Apparatus for measuring the level of a substance in a container of substantial depth which is practical, efficient, and rugged. Sonic or acoustic waves are transmitted through relatively sonic-wave propagating members, from an emitter member immersed in the substance whose level is to be determined to...http://www.google.com.au/patents/US3975958?utm_source=gb-gplus-sharePatent US3975958 - Apparatus for level measurement of free flowing substances, solid substances or combination of these in tanks, containers, reservoir or the like
