MQ135 Gas Sensor Study and Calibration Report
The MQ135 gas sensor is a versatile sensor widely used for detecting air quality and measuring various gases such as CO2, NH3, alcohol, benzene, and smoke. This report provides an overview of the sensor's functionality, calibrations for different gases, the Freundlich Absorption Theorem, and its applications.
1. Overview of MQ135 Gas Sensor
The MQ135 gas sensor operates based on the principle of changes in electrical conductivity in its tin dioxide (SnO2) sensing layer when exposed to gases. The sensor provides an analog output corresponding to the gas concentration.
Key Specifications:
- Operating Voltage: 5V
- Detection Range: 10ppm to 1000ppm (varies based on gas)
- Preheat Time: ~24 hours for accurate readings
- Output: Analog (voltage proportional to gas concentration)
2. Calibration of MQ135 for Different Gases
Calibrating the MQ135 sensor involves determining its sensitivity to specific gases. This process requires plotting the sensor's resistance ratio (Rs/Ro) against gas concentration (ppm) on a log-log scale.
Steps for Calibration:
- Preheat the Sensor: Ensure the sensor has been powered for 24 hours for stable readings.
- Determine
Ro(Baseline Resistance):- Expose the sensor to clean air.
- Measure its resistance (
Ro).
- Record
Rsfor Different Gases:- Expose the sensor to known concentrations of a specific gas.
- Measure the sensor's resistance (
Rs).
Calibration Curve:
- The calibration curve is a log-log plot of Rs/Ro (y-axis) vs. gas concentration in ppm (x-axis).
- Each gas has a unique calibration curve.
Example Calibration Data (Approximate):
| Gas | Rs/Ro (at 50ppm) | Rs/Ro (at 100ppm) | Rs/Ro (at 500ppm) |
|---|---|---|---|
| NH3 | 3.6 | 2.2 | 0.6 |
| CO2 | 4.0 | 2.5 | 0.7 |
| Benzene | 5.0 | 3.0 | 1.0 |
3. Freundlich Absorption Theorem and Its Application
The Freundlich Absorption Theorem describes the relationship between the concentration of gas adsorbed on a solid surface and the gas's partial pressure in the surrounding environment. It is expressed as:
[ x/m = kP^{1/n} ] Where:
- ( x/m ): Amount of gas adsorbed per unit mass of adsorbent
- ( P ): Partial pressure of the gas
- ( k ): Adsorption capacity constant
- ( n ): Empirical constant (( n > 1 ))
Applications of the Freundlich Absorption Theorem:
-
Environmental Monitoring:
- Used to predict the adsorption of pollutants like CO2, SO2, and NOx on activated carbon or soil surfaces.
- Helpful in designing air purification systems.
-
Catalysis and Industrial Applications:
- Evaluates the adsorption capacity of catalysts in chemical reactions.
- Optimizes adsorption systems for gas storage and separation processes.
-
Agriculture:
- Helps in understanding fertilizer and pesticide adsorption in soil.
-
Biological Systems:
- Used to model gas adsorption on biological surfaces, such as oxygen absorption in tissues.
Graphical Representation:
Below is an image representation of the Freundlich Absorption Graph:
4. Sensitivity Curves of MQ135 Sensor
The MQ135 gas sensor has different sensitivity curves for various gases. These curves represent the relationship between the gas concentration (ppm) and the sensor's resistance ratio (Rs/Ro).
Example Sensitivity Curves:
5. Conclusion
The MQ135 gas sensor, when calibrated accurately, is an effective tool for detecting multiple gases. Understanding its calibration process, sensitivity curves, and the Freundlich Absorption Theorem provides deeper insights into gas detection and adsorption dynamics. These concepts are invaluable for environmental monitoring, industrial applications, and scientific research.