Our start point is exactly the same, we would choose the fraction of oxygen (FO2), based on a safe maximum PO2 of the diluent at our Maximum Operating Depth (MOD).

So using the same depth as the previous post as an 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

PO2/P (where P is the depth as an absolute pressure) = FO2

1.0/8 = 0.125

So we would probably choose an FO2 of 0.12 (12% O2) for the oxygen in our diluent.

Having calculated the FO2 we want, the next step is to calculate how much helium we will need in the loop to meet our chosen Equivalent Narcotic Depth (END). We need to calculate the loop fraction of helium first as this will then tell us what we need to have in our diluent. Remember that because we are on a constant PO2, our loop gas will not be the same as our diluent and in fact will contain a little more oxygen than our diluent. As we are considering oxygen to be narcotic, we need to take this slight increase in FO2 in to account.

So, the fraction of helium needed in the loop (FHe loop) =

1 - (END/P) where END and P (pressure at actual dive depth) are in absolute pressure.

If we chose a 30m END = 4 Bar, and have already planned our dive depth to be 70m, P = 8 Bar

1 - (4/8) = .50

So, the FHe in the loop is .50 (or 50% He)

In order to calculate how much helium we need in the diluent we simply use:-

FHe diluent = 1 - [(1 - FO2 loop - FHe loop) + FO2 dil)]

Firstly, let us check the FO2 in the loop.

Setpoint/P will give us this so:-

1.3/8 = .1625 (rounded down to .16 or 16%)

Therefore, using the above formula, the helium we want in our diluent cylinder is:-

FHe dil = 1 - [(1 - .16 - .50) + .12]

This leads to a diluent FHe of .54 or 54% in our diluent cylinder.

Some of you might have slightly different figures depending on if or how you have rounded up or down your calculations. I tend to round down the O2 and round up the He for conservatism.

Please bear in mind that none of this is a substitute for proper CCR and trimix training.

Safe diving.

© Eau2 & Martin Robson 2017