VIBRATION MODES OF A RESONANT SILICON TUBE DENSITY SENSOR

We present an investigation of the resonance parameters for a new sens or for on-line measurements of fluid density, The sensor consists of a tube system made of single crystalline silicon, The tube system is ex cited electrostatically into mechanical resonance and the vibration is detected optically, Using a simplified theoretical analysis, the reso nance frequency can be shown to be proportional to 1/root rho, where r ho is the density of the silicon and the fluid weighted according to t heir areas in a cross section of the tube, Thus, a change in fluid den sity results in a change in the resonance frequency, This dependence i s demonstrated by measurements for four different vibrations modes, Th e quality of the vibration is also investigated through measurements o f the Q-values of the vibration modes, The tubes are made using anisot ropic silicon KOH etching and silicon-to-silicon fusion bonding microm achining techniques, The dimensions of the tube system are 8.6 x 17.7 mm with an outer tube thickness of 1 mm and a wall thickness of 100 mu m. Total tube length is 61 mm, and the sample volume is 0.035 mi, The sensor has a very good density sensitivity of the order of -200 ppm/( kgm(-3)) and a high Q on the order of 3000 for air in the tube. [142]