This site will tell you what the heck is in your tap water: EWG's Tap Water DatabaseIt’s really gross looking and makes me wonder what the heck is in my tap water.
I asked if you could define "barely", and you've defined it with "barely".When I use Zero water in that same humidifier, even after many gallons, there is barely a thin membrane of chalk which isn’t even enough to justify cleaning.
In my mind, a reading of "000" shouldn't leave a membrane of chalk, or it isn't really "000".
I haven't been able to find anything on the accuracy/fallibility of ZW's meter, and that seems to be the only device used to measure TDS. From a critical viewpoint, that doesn't seem objective or even scientific to me.
It may take longer to build up, but in time, membrane upon membrane will still build up, which doesn't happen with distilled water.
I ran a crude unscientific seat-of-the-pants cost calculation in post #20, assuming a TDS of 250. If someone had really bad water—say a TDS of 500 (rare, but not unheard of), the cost per gallon to filter tap water would increase to $1 per gallon, which is comparable to buying distilled. If RO can be gotten for $.38 per gallon (post #29) that seems more economical (out of pocket) than ZW.