The present invention is flow rate meters which are able to accurately measure the volumetric rate of flow of a liquid through a conduit without requiring foreknowledge of the physicochemical characteristics of the liquid, e.g., for the purpose of calibration of the thermal mass flow rate. One application of the flow meters of the invention is to incorporate them in a system for measuring the flow rate of urine excreted by a catheterized patient. The invention also provides methods for using the flow rate meters.