A fluid processing cassette and sensor coupling system is disclosed, comprising a cassette comprising a cap having an opening formed by an inner cylindrical wall having a first diameter, an outer cylindrical wall having a second diameter, and a contact surface connecting the inner and outer cylindrical walls. The contact surface includes a varying diameter that decreases from the second diameter to the first diameter. A sensor post comprises a ring disposed around a cylindrical body and is positioned to engage with the contact surface of the cap to form a seal.
