EC stands for electrical conductivity (as far as I know) and different ppm meters read ppm at different conversion rates. For example my bluelab uses a 500ppm to every 1EC rate while others use a 700ppm rate.
What they have in common though is the EC.
Both meters will read different values, but they will read the same EC.
So a 1.0 EC solution read by one meter will read 500ppm and one will read 700ppm.
It causes issues sometimes because in my case I not only misread the feeding amounts, but AN uses a 700 conversion and my meter is 500 so I was overshooting by a large amount.
My Tap seems to average 1EC, so that would be anywhere from 500-700ppm depending on your meter. Yay R/O