The RV Filter guy told me that the TDS units will not measure hardness - and he sells them.
See
HEREHe is usually pretty accurate although his recommendation for a magnetic water softener based on a one season very non-rigorous test is silly.
Clay (WA5NMR), Lee (Wife), Katie & Kelli (cats) Salli (dog).
Fixed domicile after 1 year of snowbirding and eleven years Full Timing in a 2004 Winnebago Sightseer 35N, Workhorse chassis, Honda Accord toad