i'm trying to calibrate my Ph meter and i'm using the solutions that came with it and following the directions. for some reason i dip the probe in the 6.8 and level the meter to 6.8 with the screw driver. i then wipe the probe off and dip it in the 4.0 solution it reads at 4.2-4.3..not 4.0 like it should..and i can wipe it off and put it back in the 6.8 solution and it reads fine. do you have any idea why this may be? could i be doing something wrong?