A sensor (100, 120, 302, 304) for measuring one or more properties of a biopharmaceutical fluid in disposable containers, comprising: (a) a source of electromagnetic radiation (244, 310), and a detector for measuring electromagnetic radiation, the source of and the detector of electromagnetic radiation are included in a reusable sensor assembly (120, 304) having a housing supporting said electromagnetic radiation source for generating a path of electromagnetic radiation; (b) a disposable flow cell assembly comprising a conduit or container (102, 302) having an interior space for a fluid, an exterior, at least one electromagnetic radiation reflective surface at an acute angle relative to said electromagnetic radiation source, and at least one optically clear portion; said optically clear portion (134, 330) is located between the interior and the exterior of said conduit or container (102, 302), and electromagnetic radiation from the electromagnetic radiation source (244, 310) reflects off said reflective surface, then reflects off one or more particulates when located in the interior of said conduit or container (102, 302), and thereafter passes through said optically clear portion (134, 330) and to said electromagnetic radiation detector; and (c) said electromagnetic radiation detector is a measuring detector for measuring electromagnetic radiation generated by said electromagnetic radiation source (244, 310) that had impinged upon a particle of a biopharmaceutical fluid in the conduit or container; characterized in that: (d) said at least one reflective surface and said at least one optically clear portion are included on an optical inset component set within the conduit or container, whereby said reflective surface is located in the interior of said conduit or container; and (e) said path of said electromagnetic radiation is generally parallel to the optically clear portion when the path is located in the interior space for a fluid of said conduit or container. (a) a source of electromagnetic radiation (244, 310), and a detector for measuring electromagnetic radiation, the source of and the detector of electromagnetic radiation are included in a reusable sensor assembly (120, 304) having a housing supporting said electromagnetic radiation source for generating a path of electromagnetic radiation; (b) a disposable flow cell assembly comprising a conduit or container (102, 302) having an interior space for a fluid, an exterior, at least one electromagnetic radiation reflective surface at an acute angle relative to said electromagnetic radiation source, and at least one optically clear portion; said optically clear portion (134, 330) is located between the interior and the exterior of said conduit or container (102, 302), and electromagnetic radiation from the electromagnetic radiation source (244, 310) reflects off said reflective surface, then reflects off one or more particulates when located in the interior of said conduit or container (102, 302), and thereafter passes through said optically clear portion (134, 330) and to said electromagnetic radiation detector; and said optically clear portion (134, 330) is located between the interior and the exterior of said conduit or container (102, 302), and electromagnetic radiation from the electromagnetic radiation source (244, 310) reflects off said reflective surface, then reflects off one or more particulates when located in the interior of said conduit or container (102, 302), and thereafter passes through said optically clear portion (134, 330) and to said electromagnetic radiation detector; and (c) said electromagnetic radiation detector is a measuring detector for measuring electromagnetic radiation generated by said electromagnetic radiation source (244, 310) that had impinged upon a particle of a biopharmaceutical fluid in the conduit or container; (d) said at least one reflective surface and said at least one optically clear portion are included on an optical inset component set within the conduit or container, whereby said reflective surface is located in the interior of said conduit or container; and (e) said path of said electromagnetic radiation is generally parallel to the optically clear portion when the path is located in the interior space for a fluid of said conduit or container.