1、TECHNICAL NOTEPCB PIEZOTRONICS, INC. WWW.PCB.COM TN-15BackgroundIn the late 1950s and 1960s, with the advent of the aerospace eraand advanced weapons development, came the requirement forhigh-frequency pressure sensors to make shock wave, blast, rocketcombustion instability, and ballistic measuremen
2、ts. Piezoelectricsensors at that time had limited frequency response, and weremainly used for acoustic and engine combustion applications. It was during this period that Walter Kistler, working closely with AbeHertzberg at the former Cornell Aeronautical Labs in Buffalo, NY,developed miniature, high
3、-frequency acceleration-compensatedquartz pressure sensors with microsecond response time. Thisresearch spearheaded the development of shock tube technology,critical for researching aerodynamic shock waves that a spacecraftmight encounter during re-entry. Other research facilities developedspecial s
4、ensors tailored to specific applications. At AberdeenProving Ground, Ben Granath designed blast pressure sensors forweapons development and a unique, tourmaline-structured, non-resonant pressure bar for reflected shock wave measurements. PatWalter, then an engineer at Sandia National Laboratories, p
5、rovidedinvaluable feedback on these early sensor designs.Need for Dynamic CalibrationAlong with the development of higher frequency sensors came theneed for dynamic pressure calibration. Since dynamic calibratorswere not commercially available until relatively recently, many labsdeveloped unique cal
6、ibration devices to suit specific needs. Theseincluded a variety of hydraulic and pneumatic shock, pulse, andsine wave pressure generators. The dead weight tester wassometimes used in a pressure release mode to generate a known,negative pressure pulse. The calibration shock tube remains themost prac
7、tical device for producing the fastest rise time over a widerange of pressures, although the pressure amplitude is not knownas accurately as with pulse calibrators. Piezoelectric Sensor CharacteristicsPiezoelectric (PE) pressure transducers are well-suited for dynamicpressure measurements. They are
8、available in high-impedancecharge output and, more commonly, Integrated CircuitPiezoelectric (ICP) designs. They are fabricated from naturalpiezoelectric quartz, natural tourmaline, or artificially polarized,manmade ferroelectric ceramics. PE sensors operate over a widetemperature range and have a w
9、ide linear dynamic range, ultrahigh-frequency response, and rise times as fast as 0.2 S. They aresmall, have flush diaphragms, and provide a clean, high-voltageoutput. A single quartz sensor may be used accurately in ranges of0 to 10 psi, 0 to 1000 psi, or any level in between. With durable,solid-st
10、ate construction, they are ideal for use in harshenvironments. They are uniquely suited for low-pressure fluidborne noise measurements under high static pressure. There is a general misperception that because PE sensors are“dependent on changes of strain to generate electrical charge, theyare not us
11、eable with DC or steady state conditions” (ref. 1). This isnot entirely correct for all PE sensors. It is true that ceramic structuredsensors do not respond to steady state conditions, and that they dorequire dynamic calibration. However, quartz charge output sensorshave very high insulation resista
12、nce of 1012ohms. When used withelectrostatic charge amplifiers, quartz sensors exhibit short-termstatic response and can be calibrated by conventional static deadweight methods. ICPpressure sensors might require dynamiccalibration, depending on discharge time constant (low frequency),which is fixed
13、within the sensor. Test results on the same quartzsensor, calibrated by five different methods over a wide range ofamplitudes and frequencies, indicate that its sensitivity is virtuallyindependent of the method used (see Figure 1). Figure 1:PCBs Model 113A24 quartz pressure sensor exhibits consisten
14、t sensitivity when calibrated by five different methods, over a wide frequency range, at amplitudes from 100 to 1000 psi.Dynamic Calibration MethodologyMethodology for dynamic pressure transducer calibration is moredemanding and less accurate than static dead weight methods,where measurement data ca
15、n be carried out to many decimalpoints. Dynamic pressure calibration is generally accomplished byDYNAMIC PRESSURE CALIBRATIONJim Lally by hydraulic impulsecomparison calibration, using a transfer standard; or with theshock tube. Depending on the type of calibrator and gas mediumused, rise times in t
16、he order of microseconds or milliseconds canbe achieved. Sensor mounting, switching transients, and readoutaccuracy are all factors. Digital signal processing has greatlyimproved measurement accuracy compared to earlier methodsthat used analog storage oscilloscopes. The “ideal” dynamiccalibrator wou
17、ld be structured to generate a precisely knownreference pressure that can be continuously adjusted over a widerange of amplitudes and frequencies. However, a device like thisdoes not exist. Traceability to National StandardsAs is the case with all calibration methods, dynamic calibrationshould be tr
18、aceable to a nationally accredited laboratory, such asthe National Institute of Standards & Technology (NIST). However,the authors are not aware of any national labs that provide“traceable” dynamic pressure calibration. In 1991, NIST held aworkshop that focused on transient pressure and temperature.
19、Technical presentations involved both current and proposeddynamic calibration methodologies. NIST-traceable dynamic pressure sensor calibration can beachieved by pressurizing a chamber with an accurately known staticpressure, as measured with a NIST-traceable DC reference gage,and quickly venting th
20、e sensor to be calibrated to this knownpressure. Signal conditioning and readout instruments would beNIST-traceable through electrical calibration. Other methodsinvolve sine or pulse comparison calibration, using a transferstandard with NIST-traceable calibration. In the early 1970s, a group of scie
21、ntists, sensor users, andmanufacturers formed a working group to develop, for ANSI, A Guide for the Dynamic Calibration of Pressure Transducers. Thisdocument has been updated and published by the InstrumentationSociety of America (ISA) (ISA document 37.16.01.2002).Why Use Dynamic Calibration?Dynamic
22、 pressure calibration is useful for several reasons. Withceramic structured sensors, it is the only way to determine sensoroutput relative to input. Some DC low-frequency sensors may notrespond the same to identical static or dynamic input pressure.The output of all pressure sensors is frequency dep
23、endent.Dynamic calibration devices, such as the shock tube, are quiteuseful for determining sensor resonance characteristics, as well asresonance in gas passages associated with recessed mounting. Gaspassage resonance is analogous to the frequency response changeresulting from the addition of an ada
24、ptor between anaccelerometer base and test structure. Development of Dynamic Pressure CalibratorsDynamic pressure calibrators have evolved over the years inresponse to specific needs at various laboratories. Caibrators varywidely in the type of pressure source used, and in their amplitudeand frequen
25、cy range. PCBadapted the best of these technologiesfor in-house sensor research and dynamic calibration. To helpcustomers understand and evaluate the characteristics of sensorsfor transient applications, several dynamic pressure calibrators arenow offered as standard commercial products. Dynamic pre
26、ssure calibrators are of two general types, periodic andaperiodic. The periodic types, such as a Pistonphone, generate adefined sine wave pressure for calibration of microphones andother low-pressure acoustic sensors. Aperiodic calibratorsgenerate a single pulse. The hydraulic piston and cylinder im
27、pulsecalibrator, developed at Sandia in the 1960s, is one of the moreversatile dynamic calibrators, with capability to calibrate over awide pressure range. Some aperiodic calibrators use an accurate DC pressure gauge toset a known static pressure in a chamber and then rapidly switchthe test sensor t
28、o this pressure using a fast-acting valve. One suchdevice is the Aronson calibrator, which incorporates a poppet-typeswitching valve. It was developed by Phil Aronson at the NavalOrdnance Laboratory to calibrate underwater pressure sensors atincremental pressures under higher static load conditions.
29、 Of the different pressure switching mechanisms, poppet valvesprovide the fastest response, usually in the 50 to 100 S range.Solenoid valves are generally not well-suited, as they tend toproduce an oscillating pressure source during the switchingprocess. In pneumatically operated calibrators, the us
30、e of heliumwill provide the fastest rise time.Hydraulic Impulse CalibratorThis versatile aperiodic calibrator is structured with a free-fallingmass dropped onto a piston and cylinder manifold to create ahydraulic pulse with a 3 mS rise time and 6 mS duration (see Figure2). A linear tourmaline transf
31、er standard, installed in the manifold,measures pulse amplitude, which is then compared with thesensor being calibrated to establish its input/output sensitivity.The drop calibrator has the capability of generating pressures from 100 to 20,000 psi, depending on the height from where the massis dropp
32、ed.A high-pressure version of the drop calibrator, PCB Model 913A10,operates from 10,000 to 125,000 psi. This unit uses an accelerometerto measure deceleration of the free-falling mass after it strikes thepiston. Deceleration of the mass, coupled with the geometries of thepiston and cylinder, determ
33、ines pressure pulse amplitude. TheModel 913A10 structure is less complex and it is easier to operate,compared to other high-pressure dynamic calibrators.DYNAMIC PRESSURE CALIBRATIONDYNAMIC PRESSURE CALIBRATIONFigure 2:PCB Model 913B02 impulse calibrator uses a free-falling mass that strikes a piston
34、 andcylinder manifold to produce a pulse waveform for pressure sensor calibration over awide dynamic range. Calibration is performed by comparing outputs from the test andreference sensors, both installed in the manifold and subjected to the same pressurepulse. Pressure amplitude is determined by th
35、e height from which the mass is dropped.Calibration Shock TubePCB Model 901A10 is a gas-driven shock tube, capable of producingshock waves with nanoseconds of rise time (see Figure 3).Depending on the diaphragm material separating the driver fromthe test section, shock waves as low as 3 psi can be g
36、enerated usingaluminum foil, and 1000 psi using sheet aluminum. Compressedgas, such as air, helium, or nitrogen, is pressurized in the driversection until the diaphragm bursts, sending a shock wave into thetest section. As a driver source, helium provides a well-formedshock wave with the highest Mac
37、h number. Amplitude accuracy ofthe shock wave (approx. 1.5%) is calculated from measurement ofthe shock wave velocity and atmospheric pressure.Figure 3:PCB Model 901A10 shock tube is useful for calibrating and testing the dynamicbehavior of high-frequency pressure sensors. Shock wave amplitude is ca
38、lculated bymeasuring shock wave velocity, temperature, and barometric pressure. Pressureamplitude is determined by selection of the diaphragm material and thickness used inthe driver section. Reflected shock waves, occurring at the end wall, will excite theresonance of most pressure sensors. This lo
39、cation is also used for testing the resonantcharacteristics of gas passages in front of recess-mounted sensors.Aronson Step Pressure GeneratorThe step pressure generator (see Figure 4) was developed by PhilAronson and R. Wasser at the Naval Ordnance Lab in the 1960s, tocalibrate underwater pressure
40、sensors at incremental pressuresunder high static loads. Phil dedicated much of his professionalcareer to the study of transient pressure measurements anddynamic calibration. Their goal was to develop an aperiodiccalibration device, capable of performing dynamic calibration withgreater accuracy and
41、ease than was possible with the shock tube.The device, using helium as a gas source, is capable of generatingknown step pressures up to 2000 psi in 50 S.The concept and operation of the Aronson Step Pressure Generatoris quite fundamental. It involves rapidly venting a precisely knownstatic pressure
42、to a sensor diaphragm by pressurizing the mainreservoir with a known static pressure, then quickly exposing thesensor being calibrated to the reference pressure by releasing thefast-acting poppet valve. The pressure drop in the main reservoirdue to the added volume between the sensor diaphragm andpo
43、ppet valve is negligible with flush diaphragm sensors, and wouldbe indicated on the digital pressure gauge that monitors reservoirpressure. The step pressure generator can be used to producepositive or negative pressure pulses of accurately knownamplitude. NIST-traceability is through the DC referen
44、ce gaugesused to set the known static pressure level to which the test sensorwill be rapidly exposed. The Pressure Division of PCBPiezotronics, Inc. specializes in the development,application, and support of piezoelectric and piezoresistive pressure sensors,transducers, and transmitters for dynamic
45、and static pressure test, measurement,monitoring, and control requirements. This product focus, coupled with the strengths and resources of PCB, permits the Pressure Division to offerexceptional customer service, 24-hour technical assistance, and a TotalCustomer Satisfaction guarantee.DYNAMIC PRESSU
46、RE CALIBRATION3425 Walden Avenue, Depew, NY 14043-2495 USAPressure Division toll free 888-684-001124-hour SensorLineSM716-684-0001Fax 716-686-9129 E-mail Web site ISO 9001:2000 CERTIFIED A2LA ACCREDITED to ISO 17025 2005 PCB Group, Inc. In the interest of constant product improvement, specifications
47、 are subject to change without notice.PCB and ICP are a registered trademarks of PCB Group, Inc.SensorLine is a service mark of PCB Group, Inc. All other trademarks are properties of their respective owners.PRS-TN-15-0205 Printed in U.S.A.Visit to locate your nearest sales officeFigure 4:This step
48、pressure calibrator generates an accurately known step pressure using a fast-acting poppet valve. This type of aperiodic calibrator is easier to operate and provides moreaccurate calibration than the shock tube, but does not operate at so high a frequency. “Pistonphone” Microphone CalibratorThe Pist
49、onphone (see Figure 5) is a good example of a periodiccalibrator. The portable, battery powered device produces a fixed 134dB amplitude sine wave at a frequency of 250 Hz for calibration ofmicrophones and low-pressure acoustic sensors. The known sine wavereference pressure level is generated by two opposed reciprocatingpistons, in a controlled volume inside the Pistonphone. The use ofprecision mounting adaptors is critical for maintaining known volumeand reference pressure when calibrating different types of sensors.Figure 5:Micro
