The performance of a pH electrode or pH meter will change with time due to aging because as the electrode is used its coating degrades, to ensure it is working correctly, regular calibration is a must. If you are using your pH meter daily, you should calibrate it at least once a week. To calibrate, use either VitaLink ESSENTIALS Buffer 4 or VitaLink ESSENTIALS Buffer 7. Using both of these buffer solutions will make your calibration more accurate.