A method for determining a scaling factor for scaling an output of a sensor comprising a first electrochemical cell having a working electrode which is in electrical contact with a conductive track, the working electrode and the conductive track formed on a common substrate, to compensate for the effect on the output caused by any variation in thickness of a working electrode from a reference value of the thickness, the method comprising: obtaining a first measured value (I meas) from a portion of an additional conductive track formed on the common substrate so as to have the same thickness and formed from the same material as the working electrode and deposited at the same time and under the same conditions as the working electrode; and wherein the first measured value is the current (I meas) obtained when a predetermined voltage is applied between the additional track and a further conductive track also formed on said common substrate; generating the scaling factor from the first measured value and a second measured value (I nom), wherein the second measured value relates to the average thickness of the working electrode of a batch of cells and is dependent upon the average electrical resistance of a plurality of additional conductive tracks, each additional conductive track of the plurality of additional conductive tracks being formed on a separate common substrate with a respective one of the cells from the batch of cells and formed from the same material as and having the same thickness as the working electrode of the respective one of the cells of the batch of cells and deposited at the same time and under the same conditions, wherein the second measured value for each additional conductive track of the plurality of additional conductive tracks is obtained by: applying a predetermined voltage between that additional conductive track and a further track formed on each separate common substrate with that additional conductive track to obtain a current value; and averaging the obtained current values to obtain the second measured value. obtaining a first measured value (I meas) from a portion of an additional conductive track formed on the common substrate so as to have the same thickness and formed from the same material as the working electrode and deposited at the same time and under the same conditions as the working electrode; and wherein the first measured value is the current (I meas) obtained when a predetermined voltage is applied between the additional track and a further conductive track also formed on said common substrate; generating the scaling factor from the first measured value and a second measured value (I nom), wherein the second measured value relates to the average thickness of the working electrode of a batch of cells and is dependent upon the average electrical resistance of a plurality of additional conductive tracks, each additional conductive track of the plurality of additional conductive tracks being formed on a separate common substrate with a respective one of the cells from the batch of cells and formed from the same material as and having the same thickness as the working electrode of the respective one of the cells of the batch of cells and deposited at the same time and under the same conditions, wherein the second measured value for each additional conductive track of the plurality of additional conductive tracks is obtained by: applying a predetermined voltage between that additional conductive track and a further track formed on each separate common substrate with that additional conductive track to obtain a current value; and averaging the obtained current values to obtain the second measured value. applying a predetermined voltage between that additional conductive track and a further track formed on each separate common substrate with that additional conductive track to obtain a current value; and averaging the obtained current values to obtain the second measured value.