An oxygen sensor (1, 2, 3, 4, 5) comprises a working electrode (WE) and a comparatively larger Ag/AgCl reference electrode (RE). A floating voltage between the electrodes (WE, RE) is temporarily retained and a measurement voltage is applied to between the electrodes (WE, RE) during a first period to cause reduction of oxygen and production of an evoked current at the working electrode (WE). The temporarily retained floating voltage is then applied between the electrodes (WE, RE) during a second period to produce a current out from the working electrode (WE). The pO2 in a liquid medium is then representative of a measured net charge to the working electrode (WE) equal to a sum of a charge transferred to the working electrode (WE) during at least a last part of the first period and a charge transferred to the working electrode (WE) during at least a last part of the second period.
