The most common reasons for pH meter error is dry storage of the electrode and electrode contamination. These cause loss of accuracy, reduced performance and slower response times. Thoroughly clean the electrode after use and always store your pH meter with the cap firmly in place. Frequency of meter calibration is crucial and will optimize your pH meter?s performance. For calibration of pH meters at pH 4.0, pH buffer 4.0 @ 25?C (77?F) based on international (APHA) standard formula.