Abstract: A method and apparatus for detecting the presence of a particular constituent in a sampled atmosphere. The sampled atmosphere is drawn along two substantially identical flow paths, each path containing a detector. The flow of the particular constituent is delayed along one path only but not the other path to separate, in time, the signals from the two detectors. Means are provided for obtaining a resultant difference signal and for utilizing the difference signal to provide a positive identification of the presence of the particular constituent in the sampled atmosphere.

Abstract: A method and apparatus for detecting the presence of a particular constituent in a sampled atmosphere. The sample is drawn through two substantially identical flow paths, each path containing a detector. The conditions prevailing in one of the flow paths is such as to convert the particular constituent into matter which will not produce a response in the associated detector. When the particular constituent is vapor originating from an explosive the conversion can be by heating the flow path to a temperature at which the vapor breaks down into components which do not produce a response in the associated detector. In the absence of the particular constituent in the sampled atmosphere the signals from the two detectors are substantially equal but the presence of the particular constituent results in a difference in signals from the two detectors.

Abstract: Baseline drift and noise can produce difficulties in registering signals quantitatively without recourse to frequent rezeroing of the registering instrument. The invention provides a circuit in which the registering instrument only functions when a required signal is to be noted, the instrument at all other times being at a zero reading. The characteristics of a signal to be registered will be known. The circuit includes a differentiator which notes the rate of change of incoming signals and operates a switching device upon detecting a required rate of change to enable the signal to pass to the registering instrument. At all other times the registering instrument is isolated by the switching device.