TVOC / NOx Sensor Calibration

TVOC and NOx sensors do not follow traditional calibration approaches. Instead, they rely on internal algorithms and a continuous learning period which the sensor uses to build a baseline and adjust its readings accordingly. 
 In practice this means that readings may fluctuate more in the beginning and be unstable until a baseline is established. Values stabilize after a learning period (which is 12 hours by default). 
 For a deeper explanation: Explaining VOCs, TVOC and the VOC Index