EC and ppm are essentially the same difference. EC is just more universal because it will be the same whether your ppm is .5 or .7 conversion. If you have an ec of 2.0, then ppm at .5 conversion will be about 1000 and .7 will be 1400 while EC remains constant. As long as you are monitoring ppm or EC you can do a rough conversion to figure out the other. If you are monitoring ppm then you need to know the conversion ratio of the meter to get a rough estimate of your EC. the conversion factors are essentiallyt he difference between standard and metric. .5 conversion is most commonly used in the U.S. , .7 was rooted first in Australia, and least common .64 is a European conversion. Different brands of meters use different conversion factors. For example, Hanna, Milwaukee 1 ms/cm (EC 1.0 or CF 10) = 500 ppm, Eutech 1 ms/cm (EC 1.0 or CF 10) = 640 ppm, Truncheon 1 ms/cm (EC 1.0 or CF 10) = 700 ppm. All brands share the EC of 1.0 but use different conversion factors for their ppm calculation. EC is more universal, but if you know the conversion factor of a ppm meter, you can calculate EC or vice versa if you want to use an EC meter and calculate your ppm. Hope this helps.