Report-MQ135_Gass_Sensor

Report on MQ135 Gas Sensor

🔹 Introduction

The MQ135 gas sensor is a widely used air quality sensor capable of detecting gases such as ammonia (NH₃), nitrogen oxides (NOₓ), alcohol, benzene, smoke, and carbon dioxide (CO₂). It is commonly used in air quality monitoring systems, IoT projects, and pollution detection devices due to its low cost and sensitivity.During my study of the MQ135 datasheet, I observed that calibration plays a key role in accuracy and reliable detection of gases.

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🔹 Specifications

• Sensor Type: Semiconductor gas sensor
• Detectable Gases: NH₃, NOₓ, alcohol, benzene, smoke, CO₂
• Operating Voltage: 5V DC
• Load Resistance (RL): Adjustable (10kΩ typical)
• Heater Resistance (RH): 31Ω ± 3Ω
• Heating Voltage (VH): 5V ± 0.2V
• Power Consumption: < 800 mW
• Concentration Range: 10 ppm – 1000 ppm
• Preheat Time: 24–48 hours for accurate results

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🔹 Working Principle

The MQ135 sensor works on the principle of changes in resistance of its sensitive layer (SnO₂ – tin dioxide) when exposed to target gases.

• In clean air, SnO₂ has higher resistance.
• When pollutant gases are present, the resistance drops due to adsorption of gas molecules on the sensor’s surface.
• This change is measured as voltage variation across the load resistance (RL), which can be calibrated to indicate gas concentration in ppm.

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🔹 Calibration

Calibration is essential because different gases produce different responses.

1. Place the sensor in a known concentration of target gas.
2. Record the sensor resistance (Rs).
3. Calculate the ratio Rs/R₀, where R₀ is the sensor resistance in clean air.

Example Detectable Levels:

• Ammonia (NH₃): Detectable from ~10 ppm
• Carbon dioxide (CO₂): Detectable from ~350 ppm
• Benzene: Detectable from ~10 ppm
• Alcohol & Smoke: Detectable from ~10 ppm

Each gas has a characteristic Rs/R₀ vs ppm curve from the datasheet.

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🔹 Freundlich Absorption Isotherm (Graph)

The sensor’s gas adsorption behavior follows the Freundlich Absorption Theorem, expressed as:

\[\frac{Rs}{R_0} = A \cdot (C)^{-m}\]

Where:

• Rs = Sensor resistance in gas
• R₀ = Resistance in clean air
• C = Gas concentration (ppm)
• A, m = Constants depending on gas type

The datasheet provides log-log plots of Rs/R₀ vs ppm for various gases, showing that sensor sensitivity decreases nonlinearly with increasing gas concentration.

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🔹 Images & Graphs

MQ135 Sensor Module

Example Rs/R₀ vs ppm Graph (from Datasheet)

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🔹 Applications

• Air pollution monitoring
• Indoor air quality control
• Industrial gas leakage detection
• Smart IoT air quality devices
• Smoke and alcohol detection systems

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🔹 Advantages

• Low cost
• High sensitivity to multiple gases
• Easy interface with microcontrollers (Arduino, ESP32, Raspberry Pi)

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🔹 Limitations

• Requires long preheating time
• Cross-sensitivity (reacts to multiple gases, not selective)
• Requires calibration for accuracy

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Conclusion

The MQ135 gas sensor is an effective solution for monitoring air quality due to its wide gas detection range and ease of use. Although it lacks selectivity for individual gases, it is widely applied in IoT, environmental monitoring, and safety systems. Calibration and interpretation of its Freundlich absorption curves are essential for accurate measurements.

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Reference

• MQ135 Gas Sensor Datasheet – SparkFun