A simple known method of measuring urine production is to collect the produced urine via a catheter in a bag.  A nurse replaces the bag with an empty bag at regular time intervals and notes the time and the volume produced since last replacement, and calculates manually a figure representing urine production.  Such a method is relatively slow, labour intensive, and inaccurate as to variations in urine production volume during the interval between replacements of the collection bag. 
Attempts have been made to improve measurements;
U.S. Pat.  No.  3,919,455 describes a device comprising a siphon chamber for the urine with a self emptying function, and wherein the urine volume is measured with the aid of an optic and/or electric sensor.  When the urine level in the siphon chamber increases, the capacitance between two electrodes in the walls of the siphon chamber changes.  In this way a signal is created that corresponds to the amount of urine in the siphon chamber.  See e.g. FIG.  4 and column 4 lines 34 to 52. 
GB2243918 describes a siphoning sensor in contact with fluid. 
A problem of prior art is how to accomplish real time measurements.  Another problem is how to provide intelligent programmed system loops that identifies prolonged emptying procedure.  Some systems that offer volume calculation were either in direct contact with the fluid or comprise an optic sensor or a scale that provides the information over time.