Flow and Pressure Sensors for Anaesthesia and Respirator Equipment
The aim of this project is to design, fabricate and evaluate new flow and pressure sensors specially adapted for use in medical equipment such as anesthesia and respirator systems.
The project has lead to the developement of the lift force flow sensor (shown in the figure below) making use of force normal to the flow for flow measurement. The lift force deflects an airfoil structure and the deflection is detected with polysilicon piezoresistors connected in a Wheatstone bridge configuration. The sensor consists of two airfoil plates connected to a center support beam where each plate has a top-area of 3 x 3 mm2 and a thickness of 8 µm. A flow sensitivity of 0.65 µV/V/(l/min)2 has been measured.
The lift force acts with different magnitude on the two airfoil plates making it possible to connect the strain gauges in a "differential mode" canceling any common mode influence e.g. acceleration. Good suppression of acceleration has been verified. Due to the symmetric design, the sensitivity is independent of the flow direction and the induced pressure drop is relatively low. In the latest sensor design the detecting strain gauges and plate suspension have been separated giving the sensor both higher sensitivity and faster time response.
- Datex-Ohmeda AB, Bromma
- Swedish National Board for Industrial and Technical Development (NUTEK)