Abstract: The apparatus comprises a flexible tube through which fluid flows from a supply, and a opaque ball which is adjustably disposed within the tube. When fluid is first fed through the tube, an air bubble forms, and the location of this bubble is set by squeezng the tube and forcing the air bubble to an upper portion of the tube. The opaque ball rests on the top surface of the fluid in the air bubble. A photoelectric detector assembly comprising a source of light and a light detector is mounted on the tube, with the beam of light interrupted by the opaque ball. The light detector is connected to an electronic circuit which is arranged to perform no function so long as the beam of light from the source is interrupted by the ball.