Abstract: A system for validating capacitance measurement characteristics for a biomass measurement device (probe), is disclosed in which a test chamber for contains a test liquid medium, and a docking arrangement enables the measurement device to be disposed in the test medium in the chamber to measure the capacitance of the medium at a measurement zone in the chamber. A capacitive agent or structure (such as a capacitive device) is positioned in the test medium in the test chamber in a predetermined manner in order to provide a permittivity at the test zone which is different to the permittivity of the media without the capacitive agent or structure present.