A Dangerous gas mixture

[Rick Murchison]

... Simon Richards states that your data implies that the isolator should be open when analyzing. My conclusion is just the opposite. To me your data clearly implies that IF the tanks have different mixes, then the most reliable way to detect this condition is to analyze with the isolator shut. Otherwise you're drawing from both tanks and will get something close to an average reading. Am I missing something??

Nope. You've got it right. If you let your tanks get filled out of your personal control, you are wise to analyse each post with the isolator closed. Just remember to open that sucker before you go diving
As an additional side note, if you ever do get different gasses in your doubles, to get them synced again treat them as independent tanks - that is, you can completely drain one, then open the isolator and get the same mix in both that way before filling with the isolator open as it should be, or with the isolator closed you can adjust one to match the other mix-wise and then open the isolator to equalize the pressure before final topping.
Rick