Introduction
The MQ135 gas sensor is a popular sensor used to detect various harmful gases in the environment, such as ammonia, carbon dioxide, smoke, alcohol, and benzene. It works by measuring changes in its electrical resistance when exposed to these gases.
Calibration for Different Gases
Calibration means figuring out how the sensor’s resistance relates to the concentration of each specific gas. The sensor resistance in clean air is called R₀, and the resistance when exposed to gas is Rs. The sensor output is given by the ratio:
[ \frac{R_s}{R_0} ]
where:
- ( R_s ) = sensor resistance in the presence of gas
- ( R_0 ) = sensor resistance in clean air
Different gases affect the sensor resistance in unique ways:
- For ammonia (NH₃), Rs/R₀ decreases sharply in the range of 10–300 ppm.
- Carbon dioxide (CO₂) shows gradual Rs/R₀ changes between 350–10,000 ppm.
- Smoke causes a quick drop in Rs/R₀, useful for fire detection.
- Alcohol and benzene cause moderate Rs/R₀ decrease as concentration rises.
By plotting ( \frac{R_s}{R_0} ) versus gas concentration on a log-log scale, calibration curves are formed. These curves help convert raw sensor data into actual gas concentration (ppm).
Freundlich Absorption Theorem Graph
The MQ135 sensor response follows the Freundlich adsorption isotherm, expressed mathematically as:
[ \frac{R_s}{R_0} = A \times (C)^{-k} ]
where:
- ( C ) = gas concentration (ppm)
- ( A ), ( k ) = constants determined experimentally for each gas
On a log-log plot, this relation appears as a straight line, making it easy to interpret sensor readings. Each gas has a unique ( A ) and ( k ), which allows distinguishing between gases based on how the resistance changes with concentration.
Summary
The MQ135 sensor detects multiple gases by measuring changes in resistance. Calibration using the ratio ( \frac{R_s}{R_0} ) and the Freundlich isotherm allows us to map sensor signals to gas concentrations accurately. This makes MQ135 ideal for air quality monitoring and environmental safety.