Electrical measurement of sediment layer thickness under suspension flows

Frans de Rooij, Stuart B. Dalziel, P.F. Linden 

Department of Applied Mathematics and Theoretical Physics
University of Cambridge, Silver Street, Cambridge CB3 9EW, UK
Tel. +44.1223.339737, fax: +44.1223.337918.
e-mail: f.derooij@damtp.cam.ac.uk.
We thank David Lipman for his technical assistance in the designing and building of the experimental apparatus. The financial support for this work from Yorkshire Water is gratefully acknowledged.

This paper reports a new technique to measure the thickness of a layer of deposited sediment as a function of time, independent of the flow conditions or presence of suspended sediment above the layer. Small electrodes on the bottom and a reference electrode in the fluid above were used to measure the resistance of the layer with a small AC current and a bridge circuit. Using a multiplexer and an Analog-to-Digital converter the growth of the layer can be accurately monitored at many locations on the tank bottom. In a trial experiment the sedimentation under a stagnant column of a monodisperse suspension was examined. The results show that changes in the sediment layer thickness of less than 0.3% can be measured for layers up to 0.2 g/cm2.