OK, now that I’m thinking about this again while sober, it finally dawned on me. It’s basically stoichyometry from high school chemistry. HCL has a molar mass of 2, and KNO3 of 5, so a 1:4 molecular ratio of them is 5L:8L volumetrically.
How about changing the electrolyzer to make up for the missing oxygen? It already corrects for the chloroplatanic acid having too much Cl in it, so why not this too?
I’m trying to remember how it used to work when it needed one more mixer and a selector tag on zero instead of one. Didn’t the bath give hydrogen instead of water, and all of the oxygen come from the electrolyzer? And didn’t you end up with some extra hydrogen and oxygen left over from water going in somewhere? Or did it just work perfectly in a closed loop?