Thermal pulsed micro flow sensor

Abstract

A thermal pulsed micro flow sensor includes thermal sensors positioned in a fluid channel at downstream positions relative to a heater. Flow rate is measured by determining the time that it takes a thermal pulse to pass between two of the sensors. Since the resolution of the measurement increases with increasing distance between sensors while the accuracy of the measurement decreases, there is a conflict between the requirements of accuracy and short response time, as well as between accuracy and resolution. By providing at least three sensors and by varying the distances between the sensors, however, it is possible to select a pair of sensors having the highest resolution required by the application in which the sensor is used, while still ensuring that the velocity measurement is within the range of velocities accurately measurable by the selected sensors. Furthermore, for sensors having predetermined overlapping velocity measurement ranges, by selecting the distance between two sensors based on a desired accuracy, and by including an adjustment factor, a desired resolution can likewise be achieved. As a result, the cross-sectional area of the channel can be made constant, and the micro flow sensing device can be can be used for different flow conditions without having to adjust the sensors, or to use sensors having different characteristics.