A novel thermal transduction method for sub-mW flow sensors

We present a novel approach to micromachined thermal flow sensors embedding four thin-film germanium thermistors in a silicon-nitride membrane. The appropriately arranged thermistors act as heat sources and as temperature sensors simultaneously. They were connected to form a Wheatstone bridge supplied with a constant electric current. Both, the bridge voltage and the voltage at the bridge supply terminals, are immediately available output signals offering high initial sensitivity or wide measurement range, respectively. The self-heating based flow transduction mechanism combine advantages of the hot-film and the electrocalorimetric flow sensors. With respect to comparable electrocalorimetric flow sensors, the power requirement is reduced by more than an order of magnitude. This feature is beneficial for remote sensing applications and crucial for measuring the flow of fluids that endure only slight temperature elevations.