With this library you can obtain ppm for every MQ135 detectable gases
The way is simple, only imports the library to your .ino source code
#include <MQ135.h>
Instantiate the object
MQ135 mqSensor(A0);/*Where A0 is the analogic input pin 0 where the output(analogic) of the sensor is A0*/
No need to calibrate, it's done when it's booting but you should have good values after a reboot and 48h
Get the resistance (in void loop method)
float resistance = mqSensor.getResistance();//resistance
Get the gas reading
float co = mqSensor.getCO(resistance);//co ppm