The description below, is what i derive from the poor datasheet of this sensor, it may be uncorrect, so if you have suggestions please leave me a feedback.
The “sensitivity characteristics of the MQ-135” figure of the datasheet, you can see it below, it is used to convert the output of the sensor to the related ppm physical physical charateristic for the gas under test.
The graphic above seems a power function
y = a*x^b
ppm = a*(Rs/Ro)^b
using power regression, we can obtain scaling factor (a), and exponent (b), for the gas we would like to measure
Ro = Rs * sqrt(a/ppm, b) = Rs * exp( ln(a/ppm) / b )
We know the current amount of CO2 gas in atmosphere http://co2now.org/, we can use this as a reference for calibration.
ppm = 116.6020682 (Rs/Ro)^-2.769034857
we also know that the current amount of CO2 gas in atmosphere is (unfortunately) 392ppm, so, heating the sensor for 24 hours, and leaving it in open air, if we measure 26954ohm as the resistance output we can Ro should be 41763.
Datasheet does not tell us much for detecting concentration scope for this gas, the figure is from 10 to 200ppm, but we can suppose a limit from 10 to 2000ppm.
The above derivation seems to works quite well, but for estimating the scaling factor, and exponent in a more precise way, i’ve logged raw adc data from MQ135 and correlated to a MHZ14 NDIR infrared sensor.
For more detail: Cheap CO2 meter using the MQ135 sensor with AVR ATmega