Pressure Instrumentation for
Mark H. Houston and Jerome M. Paros
construction activities the accurate measurement of pressure is
a key requirement
The pressure measurement may be used as primary observation data such as
in Tsunami detection, wave and tide gauges, and platform
leveling applications, or the pressure measurements may be used
as associated observation data such as in depth sensors for
ROV'S, profiling instruments, and towed arrays. In all these
applications pressure transducers employing quartz resonator
technology have been successfully used in underwater systems
that required the highest resolution, accuracy, and stability.
This paper describes the construction, operation, and
performance of the quartz resonator technology with specific
examples of underwater applications.
In addition, advances in materials and electronics
promise to extend the usefulness of these devices within virtual
at synoptic observations.
Resonant-quartz pressure transducers can effect a
typical application accuracy of 100 ppm (0.01%FS) with a
resolution of 1 x 10-8. This
remarkable performance can be achieved through the use of a
precision oscillator whose frequency varies with
pressure-induced stress on the quartz crystal resonator.
Quartz crystals make excellent sensing elements because
of quartz's inherent mechanical repeatability, stability, and
low hysteresis. The
crystal oscillations are maintained and detected with oscillator
electronics similar to those used in precision clocks and
A. Dual-Beam Resonator Construction
Figure 1. Dual Beam Quartz
1 illustrates a flexurally-vibrating, dual beam, load-sensitive
double-ended tuning fork consists of two identical quartz beams
driven piezo-electrically in 180° phase opposition such that
little vibration energy is transmitted to the mounting pads.
The high-Q resonant frequency, like that of a violin
string, is a function of the applied beam-axial load; the
frequency increases with tensile and decreases with compressive
the load resonator is insensitivitive to temperature,
differences in the thermal expansion coefficient between quartz
and other materials produce thermal effects which are
compensated for with a unique digital temperature sensor.
The resonant frequency of the piezo-electrically driven, torsionally oscillating tines
is a function of temperature. Combining the outputs of the two
sensors into a modeling equation results in
thermally-compensated, high accuracy, calculated pressure output
over a wide range of temperatures.
B. Bourdon Tube Mechanism
2. Bourdon Tube Mechanism.
Tube illustrated in Figure 2
shows one mechanical arrangement that can be used to transform
pressure to force. A
pressure applied to the tube interior generates a force across
the quartz resonator as the tube tries to unwind with applied
change in the frequency of the quartz oscillator is a measure of
the applied pressure. The
tube and resonator are enclosed within an
evacuated chamber to eliminate air damping, to maximize the
Q of the oscillator, and to provide a reference vacuum for
absolute pressure measurements.
The sensitivity and pressure range of the transducer can
be controlled by the physical parameters of the tube, i.e.
metallurgy, wall thickness, hoop diameter, etc.
Because the total mechanism movement is approximately
several microns full scale, linearity, hysteresis, and repeatability of the overall
sensor are excellent.
Resonant Quartz Sensor Performance
transducers have demonstrated a deep-sea pressure measurement
performance with a resolution of 0.15 ppm and an accuracy of
0.015% of full-scale, (0.003% corrected), for a 10,000-psi
(absolute) sensor .
Resolution, Noise, & Accuracy
ultimate resolution achievable with a transducer is limited by
its measurement noise level.
Noise for resonant quartz devices includes both
electronic and mechanical components.
For short records (less than 103 sec), the noise is
dominated by thermal and electronic "count" errors.
For stable, deep-sea environments, these errors can be
limited to less than 0.2 ppm . For longer records, the measurement noise includes
significant contributions from oscillator noise, thermal
variations, and mechanical drift.
For example, yearly drift rates of 20 to 150 ppm are
typical for the resonant-quartz Bourdon Tube sensors.
Underwater Sensor Applications Geophysical and Oceanographic
pressure measurements in the deep ocean have contributed to the
understanding of geophysical and oceanographic processes over a
wide range of time scales.
primary sensor measurements have included short-term events such
as microseisms, surface wind-driven surface waves, and tsunamis,
and long-term phenomena such as oceanic tides, planetary waves,
and other atmospheric forcing events.
For example, Weam and Baker  reported on the
geostrophic transport fluctuations of the Antarctic Circumpolar
Current using pressure transducers located in the Drake Passage.
Bernard and Milbum  deployed quartz resonator sensors as part
of a long-wave observational program, and Fox  investigated
hydrothermal venting activity on the Juan de Fuca ridge.
Measurements for Location and Positioning
second major underwater application domain for quartz-resonator
technology is the determination of depth for use in location and
positioning of instruments, sensor arrays, and underwater
first systematic sampling of ocean properties was begun in the
1860's using hydrographic wire casts with Nansen bottles and
reversing thermometers. Sample depths were calculated from measurements of the
lengths of wire-out corrected for wire angle as estimated from
the surface entry angle and from comparisons between protected
and unprotected thermometers.
Today, oceanographers use CTD instruments (conductivity,
temperature, & depth) to measure "continuous"
profiles of water parameters. With proper corrections for sea
water compressibility, quartz-resonant depth sensors can measure
oceanic depths (6000m) to an absolute accuracy of less than one
these temperature-compensated, low power sensors allow
construction of reliable, self-contained wireless devices with
internal data recording.
Towed-body Depth Sensors
many active and passive sonar systems, accurate tow depth is an
important auxiliary parameter for effective data acquisition and
example, the AMS deep-tow sonar side-scan system  uses
vehicle depth to correct the geometric distortion of the
side-scan multi-beam image of the bottom.
Accurate depth data is also required to merge adjacent,
overlapping images to form coherent sea floor topography.
Towed, passive sonar systems usually employ an array of
individual receivers can be combined into synthetic aperture
arrays and can be steered to maximize signal to noise, but only
if the receiver geometry is well defined, through accurate depth
sensors can reliably measure depths to ±10 cm to a depth
of 1000 m.
underwater vehicles dominated the initial efforts to discover
the ocean depths. Remote
Operated Vehicles (ROV's) have garnered the major share of
underwater services to support oil and gas exploration,
production, and associated construction. Both the accuracy and long-term stability of the
quartz-resonant depth sensors have played an important role in
reducing the life-time support costs and increasing the
operating usefulness of these work-horse systems.
These characteristics as well as reliability and low
power consumption will also play key roles in the practical use
of the next generation Autonomous Underwater Vehicles (AUV's).
Performance Improvements from Technology
that the quartz-resonator technology coupled with simple
mechanical structures such as the Bourdon tube currently
provides outstanding performance, where and what kind of
performance enhancements can we expect?
improvements to the sensor's performance / price ratio can be
Improving the design and manufacturability of the sensors to
the long-term mechanical stability of the devices (reduce
the oscillator electronics' resolution and accuracy; and
the sensor's communications architecture to provide
capabilities for synchronous sensor array sampling.
Figure 3. Barometer Drift Rate
3 plots the cumulative drift on three resonant-quartz barometers
(11 - 16 psia). rates range from -3 to -11 ppm per year.
The barometer design, which uses a bellows mechanism, has
a much "softer" pressure-to force mechanism than the
Bourdon tube design. Lower
pressure mechanisms, in general, will usually exhibit a lower
drift rate than higher pressure mechanisms.
review of the material properties of the components of the
deep-sea quartz-resonator sensors suggests that new materials
for the Bourdon tube and the quartz-metal attachment interfaces
may improve the long-term stability of the current sensor
of quartz transducers is a function of resonant frequency,
sampling time, and counting-clock frequency. Using high
frequency, interpolating, start-stop counters, a 10-sec
integration period will resolve the typical periods to a few
parts per billion .
DSP Processing of Stable Sine - Wave Oscillator Signals
improvements in sensor performance can be accomplished in
several ways. First,
resolution and accuracy of a measurement at a given sampling
frequency can be increased by utilization of a low-noise,
stable, sine-wave oscillator design and employment of more
sophisticated processing algorithms with accurate non-integer
cycle counting. Figure
4 shows the results of an experiment to show the feasibility of
vs. Integration Time
right-hand line of Figure 4 shows a plot of pressure resolution
in ppm as a function of clock-count integration time for
Paroscientific Intelligent Transmitters.
These devices use the signal from a digital (square-wave)
pressure oscillator to gate counts of an internal clock running
at 15 MHz. The
integration time is the span over which the reference counts are
resolution represents an uncertainty of ± l count over the
cumulative reference-clock counts. As an example, an integration time of 0.6 sec represents a resolution of 1 ppm; an
integration time of 0.01 sec gives a resolution of 60 ppm. Faster sampling yields less resolution; longer integration
time produces better resolution.
Of course the resolution does not increase monotonically
with integration time, since thermal and oscillator noise begin
to dominate the results.
left-hand line of Figure 4 shows the results of an experimental
setup with a first-cut, low noise, sine-wave oscillator.
The oscillator, although of our standard design, was
hand-built and thermally isolated to achieve the best noise
performance with reasonable effort.
The oscillator output was oversampled at 80-kHz, and then
digitally processed to extract the best-fit period.
The stability of the solution and therefore the
resolution was determined by comparing different
integration-time partitions of a continuous record span.
results demonstrate that straightforward improvements to the
sensor electronics will achieve resolution of 10 ppm for 50-Hz
pressure signals without sacrificing any DC accuracy.
However, to reach a level of 30 ppb for 1-sec sample
rates will take additional efforts to develop a more stable
Communications Architecture for Synchronous Array
majority of pressure sensors are used as point measurement
devices to sample a slowly varying pressure field.
However, the ability to sample the absolute pressure-wave
field with increased resolution from 0.0001-Hz to 50-Hz may
suggest measurement opportunities that would benefit from sensor
arrays. In order to
be able to combine sensor outputs into a steerable array,
hardware and software communications protocols need to be in
place to ensure sampling synchronization among sensors.
Some of these requirements are currently being
incorporated into industry standards for "smart
transmitters" (IEEE 1452.1).
transducers have demonstrated an excellent record of accuracy,
stability and performance in underwater activities such as
deep-sea research and development, oil and gas exploration and
production, and undersea construction.
Advances in materials and electronics promise to extend
the usefulness of these devices into virtual instrumentation
arrays aimed at synoptic observations.
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Presented at Instrument Society of
International Instrumentation Symposium, Albuquerque, NM, May