Ok so I have 3 of the popular TDS3 ppm testers.
I splashed out on some calibration fluid and all 3 failed.
The newest is about 60 out the oldest about 80 both above what they should read.
The other one is about 60 below what it should read.
Only the oldest has an adjustment screw but it doesn't seem to work very well.
So how do we know what our pure readings really are.
In all fairness I got talking to another wfp user about 6 weeks ago and we tested my water using his meter and mine and they both agreed on 2.