FLUID DENSITY SENSOR-BASED ON RESONANCE VIBRATION

This paper presents the first silicon tube resonant fluid density sensor. The sensor is based on a tube system which is vibrated in a selected balanced mixed torsion and bending mode. The resonant frequency changes with the density of the fluid in the tube due to the change of the inertial mass of the vibrating tube system. A simplified analytical analysis shows that the resonance vibration frequency depends on the weighted density, rho, as similar to 1/rho(1/2), where rho is the density of the silicon and the fluid weighted in accordance to their area in a cross section of the tube. The sensor was fabricated using double-sided silicon KOH etching and silicon fusion bonding. The silicon tube system (without the frame) is 8.6 x 17.7 mm with a thickness of 1 mm. The tube system was excited electrostatically into resonance vibration which was detected optically. The resonant frequency was measured for different mixtures of 2-propanol and water. The sensor shows very good sensitivity,-200 ppm (kg m(-3))(-1) and a high mechanical Q-value, of the order of 1500. The sample volume is only 0.035 ml.