My plants in flowering were ready for a feeding so mixed up some nutes using the RO water. pH meter was dead on after adding nutes compared to the liquid test, so that was definitly the problem. I have one of the ph600 Milkwaukees. Great little unit for a pretty decent price. I clean the probe after every use and store it wet with some storage solution. Calibrate it about once a week and its never been more the .1-.2 off.