section the housing 58 is mounted on plate which plate closes housing 58 and defines the lower extent of the manifold space 32 a the filter 34 consists of a permeable wall defining a tubular structure that sits concentrically within the housing 58 air received into the interior of the housing 58 from the plumbing fitting 32 is filtered as it passes through the permeable wall into an interior of the filter 34 the aspirator 36 rotates about a vertical axis and sits atop filter 34 to draw air from the interior of the filter a lid 60 overlies the aspirator and mates with and thereby closes the housing 58 a nipple in the form of a tubular spigot projects obliquely forward in a horizontal plane from a side wall of the lid 60 and is sealing received within an end of pipe 50 the other end of pipe 50 sealing communicates with a manifold 52 the manifold 52 communicates with the inlets 26 a 26 b and 26 c in certain embodiments the pipe 54 which connects the plumbing fitting 32 and sample air inlet 24 is long and thin to control the impedance of the sample air flow path and respective narrow apertures are positioned between filter 34 and aspirator 36 and within.