A pH sensor for measuring pH levels within a measurement environment is described. The sensor comprises a reference electrode, a pH sensitive electrode, and a controller, for measuring the potential difference between the pH sensitive electrode and the reference electrode, the measured potential difference being indicative of a pH level at the pH sensitive electrode. The controller is operable to apply a voltage across first and second electrodes to control the pH level at the pH sensitive electrode, and to measure the potential difference between the pH sensitive electrode and the reference electrode following the application of the recalibration voltage. In this way, recalibration of the pH sensor is possible within the measurement environment.
