The Hanna TDS-1 has a accuracy of +- 10 ppm (@ 20 deg C. or 68 deg F) .
When calibrated with the 800 ppm packets, your readings have a range of 20 ppm. Your distilled water that measures 0 ppm with this meter could actually have 10 ppm of Total Dissolved Solids. Also a CS solution that is actually 10 ppm may have a meter reading range from 0 to 20 ppm ( 100% error). The TDS-1 was designed to measure ppm in the range 200 ppm ( 5% error) or higher. Both the TDS-1 and the PWT are conductive meters. They function electrically identical by passing a current between two electrodes and performing temperature, range and calibration factors built into each unit. I own a TDS-1.
