The measurement range of the CO2NTROL sensor is 0.5 to 100% CO2 volume which is suitable for cell culture applications. The normal level of CO2 in air is approximately 0.04% which is technically below the measurement range of the sensor. Since the CO2 value is outside of the sensor measurement range, the error will be significant and the reading may appear to be noisy. Please note that this is acceptable and one of the reasons that air is used as a calibration point.