The most common cause for pH measurement inaccuracies is an unclean or improperly cleaned electrode. This is very important to note because, during calibration, the instrument assumes that the electrode is clean and that the standardization curve created during the calibration process will remain a valid reference until the next calibration. pH meters on the market today will allow an offset voltage of approximately ±60 mV. The deviation from 0 mV is not unusual but ideally should be no greater than ±30 mV. The calibration process compensates for the change in offset voltage. If the mV offset continues to deviate and the meter is calibrated with a dirty electrode then inaccurate readings will result.