A thermistor anemometer for low flow rate measurements

A digital anemometer using a self-heated thermistor as a probe has been developed for low flow rate measurements. The principle of measurement is based on hot-wire anemometry. To enhance the probe sensitivity to the flow rate while reducing the convection due to self-heating to a minimum, the thermistor is kept to the lowest possible temperature for exhibiting the negative resistance. The heat removed from the thermistor is analyzed and compared with the measured results, to derive the characteristic function which relates the voltage across the thermistor to the flow rate and temperature. This function is transformed from the flow rate domain to the time domain by the digital waveform synthesis. The carrier signal thus generated is pulse-width-modulated by the output of the probe, to provide the digital representation of the flow rate under measurement. The cross-sensitivity of the probe to the fluid temperature is compensated by scaling the carrier amplitude. Exempting linearization and compensation for the convection, this process of quantization makes a high accuracy flow rate measurement possible with a simple configuration