Oxygen AnalyzerOxygen is different from most other gasses in that it is Paramagnetic (attracted by a magnetic field). Oxygen will also intensify a magnetic field. Most other gasses are diamagnetic (repelled by a magnetic field).

The instrument consists of two diamagnetic spheres of glass filled with nitrogen mounted at the ends of a bar to form a dumbbell. This dumbbell is mounted horizontally on a vertical Platinum ribbon suspension.

The whole measuring cell operates inside a strong non-uniform magnetic field. The spheres are repelled from the strongest part of the field and tend to rotate the suspension to a weaker part of the field.

When oxygen content increase, the field is strengthened and the measuring cell will tend to rotate more. In practice a feedback coil hold the measuring cell in the zero position and the current required to do this is a measure of the oxygen concentration in the gas.

Zero position of the dumb bell is sensed by a split photocell receiving light reflected from a mirror on the suspension. The output from the photocell is

amplified and fed back to a coil around the dumbbell so that a restoring torque due to the feedback current balances torque due to oxygen in the sample. The measuring system is ‘Null balanced’.

There is a linear relationship between the feedback current and the susceptibility of the sample. This makes it possible to calibrate the instrument from two points only. Example: Zero can be obtained by purge nitrogen and air for setting the span of the instrument.