A simple hot wire liquid level sensor suitable for accurate detection of liquid levels is described. The sensor is characterised by a fast temporal response of typically 20 ms for slow immersion in organic liquids and 5 ms for immersion in water. Temporal response improves as the immersion speed is increased. The sensor is inexpensive, compact, of simple construction and is suitable as a level detector for a wide variety of liquids over an extensive temperature range. A smooth liquid surface may be located to within an accuracy of better than 100 mu m. Application of a combination of two level sensors to the absolute measurement of volumetric flow rates of free flowing liquids is also described.