Avoid CNS Oxygen Toxicity While Diving Nitrox

The topic of oxygen toxicity is rarely discussed during sport scuba training. The reason for this is that if we are breathing air, then oxygen toxicity doesn’t become a concern until beyond the regular sport diving depths. For sport divers who start to use nitrox, Central Nervous System (CNS) oxygen toxicity is introduced as a real concern. A CNS incident can lead to convulsions and unconsciousness, both of which are very bad news while underwater. With more oxygen in the breathing mixture, the depth at which oxygen toxicity starts to become a concern, known as the maximum operating depth, gets shallower. One of the key messages of the TDI Nitrox course is that you should never go deeper than the maximum operating depth (MOD) of your gas.

cns-oxygenOne of the main differences between sport and technical divers is that technical divers will often carry one or more separate decompression gasses for use on their decompression stop. This additional gas is intended to optimize the rate of decompression, and is achieved by using a richer nitrox mix such as 50% or even 100%. In the case of technical divers, the majority of the dive will be carried out deeper than the maximum operating depth of the deco gas. If the diver were to switch to their decompression gas at depth a CNS, an oxygen toxicity incident would be very likely. For this reason, technical divers are extremely careful to avoid switching to their decompression gas at the wrong depth. The only guaranteed way of ensuring that the diver doesn’t switch to the wrong gas is to mark up the cylinder with the MOD and to certify that their buddy double checks that they are switching to the right gas at the right depth.

In the basic Nitrox course TDI recommends the use of 1.4 as the partial pressure of oxygen to be used with 1.6 as an absolute maximum. In the more advanced courses TDI still recommend 1.4 for the main cylinder, but also allows the use of up to 1.6 partial pressure of oxygen for decompression gasses. One reason for this is that it provides more effective decompression, although for divers used to using 1.4 it sometimes causes concern that they are increasing their exposure. However, for decompression stops we rarely spend more then a few minutes at this maximum partial pressure, as we move up to the next decompression stop the partial pressure drops. The exception to this is if we use 100% oxygen. The MOD of 100% is 6m(or 20 ft) which is also usually our last stop. This means that when using 100% at 6m we are right on the 1.6 limit for an extended period of time, as the last stops are always the longest. This shows the importance of holding a stop at exactly the right depth when using rich nitrox mixes.

For a diver with a single stage cylinder, the main challenge is to ensure that they can switch to their stage cylinder while maintaining their buoyancy. If they drop during the gas switch, they may drop below the MOD for the deco gas and put themselves at risk of oxygen toxicity. Equally, they must be able to maintain their depth during the deco stop and be able to manage any tasks or issues that arise without changing their buoyancy. With multiple cylinders, the divers must also be sure to switch to the right gas at the right depth and this is the reason why stage switching protocols are such an important part of any technical diving course.

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4 replies
  1. Tony Barker
    Tony Barker says:

    I appreciate your article so much. I was diving last weekend (and will be again Sat. and Sun. this weekend) for checkout dives as a DM under my local instructor. I’ve been doing it for years and did it with this instructor’s now-deceased predecessor. Don’t worry. It was un-dive-related cancer. I miss him.

    Anyway, another helping diver pulled a navigation bouy while I (and another safety diver) had our two O/W checkout students on their first navigation dive (course out and back). Nobody’s ever in any danger save a too rapid ascent, at the most. We do our O/W checkout dives in a state park with high visibility, lots of markers and a decent understanding of our state’s navigation law requiring 100 feet from any dive flag by a powered surface vessel. And, June in the deep freshwater Lake George, NY, isn’t as cold as you’d pretend. Fortunately, this fresh water body marks a very photogenic thermocline.

    Long story, short … one of my (our) charges, panicked for not seeing a buoy that was unexpectedly removed on her first O/W navigation dive. I’d sure be scared too, wouldn’t you? Once ashore, I assured this student that we were, in fact, where we should have been (course 090 E -22 ffw Lake George, NY, USA @ Hearthstone Point), but that someone had removed our intended destination (the aforementioned buoy). Rest assured, I mentioned it to the class instructor in no uncertain terms. My point is: “Care for your divers.” … and, from Dickens: “God save us, one and all.” Happy splashes.


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