To accurately assess the severity of coronary artery disease, intracoronary pressure and flow measurements are required. In this paper, a novel flexible flow sensor, intended to be mounted on a medical guidewire, is tested experimentally. The device consists of a heating element and two thermopiles embedded in polyimide to measure flow-dependent heat transfer. The main aim of this paper is to determine whether constant temperature (CT) operation of the heater is feasible and is an improvement of the sensor response in unsteady flow compared with constant power (CP) operation. Thus, the flexible devices are glued to a surface and subjected to steady and unsteady water flows. From the relation between the sensor output and the applied shear rate it is shown that CT operation is superior over CP operation. Subsequently, a quasi-steady relation is identified and tested for coronary shear rate dynamics and found to be very accurate when the heater is operated at a CT difference of only 5 K, with an average error of 5%.