Abstract: A device for detecting the presence of air bubbles within a fluid-carrying tube, for example a blood tube used in medical procedures. The tube is placed between a light source and an optical sensor which produces signals when bubbles pass through the tube. These signals are integrated and a switch changes state when the integration reaches a predetermined value. The detector thus responds to a predetermined quantity of air, either by a large air bubble or an accumulation of small air bubbles. In a preferred embodiment, the switch releases a springloaded clamp which pinches off the tube when an excessive amount of air is detected.