The pH of your nutrient solution greatly affects the availability of nutrients within that solution. pH which is too high or too low can cause deficiencies in essential nutrients that plants need to grow. It can also negatively affect beneficial bacteria in your system or substrate.
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.
The temperature of your solution will also affect the pH value. The Plant!t Essentials 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.