An EC of 1.0 ms will be produced by 700ppm NaCl.

So if you're measuring anything but un-iodized table salt, a ppm is at best a relative measurement.

EC will also sometimes be expressed in CF ( Conductivity Factor) which is EC in ms x 10. Apparantly this decimal position is preferable for marketing purposes.:wtf:

Also, watch out- some ppm meters will express in relative concentrations of MgCl2, which will give a reading of 500ppm @ an EC of 1.0.

Ppm is a really crappy standard of measurement.