Pretty resourceful...Maybe try an empty tictac container?
Maybe a silly question: Why not invert the setup? Then there would be direct exchange across the breather bag into the test solution - no air gap. The bag does not allow water to cross, so I assume it also does not allow ions (kH) to cross either.
Not a silly question at all. The reason why I didn't want to invert the contraption is the cheesy rubber band. Pretty sure the solution would leak out/water would leak in. With right side up, air trapped under the rubber band prevents any fluid exchange.Try inverting the container so there's no air gap, just dead air above the drop checker fluid. There's no need for an air gap if you have a waterproof gas permeable membrane.