A measurement method and system in which a plurality of sensors are scattered about the system. Each flexible liner panel or tile has an elongate body and a flexible fin or flap formed integrally of elastomeric material, where the flexible fin or flap extends at an acute included angle ) to the side walls and the Shore A hardness of the elongate body 21) is equal to, or greater than, the Shore A hardness of the flexible fin or flap. Circuitry is included that monitors the phase relationship between a voltage across a secondary of the transformer and a current through the primary of the transformer.