Dynamic Pressure Calibration
High-frequency pressure sensors capable of measuring shock waves, blast, rocket combustion instability, and ballistics were initially developed by researchers for laboratory use. Here’s an overview of some of the sensor types, and associated calibration equipment, available on the commercial market.
Jim Lally and Dan Cummiskey,
It was during this period that Walter Kistler, working closely with Abe Hertzberg at the former Cornell Aeronautical Labs in Buffalo, NY, developed miniature high-frequency acceleration-compensated quartz pressure sensors with microsecond response time. This research spearheaded the development of shock tube technology crucial to studying the sort of aerodynamic shock waves that spacecraft can encounter during reentry. Other research facilities devised special sensors tailored to their specific applications. At Aberdeen Proving Ground, Ben Granath designed blast pressure sensors for weapons development and a unique, tourmaline, nonresonant pressure bar for reflected shock wave measurements. A young engineer at Sandia National Laboratories, Pat Walter, provided invaluable feedback on these early sensor designs.
Need for Dynamic Calibration
Piezoelectric Sensor Characteristics
There is a general misperception that because PE sensors are “. . . dependent on changes of strain to generate electrical charge, they are not usable with DC or steady-state conditions” . This is not entirely correct for all PE sensors. Because quartz sensors exhibit an insulation resistance >1012 (unlike ceramic types), they are suitable for short-term static (or quasi-static) measurements. This may be accomplished with an electrostatic charge amplifier, permitting calibration by conventional static dead-weight methods. IC piezoelectric pressure sensors may or may not require dynamic calibration, depending on the discharge time constant (low frequency), which is fixed within the sensor. Test results on the same quartz sensor, calibrated by five different methods over a wide range of amplitudes and frequencies, indicate that its sensitivity is virtually independent of the method used (see Figure 1).
Dynamic Calibration Methodology
Traceability to National Standards
Dynamic calibration traceable to NIST can be achieved by pressurizing a chamber with an accurately known static pressure, as measured with a NIST-traceable DC reference gauge, and then quickly venting the sensor undergoing calibration to this known pressure. Signal conditioning and readout instruments would be NIST traceable by means of electrical calibration. Other methods are based on sine or pulse comparison calibration using a transfer standard with calibration traceable to NIST.
In the early 1970s a group of scientists, sensor users, and manufacturers formed a working group to develop for ANSI A Guide for the Dynamic Calibration of Pressure Transducers. This guide has been reviewed and updated and will soon be released as ISA Guide document ISA37.16.01 2002.
Why Calibrate Dynamically?
Dynamic Pressure Calibrators
There are two basic types of dynamic pressure calibrators—periodic and aperiodic. Periodic types, such as Pistonphones, generate a defined sine wave pressure for calibrating microphones and other low-pressure acoustic sensors. Aperiodic calibrators generate a single pulse. The hydraulic piston and cylinder impulse calibrator, developed at Sandia in the 1960s, is one of the more versatile dynamic instruments with its ability to calibrate over a wide pressure range.
Some aperiodic calibrators use an accurate DC pressure gauge to set a known static pressure in a chamber and then rapidly switch the test sensor to this pressure with a fast-acting valve. One such device is the Aronson calibrator, incorporating a poppet-type switching valve. Of the various pressure switching mechanisms, poppet valves provide the fastest response, usually in the 50–100 µs range. Solenoid valves are generally not a good choice because they tend to produce an oscillating pressure source during the switching process. In pneumatically operated calibrators, helium will provide the fastest rise time.
Hydraulic Impulse Calibrator. This versatile aperiodic calibrator is configured with a free-falling mass dropped onto a piston and cylinder manifold to create a hydraulic pulse with a 3 ms rise time and 6 ms duration (see Figure 2).
A linear tourmaline transfer standard installed in the manifold measures the amplitude of the pulse, which is then compared with the sensor being calibrated to establish its I/O sensitivity. The drop calibrator can generate pressures from ~100 to 20,000 psi, depending on the height from which the mass is dropped.
A high-pressure version of the drop calibrator, PCB’s Model 913A10, operates from 10,000 to 125,000 psi. This unit uses an
Calibration Shock Tube. PCB’s Model 901A10 (see Figure 3) is a gas-driven shock tube capable of producing shock waves with nanoseconds rise time. Depending on the diaphragm material separating the driver from the test section, shock waves as low as
Aronson Step Pressure Generator. The step pressure generator (see Figure 4) was developed in the 1960s by Phil Aronson and R. Wasser at the Naval Ordnance Lab to calibrate underwater pressure sensors at incremental pressures under higher static load conditions.
Aronson devoted much of his professional career to the study of transient pressure measurements and dynamic calibration.
He and Wasser set a goal of developing an aperiodic calibration device that could perform dynamic calibration with greater accuracy and ease than was possible with the shock tube. Their device, using helium as a gas source, can generate known step pressures up to 2000 psi in ~50 µs.
The Aronson step pressure generator is quite fundamental in both concept and operation. It rapidly vents a precisely known static pressure to a sensing diaphragm by pressurizing the main reservoir with a known static pressure, then quickly exposing the sýnsor being calibrated to the reference pressure by releasing the fast-acting poppet valve. The pressure drop in the main reservoir due to the added volume between the sensor diaphragm and poppet valve is negligible with flush diaphragm sensors, and would be indicated on the digital pressure gauge that monitors reservoir pressure. The step pressure generator can be used to produce positive or negative pressure pulses of accurately known amplitude. Traceability to NIST is through the DC reference gauges used to set the known static pressure level to which the test sensor will be rapidly exposed.
“Pistonphone” Microphone Calibrator. The Pistonphone (see Figure 5) is a good example of a periodic calibrator.
The portable, battery-powered device produces a fixed 134 dB amplitude sine wave at a frequency of 250 Hz for calibrating microphones and low-pressure acoustic sensors. The known sine wave reference pressure level is generated by two opposed reciprocating pistons in a controlled volume inside the Pistonphone. The use of precision mounting adaptors is critical for maintaining the known volume and reference pressure when calibrating different types of sensors.
Jim Lally is a cofounder of PCB Piezotronics and Dan Cummiskey is Pressure Division Manager, PCB Piezotronics Inc., Depew NY; 764-684-0001, firstname.lastname@example.org, email@example.com.