Moka 1 Cup wrote:If this is correct, and it makes sense, why the TDS meter and the GH test measure anything different than zero hardness, and specifically since the solution is only distilled water + KHCO3, without presence of calcium or magnesium, what is the GH test solution measuring?

The TDS meter measures conductivity, and sodium or potassium bicarb gives you conductivity (and alkalinity) without hardness .

When the hardness is near zero the GH test can't measure anything. It starts a pale green and just gets less pale so you never see an endpoint. When you have a measurable amount of calcium or magnesium in the water the titrations look more like this:

Perfect explanation.
At this point having decided to use potassium bicarbonate either in distilled water or in water filtered with ZeroWater, I can avoid using the GH tester in the future.