(21-04-08) Today, diving with nitrox is increasingly spreading among Spanish divers, although it is not used very often. When we travel abroad, we can find a number of cruises that include this service. Divers have to pay a supplement for it in many cases.

what is NITROX?

     The word “nitrox” comes from the acronym Nitrogen and Oxygen. It is also known as “Enriched Air” or EAN (Enriched Air Nitrox). The air is mainly composed of 21% oxygen and 79% nitrogen. In natural state none of the two gas affect us. It is different when they are pressurized. From the moment we rise the percentage of oxygen the new mixture is named nitrox. Raising the oxygen means less percentage of nitrogen.

There are different methods to get this mixture that we will explain later. Oxygen limit for recreational diving is 40% (there is no legislation on the subject and it can be up to 60%, in which case it is called Nitrox B, used for 13 m. depth and partial pressure of oxygen of 1.4bar, to 16 meters and partial pressure of oxygen of 1.6). To identify this new mixture, apart from a range of labels and stickers, we use the nomenclature EAN32, where the number indicates how much oxygen is in our mix. Thus, in this particular case, nitrogen has been reduced to 68%.

a bit of history

     J. Priestley discovered O2 as a chemical element in 1774. In 1878 Paul Bert published the first studies about hyperbaric oxygen. H. Fleuss was the first in diving using nitrox, in 1879. In 1955 U.S. Navy developed deco tables to dive with nitrox. NOAA divers use nitrox from 1970.

     Since 1911 nitrox is used for diving, but only on a professional level. In the 90 it started to be used in recreational diving. Thanks to this trend, tables as reliable as those used for air have been created.

why to use nitrox?

     Many people that do not use nitrox think that using it will make our time diving longer. This is not completely true. By reducing the amount of nitrogen that is dissolved in our tissues (a lower percentage in the mixture reduces the absorption, Henry’s Law), we can spend more time at a certain level than with air, although the total time diving will be the same.

Furthermore, it is thought that in case of narcosis this would take more time to affect us due to the reduction in N2. This is completely wrong. Our time diving depends on many factors, but nitrox is not one of them.

What is true is that nitrox will increase the time we can stay on the bottom. How can this be? By reducing the percentage of nitrogen we delay the entry into decompression time, and therefore we can stay longer in the depth where the air would penalize us. I like to explain it as I was taught. It's like a cup of coffee to which we add sugar until a moment when it is not dissolved. If we had added less sugar in the same time we have spent in this case, sugar would have completely dissolved.

NITROX ADVANTAGES

     * If we use air tables:

1. More time at the bottom to the same depth
2. More time without decompression
3. More efficiency in stops
4. Higher depth for the same time diving

     * Shorter decompressions for the same depth and time
     * Less time of the interval on the surface
     * Reduction of the time to be able to fly
     * Reduction of the narcosis at a same depth
     * Better oxygenation

NITROX DISADVANTAGES

     The most significant disadvantage is the limitation in depth due to the toxicity of the oxygen. This disadvantage is not so, because EAN32 only limits to 33 m, which is deep enough for scuba diving, using 1.4 as a reference partial pressure of oxygen.

     Not every dive centre has nitrox available due to the way of mixing the gas. Mixing can be by partial pressure, where we risk the tanks to explode if they have oxygen inside, or by membrane, which is a safer method but also more expensive.

     Price of renting tanks or recharge them is a bit higher than using air.

DO I HAVE TO CHANGE MY EQUIPMENT?

     In principle, our usual regulator is fully compatible with Nitrox mixture that does not exceed 40% of oxygen. Anyway, if we want to be completely sure we can read the specifications of our regulator. The main difference is in the oxidation. When we increase the percentage of oxygen we also increase the effort that our equipment has to make.

     As far as our computer we will need it allows us to change the percentage of oxygen in the mixture. Today, most of the computers are already prepared for this configuration. It is different if we want to change the limit of partial pressure of oxygen. In this case, it is possible that we cannot do it manually, but via PC.

     Regarding the tanks, we are strongly recommended that it is exclusively used for Nitrox, and it is so primarily to avoid confusion. If our tank is not properly signed and exclusively dedicated to this mixture, it can happen that accidentally use we use it as if it were air and we go deeper than the limits permitted for the mixture and we get poisoned by oxygen.

     Therefore, our tank with Nitrox has to be signed with a sticker with the words Nitrox, Enriched Air Nitrox or Enriched Air. These stickers will be green with the letters in white or yellow if the tank is yellow, and 10 cm wide; or 15 cm. wide in green with two yellow stripes (2.5 cm each, one on the top and the other one on the bottom) if the tank is not yellow. In addition, it has to be another sticker that indicates the percentage of oxygen and high operational depth. Moreover, the diving centre is required to register each load with all its data.

EXPOSICIÓN AL OXÍGENO

     First case: We breath 32% oxygen and do a no-stop first dive to 100 fsw (30 m) for 22 minutes, wait one hour between dives and then do a repetitive no-stop dive to 70 fsw (21 msw) for 34 minutes. We do not need any decompression stops, although we can add a 3-5 minute safety stop for each dive.

     Second case: We breath air, do a no-stop first dive to 100 fsw (30 m) for 22 minutes, wait one hour between dives, and then do a repetitive dive to 70 fsw (21 msw) for 34 minutes. This plan does not require any decompression stops on the first dive (the 3-5 min safety stop is taken) but requires a very long 18 minute mandatory decompression stop for the second dive. If we did not want to conduct a decompression dive on the second dive, a no-stop dive would be only 14 minutes. This is 20 minutes less bottom time than in the first case. By using enriched air nitrox we have an 18 minute decompression advantage