Practical hot wire anemometerexcitation modes

Abstract

The operation of a thin film hot wire directional anemometer is demonstrated using three modes of operation; constant voltage, constant current, constant resistance, and the heating response and characteristics for the different excitation modes observed. Evaluation is primarily by experimental approach. The anemometer fabricated is a four element 2mm x 2mm thermoresistive sensor array mounted on a 1.5 µm silicon nitride membrane formed by bulk reverse etching. Reverse etching is used for thermal isolation of the sensor elements and allows element temperatures in excess of 500°C to be reached with an input power of 250mW and accurate lower temperature operation with element temperatures and heating powers of 65°C and 25mW respectively. Current sources are commonly used for excitation of such devices and resistance feedback often not required due to low resistance variations during operation, however high power modes of operation can lead to instability and self-destruction of positive temperature coefficient of resistance (PTCR) devices. Voltage or resistance feedback provides stable operation due its self-limiting nature in a PTCR device. Resistance monitoring provides a means to achieve stable temperatures of the heating elements and provides reduced sensitivity to fluctuations in ambient air temperatures and a more acceptable response to the incident airflow velocity.