On the measurement of solute concentrations in 2-D flow tank experiments

In this study we describe and compare photometric and resistivity measurement methodologies to determine solute concentrations in porous media flow tank experiments. The first method is the photometric method, which directly relates digitally measured intensities of a tracer dye to concentrations without previously converting 5 the intensities to optical densities. This enables an effective processing of a large amount of images to compute concentration time series at various points of the flow tank. Perturbations of the measurements are investigated and both lens flare effects and the image resolution turned out to be the major sources of error. An attached mask is able to minimize the lens flare effects. The second method for in situ measure10 ment of salt concentrations in porous media experiments is the resistivity method. The resistivity measurement system uses two different input voltages at gilded electrode sticks to enable the measurement of salt concentrations from 0 to 300 g/l. Power laws are used to relate apparent resistivity values and salt concentrations. However, due to the unknown measurement volume of the electrodes, we consider the image analysis 15 method more appropriate for intermediate scale laboratory benchmark experiments to evaluate numerical codes.