Method 1: See if oxygen lies within an expected range (no calibration required)
The output of the sensors have a gain of 100 applied to them (Allowing better resolution from the ADC). This gain is also present in the database readings, and stored in Volts. So simply take database results, divide by 100 to remove gain, multiply by 1000 to convert to millivolts, and if the reading falls between 5.5 and 8.5 mV we can assume normal concentration.
Oxygen concentration is normal if 5.5mv < ( reading in mV / 100) < 8.8mv
See the red dotted lines below indicate the upper (8.8mv) and lower (5.5mV) expected readings for a 21% oxygen level. The readings show a well ventilated room. Readings in the basement would tend towards the lower limit, if not exceed them.
Method 2: Converting to Concentration of Oxygen in Air (%) (calibration required)
Measurement of 0.72v is observed (from our databases)