NOVELTY - The microelectromechanical system (MEMS) device for determining a property e.g. density of a fluid, comprises a substrate (12), a first base (16) on the substrate, a tube structure (14) extending from the first base and spaced apart from a surface (18) of the substrate, a drive element (26) and a sensing element in proximity to the tube structure, and a temperature sensing element on the tube structure for monitoring the temperature of the fluid flowing through the tube structure. The tube structure comprises a pair of geometrically parallel tube portions lying in a plane.    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 (claimed), and useful in drug infusion systems, fuel delivery systems, fuel cell 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 for determining a property e.g. density of a fluid, comprises a substrate (12), a first base (16) on the substrate, a tube structure (14) extending from the first base and spaced apart from a surface (18) of the substrate, a drive element (26) and a sensing element in proximity to the tube structure, and a temperature sensing element on the tube structure for monitoring the temperature of the fluid flowing through the tube structure. The tube structure comprises a pair of geometrically parallel tube portions lying in a plane, a continuous internal passage defined in part within the parallel tube portions, and an inlet and outlet of the internal passage fluidically connected to the first base. The tube structure is configured for vibrational movement in the plane parallel to the surface of the substrate. The drive element is operable to induce the vibrational movement of the tube structure in the plane of the tube structure and to induce resonant vibrational movements in the tube portions. The sensing element is adapted to sense deflections of each of the tube portions when the tube structure is vibrated with the drive element. The tube structure is configured so that the vibrational movement is parallel to the surface of the substrate. The resonant vibrational movements of the tube portions are in the plane of the tube structure and 180 degrees out of phase with each other. The tube structure is enclosed in an evacuated hermetically sealed package. The drive element is magnetically or electromagnetically coupled to a magnetic field source outside of the package, is electrostatically coupled to the parallel tube portions for inducing the vibrational movement of the tube structure, comprises a piezoelectric element formed of a piezoelectric material for inducing the vibrational movement of the tube structure, and is physically attached to the parallel tube portions and electromagnetically coupled to a magnetic field source. The sensing element comprises a piezoelectric element formed of a piezoelectric material for sensing the vibrational movement. The drive element consists of conductive runners, conductive runners on circuits, and magnetic films. The magnetic field source consists of permanent magnets and electromagnets. The drive element and sensing element monitor the temperature of the fluid flowing through the tube structure. The tube structure comprises a distal portion relative to the first base. The distal portion is anchored to a second base on the substrate, so that the tube structure is supported apart from the substrate at oppositely disposed ends of the tube structure. The parallel tube portions are in fluidic parallel to each other. The drive element is disposed on a platform between the parallel tube portions. The parallel tube portions are in fluidic series with each other to define the continuous internal passage. A first of the parallel tube portions is fluidically connected at one end to the inlet and at an oppositely disposed second end to a second of the parallel tube portions. The second of the parallel tube portions is fluidically connected at one end to the outlet and at an oppositely-disposed second end to the first of the parallel tube portions. A portion of the sensing element is disposed on a platform between the parallel tube portions and on a frame surrounding the tube structure. The parallel tube portions are spaced apart from each other by a single gap. The tube structure is cantilevered, so that the distal portion is suspended apart from the substrate and enables vibrational movement relative to the substrate. The portion of the sensing element is anchored to the first base and to the distal portion of the tube structure. The first base comprises first and second portions separated by the surface of the substrate. The tube structure interconnects the first and second portions. The inlet and outlet of the internal passage are fluidically connected to the first and second portions respectively. An INDEPENDENT CLAIM is included for a method of operating a microelectromechanical system (MEMS) device.    DESCRIPTION OF DRAWING(S) - The diagram shows a plan view of a microfluidic device with a micromachined cantilevered structure.    Substrate (12)    Tube structure (14)    First base (16)    Surface of the substrate (18)    Drive element. (26)