A THERMISTOR ANEMOMETER FOR LOW-FLOW-RATE MEASUREMENTS

A digital anemometer using a self-heated thermistor as a probe has bee n developed for low-flow-rate measurements. To enhance the probe sensi tivity to flow rate while reducing convection-due to self-heating to a minimum, the thermistor is kept at the lowest possible temperature fo r exhibiting the negative resistance, The heat removed from the thermi stor is analyzed and compared with the measured results, to derive the characteristic function which relates the voltage across the thermist or to the flow rate and temperature. This function is transformed from the flow-rate domain to the time domain by the digital waveform synth esis. The carrier signal thus generated is pulse-width-modulated by th e output of the probe to provide the digital representation of the flo w rate under measurement. The cross-sensitivity of the probe to the fl uid temperature is compensated by scaling the carrier amplitude. This process of quantization including linearization and compensation funct ions makes a high-accuracy, how-rate measurement possible with a simpl e configuration.