Abstract: A system for construction of mass density sensors and mass flow sensors using modular elements to construct low cost, application optimized, sensor assemblies. The basic configuration is a mass density sensor, including a frame (13), an internal conduit (50), and a pair of integrated torquer/angular motion pickup modules (100a,b). Application specific seal modules (60a,b, 62a,b, 64a,b) are provided between the internal conduit (50) and interface ports (20a,b) on the frame (13). The internal conduit (50) is connected to the frame by a system of radial stays (90a,b) and (91a,b) and lateral stays (130a,b) and (132a,b). The pair of integrated torquer/angular motion pickup modules (100a,b) is fixed on the frame (13). Permanent bar magnets (70a,b) are fixed to the internal conduit (50). The sensor is connected to an external conduit. During operation, the internal conduit (50) is driven to oscillate in rotation relative to the frame (13).