A centrifuge for separating blood and blood components having a blood processing vessel mounted on a rotor of a centrifuge. A sensor in the outflow race of a return peristaltic pump detects air bubbles in the fluid within a return loop. The sensor may be a sonic sensor, a sonic pulse echo sensor, or capacitive plates. A pre-determined minimum bubble size or sizes or a cumulative volume may be selected, and the device operator may be warned only of the existence of bubbles that exceed a certain size or of a cumulative volume of bubbles, or the blood donation procedure may be stopped if a bubble exceeds a certain critical size or if a pre-determined volume of bubbles over a certain period or volume of fluid is exceeded.
