Paroscientific, Inc. Resonant Quartz Barometers

RESONANT QUARTZ BAROMETERS

Introduction

Accuracy, stability, and reliable performance under difficult environmental conditions are key performance requirements for meteorological instrumentation. Accuracy and stability are required to assure data quality. Instrumentation reliability directly affects data network integrity as well as operating costs.

Barometers employing quartz crystal resonator technology were developed and commercially introduced over 30 years ago by Paroscientific, Inc.¹ The design and performance requirements included: (1) Inherently 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 stability.

These 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.

Construction & Operation

The resonant 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 conditions.

The 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 counters.

Several 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 180^o 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

The 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.

Because the 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 observed values.

Resolution, Noise & Accuracy

Short-term measurements generally require high pressure resolution while longer term measurements need accuracy, stability, and insensitivity to environmental errors.

With a 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 resolution. Typical 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.

Noise levels 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

The 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.

Long-Term Stability

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

GPS Meteorology

Global 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.

GPS 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

GPS 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 Earth’s 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

The 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.

Conclusions

There are 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 demanding requirements.

References:

1.	Paros, J.M., 1973, Precision Digital Pressure Transducer, ISA Transactions: 12, p. 173-179.
2.	Agnew, D.C., 1995, Analysis of Microbarograph Comparison Data, U.S.G.S. Internal Project Report, August 24, 1995.
3.	Duan, 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.
4.	Businger, 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.
	This 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.