 Carbon dioxide sensors provide an important metric for testing environmental conditions and basic air quality. There are two common types of sensors. A true CO2 sensor, like this SCD-30, measures carbon dioxide levels by exposing air to an internal light source and recording how much infrared light of a specific bandwidth passes through. This technique is called non-dispersive infrared sensing. By adjusting which wavelength of light is detected, it can be used to measure the presence of a variety of different gases. The other kind of sensor is an effective or equivalent CO2 sensor, like this CCS-811. Note it is an ECO2 sensor. This one is much less expensive, but attempts to approximate CO2 levels by measuring organic gases. It's fine for basic indoor air quality sensing, but you may want to calibrate against a true CO2 sensor to determine if the approximation is good enough.