Nitrox is the use of oxygen enriched air for diving. Whereas in normal air there is 21% Oxygen and 79% Nitrogen, in nitox diving some of the Nitrogen is replaced by Oxygen to give different mixes. For example 27% Oxygen to 73% Nitrogen, or 32% Oxygen to 68% Nitrogen are common mixes. The less Nitrogen in the mix, the less susceptible you are to the effects of narcosis and the less nitrogen gets absorbed into the body - so decompression requirements are reduced.

This is used by some divers to provide large safety margins on their dives. Other people prefer to use it as a way of increasing their bottom time. However there is a limiting factor. Oxygen becomes increasingly more toxic as it is breathed at higher partial pressures. By increasing the amount of Oxygen in the mix you increase its partial pressure at depth so for each mix there is a maximum safe depth that you can dive to. The BSAC recommends a maximum partial pressure of 1.4 bar when diving with Nitrox, which effectively limits your maximum depth as shown in the following chart.

N.B. A 32% mix is usually referred to as EAN32 (i.e. Enriched Air Nitrox 32%). A 27% mix is EAN27 and so on ....

You may also hear some divers talking about OTU's (Oxygen Toxocity Units). If you are continually breathing Oxygen at high partial pressures the body eventually becomes saturated and this can lead to serious side effects. But this is rarely a problem for sports divers. Even if you add up all your bottom times, assume they were all at the maximum depth and there were no surface intervals you are unlikely to come anywhere close to the toxicity limit.

The club has installed equipment in the club arch to allow nitrox mixes to be prepared using the club compressor. The technique requires the use of Oxygen cylinders which are obtained from British Oxygen Company (BOC), a decanting hose and non-return valves fitted in the filling hoses. The compressor also has a double-filtration system to ensure that the air is absolutely clean. Before filling a cylinder with air a small quantity of pure oxygen is first bled into it. The cylinder is then pumped with air in the normal way to achieve the required overall mix. A simple lookup table is used to work out how much pure Oxygen needs to be bled in at the start, to achieve the desired end result.

Before you can use the club nitrox facility there are a number of additional steps you must take.

• You must be Nitrox certified i.e. have your Nitrox 'Ticket';
• You must be trained by an existing Nitrox member on how to operate the club Nitrox facility;
• Cylinders must be in test and O2 cleaned within the past 12 months;
• Demand valves must be O2 cleaned within past 12 months if they are to use EAN40 or higher concentrations of O2;
• You must update the log of O2 usage each time your cylinder is filled and pay for the O2 when asked to do so by the Treasurer;
• All fills must be checked with an O2 analyser. Each person should provide their own or have access to a shared one;
• Personal filters must be used when filling from any compressor that does not have a secondary filtration system built in.

Nitrox Blending
These instructions should be used with for obtaining Nitrox fills from the club compressor.

1. Connect O2 whip to Oxygen cylinder and dive bottle (gauge end to dive bottle, valve end to Oxygen cylinder);
2. Take accurate measure of contents of dive cylinder;
3. Calculate number of bars of Oxygen that you need (see notes below);
4. Open O2 valve and decant in the exact number of bars that you require;
5. Close cylinder valve and measure new contents of O2 cylinder. Mark it in chalk on side of cylinder;
6. Disconnect all hoses and fill cylinder with air in the normal way. N.B. The new filling leads are fitted with one-way flow valves;
7. Use Oxygen Analyser to measure Oxygen content of final mix and confirm that it is correct.

Working out how much Oxygen to decant into the bottle to achieve the desired mix sounds complicated but in reality can be determined from a simple blending chart.

Knowing the maximum depth you intend to dive to, use columns 1 or 2 to determine the Nitrox mix that is appropriate for the dive. Column 1 is the BSAC recommendation, based on a mximum partial pressure of 1.4 bar. Column 2 is based on the older standard of 1.6 bar. For example a dive to 40 metres can be done using a 28% mix (at 1.4 bar) or using a 32% mix (at 1.6 bar).

Now follow the row across the page until you get to the column for the cylinder pressure that you're going to pump to. This will tell you how many bars of Oxygen to decant in. So for a dive to 40 metres, using 28% mix (0.28 PO2), a 230 bar cylinder requires 20 bars of Oxygen to be decanted in.

The table assumes you will be pumping with air and that there is only air in the cylinder to start with. If the cylinder already contains a Nitrox mix an additional calculation is required. Use the table to look-up the equivalent number of bars of Oxygen that are already in the bottle. If previous fill was EAN32 and there is 60 bar left, the table shows this is the equivalent of already having added 8 bars of Oxygen. Subtract this from the number calculated for your required mix and you know how many bars of Oxygen you need to decant in. In our example 20 bar (required) minus 8 bar (current), gives 12 bar to decant in.