EC electrodes can develop nutrient build up over time. To minimize build up, always rinse the probe in fresh tap water after every use.
CONVERTING EC TO TDS
The Milwaukee EC60 PRO Conductivity Tester, like all conductivity meters, provides a measure of electrically charged ions in a solution and is an absolute measure of conductivity.
In Hydroponics, EC is often converted to TDS in parts per million (ppm). Different scales include the 500 scale, 650 scale and the 700 scale. However, true PPM can only be determined through chemical analysis.
Your mS/cm reading can be approximately converted to TDS (ppm) as shown in the examples below.