The Tactical Use of Dive Computers

Almost every diver uses a dive computer but unfortunately most have no idea how to dive safely while using it. The point of this article is to highlight the practical ways to dive your computer so that your risk of developing DCS is reduced.

“No-decompression limits” on dive tables and computers are the calculated maximum amount of extra N2 our bodies can tolerate. Because we do not know exactly what this limit is, different tables and computers have slightly different limits. Decompression tables are based on certain assumptions. They assume that the diver descended at a prescribed rate, that the diver spent the entire bottom time at the maximum depth, and that the diver ascended directly to the surface at a prescribed rate. In reality the diver usually descends at a slower rate, spends at least some of the bottom time at a shallower depth and usually ascends at a slower rate, sometimes adding a ‘safety stop’ before surfacing. In addition, the maximum depth of the dive usually has to be rounded down to the next deeper depth on the table, and the bottom time usually has to be rounded up to the next time on the table.

In the figure at left, the outside of the box represents the dive that the decompression table assumes the diver has done. The black area represents the actual dive and the grey area is the difference. Every minute the diver is shallower than the maximum depth, he is absorbing less N2 than the table calculates. Therefore, the grey area represents the amount by which the diver is ‘safer’ than if he had done the dive the table assumes he did. This point is critical. Because virtually every dive a person does is more conservative than the decompression table they use, diving using decompression tables has a built-in safety factor compared to the decompression model that was used to produce the tables.

When we dive using a dive computer, it keeps track of our depth and the time spent at each depth (this data is updated every second in newer computers). The computer therefore uses the “real” depth of the diver to calculate the no-decompression limit and decompression requirements. This means that all the built-in safety of the grey area on the figure is removed for every dive. So when comparing a decompression table and a dive computer using the same decompression model, divers using the computer will experience many more cases of DCS than divers using the table.

Wise Ways To Use Your Computer

For many reasons, it is difficult to reliably say that one computer is more or less conservative than another. “Bubble” decompression models are not necessarily safer than classical Haldanian “solution” models. In addition, most newer dive computers have variables that can be changed by the diver. Some allow you to add conservatism to the model while others let you add “deep stops” or change the calculations to try and minimize bubble formation. The diver usually has no idea exactly how changing these settings will change the calculations and the resulting decompression profile.

The two computers I personally have the most experience with are the Cochran and the VR3. The Cochran starts decompression stops much deeper than the VR3 but has much shorter shallow stops so that the total decompression times are similar. Which model is more conservative and which is safer? No way to tell. Both seem to work fairly well.

No matter what dive computer you use, you will probably want to make it more conservative. The objective is to restore some of the built-in safety. One way is to always stay well inside the no-decompression limit shown on your computer. Always ascend before your computer says you have no time left at that depth. Ascend and descend at slow rates, because slower rates of depth change reduce the risk of DCS.

For deeper dives, add a one- to two-minute stop at approximately one-half your maximum depth, even though your computer won’t require it. If you are doing a no-decompression dive, spend five to 10 minutes at a shallow depth (less than 30 feet) at the end of your dive before you surface.

If you are doing decompression diving, always use a slow rate of ascent (30 feet per minute), always do deep stops, and do your compression stops deeper than called for on your computer. How much deeper depends on how your computer responds to this. The Cochran computers only add a few minutes to your decompression time when you do this (if a 10-minute stop is required at 10 feet and you are at 20 feet, the 10-foot stop will clear in 13 minutes or so). Other computers become very conservative when you do this (e.g., the VR3) for reasons not completely understood. For these computers, you will have to be closer to the depth your computer wants you to be at or your decompression time will become very long.

However, the most important point after a decompression dive is staying in the water at a shallow depth, around 10 feet, for a while after the computer has cleared. The amount of time to stay depends on how much decompression you have done – the longer the decompression, the longer you should wait after your computer has cleared. Finally, the longer the decompression time, the slower your ascent from 10 feet to the surface should be. For extreme dives, some divers take 10 minutes to ascend the last 10 feet.

When diving special gas mixtures, there are a few “tricks” you can use to make your diving more conservative. With Nitrox, set the percentage of oxygen in the computer lower than the percentage oxygen you are breathing. For example, if you are diving Nitrox-36 and you tell you computer you’re breathing Nitrox-32, it will calculate your no-decompression limits and decompression profile as if you were breathing four percent more N2 than you actually are, reducing your risk of DCS. But there’s a warning – this trick also will tell your computer you are breathing four percent less oxygen so the calculated maximum safe depth and oxygen exposure will be wrong. You will have to keep track of these values yourself. The same trick can be accomplished when diving Nitrox and using an air computer, but the same warning also applies.

Make Sure You Have Backup

If you dive long enough, every piece of dive gear you use will fail during a dive. Therefore, you have to plan every dive done with a dive computer as if it will fail during the dive. If you’re diving inside the no-decompression limits of the computer and it fails, immediately ascend to less than 20 feet. If you are well inside the no-decompression limits, you can immediately surface or continue the dive at less than 20 feet for for as long as you want. If you were close to the no-decompression limit when your computer failed, spend several minutes at 10 feet before surfacing.

If you are doing decompression dives, you must have backup for your computer. If your dive profile is relatively “square,” meaning you spend most of your bottom time near your maximum depth, you can use decompression tables for backup. This is typically the case for wreck diving but if you are doing a dive at many different depths, tables are not reasonable backup. For example, one cave I dived on the north end of Vancouver Island slowly descends over the first 1,500 feet to a depth of 112 feet. If it takes 31 minutes to swim each way, your computer will probably say it is a no-decompression dive. However, if your computer fails and you switch to tables, you’ll have to use the 120 feet for 70 minutes profile, and that will require about 120 minutes of decompression. Most likely, you’re not carrying that much gas.

The only reasonable backup for your dive computer for these types of dives is a second dive computer. As long as you and your dive buddy stay together, you can each wear one dive computer, one being the backup for the other. If you two are likely to become separated, or if you are going to be diving different profiles, each of you must wear two dive computers. You don’t have to wear two of the same computers. In fact, it is safer if you wear different computers and make sure both are cleared before you surface. I often dive wearing a VR3 and a Cochran. The VR3 tells me to stop for the deep stops. Then the Cochran tells me to stop for intermediate stops. Finally, the VR tells me to stay longer at the shallow stops before surfacing.

Don’t forget that the risk of DCS increases as total decompression time increases (for every decompression model in use). Remember that the risk of DCS increases as the depth of the dive increases. Last but not least, don’t forget that the risk of DCS is low on short, deep dives but if you do get hit, it is likely to be very serious and not respond well to treatment. I should know – I did a cave diving trip last February in Florida, where I got DCS for the first time in my life. I had stopped using the methods I recommend here and probably got bent as a result. Luckily, I made a complete recovery but I now believe the ideas presented in this article are even more important now than ever.

There are many factors that influence your risk for DCS after a given dive but if you are thoroughly informed and plan your dives well with the help of your dive computer, your risk will be much lower. Dive safe.

David Sawatzky, M.D. is a diving medical specialist and has written a diving medical column in the Canadian magazine Diver for the past 10 years. A version of this article appeared in a recent issue of Diver.