Piezoelectric Pressure Transducers

Instantaneous fluid pressure acting on a surface can be measured using pressure transducers. These consist of piezoelectric crystals that get charged when their faces are deformed. The electrical signal is suitably amplified before the local pressure is measured as voltage. Pressure transducers are primarily suitable for measurement of pressure fluctuations. Their operating characteristics deteriorate as the forcing frequency approaches zero. Pressure acting on a surface can also be estimated by installing strain gauges on it. This is accomplished by determining the change in resistance of the wire due to longitudinal strain. Fluid pressure can also be measured relative to atmospheric pressure using a capacitance pick-up. Here the differential pressure changes the capacitor spacing and hence the capacitance. When this element is placed in an electrical circuit the change in capacitance is observed as a change in output voltage. The pressure difference across the faces of the capacitor is developed commonly by a pitot or a static tube. Capacitor pick-ups vary significantly in construction depending on the method used to fabricate a capacitor element. This could be a thin metal diaphragm fixed on its boundary or a thin deformable layer on a semi-conductor substrate that forms a part of an amplifier circuit. Piezoelectric transducers are also used in `lift-drag-moment' balance that directly measures forces and moments on objects placed in wind tunnels. The design of the balance is usually complicated by the fact that elaborate mechanisms are needed to transmit the load to independent piezoelectric crystals.