A NDIR (nondispersive infrared) gas sensor module is a simple spectroscopic device for gas detecton. It is simple and is very accurate and reliable and has a long lifetime because there are no consumables in the NDIR process. The NDIR modules use a broadband IR detector [thermopile, pyroelectric, lead selenide (PbSe) or MerCad Teluride] with a narrow bandpass filter in front of the detector to look at the absorption band of the gas you are interested in. They require a broadband IR source to shine through the gas.
If your choice is the complete NDIR Gas Sensor Module, you have you have the options of Precision, Compact and Low Cost.