TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Component: The first sensing device senses a predetermined gas.The second sensing device senses a different gas than the first sensing device. It comprises a reference device for compensating the first sensing device.The reflective surfaces focus portions of the light from the emitter to the sensing devices. They are separate parts that are assembled to form the reflective surfaces within the chamber. They are integrally formed.The gas entry opening includes a filter to exclude contaminants.The chamber has more than one gas entry opening.The reflective surfaces are coated with a material to prevent degrading of the reflectivity.The first and second sensing devices are on opposite sides of the emitter. They have at least partly surrounding light collectors to increase the light rays focused on the sensing devices.The light collectors are reflectors (38a, 38b), or lenses.The emitter, first sensing device or second sensing device has a lens to focus light.The emitter has a reflector around a portion of it to focus light.The gas detector may comprise a housing (12); control circuits carried by the housing; a diffusion chamber carried by the housing, and having sensing and reference portions (30a, 30b); a reflector symmetrically located relative to the portions; and a gas inflow port, on the housing, oriented to enable inflowing ambient gas to diffuse symmetrically into the portion. It can be a gas sensor comprising the housing and a gas inflow portal; or a chamber comprising the sensing devices, the emitter, the first and second concave reflective surfaces, and the gas entry opening as above.The housing defines first and second mirror image portions that abut one another along a common plane and are open to one another.The gas inflow portal is laterally located relative to the mirror image portions.The chamber can be a folded beam, gas sensing chamber comprising curved reflective surface(s), the emitter, sensor(s) (34a, 34b) of emitted light reflected just once, and first and second optical filters (36a, 36b) for producing a gas related radiant energy beam and a reference beam.The control circuits include executable instructions to compensate for undesired gas born contaminants. The contaminants comprise water vapor.The inflow port is symmetrically located relative to the common plane.The inflowing gas moves in a direction generally parallel to the common plane.The gas detector further includes radiant energy beams that extend generally perpendicular to the common plane.The beams each exhibit a single reflection.A first beam extends across and reflected in one portion with a second beam extending across and reflected in the other portion. The sensing and reference portions are symmetrical.The first light and the second light vary by the same amount due to water condensing on both concave reflective surfaces.The gas sensor includes a radiant energy source, and a filter that overlies the mirror image portions.The radiant energy source is located on the common plane. It directs radiant energy symmetrically relative to the common plane and in a plane generally perpendicular to the common plane.The radiant energy crosses the mirror image portions with only a single reflection.The filter reduces fluid flow velocity in the mirror image portions.Preferred Method: The method further includes reflecting the beams across the respective region. The beams are projected from a common location. They are sensed at two different locations. They are each filtered prior to being sensed.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Material: The reflective surfaces are formed of gold or chrome.    TECHNOLOGY FOCUS - POLYMERS - Preferred Material: The chamber parts are constructed of plastic.