1. The "Human Nose" Principle
The NOx Index does not measure an absolute concentration. Instead, it mimics how a human nose perceives odors.
Acclimatization: When you enter a room, your nose detects a smell immediately, but after a while, you stop noticing it. Your brain accepts that smell as the new "normal".
The 24-Hour Baseline: The sensor uses a moving average of the last 24 hours as its reference point (learning time).
The Offset: This 24-hour average is automatically mapped to a value of 1.
2. Why the Readings Stay at 1
Consistent Environments: If you live in an area with constant, steady pollution, the sensor "learns" this as the baseline. Because there is no change relative to the last 24 hours, the index stays at 1.
Clean Air: In environments with nearly no oxidizing gases, the index will naturally remain at its lowest point.
3. When Does the Index Increase?
The index (ranging from 1 to 500) only rises when the sensor detects a new event—a sudden spike in gases that differs from the recent 24-hour history.
Typical Triggers: Using a gas stove, being near heavy vehicle exhaust, or sudden industrial emissions.
Intensity: A value above 1 means there are more NOx compounds present now than the recent average.
4. Managing the Sensor in the App
It is important to note that the TVOC and NOx sensors operate as a paired unit within the same hardware module.
While the sensor is self-calibrating and "always learning," you have manual control over its power state.
Toggle Path: Go to Settings -> Sensors -> Advanced -> TVOC mode.
Turning "TVOC mode" OFF will disable both the TVOC and the NOx sensors simultaneously. Ensure this is ON if you wish to track NOx events.
A typical kitchen activity profile is presented, for which simultaneous monitoring of VOC and NOx indices helps to distinguish between different types of events.
