These meters give you quick and accurate readings of conductivity and TDS. No need to match colors or use dropper bottles. Simply dip in the tester and read the display. DIST1 and DIST2 give readings in parts per million of Calcium Carbonate, and DIST3 and DIST4 give readings in µS/cm (micro siemens). ATC assures that all readings are corrected to compensate for the effect of temperature. An LCD displays the digits clearly. If calibration is needed simply submerge the electrode into the calibration solution (sold separately), and turn the trimmer at the back of the tester.