What olderfart said is exactly true from what I know....Now I believe the cheap meters my not be able to calibrate and checking ppm of low ppm water in not very accurate... I calibrate my apera ec60 before each grow 3 points low, medium, and high which in running coco IMO it's kind of important cause coco is hydroponics with a twist
"Most EC meters do not read accurately below 50 PPM and RO will be <10 PPM when the membranes are good. So you cannot read the EC until it is over the threshold for your meter. PH on the other-hand should be accurate on any calibrated meter when sampling RO Water."