NOVELTY - Detection apparatus for detecting a leak of a fluid from a first vessel (10) having an outer surface, comprises an odorant material disposed in at least a portion of an annulus, at least a portion of the odorant material having a detectable odor(s).    USE - The invention is for detecting a leak of a fluid, i.e. hydrogen from a first vessel having an outer surface (claimed), or gases including nitrogen, carbon monoxide, nitrogen trifluoride, ethylene oxide, carbon tetrafluoride, and other perfluoro gases. Hydrogen gas is used in e.g. polymer electrolyte membrane fuel cells.    ADVANTAGE - The inventive odorants are not dispersed in the bulk gas but are accessible only to the leaking gas streams, thus leaving the gas uncontaminated and making leak detection by smell viable without adding odorants in the entire gas stream.    DETAILED DESCRIPTION - Detection apparatus for detecting a leak of a fluid from a first vessel having an outer surface, comprises second vessel (40) having an outer surface and an inner surface spaced apart from and surrounding the outer surface of the first vessel, thus forming an annulus between the outer surface of the first vessel and the inner surface of the second vessel; and an odorant material disposed in at least a portion of the annulus, at least a portion of the odorant material having a detectable odor(s).    An INDEPENDENT CLAIM is also included for a method of detecting a flow of a fluid from a first vessel having an outer surface, comprising providing a second vessel; providing an odorant material disposed in at least a portion of the annulus; providing a pressure relief device in fluid communication with the annulus; transmitting the flow of the fluid from the first vessel to the annulus where the fluid has an annulus pressure, where the flow of fluid picks up and transmits a portion of the odorant material having the detectable odor into an atmosphere surrounding the outer surface of the second vessel, via the pressure relief device when the annulus pressure reaches or exceeds a predetermined pressure; and detecting the detectable odor in the surrounding atmosphere.    DESCRIPTION OF DRAWING(S) - The figure is a simplified, partial, cross-sectional view of a vessel.    First vessel (10)    Gas (15)    Inner wall (18)    Odorant layer (30)    Second vessel (40)