How to choose your first computer
Rather like their desktop equivalents, dive computers have evolved from the expensive preserve of the minority to affordable, almost essential items within the budget of most regular recreational divers. They range in price according to their design and application, from high-end devices priced upwards of £1,500 to entry-level computers available for as little as £125.
Why Buy a Computer?
Much as I love dive tables, computers have effectively rendered them obsolete (although I still encourage divers to learn about them). The No Decompression Limits (NDLs) displayed on tables are based solely around your maximum depth, regardless of your actual dive profile, whereas a dive computer continuously tracks changes in your depth and adjusts your NDL to suit, often significantly extending your dive time as a result.
Computers more accurately calculate your surface intervals and no-fly times – especially useful for the typical multi-day, multi-dive holiday – and all this essential information is presented in a single, digital display.
Dive computers give far more accurate readings than their analogue counterparts and will generally retain that accuracy during their lifespan. Analogue gauges can be off by as much as +/- 10% even when new, and their accuracy decreases over time as repeated use wears out their mechanical components (although it has to be said that 'repeated use' means several hundred dives).
It should be noted that dive computers do not automatically make your diving safer – they will still allow you to do some seriously stupid things, especially if you don’t understand the display correctly. You still need to follow traditional safe diving practices.
First consideration – price and purpose
In the article about regulators I mentioned that it doesn’t really matter how much money you spend, they all do their job and are similarly easy to use. Although all dive computers will work in a recreational dive context, I do consider that the complexity of some high-end computers – geared towards technical diving – may hamper the recreational diver somewhat, simply because they come with features that are above and beyond recreational training, and may lead to some confusion if those features are not properly understood. Some computers can switch between recreational and technical modes, but unless you’re already on the path to technical diving, features such as gas switching, trimix and rebreather compatibility are redundant in the standard recreational environment. For that reason, I’m inclined to recommend a more basic computer even to divers who might have just won the lottery!
Standard dive computer features
All dive computers show depth, dive time, your current NDL (or by how long you might have exceeded it), and have a visual ascent rate monitor. Most will have audible alarms for exceeding ascent rates or NDLs, a critical air supply warning for air-integrated computers, and user-programmable alarms for personal depth and time limits. Surface interval and no-fly times also come as standard, as do automatic safety stop features, which start counting down from 3 minutes once you reach safety stop depth. Should you exceed your NDL, a dive computer will provide information as to how to deal with it, and have you make an extended safety stop – or 'emergency decompression stop' – as necessary, or lock in error mode and prevent you from diving if you’ve gone too far over the limit.
If a computer does not have one or more of the above, then I can’t see much point in buying it. Other features such as planning modes, temperature displays and a backlight, the ability to increase the computer’s level of conservatism to reduce nitrogen exposure for personal or medical reasons, and a function to compensate for diving at altitude are all fairly standard, if not universal, but again, I can’t think of any reason to buy a computer which does not have them.
All current models of recreational dive computer are nitrox compatible, although there may be a few older computers on the market that are not. Nitrox pricing and availability varies, but as it becomes increasingly commonplace, with its inherent benefits in terms of extended bottom time and reduced nitrogen exposure, I can think of no reason to buy a computer which is not nitrox compatible.
The 'Deep Stop' feature – making a short stop at around 15m after a deep dive – is sometimes used as a selling point, but for the bulk of easy recreational diving, following a safe, conservative dive profile eliminates the need.
Some computers are able to connect to a home computer so you can download and analyse your dive profiles, with the associated cable and software sold as optional extras.
A note on algorithms
Algorithms are the mathematical models used to calculate nitrogen exposure based on your depth and time underwater. You might see references to Haldane, Bühlmann, VPM and RGBM, but for the recreational diver, it really doesn’t matter. The only real practical difference is that some models (especially in older computers) may give slightly longer bottom times than others, and slightly different ascent rates. The golden rule of team diving with computers is to follow the most conservative computer and plan your dive profile to suit, but some 'ahem' advanced divers insist their computer is 'better' because it lets them push their limits for longer. This is a quite ridiculous argument.
Computers are broadly split into wrist-mounted devices, or console versions that come packaged with an analogue SPG, with some models available in both designs. There are air-integrated versions of both, but I’ll come to that shortly. Which you choose is really down to personal preference: consoles are bulkier and, if improperly secured, may bump into things; wrist-mounted computers are less bulky, but might slip off and plummet to irretrievable depths. Proper care and attention prevents these things from happening, and there’s no significant advantage or disadvantage to either design. Some wrist-mounted models are the same sort of size as a regular wristwatch, wearable both underwater and to the post-dive party, but divers may find the smaller displays much less easy to read. Entirely up to you.
The increasingly popular air-integrated (AI) dive computers are available in two types – hosed or wireless. The hosed version connects to the high-pressure port of your regulator’s first stage and effectively replaces the analogue SPG; the wireless versions have a transmitter which connects to the HP port instead of the hose, beaming information about your air supply to the computer on your wrist. Wireless transmitters are often sold as an optional extra, so check the product description carefully before investing.
AI computers are a hotly debated topic on internet dive forums, and although they have some excellent features, there are some drawbacks. An AI computer will tell you exactly how much air is in your tank, rather than less precise SPGs, and they also have the capability to measure your breathing rate and, therefore, calculate your remaining dive time based on how much air you currently have, and how quickly you’re using it, along with displaying your NDL.
I think that AI is one of those Marmite 'love-it-or-hate-it' concepts, partly due to personal preference, and perhaps partly psychological, in terms of the way different people interpret how information is displayed. My own amateur explanation is something like this: digital readings tell you exactly what you’ve got; analogue gauges provide a visual reference as to where you are. An AI computer will show that you have exactly 60 bar in your tank; analogue SPGs show that you’re approaching the red zone.
In that respect, I vastly prefer analogue SPGs, but I prefer a digital reading of depth – only you can decide which is best for you, and I do feel that AI is something you should try before you buy, if possible. As always, if you’re uncertain, then stick with what you know.
One drawback of AI computers is that they are a single point of failure for all the information you need underwater, which is why I strongly recommend that divers have a backup analogue SPG, either as part of their setup or in their spares kit. If your computer fails underwater then you can often work around an inability to measure dive time and depth (you will have a buddy, after all), but if you can’t measure your air supply then you can’t dive. The most common cause of failure is dead batteries, although, in my experience, this mostly resulted in inconvenience rather than a serious threat to life. If you buy a dive computer with user-changeable batteries, please carry some spares! Which brings me nicely to the next section.
User changeable batteries
If your dive computer is marketed as having a user-changeable battery, it’s usually pretty easy to do with simple tools, a replacement battery kit, and a bit of silicone grease. If you don’t want to do it yourself, there’s probably a handy dive instructor somewhere nearby(!) and if you have to take it to a shop, it only takes a matter of minutes, and you probably won’t get charged for the labour.
If the battery is not user-changeable then it has to go to a technician who has the correct tools to remove the battery cover, replace the seals and then be able to pressure-test the device. This will almost certainly be chargeable and if you do not take it to an authorised technician, your warranty will be voided, so if the computer floods next time you use it, then you will be significantly out-of-pocket.
I think most manufacturers suggest changing the battery every year or after 100 dives, whichever comes first. It might last longer, but if you want to avoid a lot of inconvenience, please follow that rule!
As a final point, when you do buy a computer, please make sure you RTFM – that means Read The FULL Manual – before you take it underwater. Improper use and failure to follow the manufacturer’s recommendations or understand the display can be dangerous but correctly used they are one of the most valuable tools available to recreational divers.