Flow measurement in micromachined orifices

The flow conditions in micromachined orifices and gaps are discussed for cross section areas in the range of 0.5 to 1 mm(2). Aligned to a small suspension beam, different plates with geometries of 650 mu m x 450 mu m and 700 mu m x 700 mu m reduce the cross section of the flow channel. The mass flow of water is calculated by an analytical model and by FE simulation. A good agreement with the experimental data is shown. The flow induces a plate deflection and a mechanical stress in the suspension. The mechanical stress is measured via an integrated piezoresistive Wheatstone bridge offering full scale output signals up to 120 mV V-1. Corresponding to the models, the output signal is proportional to the pressure difference and the square root of the mass flow. A sensitivity of 0.03 mV V-1 Pa-1 is calculated for air flow rates from 0 to 4000 sccm/min. Different mass flow measurements of air and water are presented and aspects of different viscosity and plate geometries are discussed. Therefore, design scaling rules can be deduced for different applications.