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.
The temperature of your solution will also affect the pH value. Essential pH Meter has automatic temperature compensation. When calibrating, it is necessary that your pH meter and your calibration fluid have the same temperature. To achieve this, place them in the same room several hours prior to performing calibration.