One of the most important points we learn in the Enriched Air Diver course is that we must always analyse our nitrox tanks in order to verify the oxygen content. We are taught this very important practice in order to avoid some well-known risks, such as CNS oxygen toxicity and decompression illness, but there are other factors we should be considering. In many cases we are unaware of the filling process and content of our scuba tanks, the condition of the compressor system, or the purity of our breathing gases, nonetheless we mostly trust our dive operators to manage and control these factors to ensure our safety. But aren’t we as divers also partially responsible?
For many newly nitrox certified divers, the most apparent reason for analysing their gas is to avoid central nervous system oxygen toxicity, which is caused by a high partial pressure of oxygen in the body from breathing gases. The deeper the dive, the higher the atmospheric pressure, which equates to a relatively higher partial pressure of breathing gas. Oxygen toxicity becomes a higher risk at a partial pressure above 1.4ATA, which is the equivalent of breathing a nitrox 40 at 25m/82ft.
Oxygen toxicity, however, isn’t the only major risk involved in nitrox diving. The benefit of diving a partial pressure of oxygen higher than 21% is that the remaining percentage (which in a nitrox tank is occupied by nitrogen) is decreased – which increases the limits of no-stop diving compared to normal air (all other things being equal). The dive planning, or in most cases dive computer algorithms, are adjusted to the specific percentage of nitrox, which helps the diver gain the most benefits of nitrox diving on the dive itself. If the dive computer is programmed to a nitrox setting of 32%, a common nitrox mix in most dive centres, however the gas that the diver is actually breathing is air (21%), and the diver follows their plan/computer close to the no-stop limits of nitrox 32, they would have a substantially increased risk of decompression illness, most likely missing unknown mandatory decompression stops.
All nitrox divers should be aware of these risks, however when diving with air we are not taught the need to analyse our tanks at all, which poses some interesting but scary possibilities. For example, it is possible that the air tank you are given on a dive boat is actually a nitrox tank that has not been labelled correctly. In this case your risk of decompression illness may decrease, as you are following the no-decompression limits of a lower oxygen content, but your risk of oxygen toxicity would certainly increase the deeper you dive, as your partial pressure of oxygen will be higher than planned.
Recreational dive centres should not have scuba tanks filled with a higher percentage than 40% oxygen, as these are the maximum limits of recreational nitrox standards, but any dive centre that caters to technical divers could have scuba tanks with up to 100% oxygen content, and possibly even tanks with less than 21% oxygen (a hypoxic mix). The risks of breathing a scuba tank with an unknown content suddenly become far more apparent, and far more damaging, possibly being deadly.
Some dive centres will have a technician on hand to fill your cylinders on-demand, and some will possibly outsource their scuba tanks from a third party. Luckily, in most dive centres all tanks are managed and filled safely, and with the correct labelling, however it only takes one mistake by a dive centre employee or a fellow diver on your trip to cause a potential problem. The most obvious and easiest way to ensure you are breathing the correct blend of oxygen and nitrogen, even if it’s air, is to analyse the tank yourself.
An instructor in Thailand was teaching a TDI Advanced Nitrox Deco Procedures course to a student and they were discussing this very subject – when to analyse an air tank – so to follow through with his principles he took the student to the compressor room, where the tanks are filled, in order to demonstrate. This compressor room keeps air tanks all in one corner, nitrox tanks in another corner, and tech/trimix tanks in another area, so the instructor grabs a tank from the ‘air corner’ and proceeds to let the student analyse the scuba tank. He’s surprised when the analyser reading shows 15% when he is expecting it to read 21%. Confused about why this might be, he calibrates the analyser a second time and analyses the tank himself, and again it reads 15%. He immediately approaches the dive centre manager, suggesting that the analyser cell might be damaged, so the manager insists to analyse the tank himself.
After much confusion and many double-checks, it turns out that a group of hypoxic trimix divers had checked out that morning, and after removing their own labels, had left their cylinders (some containing hypoxic trimix of 10/70) outside the compressor room. The dive centre staff, seeing unlabelled cylinders, had just been doing their job when they rinsed the tanks and carried them in to the compressor room, assuming they were air tanks as they were not labelled. They were then topped up with air from the compressor and left in a designated area to be used the following day by recreational air divers. After being mixed with air, the content of oxygen in the cylinders rose from 10% to 15%, but it was still dangerously hypoxic.
Needless to say, that dive centre took precautions to make sure this mistake never happened again, however they had narrowly avoided a potential incident simply by the teachings of this very topic.
Make sure that the analyser you are using has been calibrated properly, and is reading 21% when exposed to the air around you. Don’t use your breath for calibration, as the moisture can damage the O2 cell inside, and definitely don’t use an air tank for calibration, as there is no real way of knowing it contains 21% oxygen without analysing it first. The O2 cell is the main working part of the analyser, which directly measures an electrical current to determine the reading of oxygen. For this reason it also needs to be regularly calibrated using 100% oxygen, to make sure the O2 cell can react with high oxygen contents, and the O2 cell itself needs kept stored in a cool, dry environment and replaced as soon as it can no longer read up to 99%.
Be aware that flow rate can affect the electrical current, so when you are calibrating or analysing a gas make sure that the pressure is not too high, as the reading may not be accurate. Other factors that may affect the reading are temperature and humidity, so also make sure to follow the instructions in the analyser manual in case you need to adjust it accordingly.
BETTER SAFE THAN SORRY
Of course, you’ll need the proper equipment and a little extra effort, but considering the huge benefit you will get by doing it – a safe and known breathing gas – analysing your air tanks just makes sense. I tell all my students that they have the right to analyse any cylinder they are given by a dive operation, and if they are refused then they should be suspicious. It’s your choice to trust that dive operator, so being aware of the risks is important in order to make your decision. Ultimately, every diver is responsible for their own equipment, but as you know every choice you make can also affect your dive buddy, so next time you go air diving ask yourself – and your buddy – whether you can say for certain what is really in your scuba tank.