Abstract
A thermal flow transduction method combining the advantages of calorimetric and hot-film transduction principles is developed and analyzed by Finite Element Method (FEM) simulations and confirmed experimentally. The analyses include electrothermal feedback effects of current driven NTC thermistors. Four thin-film germanium thermistors acting simultaneously as heat sources and as temperature sensors are embedded in a micromachined silicon-nitride membrane. These devices form a self-heated Wheatstone bridge that is unbalanced by convective cooling. The voltage across the bridge and the total dissipated power are exploited as output quantities. The used thin-film thermistors feature an extremely high temperature sensitivity. Combined with properly designed resistance values, a power demand in sub-1mW range enables efficient gas-flow transduction, as confirmed by measurements. Two sensor configurations with different arrangements of the membrane thermistors were examined experimentally. Moreover, we investigated the influence of different layouts on the rise time, the sensitivity, and the usable flow range by means of two-dimensional finite element simulations. The simulation results are in reasonable agreement with corresponding measurement data confirming the basic assumptions and modeling approach.
Highlights
During the last decade, a growing demand for miniaturized flow sensors in industrial, automotive, medical, and domestic appliances has evolved
Contrary to the simple calorimetric concept with an active thin-film heating resistor and symmetrically positioned temperature sensors [11,12], we study in this paper a thermal flow sensor that incorporates advantages of both the calorimetric and the hot-film transduction principle in a single device
In the case of flow sensors, these parameters are subjected to significant change due to convective heat transfer, which impedes the design of the series resistance for safe operation
Summary
A growing demand for miniaturized flow sensors in industrial, automotive, medical, and domestic appliances has evolved. Contrary to the simple calorimetric concept with an active thin-film heating resistor and symmetrically positioned temperature sensors [11,12], we study in this paper a thermal flow sensor that incorporates advantages of both the calorimetric and the hot-film transduction principle in a single device This device is based on an array of four thin-film germanium thermistors embedded in a silicon membrane and connected to form a Wheatstone bridge. A special feature of our design results from the negative temperature coefficient of resistivity (NTC) and the applied constant current supply of the Wheatstone bridge In this case, convective cooling of the thermistors increases the Joule heat dissipated in these devices. The presented approach excels with a combination of high sensitivity, low power consumption, and low excess temperatures required for operation
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