High Performance Sensor for Angular Resolved Flow Measurement

Circular arrays of amorphous germanium thermistors (TCR = -1.8%/K) which are placed on a 1.3 mum thick diaphragm offer a high resolution method for direction measurements of gas flows. The direction is extracted from the difference signals of two pairs of opposite thermistorsoriented at right angles to each other. The normally applied evaluation uses the arctangent function to calculate the flow direction assuming a sinusoidal directional signal of the single thermistor pairs. However, for real devices the directional characteristic is not exactly sinusoidal and also influenced by the flow velocity. This limits the accuracy of angular measurements. By means of a finite element approach the distorted characteristic is modeled and compared to the measurements of the micromachined device. Additionally, a ring shaped structure is evaluated as an alternative heater geometry. It reduces the deviations from the simple sinusoidal model by more than 50%.