I'm using a few gas sensors (MQ2, MQ3, MQ4, MQ5, MQ6, MQ7, MQ8, MQ9, and MQ135) to measure the air quality inside my home. These sensors have been calibrated to work within a specific concentration range (i.e. 200 - 10000ppm). Here's an example model from one of the datasheets:
Rs = Resistance of the Sensor
Ro = Resistance of the Sensor in Clean Air
An Arduino reads the values from the sensor. I then convert the Rs/Ro value to PPM using y = mx + b, where y = Log(Rs/Ro) and x = Log(PPM).
When concentrations are too low (<200ppm) or too high (>10000ppm), the Rs/Ro value falls outside the model. What should I do with the values that fall outside the model's range?