Measurement and simulation of the frequency response of a thermal flow sensor at different flow speeds

This contribution presents the complete (amplitude and phase) frequency response of a calorimetric flow sensor in a free, laminar air stream at different flow speeds ranging from 0 to 5 m/s. Given the geometry of the sensor, which features a central thermistor closely surrounded by a heater element, the complexity of the resulting frequency response is reduced by applying a simple transformation. This transformation involves taking the ratio between the temperature oscillations at the surrounding thermistors and the central thermistor. The resulting frequency response shows a second order transfer function of real poles. The parameters of this transfer function are used to analyze the effect of the flow speed on the frequency response. Results show that for air, the position of the first characteristic pole remains constant at the downstream thermistor for flow speeds up to 1 m/s. Experimental and simulated results are consistent, which allows the further study of the sensor's response by means of numerical simulations in order to determine the effect of the thermal properties of the fluids on the frequency response. The ultimately goal is to achieve fluid independent flow measurement. (C) 2012 Elsevier B.V. All rights reserved.