pH Sensor and Calibration method

A pH sensor 10 for measuring pH levels within a measurement environment comprises reference electrode 40, pH sensitive electrode 30, and a controller for measuring the potential difference between the pH electrode and the reference electrode, the measured potential difference indicating pH level at the pH electrode. The controller is operable to apply a calibration voltage across first and second electrodes 50, 60 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. Recalibration of the pH sensor is possible within the measurement environment. One or both of the electrodes 50, 60 may be the pH electrode and reference electrode. The sensor may be combined with a dissolved oxygen sensor where one of the first and second electrodes are used as a working electrode.