Two dive buddies look at their computers underwater. One dive computer indicates that it’s time to ascend; the other diver shakes his head and suggests they remain where they are a little longer. Who is right? In most open-water courses, new divers learn to follow the most conservative computer in the group. Generally, that is good advice. However, few new divers learn why one dive computer gives less time than the other. Here, we’ll answer that question – why some dive computers are more conservative than others — and how can choose one to suit your diving style.
A bit of dive-planning history
Long before recreational diving became a popular hobby, John Scott Haldane developed the first science- and evidence-based diving tables for the British Royal Navy. He published his Haldanian decompression model in 1908 and, while it has been modified substantially since then, neo-Haldanian dive tables or decompression models still make up the “brains” of many modern dive computers.
The 1980s saw the advent of a decompression calculation based on bubble formation in the diver’s body and how to control this process. These “dual-phase bubble models” became popular for a time, especially among technical divers. Just like the earlier models, they changed over time and eventually scientists combined characteristics of both types. One of the most popular manufacturers of recreational dive computers currently uses the result of that combination, the Reduced Gradient Bubble Model (RGBM).
So, how does this apply to the average recreational diver?
Same dive computer, different dives
This is perhaps the most straightforward scenario: you and your buddy own the same model of dive computer, but you have done different dives over the past few days. Your friend may have just finished a week of liveaboard diving, whereas you only just arrived at the dive center. The likely scenario underwater will be that your friend will have shorter no-decompression times as the computer is making allowances for previous dives. The same would happen if one of you were to complete three dives per day for the duration of the vacation while the other diver only did one or two.
In fact, if you read the instructions of most recreational dive computers, they recommend taking a day off every five days or so to allow your body to off-gas. Following these rules will not only reduce your risk of decompression sickness, but also make for a more relaxed vacation.
Different computers, same dives
Remember what we said about different ways of calculating decompression above? All dive computers will allow less time the deeper you go and more time if you stay shallower. But the details of the calculations will be different since different brands use different decompression algorithms to calculate your dive time.
None of these are by definition any more or less safe, but some will give you more time underwater than others. Scubapro, for example, use a neo-Haldanian algorithm, which is programmed to allow a relatively long bottom time and give a comparably large amount of credit for a one-hour surface interval. This suits many recreational divers as it allows them a lot of freedom underwater.
By comparison, Suunto computers use the RGBM algorithm mentioned above and are generally known for being conservative. Compared to their neo-Haldanian counterparts, they tend to offer slightly less no-decompression time at depth and less credit for short surface intervals. In practice this usually means the diver on a Suunto computer will signal to go shallower a few minutes earlier than other divers on the first dive of the day. The difference tends to be more pronounced on repetitive dives, especially if the last dive was done close to no-decompression limits and the surface interval was relatively short.
Is that good or bad? Underwater, things aren’t black and white, most of the time. Less time at depth means a diver’s body absorbs less nitrogen. Combined with a slow, controlled ascent that does mean a reduced risk of decompression sickness. Generally, being conservative means being safe.
So, which computer should I buy?
This is the $64,000-dollar question. Which computer suits you will depend on any number of criteria.
First, think about the diving you would like to do, now and in the future. Are you planning to dive a day at a time, for example on weekends? Or will you be joining lots of liveaboards, offering four or five dives per day? What about support and servicing for your computer? Some of them offer user-changeable batteries. Others not only need special batteries, but you may also void your warranty by attempting to replace them yourself
Are you going to spend time in remote areas? If so, then it’s especially important that you can either get support or spare parts. As a rule of thumb: the bigger the brand, the more international support is available.
What’s your budget?
Then there is the question of budget and whether one of the cheaper models will suffice. It all depends on how complex your diving is likely to be. All of today’s computers accommodate basic air and nitrox diving. Watch-size computers are easier to use day-to-day, even when you are not in the water, but their displays are smaller and might be harder to read in low visibility.
Many basic dive computers allow users to switch between different gases on one dive, which is usually associated with technical diving. If you’d like to do that in the future, then your computer-buying considerations change fundamentally. You will now be looking for a dive computer specifically designed to accommodate decompression diving. Any of the types described here would be suitable for most divers; ultimately the question of which dive computer is right for you comes down to a combination of price and planned use for the computer.