Sensitivity improvement of integrated thermal anemometers obtained by jet flow impingement

A typical integrated hot wire flow sensor, fabricated by means of micromachining, has been characterized in order to evaluate the low flow detection capabilities. The lowest detectable flow has been related to a sensitivity threshold, due to the forced to free convection transition. A simple lumped element model have been used to show the dependence of the sensitivity threshold on the sensor dimensions and the physical parameters of the gas. The resort to the high heat transfer coefficients of fluid jets has been presented as a valid method to improve the sensitivity of the devices. Measurements of the sensor response for various jet diameters and incidence angles have been reported and discussed.