Normoxic Trimix
Why Trimix?
Now my CCR ticket gave me a lot of enjoyable diving but I started to hit some snags. The first is that managing faults on a rebreather can be complicated and I must admit that I nark badly. Now admitting you suffer from Nitrogen Narcosis in some circles is like admitting that you can't hold your beer but that's just me. I don't want to hit a snag with half my brain off line so I started by restricting my depth and then, as the suitability of the rebreather forced it to be instrument of choice for most of my diving, I started to play with the diluent.

The best way to deal with narcosis is to reduce the amount of nitrogen in your head. You can't replace it with oxygen as the rebreather is already feeding you as much as I am comfortable with so you have to trade out some nitrogen gas for the far more biologically inert helium.

Mathematically we calculate an Equivalent Narcotic Depth as being the depth that you would be breathing the same amount of nitrogen if you were on air. For all that we say air is a bad gas to dive we have all done enough of it to know what we are talking about.

So as the ppN2 = FN2 * depth (all depths in bar absolute so meters*10+1)
Equating ppN2 for the Trimix to the ppN2 at the END for air we get
FN2 * depth = 0.79 * END
so
END = FN2 * depth / 0.79
or
FN2 = END * 0.79 / depth
I'm not narked to the point of significant loss of skills at 20 meters, I'm beginning to go a 30, at 40 I'm wobbly and at 45 I'm gone. For me the standard 40meter limit on the ticket is good. However a 20/30 (20% oxygen, 30% helium and by implication the last 50% nitrogen) mix will give me at 50 meters depth and END of 27 meters. That is my body may be 50 meters down but my brain is responding as if it were at 27 meters. I can cope with that.

The other problem is that the sort of diving I wanted to do involved fascinating wrecks and I wanted more time than 5 minutes deco limit was allowing me. Add to this that helium is a fast gas and you gas on and off large quantities so it is a very unforgiving gas as far as missed deco is concerned. A couple of minutes of missed stops on Nitrox is stupid but breathing O2 on the boat will probably leave you none-the-worse but the same error on helium will probably bend you.

I started blending myself mixes with helium in but drew myself up a set of no-stop tables. Dealing with the narcosis problem had made the time problem worse. I upgraded to a computer that understands helium and rebreathers so I get a better no-stop time but the gain isn't much. Clearly I needed more skills for the diving I wanted to do so this lead me to the mod 2 course.