In a previous blog post ‘Choosing OC Trimix’ I referred to some of the well established limits for the PO2 of a gas, be it for your bottom mix or decompression gas. In that post I said that we could consider a PO2 of 1.4 as our ‘working’ limit and the start point for calculating the FO2 for the best mix for our trimix dive.
When diving on OC, the moment we get shallower, the PO2 will fall with a consequent reduction in CNS% per minute and OTUs per minute.
That is not the case with CCR. On a constant partial pressure of oxygen our CNS% per minute and OTUs per minute will also remain constant irrespective of a reduction in ambient pressure. This means as CCR divers we are exposed overall to a higher or potentially very high CNS% and OTU loading by the end of the dive.
This is one reason that our start point for the PO2 of our diluent at maximum operating depth (MOD) is much lower than on OC.
Another reason is that if we have a higher FO2 in our diluent, for example one that matches our setpoint at 1.3, it would be difficult if not impossible to flush the loop down should you need to reduce the loop PO2 and/or check the response of your cells.
Depending on which manual you might read or one particular training organisation’s preference compared to another it is generally considered to safe to apply one of the following rules:-
Max PO2 of diluent at MOD ≤ 1.0 - 1.1
Max PO2 of diluent at MOD = Dive setpoint - 0.2
Most of us probably dive with a setpoint in the region of 1.3 - 1.2 PO2 so either rule will put us roughly at the same start point.
So, for example, on a dive to 70m and a maximum PO2 of our diluent at our MOD of 1.0 we would have a FO2 of: -
70m = 8 Bar
1.0/8 = 0.125
So we might well choose an FO2 of 0.12 (12% O2) for the oxygen in our diluent.
The next step is to choose the level of narcosis with which we are comfortable. In this example I am going to elect that oxygen is not narcotic. In a previous blog post, Is Oxygen Narcotic I discussed this and of course it is a personal decision and divers can factor in oxygen as causing narcosis if they prefer.
As outlined in the post about choosing OC trimix, what we are actually doing is calculating the amount of nitrogen (N2) we want in the mix and whatever is ‘left over’ is the helium (He) content.
So as an example let us have an equivalent narcotic depth (END) of 30m.
The FN2 in air is 0.79 so the PN2 at 30m (4 bar) is:-
0.79 x 4 = 3.16
If we want to keep the same PN2 at our Mod of 70m we simply divide 3.16 by the absolute pressure at 70m so:-
3.16/8 = 0.396
So, the fraction of nitrogen in the mix should be 0.395 (we can round it to 0.40). We now have:-
FO2 = 0.12
FN2 = 0.40
The remaining 0.48 (48%) is made up with helium. So our onboard trimix diluent is going to be 12/48.
I hope this post is useful. If you have any questions or I can help in any way you are very welcome to email me at firstname.lastname@example.org
© Eau2 & Martin Robson 2017