by Lauren Kieren
Labeling and Identifying Nitrox Cylinders
Any time you fill a tank with nitrox, it must be identified as such. This will help to prevent accidents in the event that someone uses a tank filled with nitrox without taking the proper precautions. The industry standard for tanks filled with nitrox is to mark the tank with a tank wrap, as well as identify the mixture with a label or tag.
A nitrox tank wrap is an adhesive decal, generally 10-13 cm. (4-5 in.) in width, that is designed to completely encircle the diameter of the tank. The decal is usually printed in yellow and green with the word Enriched Air and/or Nitrox printed continuously on it in bold green or yellow letters. This tape makes it quick and easy to spot a nitrox bottle in a group of tanks.
It is standard practice to identify the actual nitrox mix currently contained within the cylinder and note it on a Contents label. This information can be found by analyzing, or watching someone analyze, the cylinder for oxygen content using a properly calibrated and functioning analyzer. At a minimum the information recorded on the label should include; oxygen content, maximum operating depth (MOD) of the gas you will be breathing, the name / initials of the person who analyzed the mixture, and the date it was analyzed.
After you analyze the cylinder and find the oxygen content, the rest comes pretty easy. The only additional “how to” necessary is calculating the Maximum Operating Depth (MOD).
To calculate the MOD for a specific ppO2 and percentage of oxygen (FO2) the following formula is used:
MOD = 10metres X [(ppO2/FO2) - 1]
For example, if the gas contains 36% oxygen (FO2 = 0.36) and the maximum ppO2 is 1.4 bar, the MOD (m) is 10 metres x [(1.4 / 0.36) - 1] = 28.9 metres
MOD = 33feet X [(ppO2/FO2) - 1]
For example, if the gas contains 36% oxygen (FO2 = 0.36) and the maximum ppO2 is 1.4 bar, the MOD (fsw) is 33 feet x [(1.4 / 0.36) - 1] = 95.3 feet.
The additional information on the Contents label concerning nitrogen and helium is primarily intended for use by certified TDI Technical Divers trained to dive with helium in their breathing gas mixtures.
Finally, it is important to note that although the industry standard is described and pictured here, some countries require different looking labels by law. A proper nitrox diver course will teach you these procedures and allow you to practice until you are comfortable analyzing and properly labeling a nitrox cylinder.