Abstract: Method of continuously monitoring an atmosphere to detect the presence of a combustible gas or vapor in the atmosphere which comprises testing the atmosphere with a detector including a transducer which responds to the presence of the combustible gas or vapor and continuously supplying to the vicinity of the transducer a controlled small amount of a combustible gas or vapor to which the transducer responds so that the detector normally indicates the presence of at least the said controlled small amount of combustible gas or vapor, inoperativeness of the detector owing, e.g. to malfunctioning of the transducer being observable by the detector ceasing to indicate the presence of at least the said controlled small amount of combustible gas or vapor.