A medical fluid machine includes an enclosure a disposable unit accepted by the enclosure, the disposable unit including a conduit through which a medical fluid can flow, the conduit including a viewing portion a light source configured and arranged to emit light onto the viewing portion of the conduit a camera focused on the viewing portion of the conduit and a processor configured to determine a speed of a particle entrained in the medical fluid based on at least two images of the particle in the viewing area taken by the camera.
