The most common pH meter calibration is the two point calibration and is best suited when you have a range of pH samples. The buffer solutions should bracket your expected pH sample. In this process the pH meter determines the slope and offset error for the actual pH electrode in use. This information then allows the meter to adjust the mV/pH equation of the pH meter to match the characteristics of the electrode in use. http://www.yinmik.com/ph-meter/manual-calibration-ph-meter/