An oxygen sensor arrangement for sensing the oxygen in a breathing loop of a breathing apparatus, which oxygen sensor arrangement comprises: - at least one primary oxygen sensor (30, 31) arranged to operatively measure the oxygen in the breathing loop, - at least a secondary oxygen sensor (31, 32) arranged to operatively measure the oxygen in the breathing loop, and - a control arrangement for obtaining measurement data from said oxygen sensors, wherein: - a test channel arrangement (15, 362) is adapted to operatively provide a first gas having a first known fraction of oxygen from a first gas supply to said primary oxygen sensor (30, 31) at a position (267, 267') adjacent to or directly adjacent to said primary oxygen sensor, and - at least a first test valve arrangement is arranged to operatively open and close the flow of said first gas through said test channel arrangement (15,362), - said control arrangement is arranged to operatively actuate said first test valve arrangement so as to provide an amount of said first gas to said primary oxygen sensor (30, 31) via said test channel arrangement (15, 362), and - said control arrangement is arranged to operatively obtain measurement data from said primary and secondary oxygen sensors (30, 31, 32) characterized in that - said control arrangement is arranged to operatively obtain measurement data from the secondary oxygen sensor (31, 32) and the primary sensor (30, 31) when no test valve arrangements (41, 46) are actuated to provide the first gas onto the primary sensor (30, 31), and - said control arrangement is arranged to operatively obtain a fitted calibration curve for said secondary oxygen sensor (31, 32) using said measurement data operatively assembled from the secondary oxygen sensor (31, 32) and the primary sensor (30, 31). - at least one primary oxygen sensor (30, 31) arranged to operatively measure the oxygen in the breathing loop, - at least a secondary oxygen sensor (31, 32) arranged to operatively measure the oxygen in the breathing loop, and - a control arrangement for obtaining measurement data from said oxygen sensors, wherein: - a test channel arrangement (15, 362) is adapted to operatively provide a first gas having a first known fraction of oxygen from a first gas supply to said primary oxygen sensor (30, 31) at a position (267, 267') adjacent to or directly adjacent to said primary oxygen sensor, and - at least a first test valve arrangement is arranged to operatively open and close the flow of said first gas through said test channel arrangement (15,362), - said control arrangement is arranged to operatively actuate said first test valve arrangement so as to provide an amount of said first gas to said primary oxygen sensor (30, 31) via said test channel arrangement (15, 362), and - said control arrangement is arranged to operatively obtain measurement data from said primary and secondary oxygen sensors (30, 31, 32) - a test channel arrangement (15, 362) is adapted to operatively provide a first gas having a first known fraction of oxygen from a first gas supply to said primary oxygen sensor (30, 31) at a position (267, 267') adjacent to or directly adjacent to said primary oxygen sensor, and - at least a first test valve arrangement is arranged to operatively open and close the flow of said first gas through said test channel arrangement (15,362), - said control arrangement is arranged to operatively actuate said first test valve arrangement so as to provide an amount of said first gas to said primary oxygen sensor (30, 31) via said test channel arrangement (15, 362), and - said control arrangement is arranged to operatively obtain measurement data from said primary and secondary oxygen sensors (30, 31, 32) - said control arrangement is arranged to operatively obtain measurement data from the secondary oxygen sensor (31, 32) and the primary sensor (30, 31) when no test valve arrangements (41, 46) are actuated to provide the first gas onto the primary sensor (30, 31), and - said control arrangement is arranged to operatively obtain a fitted calibration curve for said secondary oxygen sensor (31, 32) using said measurement data operatively assembled from the secondary oxygen sensor (31, 32) and the primary sensor (30, 31).