The sensing device includes a first vertical tube having a length substantially equal to the diameter of the tank being tested and which is sealed at the bottom and has connected at the top, a smaller diameter second tube adapted to be positioned vertically for measurement of the height or level of liquid...http://www.google.com.hk/patents/US4679425?utm_source=gb-gplus-share專利 US4679425 - Liquid storage tank leak detection system
