Harwood
Engineering Company, Inc.
A 150,000 psi Dynamic Pressure Calibrator
by Arpad A. Juhasz, Donald H. Newhall, Charles D. Bullock, James O. Pilcher
and Melinda B. Krummerich
Approved for Public Release, Distribution Unlimited by U.S. Army Ballistic
Research Laboratory, Aberdeen Proving Ground, Maryland, U.S.A.
Introduction
Although ballistic pressure transducers (gages) are used to measure dynamic
events which occur in milliseconds, the determination of their response
characteristics is routinely limited to static calibration against a deadweight
pressure standard. The strength of this method is that the deadweight device
is a primary standard. Its weakness lies in the assumption that the static
and dynamic responses of the gage in question are equivalent. Differences
in gage response to static and dynamic events, however, can lead to serious
measurement errors. There has been general agreement in the measurement
community that dynamic techniques are needed to supplement current static
calibration methods. Several techniques have been developed to address
this problem.
1. Ballistic Pulse Method
In one version of this technique, the gage is mounted at the end of a tube
in contact with a hydraulic fluid confined by a movable piston. The tube
guides a projectile which impacts the piston to create a pressure pulse
in the fluid. Different pressures may be achieved by varying the compressibility
of the fluid, the mass of the piston, and the mass and velocity of the
projectile. The pulses rise within milliseconds and mimic the characteristic
rising and falling of a ballistic pressure pulse. One such device, capable
of operating to a pressure of 100,000 pounds per square inch, is operational
at the Combat Systems Testing Activity (CSTA), Aberdeen Proving Ground,
Maryland.3
This method is quite useful for dynamic comparison of several different
pressure gages; however, variations in projectile velocity, frictional
effects on the moving piston, and other energy losses make it difficult
to accurately computer the actual delivered pressures. Because a projectile
is fired during the calibration process, this method requires more extensive
safety provisions than are readily available in most laboratories.
2. Shock Tube Method
Two general approaches of shock tube calibration are followed. In the first,
the test gage is mounted in the end wall of a tube and subjected to a reflecting
shock wave. The gage output is monitored as the shock front arrives at
and reflects from the end wall. In the second approach, the gage is mounted
in the side wall of the tube and its output is monitored as the shock front
passes. Both methods generated rapidly rising pressure pulses that are
readily calculated by thermodynamic principles using velocity measurements
and gas properties.
Shock tube methods are useful in establishing the dynamic response characteristics
of pressure gages. However, calibration is generally limited to pressures
below 1000 pounds per square inch, whereas ballistic applications require
far higher pressures.
3. Negative-Going Pressure Step Method
In this technique, the gage is exposed to a given pressure under static
conditions using a hydraulic fluid. The pressure on the gage is then relieved
using a fast acting dump valve, bringing the system to atmospheric pressure.
The gage output obtained during the depressurization is assumed to be the
inverse of the corresponding positive pressure step.
This method's strengths include its relative simplicity and suitability
for use in calibration facilities. The response of the negative step calibrator
can be very quick, 100 microseconds or less. However, the major assumption,
that the positive response of the gage is equal and opposite to the negative
response of the gage, is not completely accurate; pressure preloading of
the gage-to-mount interface and hysteresis can cause significant differences
between the pressurization and depressurization pulses.
4. Positive-Going Pressure Step Method
In this technique, the gage, initially at atmospheric pressure, is subjected
to a pressure increase by the opening of a fast-acting valve. Because the
final pressure value is held, the method suitable for obtaining calibration
response data. Although this technique holds a great deal of promise, the
engineering details of creating a working device can be formidable. Johnson
and Cross of the National Bureau of Standards had designed a 50,000 pounds
per square inch step calibrator in the 1950's. Smith4 and Dykstra5
described low pressure versions of this device. These calibrators were
successfully used to generate positive pressure steps up to 5000 pounds
per square inch in less than one millisecond.
Building on these ideas, we have developed a device capable of generating
precisely known positive pressure steps up to 150,000 pounds per square
inch in less than one millisecond. The step calibrator may be safely operated
in a laboratory environment and can compare the response of several transducers
to a common pressure step. Additionally, it is economical to use as a routine
laboratory tool for gage calibration and screening.
The discussion that follows describes the pressure step calibrator and
presents several examples of its operation.
Discussion
1. Design Concept
Figure 1 illustrates the basic configuration of
the device, details are given in Figure 2. A large
pressure reservoir is connected to a much smaller test chamber by a fast
acting ball valve. The test chamber is equipped with several gage ports
and a vacuum port which aids in filling the reservoir and setting the baseline
chamber pressure. The large ratio of reservoir to test chamber free volume
reduces overall system pressure drop while generating the pressure step.
Reservoir pressure is provided by a conventional hydraulic high pressure
panel and monitored by a high quality static reference gage. The maximum
operating design pressure is 150,000 pounds per square inch and the specified
action time (10% - 90% of peak pressure) is under one millisecond.
Outputs of both the static reference gage and the ballistic test gages
are monitored during the course of the test. The final steady state output
of the reference gage is taken as the true value of the pressure step maximum.
The short- and long-term monitoring of the test gage outputs establish
the relationship between dynamic and steady state response behavior.
2. Device Description
The device, illustrated in Figures 2, 3,
and 4, consists of a large pressure reservoir (1)
opening into a short wide channel (2) which terminates in a very small
cylindrical test chamber (3). Located in the test chamber (3) is a ball
valve (4) which provides a high pressure seal at either of the valve seats
(5) located at each end of the test chamber (3). Located in the side wall
of the test chamber (3) are four gage ports (6) and one vacuum line port
(7). The channel (2), test chamber (3), gage ports (6) and vacuum line
port (7) are contained in the test head (8), a monolithic assembly shown
in lateral cross section in Figure 3. The test
head (8) is readily removed from the assembly, allowing changing of the
gage ports (6) and the test chamber (3). The ratio of the reservoir (1)
volume to the test chamber (3) free volume is 197:1. The channel (2) is
kept short and wide to minimize retardation of fluid flow during the operation
cycle. The end closure of the test chamber (3) is formed by the ball valve
actuator piston (9) and the piston guide bushing (10). The piston is actuated
by a quick release top-dead center mechanism which consists of three pin
joints (11), and air controlled trigger mechanism (12), a hydraulic jack
(13) and a limit stop/buffer (14). The system shown in Figure
2 is in the cocked position with the ball (4) pressed against the seat
(5) that isolates the test chamber (3) from the reservoir (1). The jack
(13) is pressurized to provide sufficient force to seal the reservoir (1)
from the test chamber (3). The ratio of reservoir (1) pressure to jack
(13) pressure is approximately 100:1.
A hollow stem valve (15) connected to a vacuum/drain line is located
at port (7). See Figure 3.
A high pressure line (17), (Figure 2), is connected
to port (16) at the upper end of the reservoir (1). This line connects
the pressure generation system to the measurement system.
Prior to operation, both the test chamber and the reservoir are filled
with fluid at ambient pressure. During the pressurization the reservoir
is sealed off from the test chamber. The actual movement of fluid through
channel (2) is minimal, consisting of the volume required to displace the
movement of actuator piston (9) in the test chamber (0.2 cc) and the amount
due to compression of the fluid originally in the test chamber.
Figure 4 shows the device after the trigger
mechanism (12) is released. The trigger (12) forces the middle pin joint
of the top-dead center mechanism against the limit stop/buffer (14), relaxes
the force generated by the jack (13) and withdraws the ball valve actuator
piston (9) into the piston guide bushing (10). Differential pressure between
the reservoir (1) and the test chamber (3) forces the valve ball (4) against
the lower seat (5) on the piston guide bushing (9). This action forms a
new seal at the piston end of the test chamber (3) and allows fluid to
flow from the reservoir (1) to the test chamber (3) causing the pressure
in the chamber to rise to approximately 98% of the original reservoir pressure.
3. Device Operation
Starting with a drained system, the top-dead-center mechanism is placed
in the release position as shown in Figure 4. The
high pressure line (17) is closed off and the vacuum valve (15) is opened.
Gages are mounted in the gage ports (6), and the system is evacuated to
a pressure of 2 Torr and the vacuum valve (15) is closed. A 50% solution
of water and glycol with a rust inhibitor enters the system through the
high pressure line (17). Liquid is used rather than gas to minimize the
level of stored energy. When the system is filled and stabilized at atmospheric
pressure, the top-dead-center mechanism is cocked as shown in Figure
2. The hydraulic jack (13) is pressurized to approximately 1% of the
desired reservoir pressure. Monitored by the static reference gage, the
pressure generation system pressurizes the reservoir (1). Once the desired
reservoir pressure has been established, the high pressure line (17) is
closed off from the standard gage and the pressure generation system by
a constant volume valve. The device is now ready to be triggered.
Activation of the top-dead-center mechanism shown in Figure
4 initiates the event and the recording system. Pre-trigger delay features
permit the recording of initial baseline pressures, the rising portion
and final steady state values of the pressure-time curve. One gage with
exceptionally good response characteristics and known history is used as
an informal laboratory standard. The output from this gage is monitored
for at least 10 seconds after the trigger event to observe system behavior,
particularly possible pressure losses from leakage. Soon after the system
is triggered, the constant volume valve to the static reference gage is
reopened and the reservoir pressure measured. Thus, the speed of the step
can be measured using the timebase of the recording system and the magnitude
of the dynamic response can be checked against the response of the static
reference gage (a secondary standard).
At the completion of the test, the top-dead-center mechanism is recocked,
the test chamber (3) is drained through the vacuum valve (15), and the
pressure is relaxed in the reservoir (1). The gages can now be replaced
for further testing.
4. Prototype Performance
Examples of both short- and long-term responses of a commercial piezoelectric
pressure gage to a positive-going pressure step are presented in Figures
5, 6, 7,
and 8. Figure 5 shows a
typical pressure versus time history for a 75,000 pounds per square inch
pulse acquired over 10 seconds. Monitoring the test gage response for 10
seconds allows correlation to the steady state response of the static reference
gage. Figure 6 shows a 20 millisecond window of
the same 75,000 pounds per square inch pulse; Figure
7 and Figure 8 present progressively shorter
windows. Overlaying the traces indicates that the steady state value of
the pressure step is rapidly achieved and held after initial oscillations
have died out. Similar oscillations have been reported in lower pressure
devices by both Smith and Dykstra; one may conclude that these oscillations
are caused by actions within the pressure generation system and the mount,
not by the gage itself.
One important application of the step calibrator is comparing the responses
of different types of pressure gages to the same input. Figure
9 and Figure 10 show the pressure versus time
histories of both a piezoelectric gage and a strain-type gage measuring
a positive-going 100,000 pounds per square inch pressure step. The traces
are virtually identical, exhibiting initial system oscillations which quickly
decay.
The calibrator may also be used to analyze the behavior of experimental
gages by comparing their output with a known standard. Figure
11 shows the pressure versus time history of a developmental pressure
gage subjected to a 125,000 pounds per square inch pulse. This curve exhibits,
as expected, a smooth pressure rise and stable output after the peak pressure
has been attained.
The positive step calibration device in conjunction with conventional
deadweight calibration methods can be useful in tracing dynamic gage response
problems. Figure 12 exhibits the pressure history
of a developmental gage exposed to a 130,000 psi pressure step. The trace
indicates a clear upward drift after the step is complete. The same gage
had exhibited good response behavior on static calibration using the deadweight
system. In gun tests, however, the device read five percent low. From the
dynamic calibration tests it became clear that it took tens of milliseconds
for the gage output to reach maximum. This could not be detected by the
static calibration technique. In point of fact, on dissecting the gage
it was found that the bond between the strain patch ad the gage body was
faulty. The defect of the type noted could well account for the differences
in static vs dynamic behavior noted in testing.
Conclusions
The positive-going step calibration device described allows the accurate,
safe and simple dynamic calibration and evaluation of ballistic pressure
gages. The gage response obtained can be related to its static deadweight
behavior. Several gages may be evaluated simultaneously relative to a common
dynamic event. The calibrator may also be used as a diagnostic tool in
analyzing and developing experimental pressure gages.
References
1. National Bureau of Standards Report 4440, Measurement of High Pressure:
Bibliography, Index, and Preliminary Survey, W.G. Brombacher, U.S. Dept.
of Commerce, National Bureau of Standards, October, 1955.
2. NBS Technical Note 914, A New Dynamic Pressure Source for the Calibration
of Pressure Transducers, Carol F. Vezzetti, John S. Hilton, J. Franklin
Mayo-Wells, and Paul S. Lederer, U.S. Dept. of Commerce, National Bureau
of Standards, June 1976 (and references therein).
3. Personal communication with J.D. Dykstra of the Combat Systems Testing
Activity, Aberdeen Proving Ground, Maryland.
4. Smith, R.O., A Liquid Medium Step Function Pressure Calibrator,
ASME Paper Number 63-WA-263. The American Society of Mechanical Engineers,
345 East 47th St., New York, NY.
5. Dykstra, J.D. "Evaluation of Pressure Transducer Response with a
Pressure Step Generator," Bulletin of the Eleventh Meeting of the Joint
Army-Navy-Air Force Solid Propellant Rocket Static Test Panel, SPIA Publication
SPSTP/11, September, 1962.
Harwood Engineering Company, Incorporated – © June, 1999
455 South Street, Walpole, Massachusetts 02081
phone: 508-668-3600
fax: 508-660-2276
http://www.ultranet.com/~harwood/
email: harwood@ma.ultranet.com
