A housing assembly for a fluid sensor assembly includes a housing having a first and second sensing volumes. A fluid port attached to the housing has a porous membrane covering first and second fluid apertures. The first sensing volume included a vertically-oriented waveguide and is fluidly coupled to an exterior of the housing through the first fluid aperture, where the first aperture port has an area smaller than an area of a cross section of the waveguide. The second sensing volume is coupled to the exterior of the housing through the second fluid aperture, the second fluid aperture having an area larger than a cross section of the second sensing volume. While the first sensing volume and the second sensing volume are both in fluid communication with an exterior of the housing, the fluid port and porous membrane internally isolate the first sensing volume from the second sensing volume.