Theoretical and experimental investigations of a wide-range thermal velocity sensor

This paper describes a two-wire thermal velocity sensor operating with an e lectrically heated upstream wire and a downstream wire acting as a resistan ce thermometer. The sensor can be operated in such a way that the phase shi ft between the periodic (sinusoidal) voltage that drives the first wire and the detected second signal is controlled by a combination of convection, d iffusion and the finite thermal response time of the wires. This yields an effective operating range of 0.05 m s(-1) less than or equal to U less than or equal to 25 m s(-1) and corresponds to a bandwidth of 1-500. This wide velocity range is confirmed theoretically and experimentally for a number o f sensors. The paper provides detailed analytical and numerical investigati ons which are well verified by experiments. The resultant design work also aimed at a sensor that can be manufactured i ndustrially and can therefore be produced at low cost. Our immediate applic ation is the measurement of flow velocities (or volume flow rates) in slowl y changing unidirectional flows. This sensor design is briefly described an d the description involves the mechanical part of the sensor as well as the electronics. Calibration of the first automatically manufactured sensors i s shown. It is demonstrated that their performance is as good as those of t he sensors that. the authors have designed and built during their initial d evelopment work.