Abstract: A sensor system for detecting the absence of substantially air-free blood in a blood chamber of an artificial kidney dialysis system. The system includes a first and a second radiation source and sensor means responsive to said radiation. The sensor means is aligned with and intersects the axis of the first source, and is not aligned with and does not intersect the axis of the second source. Perception of sufficient radiation from the first source indicates absence of blood in the chamber. Perception of sufficient radiation from the second source indicates foam in the blood chamber. The sensor system can control valve means, and preferably can selectively be by-passed, and automatically be adjusted for sensitivity.