Calibration of the MQ135 Gas Sensor
GitHub: Matte0Sacc0 -> https://github.com/Matte0Sacc0
The sensor needs to be pre-warmed up for at least 24 hours before use, as specified in the specs. To warm up the sensor plug it into the current with a power supply module and leave it running without touching it for 24h (36h or 48h are better if possible, if th room in which the sensor is left has an high humidity this time could be necessary), in this phase just plug in VCC and GND pins and do not connect it to any board to be safe.
After many attempts I have found that the potentiometer on the sensor needs to be placed at around 3/4 of the maximum sensitivity (clockwise rotation)
After the calibration has been done the regulation potentiometer must not be rotated again
Define the values of: int DEFAULT_TEMPERATURE = 20; int DEFAULT_HUMIDITY = 33; before running the sketch otherwise the sensor will be calibrated assuming room's temperature of 20°C and room's relative humidity of 33%. These are the default value used by the function "getRZero();", but in this sketch is otherwise used the more specific function "getCorrectedRZero(DEFAULT_TEMPERATURE, DEFAULT_HUMIDITY);", in which these 2 values matter. I've initialized them as the default ones so if you can't define them the sketch will be executed as default.
- Warm up the internal coil everytime you power up the sensor -> (2 minutes after the sensor has been powered up)
- Define the gasSensor() -> "MQ135 gasSensor = MQ135(SENSOR_PIN);"
- Begin the RZERO reading from the sensor until it begins to stabilize (around 30/40 minutes)
- Third the value of he sum of each RZERO has to be divided for the value of count to obtaint the sensor's specific RZERO (...Read the advice below to obtain a better calibration...)
To obtain a better calibration of the RZERO value it's better to discard the first counts of RZERO readings to do that open the Serial monitor on (115200 baud, or initialize it differently) and read the value of count at the round just before the value begins to stabilize. After that subtract the read RZERO sum to the total sum and the read count to the total count and use the new values to make the corrected division for the calibration. (...an example is shown in the 'MQ135_calibration.txt' file...)
After the sensor has been calibrated you can save the obtained RZERO as OBTAINED_RZERO and define the sensor as "MQ135 gasSensor = MQ135(SENSOR_PIN, OBTAINED_RZERO);" in every sketch where you use it. Remember to always warm up the sensor for 2 minutes after you plug it into the current because the internal coil needs to be dryed from the humidity of the room before readung the values.