Apparatus for measuring an amount of ablation fluid circulating in a system for ablating tissue comprises a plurality of electrical contacts located within an ablation fluid receiving chamber of the system, the contacts being located at various heights and a data processing arrangement coupled to the contacts to receive data corresponding to electrical conductance between pairs of the contacts, the data processing arrangement determining a height of fluid in the chamber based on the measured conductance levels and determining based on the determined height, whether an amount of fluid circulating through the system is within a desired range.