stability, and reliable performance under difficult environmental
conditions are key performance requirements for meteorological
and stability are required to assure data quality.
Instrumentation reliability directly affects data network
integrity as well as operating costs.
employing quartz crystal resonator technology were developed and
commercially introduced over 30 years ago by Paroscientific, Inc.1
The design and performance requirements included:
digital outputs, (2) Accuracy
comparable to the primary standards, (3)
Highly reliable and simple design, (4)
Minimum size, weight and power consumption, (5)
Insensitivity to environmental factors, and (6) Long-term
barometers are used in laboratory and field pressure standards of
remarkable resolution, stability, and accuracy.
Other meteorological applications include use on marine data
buoys, atmospheric wave and turbulence detectors, and altimeter-setting
indicators. More recently,
the technology has been incorporated into automated surface observation
systems that estimate atmospheric precipitable water vapor in
conjunction with GPS (Global Positioning System) geodetic networks.
quartz crystal barometers are designed to have resolution better than
one microbar (<0.1 Pa) and a precision of better than 0.01% of
reading (<0.1 hPa) maintained even under difficult environmental
remarkable performance is achieved through the use of a precision quartz
crystal resonator whose frequency of oscillation varies with pressure
induced stress. Quartz
crystals were chosen for the sensing elements because of their
remarkable repeatability, low hysteresis, and excellent stability.
The resonant frequency outputs are maintained and detected with
oscillator electronics similar to those used in precision clocks and
flexurally-vibrating, single or dual beam, load-sensitive resonators
have been developed. The
Double-Ended Tuning Fork consists of two identical beams driven
piezoelectrically in 180o phase opposition such that very
little energy is transmitted to the mounting pads.
The high Q resonant frequency, like that of a violin string, is a
function of the applied load; increasing with tension and decreasing
with compressive forces. The
digital temperature sensor consists of piezoelectrically-driven,
torsionally oscillating tines whose resonant frequency is a function of
temperature. Its output is
used to thermally compensate the calculated pressure and achieve high
accuracy over a wide range of temperatures.
Figure 2. Barometer Mechanisms
barometer mechanisms employ bellows as the pressure-to-load generators.
Pressure acts on the effective area of the bellows to generate a
force and torque about the pivot and compressively stress the resonator.
The change in frequency of the quartz crystal oscillator is a
measure of the applied pressure. Temperature
sensitive crystals are used for thermal compensation.
The mechanisms are acceleration compensated with balance weights
to reduce the effects of shock and vibration.
The transducers are hermetically sealed and evacuated to
eliminate air damping and maximize the Q of the resonators.
The internal vacuum also serves as an excellent reference for the
absolute pressure transducer configurations.
Since any changes in the reference vacuum directly affect the
barometric output, great care is taken to ensure that there are no leaks
and minimal outgassing in the evacuated housing.
quartz crystal constrains total mechanism movement to several microns
full scale, reproducibility is excellent.
Pressure hysteresis tests on a group of 23 standard production
Paroscientific barometers showed no measurable hysteresis when cycled
over pressures from 827 to 1069 hPa.
The mean observed hysteresis was 0.001 hPa and the largest
observed value was 0.0077 hPa. The
estimated measurement uncertainty of 0.008 hPa, was greater than all
Noise & Accuracy
measurements generally require high pressure resolution while longer
term measurements need accuracy, stability, and insensitivity to
sensor of inadequate resolution, real signals can be obscured by noise,
or sensor noise can be misinterpreted as real signals.
The resonant quartz crystal barometer mechanisms, oscillator
circuits, and digital interfaces are carefully designed for high
delivered resolution is better than one part per million, and under
stabilized laboratory conditions, resolution can approach one part per
billion. Applications where
it is important to measure small pressure changes include measurements
of wind-shear, wake turbulence, and atmospheric shock waves.
as a function of frequency are generally expressed as spectral
densities. Plots of this
type are used to determine whether a sensor can measure a desired
signal. The goal is to have
the sensor noise levels much smaller than the expected real signals at
all frequencies of interest.
Data from an
evaluation by Hutt, Holcomb, and Agnew2 of high quality
sensors for use in atmospheric seismic studies showed that the resonant
quartz barometers had power spectral density noise levels a factor of
100 lower (20 db) than the next best transducer.
Figure 3. Noise Versus Record Length
ultimate resolution achievable with a transducer is limited by its noise
level. Typically, the rms
noise increases for longer data records because of sensor drift and
because temperature and other environmental contributors to noise tend
to vary more over a longer period of time.
Typical rms noise levels for the quartz transducers are shown in
Figure 3. For records
shorter than about 1 hour, the rms noise level is less than 1 part per
million (< 0.1 Pa). The
rms noise rises slowly with record length, reaching approximately 10 ppm
for records several years long.
Generally, users who care about absolute accuracy
also need to know how well the transducer holds its accuracy over time
(long-term stability) and how sensitive it is to the effects of
temperature and other environmental factors. Less stable devices
need to be calibrated more often or may be incapable of performing
adequately under field conditions.
Paroscientific pressure transducers typically deliver
accuracy better than 100 parts per million of full scale pressure over a
wide temperature range and maintain this accuracy for a long time.
Figure 4 shows cumulative drift on three resonant quartz barometers.
Drift rates range from -4 ppm to -10 ppm per year, with a median drift
rate of -7 ppm (0.007 hPa) per year.
Thousands of resonant quartz
crystal barometers are used in applications where long-term stability is
critical including transfer standards, digital altimeter setting
indicators, air data computers, calibration systems, remote sensing
stations, and drifting data buoys. An important new application is
the determination of precipitable water vapor using GPS Meteorology.
|Figure 4 - Long-Term Stability
Positioning System (GPS)
Meteorology is the application of GPS data to the monitoring and
analyses of atmospheric conditions. Accurate, frequent, and dense
sampling of water vapor is needed for operational weather forecasting as
well as for weather and climatic research.
satellites transmit radio signals that can be inverted to measure
atmospheric profiles of refractivity. The refractivity profile can be
transformed to profiles of tropospheric humidity given a temperature
profile. Ground-based GPS receivers at fixed locations with accurate
surface barometric pressure measurements can be used to gather data to
determine vertically integrated Precipitable Water Vapor (PWV).3,4
satellites transmit atomic-clock controlled L-band signals to receivers
on the earth. Time delays
of the signals can be directly attributed to the passage of the signals
through the Earths ionosphere and neutral atmosphere. The ionospheric
delay is dispersive (frequency dependent) and can be determined by
observing both of the frequencies transmitted by the GPS satellites (L1
& L2) using a dual-band GPS receiver. The neutral delay, is not
dispersive and can be decomposed into a hydrostatic delay
associated with the dry atmosphere and a wet delay
associated with the permanent dipole moment of water vapor. The Zenith
Hydrostatic Delay (ZHD) has a magnitude (equivalent GPS phase delay
length) of about 2300 mm at sea level. The Zenith Wet Delay (ZWD) can
vary from a few millimeters in
desert conditions to more than 350 mm in very humid conditions. It is
possible to predict the ZHD to better than 1 mm given surface pressure
measurements accurate to 0.3 millibar or better. The wet delay may be
estimated using Water Vapor Radiometers; however, these instruments are
subject to rain spike contamination, expensive,
and difficult to calibrate with sufficient accuracy.
Most geodesists prefer to measure the hydrostatic component of
the neutral delay using barometers and estimate the remaining wet delay
during inversion of the GPS observations.3
important ground-based measurements of barometric pressure, temperature,
and humidity necessary to determine precipitable water vapor can be made
with the Paroscientific MET3 Meteorological Measurement System which
uses a resonant quartz barometer. This GPS Meteorological technique can
recover precipitable water vapor with an rms error of 1.0 to 1.5 mm and
represents a milestone improvement
in environmental sensing technology.
More accurate prediction of storm systems will improve surface,
coastal, and air travel safety. Agriculture
and farming will greatly benefit from these models by improving crop
yields and better understanding micro-climates.
many important meteorological measurements that need high precision
pressure sensors. Resonant quartz crystal barometers with high
resolution, accuracy, and excellent long-term stability meet these
|| Paros, J.M., 1973, Precision Digital Pressure Transducer,
ISA Transactions: 12, p.
D.C., 1995, Analysis of Microbarograph Comparison Data, U.S.G.S.
Internal Project Report, August 24, 1995.
J., M. Bevis, P. Fang, Y. Bock, S. Chiswell, S. Businger, C. Rocken, F.
Solheim, T. van Hove, R. Ware, S. McClusky, T. Herring, and R. King,
1996: GPS Meteorology: Direct Estimation of the Absolute Value of
Precipitable Water, Journal of
Applied Meteorology, Vol. 35, No. 6, p. 830-838.
S., S. Chiswell, M. Bevis, J. Duan, R. Anthes, C. Rocken, R. Ware, M.
Exner, T. van Hove, and F. Solheim, 1996,
The Promise of GPS in Atmospheric Monitoring, Bulletin
of the American Meteorological Society, Vol. 77, p. 5-18.
updated report is based on the paper "High Precision
Instrumentation for Meteorological Applications" by Jerome M. Paros
and Mark H. Houston presented at World MeteorologicalOrganization
TECO-98, Casablanca, Morocco, May 1998.