Abstract: A gas detector (1) to detect a predetermined gas of interest within an air sample, the detector (1) comprising a sampling tube (12), an infra-red radiation source (13) located at one end of the tube (12), an infrared sensing means (17) located at the other end of the tube, an optical filter (16) appropriate to the gas of interest located in the infra-red radiation beam, either adjacent the infra-red source (13), or adjacent the infra-red sensing means (17), an air sample inlet located at or towards one end of the tube (12) and an air sample outlet located at or towards the other end of the tube (12), with the detector (1) capable of emitting an electrical signal or a changed electrical signal in accordance with whether the gas of interest is or is not detected, a pure, reference gas source (26-30) and valve means (31, 32) to control admission of the reference gas to the sampling tube (12) at prescribed intervals of time.