Design and Use of Piezoelectric accelerometers Measuring acceleration Piezoelectric industrial accelerometers consist essentially of three basic elements: the sensor body, the piezoelectric sensing element and the seismic mass. Initially piezoelectric accelerometers incorporated a compression design whereby the compression cut, quartz crystal sensing element is preloaded between the base plate and seismic mass. Because of the constant seismic mass, the force acting on the measuring element is proportional to the acceleration in accordance with Newton's first law: F = ma. An electrical charge is generated proportional to the force (and hence the acceleration). Because they are basically AC coupled devices, piezoelectric accelerometers are not suitable for measuring constant (DC) accelerations like those generated in a centrifuge. For "true" DC acceleration measurement, refer to Kistler K-Beam accelerometers with variable capacitance sensing elements. Although the compression cut quartz design was widely accepted with it's inherent characteristics of long term stability, low mass, high rigidity and subsequent high resonant frequency, Kistler has focused on accelerometers which utilize a shear mode quartz element that is sensitive to imposed shearing forces and unaffected by other orthogonal force components. In addition, the primary charge sensitivity of shear mode quartz is twice that for compression mode quartz. This results in a smaller seismic system design in shear mode units and thereby reduces their overall size and mass. As in the compression design, the force acting on the element is proportional to the acceleration in accordance with Newton's first law: F = ma and an electrical charge is generated proportional to the acceleration. 6 www.kistler.com