Measurement of O₂ Concentration
Plug the O₂ sensor into the slot #5 (by default) and press keypad "D4" to measure.

Hints on calibration of the O₂ sensor:
After warming up for 2-3 minutes in normal air, select Calibrate from the lower right menu (or press keypad "D#") and put a value of around 21 (or other known value) for the usual 21 Volume % of oxygen in air.