Gas and air quality sensors are widely used in indoor monitoring, smart home systems and environmental logging. However, many low-cost modules are often misunderstood and used incorrectly.
This article explains how gas sensors work, what they can realistically measure, and the limitations of popular modules such as MQ series sensors and modern integrated sensors like BME680.
How Gas Sensors Work
Most low-cost gas sensors are based on a heated sensing element that changes resistance in the presence of certain gases.
- A heater raises the sensor to operating temperature
- Gas exposure changes the resistance of the sensing layer
- The change is measured as an analog or digital signal
This method is simple but not highly selective.
MQ Series Gas Sensors
- Examples: MQ-2, MQ-3, MQ-135
- Output: analog and/or digital
- Require heating element
MQ sensors are very common and inexpensive, but they have important limitations.
What they can do:
- Detect presence of gases
- Show relative changes in air quality
What they cannot do reliably:
- Measure exact gas concentration
- Distinguish between different gases
Important considerations:
- Require warm-up time (minutes to hours)
- High power consumption due to heater
- Calibration is difficult
BME680: Integrated Environmental Sensor
- Measures temperature, humidity, pressure and gas resistance
- Interface: I2C or SPI
The BME680 provides a more integrated approach to air quality sensing.
Important note:
The gas sensor does not measure specific gases. It provides an index based on resistance changes caused by volatile organic compounds (VOCs).
Advantages:
- Compact all-in-one sensor
- Lower power consumption than MQ sensors
- Stable and easy to integrate
Air Quality Interpretation
Air quality sensors typically provide:
- Relative air quality index
- Trend over time
They do not provide laboratory-grade measurements.
For example:
- Increase in VOCs → lower air quality index
- Improved ventilation → better readings
Comparison Overview
| Sensor Type | Selectivity | Power | Accuracy | Recommendation |
|---|---|---|---|---|
| MQ sensors | Low | High | Low | Basic detection only |
| BME680 | Moderate (VOC index) | Low | Moderate | General air quality |
Environmental Factors
- Temperature and humidity affect readings
- Sensor aging changes performance
- Contaminants can damage sensors
Consistent conditions improve measurement reliability.
Power and Integration
- MQ sensors require significant current (heater)
- BME680 operates at low power
- I2C is commonly used for modern sensors
Common Mistakes
- Expecting ppm-level accuracy from MQ sensors
- Using gas sensors without calibration
- Ignoring warm-up time
Practical Recommendations
- Use MQ sensors for simple presence detection
- Use BME680 for general air quality monitoring
- Use professional sensors for precise measurements
Conclusion
Low-cost gas sensors provide useful trend information but are not suitable for precise gas analysis. Understanding their limitations helps avoid incorrect conclusions.
For most projects, integrated sensors like BME680 offer a practical balance between functionality, cost and ease of use.