DIGITAL QUARTZ PRESSURE TRANSMITTERS  
FOR ACCURATE WATER LEVEL MEASUREMENTS

Digital pressure transmitters have been developed for accurate, reliable, water level measurements.  They upgrade and replace expensive, mechanically complex, high-maintenance, mercury manometers in gas bubbler systems.  The transmitters use vibrating quartz crystal sensors and microprocessor based intelligent electronics to provide fully temperature-compensated, linearized, pressure output information in standard RS-232 format.  Previous PS-2 models had SDI-12 interface.  This interface was defined and developed as an industry standard by transducer and data logger manufactures in response to needs identified by the United States Geological Survey.  The construction, operation, performance, and water level measurement applications of these transmitters are described below. 

The accurate measurement, recording, transmission, and analysis of water levels are of vital importance to the United States Geological Survey.  As described in Reference 1, the Water-Resources Division of the USGS introduced a bubbler-gage manometer system in 1956.  A number of subsequent design changes were made by the USGS to improve reliability and reduce cost. 

A typical first generation bubbler-gage manometer system is shown in Figure 1.  It consists of a gas bubble generating system, feeding pressure in parallel to a mercury manometer and a tube/orifice placed in the water. 

The gas pressure necessary to balance out the water stage (height or head of water) also displaces the free surface of the liquid mercury in the pressure-cup reservoir of the manometer.  A float switch and servo amplifier/control/servo motor unit drives the mercury reservoir up or down a threaded shaft (or roller chain, cable belt and sprocket drive system) until there is sufficient head of mercury to balance out the gas pressure.  Because the density of mercury varies with the temperature, all uncorrected mercury manometers used by the Geological Survey will have an error of 0.01 percent per degree Fahrenheit.  These thermal errors can be reduced through an optional correcting device, that controls the angle of the manometer relative to vertical as a function of ambient temperature.  Analog and digital water-stage shaft and punched paper tape recorders, can be mechanically driven by sprockets on the manometer unit.

The desire for improved reliability and the need for a modern electrical interface prompted the Water Resources Division to search for an alternative system.  The Hydrologic Instrumentation Facility evaluated high accuracy DIGIQUARTZ® Pressure Transmitters made by Paroscientific, Inc. of Redmond, Washington.  

References 2, 3 and 4 describe the construction and operation of these vibrating quartz crystal sensors.  The basic gauge pressure transducer design is shown in Figure 2.

FIGURE 2:  GAUGE PRESSURE TRANSDUCER

Gas input P1 from the bubbler system is at a pressure equal to the water stage plus ambient atmospheric pressure.  Input P2 is at outside atmospheric pressure.  The opposing bellows arrangement cancels out atmospheric pressure and results in a net force on the lever arm proportional to the water level.  This force is transmitted through the lever arm to a load-sensitive vibrating quartz crystal (Figure 3). 

The central beam of the crystal is piezoelectrically induced to vibrate in its fundamental resonant mode.  An integral isolator mass/spring system ensures high Q operation in the internal vacuum of the housing.  A change in pressure at the bellows pressure port changes the axial load applied to the vibrating quartz beam and therefore changes its resonant vibrational frequency.  An oscillator circuit tuned to the resonant frequency of the quartz beam tracks changes in the beams vibrational frequency with time.  Fluid head can therefore be calculated from a measurement of the output frequency or period of the oscillator circuit.

Each transducer also contains a second quartz crystal sensor, which is used for precise temperature compensation of the pressure output.  The temperature sensing crystal consists of two torsionally oscillating tines connected to a mounting pad through a mechanical isolation system (Figure 4). 

Current PS-2 models have only standard RS-232 interface with new commands to support new features.  The transmitter electronics measures the periods of the two transducer signals and calculates fully temperature-compensated pressure or depth output.  The interface board has a microprocessor –controlled counter and an RS-232 communication port.  The microprocessor-operating program is stored in permanent memory (EPROM).  User settable parameters are stored in EEPROM.  The user interacts with the transmitter via the two-way RS-232 port.  Please see the programming and operation manual for a detailed list of commands available.

Users can select outputs in eight standard sets of engineering units or in user definable units.

Integration time is user settable over the range from 0.003 seconds to 47 seconds.  A longer averaging time could be used to smooth over surface waves. 

The transmitter supports much higher resolution than the standard USGS increment of 0.01 foot.  Users can select additional digits of precision as needed. 

The transmitter can be used with a standard RS-232 port.  

An optional six-digit LCD digital display is available. 

The transmitters can be used with data loggers.

Special commands are provided which make recalibration a simple field procedure.   

Figure 6:  SDI-12 WATER LEVEL SYSTEM

The Bubbler system generates static gas pressure that is approximately the same as the water pressure at the orifice of the gas line. 

The digital quartz pressure transmitters are sensitive enough to view bubble formation and release.  Figure 7 shows high-resolution measurements taken with a transmitter on a gas bubbler system.  Formation of individual bubbles is readily seen.  The pressure rises slightly during bubble formation and then drops abruptly as the bubble breaks free from the orifice.  The total effect is approximately 0.009 feet of water.  The most accurate measurement is obtained be averaging the pressure over a number of bubble cycles. 

The standard bubble rate in the USGS furnished bubbler system is 60 bubbles per minute in a sight glass.  This low rate conserves gas supplies and keeps friction in the polyethylene tubing to less than an equivalent 0.01 ft. of water in feed lines as long as 250 feet. 

Figure 8 shows the resolution that can be obtained with a 10 second averaging time.  In this mode, the stability approaches 0.0001 foot or one thousandth of an inch.  Every few  minutes, the water level was increased by 0.0005 foot by pouring a small amount of water into the water tank.

Extensive first-article testing for the U.S. Geological Survey demonstrated that the transmitters measured water levels from 0 to 50 ft. with an accuracy of better than 0.01 ft. under extreme environmental conditions.  Testing simulated the worst-case environment that might be encountered in a remote location. 

Temperatures from –20 to 55 deg C

Humidity from 5% to condensing

Diurnal temperature variations

Storage temperatures or –40 and 60 deg C

Instantaneous temperature shock from +50 to –8 degrees C

Overpressure to 60 psig

Low external pressure of 572 mbar

Electromagnetic radiation between 500 kHz and 1 GHz

Vibration and shipping/handling conditions

1. Craig, J.D., Techniques of Water-Resources investigations of the United States Geological Survey, Book 8, Chapter A2, Installation and Service Manual for U.S.  Geological Survey Manometers  (1983).  Distribution Branch, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304. 

2. Busse, D.W.,  “Quartz Transducers for Precision Under Pressure”,  Mechanical Engineering,  Vol. 109, No. 5, May 1987.

3. Busse, D.W. and Wearn, R.B.,  “Intelligent Digital Pressure Transmitters for Aerospace Applications”,  Measurements and Control, February,  1978.

4. Wearn, R.B., and J.M. Paros, “Measurements of Dead Weight Tester Performance Using High Resolution Quartz Crystal Pressure Transducer,” presented at Instrument Society of America, Aerospace Industries and Test Measurements Divisions, 34^th International Instrumentation Symposium, Albuquerque, New Mexico, May 2-5, 1988.

5. Specification for Non-submersible Hydrostatic Pressure Sensors, HIF-S-02, April, 1989 Department of the Interior, U.S. Geological Survey, Water Resources Division, Hydrologic Instrumentation Facility, Stennis Space Center, Mississippi