Well Ken, I was looking at their mix and they had 10/20 entered which I figured was a little on the hypoxic side for 218 fsw. Rob says that the Helium content was actually 50% not 20%. So I thought that maybe the Oxygen content was more like 18% or 15%? Which should push up the CNSO2
When you're diving a rebreather, the computer is reading your actual PO2, regardless of what you have entered in the computer. Running a diluent of 10/50 allows you to do a better loop flush in case your PO2 gets too high.