Brigadoon - An Occasional Corner on the Internet
Air Quality Sensing (Overview)
Designed by Mark Little
The gas sensor module consists of a steel mesh cover under which the sensing element is housed. This sensing element is subjected to heating via a current passed through the sensor. The gases in close proximity to the sensing element get ionised and are absorbed by the sensing element. This changes the resistance of the sensing element, altering the current flowing through the sensor. A load is connected between the sensor output and ground to generate a voltage which is proportional to the change in current caused by the sensor detecting gases to which it is sensitive.
A sensor commonly used for measuring air quality is the MQ135 gas sensor. This sensor is suitable for detecting NH3,NOx, alcohol, Benzene, smoke, CO2, etc. Typical detecting concentrations for a few gases are:
Gas Detecting Concentration
NH3 (Ammonia) 10-300ppm
Benzene (carcinogenic hydrocarbon) 10-1000ppm
Alcohol 10-300ppm
Disadvantages of the this type of sensor include that its preheat time is over 24 hours and that heater current is about 150mA. These factors make it unsuitable for battery operated test equipment that is used intermittently. The other disadvantage is that calibration using specific concentrations of gas is usually outside of the scope of home experimenters. As a result, in practical terms, this sensor is more suited to permanent installation and the  measurements obtained from these sensors can only provide relative measurements.

For example, it is relatively ease to take a series of measurements that provide an indication of the state of the local atmosphere (higher, average or lower than normal), but that will probably not be able to translated into a specific gas concentration.

Most of the gas sensors available to hobbyists have a similar appearance to the Waveshare gas sensor show above. The four pin connector will normally have the following outputs.
Pin Function
VCC Supply Voltage to the Senor. Typically +5V, but check the datasheet
GND Ground or Earth connection for the supply voltage and the sensor output
AOUT Analogue Output - Voltage proportional to the gas concentrations being measured
DOUT Digital Output - Digital output that goes high when the sensor reading exceeds a value determined by the setting of the potentiometer on the other side of the board.
Mounting Gas Sensors

Mounting pre-assembled gas sensors can present a problem as they are normally designed for bench top experimentation, rather than permanent installations.

The first thing to note is that if the sensor is mounted the wire mesh facing up, dust will tend to settle on the mesh, blocking the air flow to the sensor. If the sensor is mounted with the mesh pointing down, the dust is less likely to settle on the mesh.

The next thing is that two of the mounting holes are very close the body of the sensor and this can make fitting mounting screws difficult.

Thirdly, if you go with using a press fit hole to hold the sensor in place, be aware that some adhesives may contain silicone. If silicone vapors adsorb onto the sensing element surface, the sensing material will be coated, irreversibly inhibiting sensitivity.
Also the sensor's characteristics may be changed due to soaking or splashing the sensor with water.
There are likely to be more issues to consider when mounting your gas sensor, so it is recommended that the manufacturer's data sheets are consulted to ensure that the sensor is not damaged in the mounting process.