I cleaned out my tank today, and did a background reading on fresh water, the meter read 0.1EC so thats 50ppm, after filling up with fresh nutes, it rose to 3.EC which is 1700ppm after around four hours its up to 4.4EC which is 2200ppm so I guess the meter is working fine! Plants are doing extremely well, gettin a little bit big though but they've slowed down loads in the last week so shouldn;t get much bigger.