Oxygen analysers

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Chemical

The teflon is semipermeable letting in gasses and not moisture.
The instrument functions by the oxidation of the electrolyte which increases or decreases the conductivity of the cell depending on the oxygen content.

Paramagnetic (Servomex)

The off centre ball is further forced off centre when the oxygen moves in the lines of the magnetic field. This movement is detected.

A mirror is mounted on the arm. A light source is reflected off the mirror to a pair of photelectric cells. The position of the arm governs the light falling on the cells. The unit balances the light by adjusting the current in the righting coil. This current forms the measured value and is directly related to the oxygen in the sample.
Dirt and moisture can be extremely destructive for these instruments and as such should be left on rather than switched off. For fixed installations the cell is kept at 60'C