I literally orderd this ph meter/ppm meter on ebay a week ago, it immediately had off readings compared to this 20$ cheap [similar quality] ppm meter i orderd on amazon. Ppms where off by 300-400 with the ppm meter that apear to be identical to yours. Maybe imreading on diff scale 500, 700, theres no setting or function im aware of that specifies wether its on the 500, 700, etc scale, or perhaps im misinterpreting scales reguarding ec, and wether or not ppm meters have that option, or is it a formula i must calculate on my own?
the one i got on ebay, stated it was factory callibrated, not sure if i can callibrate these china ones?