It is essential to calibrate using at least TWO pH buffers. These need to differ in pH by at least three pH units over the pH range being used. For nutrient solutions, pH buffer 4.0 and 7.0 are the ideal calibration standards. Some manufacturers claim that their pH meters require calibration using only pH buffer 7.0. Professional practice however, stipulates using two buffers because this will confirm or otherwise that the correct calibration “slope” is being obtained i.e. that both the electrode and meter are ranging correctly.