist-mounted?
12: Will its strap reach around your wrist, especially when wearing a bulky drysuit?
13: How good and how accessible is its log-book facility?
14: Can you download your dives on to a PC?
15: What is the cost of the interface and software needed to do that?
16: Is it safe for repeat diving with short surface intervals?
17: Can you use it when changing to different, richer nitrox mixes for decompression during an ascent?
18: Can you change the battery yourself?
19: Can you afford it?
20: Finally, and most important of all, can you trust it?

Every computer comes equipped with its own unique selling proposition. On the tables we list the main features of most of the major computers available in the UK.

Table 1 £145-£269
Table 2 £269-£425
Table 3 £459-£906

Oceanic is one maker that allows the user to replace the battery


a stick-on guard provides protection for the screen


on many computers push-buttons have now replaced wet contacts


Radio-linked gas integration allows a computer to take your gas consumption into account


wet contacts are still favoured by some manufacturers


a multi-mix nitrox computer provides added flexibility


A display should be easy to read under water as well as in the shop


this gas-integrated computer on a console uses a hose connection rather than radio signals


Relive your dive! It can be downloaded straight to computer with a PC connection


this is the 3D profile you get as a result from a Dive Rite Nitek 3 computer

GLOSSARY OF TERMS Algorithm: The mathematical model used by the computer to make its calculations. Ascent Rate: Either fixed at, say, 10m/min or variable from 18m/min to 6m/min according to depth zone, this is calculated to allow the body to off-gas absorbed nitrogen safely. Ascent Time: The total time to the surface at the acceptably safe rate of ascent, including all the deco stops required. Breathing-Gas Integrated: Displays tank pressure and estimates, from your breathing rate and the ambient depth, your remaining air-time. Bühlmann: The Swiss physiologist who wrote many currently used algorithms. Deco Ceiling: A point in the water column beyond which it is unsafe to pass upwards at a given time. Decompression Illness: DCI, whether permanent or temporary, is caused by gas coming out of solution within the tissues and not being ventilated out through respiration. Deco Stop: A point in the water column at which it is necessary to pause to allow the body to off-gas previously absorbed nitrogen. Dive Plan: Schedule of no-stop times for different depths based on previously conducted dives. Dive Simulation: Allows practical experience of computer display in dry conditions. Gauge Mode: Ability to abandon deco-algorithm so that a computer can be used as a depth-gauge and dive-timer only. Gas-Switching: The computer calculates decompression status allowing for different mixes of nitrox that may be switched to by a diver at stages during a dive. Haldane: A British pioneer who came up with the term "theoretical tissue half-time" and on whose original research many decompression theories are still based. Illuminated Display: Illumination independent of any outside source. Log-Book: Details of dives memorised and displayed for later use. MOD: Maximum Operating Depth with a particular nitrox mix. Microbubble: Asymptomatic bubbles that may be present within the body but which can be added to and grow during an ascent during a serial dive, causing DCI. Model Tissue Half-time: The time it takes a theoretical body tissue to half saturate/desaturate exponentially. Multi-level: Any dive which does not have a square profile. Nitrox-Compatible: A computer able to calculate decompression status in consideration of reduced levels of nitrogen in air that is enriched with oxygen. No-Deco Time: Time available to a diver at a given depth without making pauses or deco-stops during the ascent. Oxygen Clock: Nitrox-compatible computers also calculate for build-up of oxygen toxicity that might occur while breathing oxygen-enriched air. PC Interface: Hardware needed to link a diving computer with a PC. PO2 Setting: Maximum partial pressure of oxygen in a nitrox gas allowable at any depth. Some nitrox computers are pre-set, while others allow the user to select a level between 1.2 and 1.6 bar. RGBM: Reduced Gradient Bubble Model. An algorithm that takes account of the formation of micro-bubbles and their effects in repeat diving scenarios. Safety Stop: Time spent paused between 5-3m (usually three minutes) in addition to that time mandatory for safe decompression, even after a no-stop dive. Screen Guard: A clear plastic screen that covers the primary screen and which can be easily replaced if too badly scratched or damaged. Square Profile Dive: A dive on which the diver descends, stays at one maximum depth, and then ascends. Water-Activated: A computer that automatically enters "dive-mode" once under water and beyond a certain depth. Wet Contact: Surface functions are activated by bridging a particular pair of contacts with wet fingers. ZH-L: A system of decompression schedule calculations since modified and used for the basis of the algorithm (ZH-L16, ZH-L8 ADT, ZH-L8 MB) used in a number of different diving computers.