Abstract: Liquid level monitoring apparatus for monitoring the level of a liquid in a container having a wall with an exterior surface and an interior surface and formed of a material which is substantially transparent to optical energy and which has an index of refraction different from that of the liquid. The apparatus comprises an optical sensor holder and adhesive means adapted to secure the sensor holder to the exterior surface of the container at a predetermined level. A sensor head is provided. An attachment mechanism is carried by the sensor head and the sensor holder for detachably mounting the sensor head on the sensor holder. A self-contained power supply is provided which is coupled to the sensor head.