Another vote for Nitrox
September 24th, 2007 Filed Under Article
Fast forward several months and several dives. I have since completed the PADI Divemaster certification. I have also been doing more diving. My recent dives have been:
- The dives for the Divemaster course
- Four dives in Cabo San Lucas
- Eleven dives on a 3-day live-aboard trip in the Channel Islands (Souther California)
- Seven dives in San Diego’s “Wreck Alley”
- Two dives in Monterey, CA.
The two Monterey dives and five of the seven Wreck Alley dives were on nitrox, the other dives were on air. I have noticed that I seem to have overcome the headaches and other CO2 buildup symptoms I had before. Two of the Cabo dives were pretty strenuous. We were fighting a strong current to get back to the boat. I thought for sure the headaches and such were going to set in, but they didn’t. The Channel Islands dives all went fine as well.
Having said that, I am even a bigger supporter of nitrox for recreational diving now that I was before. I was influenced by:
- Learning more about the physiological aspects of diving from the Divemaster course
- Conversations with a tech/cave/deep dive/trimix instructor
- My own increased focus on safety after a recent motorcycle accident
- Reading stories of divers getting DCS (the bends) while diving within recreational limits
Diving withing recreational (rec) limits involves nitrogen loading. Even though rec limits are no (mandatory) decompression diving, the more nitrogen a person has in their system, then the more opportunity for them to be one of the cases of people who get DCS even while following rec diving tables. The no decompression Recreational Dive Planner (RDP) tables are a model for a large cross section of people. They are not an actual representation of what exactly is going on in any human body, let alone your body. The less nitrogen you have trying to get out of your system, and the slower the pressure change from depth to surface (i.e. a slow ascent), the less likelihood of a person getting DCS (all other things being the same…because there are other things that lead to DCS, like exertion level, being cold and hydration).
From what I have learned, using nitrox reduces the amount of nitrogen being absorbed by the body when compared to using air for the same dive profile. The less nitrogen absorbed, the easier it is to get rid of it and the less chance of DCS. Another safety feature I see is in the case of making an emergency ascent. The less nitrogen I have taken on during the dive, the less likelihood of DCS being an issue after an emergency ascent with a missed safety stop.
One thing to keep in mind with nitrox is the max operating depth (MOD) of a given nitrox mix wherein the oxygen partial pressure in the body becomes toxic. The higher the oxygen percentage, the shallower the MOD. This mean you have to have an idea of the dive profiles you’ll be doing in order to get the appropriate mix percentage.
For a given dive profile within the no deco limits of the RDP air table, nitrox will give an increased level of being conservative with regards to nitrogen loading. If a person dives to the “edge” of the nitrox RDP tables (tables adjusted for the reduced nitrogen loading of a given nitrox percentage), then the only advantage nitrox is offering is either a longer bottom time or shorter surface interval than air, and there is no increased level of conservatism for avoiding DCS since the diver is pushing the limits.
The way I have been diving is to set my computer to nitrox mode and setting the conservative variable on my computer to its max value. One issue with using the conservative factor on the computer is that I can be well within no deco limits, but because of the conservative setting on the computer I end up in deco mode with the accompanying red flags and longer no fly penalty. This recently happened while doing a nice long and slow ascent from a 95+ foot dive. The computer didn’t seem to credit me with the slower than 33 feet per minute ascent (from around 75% of my max depth), so according to the computer I had an eight minute deco obligation between 10 - 12 feet until I could finish the dive. Sure doing this “deco stop” was extra conservative and didn’t hurt, but I would rather have more control over the planning of my conservative ascent and extra safety stop time. I think from now on I will leave my computer conservative variable turned off and just end the dive well withing the no deco time limit of the gas mix I am diving. Even though my computer does not credit me for a deep stop or a slow (10 foot per minute) ascent, I will still continue to incorporate a slow ascent in to my dive plan and just know that the ascent is just another area of conservatism for me while staying within the no deco time of the computer.
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