Linearisation of a urinary flow transducer

Abstract

URODYNAMICS IS a subdiscipline of urology concerned with the function of the lower urinary tract on the basis of information in physical signals such as the urinary flow rate, pressures measured in the bladder and the rectum (or abdominal cavity), as well as voltage variations reflecting pelvic floor muscle activity (EMG) (GRIFFtTHS, 1980). This work focuses on the measurement of the urinary flow rate. tn urodynamics, at least two different types of flow meters are in use. The first type simply measures the weight of the voided volume as a function of time. Differentiation of this signal provides an assessment of the urinary flow rate (COOLSAET and VAN DUYL, t981; ROLLEMA, 198t). The essential part of the second type of flow meter is a rotating disk. This disk is kept at a constant angular velocity by means of a feedback control system. Urine that hits the disk is accelerated to the circumferential speed of the disk. The extra energy necessary for this acceleration is proportional to the mass of the urine that 'flows through' the device per unit of time. The rotating disk technique has important advantages over the other type of flow meter (TAMMEN, 1971a, b; ROWAN et al., 1977). As rotathag disk flow transducers have a nonlinear transfer fianction, the output signal should be linearised. In commercial umdynamic measurement systems, such a linearisation is built in by the manufacturer. These complete measurement systems are very expensive and not very atwactive to research institutions; real-time output signals are usually not available and the possibilities for signal processing are limited. We have therefore decided to develop a urodynamic measurement system using a personal computer, an analogue digital convertor and a flow transducer. This work describes a simple linearisation procedure for this type of flow transducer. The method has been tested on six flow trmasducers.