ec/cf/tds question

hi guys n gals
i bought this type of pen and it says US on it instead of tds , on the chart i will post what is the correct measurment i should be using.

those are the conversion factors various manufacturers use. to get ppm pens actually use EC and convert to ppm but not all conversions are the same. the pen you have says to use US conversion. i'll have to look it up but it's the lowest of the conversions i think
USA 1 ms/cm (EC 1.0 or CF 10) = 500 ppm

European 1 ms/cm (EC 1.0 or CF 10) = 640 ppm

Australian 1 ms/cm (EC 1.0 or CF 10) = 700 ppm

http://www.kindgreenbuds.com/marijuana-grow-guide/difference-between-ppm-and-ec/