Hello, according to the datasheet of MQ135 sensor this sensor can be used for: NH4, NOx, smoke, CO2, CO and others. But in the graphs Rs / Ro vs ppm, are air, CO2, CO and NH4, (I do not understand the other symbols), can someone tell me how the concentration in ppm for NOx? Thank you!
(Datasheet: https://www.elecrow.com/download/MQ-135.pdf)
Those sensors are not calibrated by the manufacturer and they are not very accurate.
The graphs in the data sheet are approximate and only representative.
If you want to measure concentrations of a specific gas, then you must calibrate the sensor yourself by taking readings in mixtures containing known concentrations of that gas (and none of the other to which it is sensitive).