On the design of electrical conductance probes for foam drainage applications: Assessment of ring electrodes performance and bubble size effects on measurements

This work examines critical design aspects of electrical conductance probes for measuring the liquid fraction in foams. These include the electrodes’ size, shape, separation distance and intrusiveness as well as issues such as the excitation current frequency, multiplexing, data reduction to liquid fraction, etc. Measurements are performed with ring type electrodes, flush mounted on a test vessel wall at different heights to provide a measure of the longitudinal variation of liquid fraction in the foam. Runs are also performed with traditional disk and rod type electrodes immersed in the foam. The electrically determined local liquid fraction is compared with the photographically determined local bubble size distribution at the wall as well as with the volumetrically determined global liquid fraction in the entire vessel. For the examined protein-polysaccharide stabilized wet foam, drainage is negligible for a substantial initial period of time and only later a liquid fraction gradient emerges. In this no-drainage period, comparison of the local electrical signal with the local bubble size distributions reveals that the bubble size affects the liquid fraction. Moreover, a strong correlation is found between bubble size and onset of drainage.