Various types of meters may be used for measuring the oxygen content. A continuous reading type is one in which platinum wire elements’ are mounted in two chambers, one the measuring chamber and the other, the reference chamber.
Oxygen is paramagnetic, i.e. it is attracted to magnetic fields. Thus one filament has a magnetic field, while the reference filament has no field, and attracts only the air. The circuit forms part of a Wheatstone’s Bridge. The filtered and dried gas is drawn across the elements and the difference in thermal conductivity of the O2′ relative to air, causes a temperature difference in the wires.
This changes the wire resistance, and unbalances the Wheatstone’s bridge circuit, generating a resultant current, which is proportional to the percentage of oxygen in the sample. False readings are likely if the gas sample contains another paramagnetic gas such as NO2.
Zero position check : ‘Zero’ position setting can be done by using a pre-calibrated sample, and then setting the span of the instrument. Test with 100 % Nitrogen. [ CO2 may be used in emergency.] Open control valve for 3 minutes, to obtain zero reading. Now test with atmospheric air to obtain 20.8 % reading for which span control can be adjusted if necessary.