With Flairform's analytical solutions, my tools are going an extra week between calibrations!
pH meters must be regularly 'calibrated' (i.e. adjusted for accuracy) using standard pH buffer solutions which are made to an internationally agreed recipe:
*NIST: National Institute of Standards and Technology
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.