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In  order  to measure the amount of oxygen is a sample from the atmosphere of a closed space or from flue gas, there must to be some way of isolating it from the rest of the sample.  One physical property which distinguishes oxygen from most other common gases is its para-magnetism.  Faraday discovered that oxygen was para-magnetic and was, therefore, attracted by a magnetic field.  The field will also induce magnetism in the oxygen, i.e. a magnetic field is intensified by the presence of oxygen and its intensity will vary with the quantity of oxygen.

Most gases are slightly diamagnetic ,that is ,  they  are  repelled  by  a  magnetic field. Thus  glass spheres filled  with  nitrogen and  mounted  at  the  ends  of  a  bar  to  form a  dumb-bell will  tend  to  be  pushed  out  from  the  strong symmetrical  non-uniform  magnetic  field in  which  they are  horizontally  suspended. When  the  surrounding  gas  contains  oxygen, the dump-bell spheres are pushed further out of the field due to the change produced by the para-magnetic oxygen. Torque acting on the dump-bell is proportional to the oxygen concentration and therefore the restoring force necessary to bring the dumb-bell back to the zero position is also proportional to the oxygen concentration

Zero position of the dumb-bell is sensed by twin photocells receiving light reflected from a mirror on the suspension.  The output of the photo-cell is amplified and fed back to a coil wound on the dumb-bell so that the torque due to oxygen in the sample is balanced by a restoring torque generated by the feedback current.  Oxygen percentage is read from the meter which measures the restoring current.  This is scaled  to give percentage oxygen directly.  Accurate calibration is obtained by using pure nitrogen for zero and normal air for setting the span at 21% oxygen.

False readings are  obtained if the gas being sampled contains another para-magnetic gas.  The only common gases having comparable susceptibility are nitric oxide, nitrogen dioxide and chlorine dioxide.

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