When we go under pressure, our bodies start to absorb the inert part of the air we breathe, the nitrogen. At normal atmospheric pressure we are saturated with nitrogen but by going underwater breathing compressed air, we allow our bodies to soak up more.
Provided we stay no longer at depth than a slow ascent to the surface can give time for our bodies to off gas, we experience no problems. If we exceed these ‘no-stop’ times, we need to make stops at points during the ascent to allow our bodies to ‘catch up’ with this process.
All certified divers should know this and nowadays most wear a computer to monitor the potential state of decompression during and after each dive. I say ‘most’ because I have recently heard of cases of individual divers who eschew a computer saying ‘they know how deep and for how long they can stay’. This is very dangerous thinking.
Of course, many years ago when computers were in their infancy, conservative divers refused to use them, believing a watch and depth-gauge combined with a decompression table was safer. They might have been safe if their watches and depth-gauges were accurate and they were disciplined in their use.
One way to make leisure diving ‘safer’ in this regard is to breathe a gas with less nitrogen in the mix – nitrox – but it is only safer if you don’t take advantage of the longer no-stop times available. Stay longer and you still soak up just as much nitrogen as you would breathing air for the shorter no-stop time mandated.
Computer manufacturers try to make things as safe as possible to keep them away from possible litigation. That’s why they build in a few precautions that sometimes casual users fail to comprehend.
If you dive with your computer set in ‘air’ mode, it will not allow you to switch to another mode such as ‘nitrox’ until a sufficient period has passed – usually twenty-four hours. If you want to switch between air and nitrox, it’s important to start off in nitrox mode, setting air as nitrox 21 (which it is).
For the same reason, if you want to use your computer simply in ‘gauge’ mode, reading only depth and time, it will not be able to calculate your residual nitrogen levels should you wish to then switch to a nitrox or air diving mode, so it will lock you out for a period, up to forty-eight hours if you have been diving deep.
Diving computers have a sampling rate typically of every 10, 20 or 40 seconds. Normally the 20-second setting is the default setting. During a leisurely dive this is entirely practical but it is not suitable for free-diving.
Some computers have a mode specifically for free-diving when the sampling rate is much more often, even every second. This is because if you swim down to, say, 20-metres deep, a less frequent sampling rate might make a sample point at ten-metres on the way down and the next at 15-metres on the way up, totally missing the fact that you went to 20-metres in between. So gauge mode is unsuitable for use by free divers. You may need a computer with a ‘free-diving’ mode. Choose a computer that has the modes you require.
Some of our customers tell us they want to free-dive between scuba dives. Current medical thinking believes this to add a degree of hazard to the activities because the scuba diver’s body will still be loaded with residual nitrogen at this time and that will be recompressed during a breath-hold dive. No computer can calculate for these short bounces while in diving mode because of the aforementioned sampling rates. For this reason, no computer should allow you to switch to free-diving mode while it is still calculating nitrogen levels during a surface interval.
Some foolish divers will leave their computer to ‘off-gas’ at the surface while they go for a swim, ever tempted to duck-dive below the surface. That is a silly as leaving a computer tied off to a rope at the last decompression stop to ‘offgas’ while the diver climbs back on board. We positively do not recommend this. Nor do we suggest you buy a second instrument and switch between them during a day’s diving. That is the road to decompression illness.
A diving computer can only monitor the nitrogen loading of your body if it is attached to you while you on-gas and off-gas. Use it properly and it will keep you safe – although, since everyone is physiologically different and the computer’s algorithm was written for a theoretically typical person, no computer manufacturer can guarantee this.
Always read the instruction manual and be familiar with what you computer displays. Too often people go into decompression status during dives, especially where the water is warm and clear, and fail to understand that this is what their computer is telling them.