Abstract: An apparatus (1) for determining the concentration of a critical first component, such as hydrogen, of an atmosphere or environment (E), wherein the first component is capable of forming a combustible mixture with a second component, such as oxygen; the apparatus comprises: (A) a sampling unit (10) including: (A1) a small measuring chamber (12) operatively connected with an ignition means (135) capable of being operated so as to initiate in said chamber a combustion of said critical first component by reaction with said second component, and (A2) at least one sensor (15) operatively connected with said measuring chamber for generating a signal formed by essentially proportionate contributions from all significant components contained in said measuring chamber; (B) at least one flame arrestor (14) intervening between said measuring chamber (12) and aid environment (E) of interest; and (C) a control unit (16) capable of imposing a mode of operation in at least two distinct phases wherein, during a first phase,