At which frequency should I calibrate by pH metre?

As with any other instrument (unless stipulated otherwise in the regulations), it is up to the user to determine the frequency of calibration, according to the requirements that must be met. Users can refer to the information in the test or product standards, or to the manufacturing specifications. Afnor document FDX07-014 "Optimisation des intervalles de confirmation métrologique des équipements de mesure" (Optimisation of the intervals between metrological confirmations of measuring equipment) describes a number of methods to determine these intervals.