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PCB PIEZOTRONICS INC 716 684 0001 Fax 716 685 3886 For Additional Specification Information Visit www pcb com 154 Technical Information Pressure Introduction To Dynamic Pressure Sensors Piezoelectric Pressure Sensors measure dynamic pressures They are generally not suited for static pressure measurements Dynamic pressure measurements including turbulence blast ballistics and engine combustion under varying conditions may require sensors with special capabilities Fast response ruggedness high stiffness extended ranges and the ability to also measure quasi static pressures are standard features associated with PCB quartz pressure sensors The following information presents some of the design and operating characteristics of PCB pressure sensors to help you better understand how they function which in turn helps you make better dynamic measurements Types Of Pressure Sensors This catalog describes two modes of operation for pressure sensors manufactured by PCB Charge mode pressure sensors generate a high impedance charge output ICP Integrated Circuit Piezoelectic voltage mode type sensors feature built in microelectronic amplifiers which convert the high impedance charge into a low impedance voltage output ICP is a registered trademark of PCB Group Inc Figure 32 Illustrates the cross section of a typical quartz pressure sensor This particular sensor is a General Purpose Series with built in electronics Figure 32 Typical ICP Quartz Pressure Sensor Sensor Construction Piezoelectric pressure sensors are available in various shapes and thread configurations to allow suitable mounting for various types of pressure measurements Quartz crystals are used in most sensors to ensure stable repeatable operation The quartz crystals are usually preloaded in the housings to ensure good linearity Tourmaline another stable naturally piezoelectric crystal is used in some PCB sensors where volumetric sensitivity is required Polarity When a positive pressure is applied to an ICP pressure sensor the sensor yields a positive voltage The polarity of PCB charge mode pressure sensors is just the opposite when a positive pressure is applied the sensor yields a negative output Charge output sensors are usually used with external charge amplifiers that invert the signal Therefore the resulting system output polarity of a charge output sensor used with a charge amplifier will produce an output that is the same as an ICP sensor Reverse polarity sensors are also available High Frequency Response Most PCB piezoelectric pressure sensors are constructed with either compression mode quartz crystals preloaded in a rigid housing or unconstrained tourmaline crystals These designs give the sensors microsecond response times and resonant frequencies in the hundreds of kHz with minimal overshoot or ringing Small diaphragm diameters ensure spatial resolution of narrow shockwaves High frequency response and rise time can be affected by mounting port geometry and associated electronics Limitations of driving long cables at high frequencies are discussed on page 148 Check all system component specifications before making measurements or contact PCB for application assistance Why Only Dynamic Pressure Can Be Measured With Piezoelectric Pressure Sensors The quartz crystals of a piezoelectric pressure sensor generate a charge when pressure is applied However even though the electrical insulation resistance is quite large the charge eventually leaks to zero The rate at which the charge leaks back to zero is dependent on the electrical insulation resistance In a charge mode pressure sensor used with a voltage amplifier the leakage rate is fixed by values of capacitance and resistance in the sensor by low noise cable and by the external source follower voltage amplifier used In the case of a charge mode pressure sensor used with a charge amplifier the leakage rate is fixed by the electrical feedback resistor and capacitor in the charge amplifier In a pressure sensor with built in ICP electronics the resistance and capacitance of the crystal and the built in ICP electronics normally determine the leakage rate 9 T Mcatalog 2011 Seite 125 166 SYN G500 21 04 11 18 04 Seite 160