DavecUK For Zeroing I’d personally still use 0TDS water. 30ppm is way outside of the useful coffee scale for refractometers, pen style TDS meters tend to be more accurate for determining 0TDS (but not for coffee TDS, which usually starts around 10,000ppm for weak brewed coffee). But it’s up to you, I guess as long as you state the water used for zeroing.
