Dynamic Pressure Callibration

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,

PCB Piezotronics Inc.

In the 1950s and 1960s, with the advent of the aerospace era and weapons development came a requirement for high-frequency pressure sensors capable of measuring shock waves, blast, rocket combustion instability, and ballistics. Piezoelectric sensors available at the time had limited frequency response and were used mainly for acoustic and engine combustion applications. 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

The development of higher frequency sensors increased the need for dynamic pressure calibration. Since dynamic calibrators were not commercially available until relatively recently, many labs developed calibration devices to suit their specific needs. These included a variety of hydraulic and pneumatic shock, pulse, and sine wave pressure generators. The dead-weight tester was sometimes used in a pressure release mode to generate a known, negative pressure pulse. The calibration shock tube remains the most practical device for producing the fastest rise time over a wide range of pressures, although the pressure amplitude is not known so accurately as with the pulse calibrators. 

Piezoelectric Sensor Characteristics

Piezoelectric (PE) pressure transducers are well suited for dynamic pressure measurements. Fabricated from natural piezoelectric quartz, natural tourmaline, or artificially polarized manmade ferroelectric ceramic materials, they are available in high-impedance charge mode designs and, more commonly, with integral electronics. PE sensors have a wide linear dynamic range, ultrahigh frequency response, and rise times as fast as 0.2 µs, and they operate over a wide temperature range. They are small, have flush diaphragms, and provide a clean high-voltage output. Quartz sensors can be linear and accurate over all, or just a fraction of their wide dynamic range. Their durable solid-state construction lets them stand up to tough environments. They are well suited for low-pressure fluid-borne 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, 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 bellow).

Figure above. PCB’s Model 113A24 quartz pressure sensor exhibits consistent sensitivity when calibrated by five different methods over a wide frequency range at amplitudes from 100 to 1000 psi.

Dynamic Calibration Methodology
Dynamic calibration of pressure transducers is more demanding and less accurate than static dead-weight methods, where measurement data can be carried out to many decimal points. Dynamic calibration is generally accomplished by venting the sensor being calibrated to a known static reference pressure by means of a fast-acting valve; by hydraulic impulse comparison calibration using a transfer standard, or with a shock tube. Depending on the type of calibrator and gas medium used, rise times on the order of milliseconds or microseconds can be achieved. Sensor mounting, switching transients, and readout accuracy all enter into the equation. Digital signal processing has greatly improved the measurement accuracy as compared with earlier methods that used analog storage oscilloscopes. The ideal dynamic calibrator would be structured to generate a precisely known reference pressure that could be continuously adjusted over a wide range of amplitudes and frequencies. But no such device exists. 

Traceability to National Standards
As is the case with all calibration methods, dynamic calibration should be traceable to a national laboratory such as NIST, but there do not appear to be any labs that provide traceability in this area. In 1991, at a NIST workshop focused on transient pressure and temperature, technical presentations addressed both current and proposed dynamic calibration methodologies.

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 calibration is useful for several reasons. With ceramic sensors it is the only way to determine sensor output relative to input. Some DC low-frequency sensors may not give the same response to identical static or dynamic input pressure. The output of all pressure sensors is frequency dependent. Dynamic calibration devices, such as the shock tube, are quite useful for determining sensor resonance characteristics as well as resonance in gas passages associated with recessed mounting. Gas passage resonance is analogous to the change in frequency response that results from adding an adaptor between an accelerometer base and the test structure.

Dynamic Pressure Calibrators
Dynamic pressure calibrators have evolved over the years in response to specific needs at various laboratories. The instruments vary widely in the type of pressure source used and in their amplitude and frequency range. PCB adapted the best of these technologies for in-house sensor research and dynamic calibration. To help customers understand and evaluate the characteristics of sensors for transient applications, several of the dynamic pressure calibrators are now offered as standard commercial products.

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 bellow)

Figure above. PCB’s 913B02 impulse calibrator uses a free-falling mass that strikes a piston and cylinder manifold to produce a pulse waveform for calibrating pressure sensors over a wide dynamic range. Calibration is performed by comparing outputs from the test and reference sensors, both installed in the manifold and subjected to the same pressure pulse. Pressure amplitude is determined by the height from which the mass is dropped.

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 accelerometer to measure the deceleration of the free-falling mass after it strikes the piston. Deceleration of the mass, coupled with the geometries of the piston and cylinder, determine the amplitude of the pressure pulse. The calibrator has a simplified structure and is easier to operate than other high-pressure dynamic calibrators. 

Calibration Shock Tube. PCB’s Model 901A10 (see Figure above) 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
3 psi can be generated using aluminum foil, and >1000 psi using sheet aluminum. Compressed gas—such as air, helium, or nitrogen—is pressurized in the driver section until the diaphragm bursts, sending a shock wave into the test section. As a driver source, helium provides a well-formed shock wave with the highest Mach number. The amplitude accuracy of the shock wave (approximately ±1¹/₂%) is calculated from measuring the shock wave velocity, the temperature, and the atmospheric pressure. 

Aronson Step Pressure Generator. The step pressure generator (see Figure bellow) 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 bellow) 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.

Summary.

Several different types of high- and low-pressure calibrators are commercially available to assist engineers with dynamic calibration and evaluation of frequency response associated with sensor recess mounting. Calibrators vary in the amplitude, frequency, and type of waveform generated. As a general guide, the calibration methodology should most closely approximate the measurement application in which the sensor will be used.