ChemicalThe 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.
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