Report on MQ135 sensor
The MQ135 gas sensor is used to detect a wide array of toxic gases. MQ135 is one of the best gas detectors currently in use. It can be tweaked to have a high sensitivity, meaning it can detect gasses even in very tiny concentrations.
Here is the basic circuit of the MQ135 gas sensor:
We could caliberate the gas sensor into multiple modes. This is done to get an accurate reading with respect to the concentrations of the toxic gas we are detecting. We have multiple different configurations, but, we mainly speak about two of them. Baseline and Target Gas calibrations.
Baseline calibration:
We use the Baseline calibration to determine the sensor's reference resistance (R naught) in clean air. This resistance gives us the reference to compare toxic air as compared to clean air. Each sensor's output value is different. This is because of the the temperature, tolerance and humidity of the environment when manufacturing the sensor.
Steps we follow, to achieve baseline calibration:
- warm up period:
we let the sensor preheat for 24 to 48 hrs. This stabilizes the internal components and eliminates the initial drift in sensor readings.
2)preparing a clean air chamber:
we find either a concealed chamber with clean air or, a ventilated area with low concentration of pollutants.
3)Measure the sensor resistance (R naught)
connect the sensor to a 5V battery and a multimeter or a microcontroller. measure the output. Calculate the sensor resistance using the formula:
R~s~ = /frac{R~L~ (V~CC~ - V~out~)} {V~out~}
This is our reference resistance for clean air.
Now, the sensor is calibrated. We will need to periodically recalibrate it though. The temperature, humidity usually tells you how often you need to recalibrate it.
How does the sensor calculate how toxic the air is?
The sensor makes use of the ratio : Rs/R~0~.
Low ratio indicates: higher conc of toxic gas
High ratio indicates: lower conc of toxic gas
Target gas calibration:
This involves tweaking the sensor to accurately detect a particular target gas in a known concentration to map its response. This is very important because the MQ135 acts in different ways when exposed to different gasses.
Then we calculate the resistance the sensor offers when subjected to that environment. The sensor comes with a datasheet, which gives us two constant values 'a' and 'b'. Using these constants, and the formula:
Concentration (in PPM)=(/frac{a R~s~}{R~0~})^b^
We find the concentration of the gas required in ppm.
The above two are the main types of calibrations for the mq135, we also have some other calibrations that are slight variations from the above two.
Dynamic Calibrations:
Dynamic calibrations is a fancy name given to adjusting the sensor to the local environment, accounting the temperature, humidity and other factors. To do this, we use additional sensors to measure the temperature, humidity, and other such factors. We also record how the change in these factors affect the sensor and just tweak it to counter these changes.
Span calibration:
This calibration allows for the sensor to measure accurately over a huge range of concentrations. This is important as, if not done, we could be able to accurately measure only either high or low concentrations.
Freundlich adsorption isotherm graph
Q~e~=/frac{K C~e~}{n}
Where:
- Ce: Equilibrium concentration of the solute in the solution (mg/L).
- K: Freundlich constant, indicating the adsorption capacity (mg/g).
- n: Freundlich constant, representing the intensity of adsorption (dimensionless).
We have a logarithmic version of this formula too:
log(Q~e~)=logK+ (/frac{1}{n})logC