PH meters should be calibrated as a rule. Failure to do so may result in incorrect readings. For regular daily use, weekly calibration is recommended. The essence of calibration is to give the instrument the measured potential and pH conversion rule. This is done by inserting the meter into a solution of known pH, it measures the potential between the pH and reference electrodes and stores its value in its memory. The electrodes are then placed in a solution of a different pH and the potential is measured again and stored in memory. From these 2 measurements, the instrument can now calculate the pH value of the solution from the potential measured in any other solution. For the calibration, buffer solutions are used whose pH values are known exactly, for example, pH= 4.01, pH=6.86, pH=9.18. For accurate measurements, at least 2 accurate calibrations must be made.
The majority of horticultural pH measurements are made between pH 5 and 7, therefore it is recommended to calibrate with buffers pH=6,86 and pH=4,01.