I use EC simply because it was the first way I was told about it AND the first pen I bought measured EC
(essentials EC pen, pH pen, pH 7.0 and EC 2.8 Calibration fluids, pH up and pH down).
I have learned the equations to turn EC to CF, CF to PPM an PPM to EC....
EC x 10 = CF
CF x 50 = PPM
PPM...