Paramagnetism is a tried and tested method for measuring oxygen.  It uses a dumbell filled with nitrogen is placed in a magnetic field and it's position determined using a light beam and a mirror mounted on the dumbell. Oxygen molecules are drawn into the magnetic field, acting to displace the dumbell. A current is generated within the detector to vary the magnetic field to move the dumbell back to its original position and this current is then used to derive the concentration of oxygen present in the sample. This is a very dependable method for detection which is not sensitive to pressure variation and can be used to measure oxygen from ppm up to percentage concentrations.