Gas detector and mining machine

A mining machine provided with at least one rotary cutting head, and connected to a gas detector to detect a predetermined gas of interest within an air sample, said detector comprising an air sampling tube, an infra-red radiation source located at one end of said air sampling tube to emit an infra-red radiation beam along said air sampling tube, an infra-red sensor located at the other end of said air sampling tube, an optical filter appropriate to said gas of interest located in said infra-red radiation beam, an air sample inlet located adjacent one end of said air sampling tube and an air sample outlet located adjacent the other end of said air sampling tube, an air flow detector located at said air sample outlet, with said gas detector capable of emitting an electrical signal, or a changed electrical signal, in accordance with whether said gas of interest is or is not detected within said air sampling tube, a pure, reference gas source, valve means to control admission of reference gas from said reference gas source to said air sampling tube at prescribed intervals of time, and a timing device serving to control admission of reference gas to said air sampling tube automatically, at preselected levels of time.