Norcotic Trimix?

Here's a little something I've been thinking about for a couple of years. Many will say it's a ludicrous idea, but to me it's something worth playing with. Recent deep diving fatalities have urged me to put pen to paper.

It all started maybe five years ago when a few of us gave heliox a try as a diluent in our closed circuit rebreathers on 50 and 75 metre dives. What we found, like others around the world, was that with a pure heliox mix, on the bottom we would get what is termed 'The Willies'. The willies is an uneasy feeling in your mind that things just aren't quite right, everything is just a little bit scary. On the bottom this manifests itself as a feeling of 'What the hell am I doing here?'.

Most of us gave up on heliox as a diluent, and went back to the usual trimix, or more usually heliair diluents.

An Idea....

About two years ago it hit me. Being on heliox is said to give the diver an END, (Equivalent Nitrogen Depth, or Equivalent Narcotic Depth) of zero metres. BUT, this isn't actually the case. Heliox actually gives an END of less than zero because an END of zero would be a PPN2 of 0.79 ATA. Think of it this way: At the surface breathing air in our day to day lives, we are breathing Nitrogen at 0.79 atmospheres (0.79 ATA), so we are in fact ALWAYS narked to the tune of 0.79 ATA. This 0.79 narkedness keeps us sane, tempers our minds if you like. When you breath heliox, you take away that little bit of 'psycosis padding'.

To combat this, I came up with the idea of 'Norcotic Trimix'.

The Reason

Norcotic Trimix is a mixture of Oxygen, Helium, and Nitrogen, in which the Nitrogen content will provide 0.79 ATA at the bottom depth. The idea is that this will give the same narcosis level as breathing air at the surface, or normal narcotic. Hopefully this will get rid of the 'Helium Willies', but allow divers to have the other benefits that heliox provides.

Of great concern to deep rebreather divers, and to some extent to deep open circuit divers as well, is the work of breathing (WOB) at depth. Norcotic trimix provides the lowest WOB for a trimix. This decreased WOB should be instrumental in avoiding CO2 build-up problems at depth. The column 'Equivalent Air Density Depth' (EADD) is the depth you would be at on air to give the same WOB as at depth on the given mix. The shallower the better for WOB considerations.

Below is a table of Norcotic Trimixes with a PPO2 of 1.3. Note that on this table, an END of 0 is not zero narcosis, but surface equivalent narcosis, or a true zero metres.

Comparison

Now let's do a comparison:
We'll take a typical dive to 80 metres for 20 minutes.
We'll use ZPlan, with 2 minute Pyle Stops, and no conservatism.
The outcome is almost identical for Open Circuit using EAN50 and 100% O2, and for CCR running mix diluent all the way to the surface and a setpoint PPO2 of 1.3 ATA.

Observations

As you can see, pure heliox is best for WOB reasons, but 'The Willies' can be a problem.
Trimix 10/50 is best for shortness of deco time, but with high WOB.
Norcotic Trimix is a very good middle ground, providing reasonably short deco, a clear head, and hopefully, no Willies.

Recent Events

As an aside, if you look at a recent very deep rebreather dive that resulted in fatality due to CO2 problems, then the use of Norcotic Trimix of 4.6/92.5 would have given a WOB equivalent of 43m instead of 74m with the gas used, Trimix 4/80.