How to Buy a Scuba Diving Computer

In any Open Water Course that you take, you'll learn how to use a dive table. It's actually quite simple to use once you get the hang of it, so why don't more people use them? Dive computers and dive tables are based on the same science, so they're equally safe in terms of your risk of getting decompression illness.

However, there are a few disadvantages that dive tables have, as opposed to dive computers:

1. Dive computers don't round. They are exact. You probably noticed when using the dive table that you often had to round down to the more conservative number. The result is that dive computers will usually give you a longer dive for a given depth than a dive table will.

2. A dive table is equivalent to a dive computer if you quickly go down to a given depth, stay there for the duration of the dive and then ascend in a vertical line to the surface. But, people don't dive like this! They explore a shipwreck or coral reef that has many levels. For this kind of dive, a dive table is useless, but a dive computer will allow you to dive until you are low on air.

3. Dive computers don't make mistakes. Some people are not strong at math and can easily make mistakes when using the dive tables (as you probably discovered in your open water course!) Making a large error can lead to serious problems.

The 2 basic types of dive computers are the wrist-mounted and the "puck" or instrument console style that is attached to your pressure gauge (and often a compass). I only recommend the wristwatch style for the following reasons:

1. If you're in the pool, you don't need to use your dive computer. However, if you have the instrument console style that's attached to the first stage on your regulator, it will be activated and it's impossible not to bring it in. This will suck up a lot of batteries unnecessarily.

2. It's easier to look at your wrist to see your depth/deco time instead of finding your computer down at your waist somewhere. Think about trying to do a slow descent, or a safety stop with one hand holding your computer. This leaves only one hand to hold a line, release air out of your BCD, hold a camera and help a buddy with something.

3. You can use a wrist-watch style computer for other purposes such as wearing around the town, as an alarm, or when free-diving.

One of the latest features available in dive computers is an "air-integration" option. This means that there is a transmitter attached to the first stage of your regulator which sends a signal to the receiver in your dive computer to measure your air. You might use an SPG as a back-up, but it's really not necessary.

I recommend this option for any diver for the following reasons:

1.It's convenient and easy to use. You can look at a single screen on your wrist to measure total dive time, depth, air in tank, and deco time. There is no hassle to try to find your SPG and to have to keep track of 2 things.

2. It helps reduce your drag in the water. There is no SPG hanging down from your waist when you're swimming.

3. The computer can measure air consumption rate and make recommendations for you. If you're working very hard, at a deep depth and using lots of air, the computer will tell you the air-time you have remaining if you stay at that same workload. Even when you're at "0" air-time, it means that you still have enough air to make it to the surface safely. This is extremely helpful on deep dives in strong currents.

These days, it's becoming more and more common to dive using enriched air. This means that the percentage of Oxygen is increased and the amount of nitrogen in your gas mix is decreased. Nitrogen in your bloodstream is what causes decompression sickness so when you dive, this is what your dive computer or table has to control. When you use Nitrox, your body is taking in less Nitrogen, which means that you can dive for a longer time at certain depths.

When you buy a dive computer, make sure it has a setting that allows you to use it with Nitrox, even if you're not certified to use it now. If you're a serious diver (or planning on becoming one), you'll definitely require this at some point. And remember to take the Nitrox Course to get certified! There are some dangers to Nitrox diving including seizures caused by excessive oxygen at certain depths.

You've probably noticed that there is a huge range of sizes of wrist-mounted dive computers. Some look like normal watches you would wear around town, while others are about 15 times that size! I have no specific recommendation, but some advice regarding your own situation.

If you plan to dive in low-light, low-visibility conditions, a larger computer is perfect for you. It will be much easier to read when conditions are not ideal. Or, if you have have poor eyesight, or wear glasses on land but not in the water, a computer with larger numbers could be very helpful.

If your eyesight is normal, and you plan on diving in mostly good-visibility conditions, then a smaller computer should be fine. The big advantage to this one is that you can leave it on your wrist at all times, so you'll be less likely to misplace it.

Many computers offer the chance to download your dive profile information to your MAC or PC. This isn't for everyone and most people don't bother to do it. However, there are a few devoted people who are fascinated by things like this. Beware: there are some good programs and bad programs. Check around on the internet forums for the model you're thinking of buying and see if they have a user-friendly computer program.

Keep in mind though, for the average person this really doesn't matter (I've never done it!)

I would never buy a computer that I couldn't easily change the battery myself. Some models of dive computers require you to send your computer to the factory for servicing every time you need a new battery. It's an expensive hassle, especially if you're a dive professional. And when you're on vacation, this is totally impractical. I usually carry a spare battery for my dive computer, so I can change it in the middle of a dive vacation if necessary.