NDIR against Metal Oxide Films in CO2 Measurement
NDIR sensors provide definitive CO2 accuracy through direct mid-IR light absorption, while low-cost metal oxide films infer CO2 via prone cross-reactive surface chemistry.
Metal Oxide Sensor
Semiconductor Interaction
Chemiresistive gas detection utilizes a metal oxide sensor to measure chemical concentrations through variations in electrical conductivity. Oxygen ions adsorb onto the surface of the heated metal oxide film and create a depletion layer that limits electron flow.
Resistance Change
Reducing gases like carbon monoxide react with the adsorbed oxygen and release electrons back into the conduction band. This reaction decreases the electrical resistance of the film. The magnitude of the change corresponds to the concentration of the target gas in the surrounding air.
Measuring the current at a constant voltage allows the system to calculate the gas density.
Gas Specificity
Different oxides like tin dioxide or tungsten trioxide show varying levels of sensitivity to specific molecules. Selecting the right material and operating temperature improves the ability to distinguish between gases. Catalytic additives are often used to lower the required energy for the reaction.
Response Time
Heating elements maintain the sensing film at a high temperature to ensure fast reaction kinetics. The time it takes for the resistance to reach a stable value is limited by the diffusion of gas into the porous structure. Recovery occurs when fresh air restores the oxygen layer on the surface.
Filters are placed over the sensor to block interfering chemicals.