NOVELTY - The microelectromechanical system (MEMS) device (10) for determining a property e.g. density of a fluid, comprises a substrate (12), a structure on the substrate, a cantilevered member (30), a drive electrode (26) and sensing electrode (28) on a substrate surface (18), and a unit for determining the property of the fluid from the outputs of the sensing electrodes. The structure comprises a base and a U-shaped, C-shaped or linear tube, which extends from the base and is spaced apart from the substrate surface for vibrational movement in a plane normal to the substrate surface.    USE - The microelectromechanical system (MEMS) device is useful for determining a property such as mass flow rate, density, specific gravity, volumetric flow rate, chemical concentration, viscosity and lubricity of a fluid, and useful in chemical concentration sensors, fuel deliver/handling systems, fuel cell systems, drug delivery systems (all claimed), drug infusion systems and drug and chemical mixing systems.    ADVANTAGE - The microelectromechanical system device is capable of precisely determining the property of the fluid with high performance, and can be economically manufactured with high sensitivity.    DETAILED DESCRIPTION - The microelectromechanical system (MEMS) device (10) for determining a property e.g. density of a fluid, comprises a substrate (12), a structure on the substrate, a cantilevered member (30), a drive electrode (26) and sensing electrode (28) on a substrate surface (18), and a unit for determining the property of the fluid from the outputs of the sensing electrodes. The structure comprises a base and a U-shaped, C-shaped or linear tube, which extends from the base and is spaced apart from the substrate surface for vibrational movement in a plane normal to the substrate surface. The tube comprises a continuous internal passage, a fluid inlet and outlet of the internal passage fluidically connected to the base, and a distal portion relative to the base. The cantilevered member is attached to the distal portion of the tube and configured for vibrational movement relative to the distal portion and in the plane normal to the substrate surface. The drive electrode is adjacent to the cantilevered member or the distal portion of the tube, and is operable to induce the vibrational movements of the tube and the cantilevered member. The sensing electrodes are adapted to sense deflections of the tube when vibrated with the drive electrode and to produce outputs corresponding to the sensed deflections. The property of the fluid flows through the internal passage. The cantilevered member comprises a reduced section between the cantilevered member and the distal portion. The tube, the cantilevered member and the reduced section are configured so that flexure during the vibrational movement of the cantilevered member is concentrated in the reduced section. The tube and the cantilevered member are configured so that the vibrational movement of the cantilevered member is 180 degrees out of phase with the vibrational movement of the tube. The cantilevered member and the tube have equal masses. The vibrational movement of the cantilevered member counteracts the vibrational movement of the tube to minimize clamping losses dissipated to the substrate. The drive electrode is disposed beneath the cantilevered member and/or the distal portion on the substrate surface, and is adapted to directly induce the vibrational movement of the cantilevered member and/or the tube and indirectly induce the vibrational movement of the tube and/or the cantilevered member. The drive electrode is operable to induce the vibrational movements of the tube and the cantilevered member either electrostatically, electromagnetically, piezoresistively, piezoelectrically, thermally or optically. The MEMS device further comprises a film on the cantilevered member or the distal portion of the tube for electromagnetically or piezoelectrically coupling with the drive electrode to electromagnetically or piezoelectrically induce the vibrational movements of the tube and cantilevered member. A large gap exists between the tube and the substrate to reduce squeeze film damping of the tube, which is not enclosed in a vacuum package. Oppositely-disposed ends of the tube are attached to the spaced-apart portions of the base, the distal portion of the tube is midway between the ends of the tube, and the cantilevered member extends perpendicularly from the distal portion. The base and the tube define and surround an opening in which the cantilevered member is disposed. The cantilevered member comprises an internal chamber, which is fluidically coupled to the internal passage. An INDEPENDENT CLAIM is included for a method of operating a microelectromechanical system device.    DESCRIPTION OF DRAWING(S) - The diagram shows a plan view of a microfluidic device with a resonating micromachined tube, a cantilevered member and a drive electrode beneath the cantilevered member.    Device (10)    Substrate (12)    Substrate surface (18)    Drive electrode (26)    Sensing electrodes (28)    Cantilevered member. (30)