Experimental procedures and cavitation mechanisms in tensiometer measurements

In recent years tensiometers for direct measurement of matric suction have been developed at Imperial College and later on at the University of Saskatchewan. The major drawback of these instruments is water cavitation which may occur before pressure equalisation. A better understanding of the mechanisms that control cavitation inside the tensiometer may therefore help optimise their design and define adequate experimental procedures. This paper presents some of the experiences gained over the past four years using three tensiometers manufactured by Imperial College. Some of the anomalous responses recorded by tensiometers are first discussed. These were probably due to inadequate saturation of the porous ceramic rather than malfunctioning of the instruments. The results from a series of tests conducted with two or three tensiometers positioned on the same sample are then presented. These were helpful in defining suitable experimental procedures. Finally, the history of tension breakdown of each tensiometer is examined to identify some of the factors that affect the maximum sustainable tension as well as measurement duration. On the basis of data recorded before and after cavitation, a possible mechanism of cavitation in a tensiometer is described.