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