Scanning recording system for multiple capacitive water-depth transducers

Journal o f Hydrology, 79 (1985) 311--318

311

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

[1] SCANNING RECORDING SYSTEM FOR MULTIPLECAPACITIVE WATER-DEPTH TRANSDUCERS

H.R. HOLBO 1 , E.L. MILLER 2 and R.C. SIDLE 3 1 Comstock Environmental Sciences, 4785 Southwest Riverside Drive, Albany, OR 97321 U.S.A.) InnovaTech, 2912 Northwest Arthur Avenue, Corvallis, OR 9 7330 (U.S.A.) 3 USDA Forest Service, Forestry Sciences Laboratory, P.O. Box 909, Juneau, A K (U.S.A.)

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(Received January 3, 1983; revised and accepted February 21, 1985)

ABSTRACT Holbo, H.R., Miller, E.L. and Sidle, R.C., 1985. Scanning recording system for multiple capacitive water-depth transducers. J. Hydrol., 79: 311--318. An electronic water-depth measuring and recording system is described, which scans transducers hourly with exceptional depth and time accuracy. Transducer design is simple and rugged. Cable lengths between transducers and recorder are not critical. The system, which has performed reliably for several storm seasons in coastal Alaska, requires very little power and operates from a battery supply for extended periods. This design has proven cost~effective for continuous monitoring of well water levels and could easily be adapted for measuring rainfall and streamflow.

INTRODUCTION W a t e r d e p t h is universally m e a s u r e d in w a t e r s h e d h y d r o l o g y t o define s t r e a m f l o w , rainfall, a n d g r o u n d w a t e r regimes. H o w e v e r , s y s t e m s designed to m o n i t o r w a t e r level in o n e o f t h e s e s i t u a t i o n s are o f t e n u n s u i t e d t o t h e others. C o n s e q u e n t l y , a diverse a r r a y o f i n s t r u m e n t s m a y h a v e t o b e e m p l o y e d , a n d t h e r e s u l t a n t d a t a m a y be d i f f i c u l t to c o m b i n e i n t o a d a t a base f r o m w h i c h t h e h y d r o l o g i c a l b e h a v i o r o f a w a t e r s h e d c a n be described. O f t e n , e x p e n s e limits t h e n u m b e r o f s a m p l e p o i n t s w h i c h c a n be m o n i t o r e d a n d t h e f r e q u e n c y o f o b s e r v a t i o n , resulting in i n a d e q u a t e e x a m i n a t i o n of some parts of a watershed. The more remote a watershed, the more likely t h a t d a t a are limited. F o c u s i n g o n smaller p o r t i o n s o f a w a t e r s h e d , s u c h as areas t h a t are n e w l y r o a d e d , p r o n e t o slope failure, or geologically diverse, also is b e c o m i n g i m p o r t a n t , b u t c o s t m a y restrict s a m p l i n g t h o s e areas a t t h e d e n s i t y n e c e s s a r y to c h a r a c t e r i z e w a t e r s h e d r e s p o n s e t o land-use practices. M o r e o v e r , as t h e s t u d y area b e c o m e s smaller, f o r m e r l y suitable i n s t r u m e n t s m a y n o t be useful b e c a u s e t h e y were designed to m o n i t o r large-scale processes. T h e c o m b i n e d e f f e c t o f t h e s e limiting f a c t o r s c a n

312 severely compromise the data base upon which important land-management decisions m a y be made. Clearly, there is a need for low-cost instruments that can effectively acquire this data. Instruments must be able to acquire more data reliably, rapidly, and accurately, be capable of deployment in remote areas, and operate unattended for prolonged periods of time. The instrument system described in this paper is the p r o d u c t of our efforts to alleviate the problems just noted. Our particular objective was to develop an automated recording system which could monitor the water level in several groundwater piezometers in small, rapidly responding catchments but which could easily be adapted to monitor streamflow, rainfall, or combinations o f both.

TRANSDUCER SELECTION AND DESIGN Many different techniques can be used to determine water level, ranging from manual devices like dipsticks or floats to sophisticated electronic devices employing pressure, thermal, conductive, or capacitive properties of the liquid. Some use acoustical or optical reflectors to remotely sense liquid level. However, most are too complicated or expensive for d e p l o y m e n t in field locations, where AC line power is lacking and the system may be vandalized. An earlier a t t e m p t to address water-level measurement needs in watershed hydrology (Holbo et al., 1975) was only partly successful because of the electrical and structural complexity of the transducer. This design determined water level through the electrical conductivity of water when in contact with numerous (32) vertically spaced electrodes. Although the system was accurate and fast and met the requirements for reliable, unattended operation, a simpler, lower-cost system was still desired. Capacitive liquid-level transducers have been in use for many years. Revesq (1958) describes their theoretical basis of operation and how they could be applied to industrial level measurement problems. Even as early as 1951, Dash and Boorse used the basic principles to perform measurements in cryogenic research. But the use of capacitive transducers in watershed hydrology has not become widespread. The capacitive transducer can be designed fairly simply for hydrological applications, adapted to different depth ranges, and made structurally rugged. A novel system described by Hinson (1971) allows water levels to be measured w i t h o u t the disturbing, limiting effects of lead-wire capacitance. Thus, cables leading to the measurement location can be any convenient wire gauge and length; ordinary 22-gauge, 3-conductor, polyvinyl chloride (PVC) plastic-covered cable would suit most applications. Hinson made cable length noncritical b y using a carefully balanced AC excitation signal while measuring only the DC c o m p o n e n t of current flow through the transducers (see Fig. 1). This DC current is obtained by rectifying the signal at the

313 TRANSFORMER

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transducer. Only the effect of water level on transducer capacitance influences this DC current. Stray capacitance resulting from the cable produces AC current and does n o t affect the rectified (DC) measurement. The transducers used in the present system are an adaptation of Hinson's (1971) design. They use insulated electrodes m o u n t e d within a section of PVC pipe on solvent-welded PVC supports. The assembly is sturdy; the insulation protects the electrodes from corrosion; the PVC is, of course, ideally suited to continual immersion in water. The pipe is available in a variety of diameters and can be cut to any length. When a capacitive transducer with an insulated electrode is placed in a conductive liquid, the liquid functions essentially as a capacitor plate whose area varies according to liquid level. The electrode's insulation is the dielectric of the capacitor, and the electrode itself is a constant-area second plate of the capacitor. Thus, the bulk capacitance of the assembly is proportional to liquid level. If the liquid is not a good conductor, a resistive c o m p o n e n t is added in series with the variable plate formed by the liquid. (Natural waters vary widely in conductivity.) Because of the way resistive and capacitive (reactive) elements influence the operating impedance of a circuit, the relative importance of these two elements will vary as the square of their ratio. If the resistance of the liquid is less than one-tenth the capacitive reactance of the transducer, then liquid conductivity will influence signal o u t p u t less than 1%. We found this to be the case for our transducer design when water conductivity was greater than 30 pmho cm, but o u t p u t was reduced when conductivity was 1 0 p m h o cm. However, because capacitive reactance is frequency-dependent, a change in probe excitation frequency can be used to achieve o p t i m u m operation. In addition, because water has a high dielectric constant, allowance must be made for the bulk capacitance of the transducer when operated in low-conductivity liquids. These design factors are discussed in more detail by Revesq (1958).

314 MULTITRANSDUCER SYSTEM

Figure 2 is a schematic diagram of our multitransducer recording system. We began its design by considering how best to capitalize on the transducer's o u t p u t signal and on the types of recorders suitable for battery operation. Analog recorders were preferred in this case because their records can be examined in the field. The clear choice for the recorder was the Rustrak* (Manchester, N.H.). This type of recorder requires a signal current to drive its pen, rather than a voltage, ideally suiting it to directly accept the current o u t p u t from the probe w i t h o u t any additional circuitry and its attendant problems. Rustrak recorders produce a strip-chart record using an inkless, impact mechanism and are known for their dependability in the field. To log data from multiple transducers required either using one recorder per transducer or scanning several transducers into a single recorder and using a slow scanning rate. To keep costs down, we elected the latter. Monitoring each transducer continuously for an interval of each scan cycle would produce a succession of traces on the chart, each representing the water level at a different transducer. We recognize that data generated on such charts are harder to read and that some information m a y be lost if water levels at individual transducers change rapidly during a scan.

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315 As part of the circuit design process, we also evaluated how long the scan interval should be with respect to the number of transducers and how transducers should be scanned. We opted for electromechanical rather than semiconductor switching because the former is more rugged and forgiving of abuse and environmental electrical hazards. Because two leads from each probe would need to be switched, a two-pole switch would be required; a small stepping switch relay (Schrack Co., New York, N.Y.), available for 12-V DC drive and able to switch 12 transducers, was chosen. Monitoring each transducer once an hour and running the Rustrak chart at 25.4 mm h-1 would produce 2.1 mm traces, giving a chart life of about 1 m o n t h . Experience showed this trace length to be readable and this scanning rate to be satisfactory for piezometers. Transducer excitation and pulses to drive the electromechanical switch for scanning at 5-min intervals are both derived from a low-frequency (16.384 KHz) crystal oscillator (system clock). A divider chain, implemented with low-power integrated circuits, produces switching pulses every 5 min. The required sinusoidal excitation waveform for the transducer is synthesized from the square-wave pulses by filtering an eight-step, weighted sinewave approximation. This signal is applied to the primary winding of a transformer with two identical secondary windings. The two secondaries are then connected to the scanner switch, to the galvanometer in the Rustrak, and to the cables going to the transducers. The transducer excitation we used was chosen to give an o u t p u t of 50 pA with an excitation of about 10 V peak-topeak at 2048 Hz and 100 cm water depth. The amplitude of the excitation signal may be adjusted to facilitate calibration or changes to other depth ranges. As an added feature, a touch-grid switch circuit enables the operator to advance the scanner switch to any desired position at a 1 Hz step rate. The relay can be set to any transducer, synchronizing transducer scanning with local civil time. All circuit components were assembled on circuit boards and m o u n t e d within the case, on the rear panel, of the Rustrak recorder. Average current drain is 15 mA, or 11 A h per month. The system has operated reliably in the field for several m o n t h s when powered by an ordinary lead-acid automobile battery. Because the time accuracy of the system clock is much better than that of the chart drive motor, we relied on it rather than on the preprinted chart markings to determine the monitoring times. This is done by designating one transducer as a reference. It is sealed at a known, fixed water level to serve as a marker for each scan interval on the chart and also to provide a measure of system performance over time, temperature, and battery condition. Even as the remaining transducers vary in water depth, the standard one provides a basis for identifying each scanned record sequence. It is, of course, necessary to write time, date, and recorder location at the beginning and ending of each chart. Clock accuracy can be verified by noting the difference in beginning and ending times and by counting the number

316

of scans. Time errors will be negligible because of the crystal-controlled oscillator. When temperature or other external factors cause apparent changes in the standard well level, depth measurements in the other wells can be adjusted, enhancing measurement accuracy. Changes may be due to the temperature coefficient of the transducer or of its PVC housing. Whether a correction is required depends upon the sensitivity of subsequent analyses to absolute water depth.

SYSTEM PERFORMANCE

We calibrated the 12 transducers (with cables 5--30 m long) to assess the linearity of their individual responses to water depth and their uniformity from one transducer to another. Linearity errors, indicated by best-fit linear regressions for individual transducers, were less than 1 cm of depth. Even with a single regression line fit to data from all transducers, errors were, with one exception, less than 1.5cm; and that exception was still within our design goal for overall system error of 3 cm. An example of data obtained by our system is shown in Fig. 3. In this example, the full-scale depth range is 100 cm, or 2 cm per division of the recorder chart (Rustrak Style A). The initial step in processing this data is to transfer chart measurements to tables or a computer. It is usually convenient to plot the depth measurements as continuous functions of time because examining the traces directly on the recorder chart is difficult. Inspecting the features in such plots may influence the type of data analysis. Should water-depth changes occur rapidly, it is important to remember that scanning is sequential. The 5-min time shift may bear critically on comparisons of rates of depth change with some other variable, such as precipitation. As described here, the multitransducer system has performed reliably in watershed studies during the fall rains in southeast Alaska (Sidle, 1985),

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protected only from direct rainfall and dampness. Effects of ambient temperatures, which varied from -- 13 ° to 29°C, were accounted for by reference to a standard transducer. Cost of the system was about $200 per well, or less than one-tenth of the cost of implementing a similar monitoring program with conventional instruments.

S Y S T E M A P P L I C A T I O N : AN E X A M P L E

We illustrate the usefulness of our system by presenting the precipitation response of groundwater as gauged by five shallow piezometers within a small catchment in the Kennel Creek drainage on Chichagof Island, off the Alaska coast (Fig. 4). The piezometers were covered to prevent introducing rain directly into the wells. The multitransducer system made measurements over three days, beginning before a storm on August 16, 1980, and continuing until the storm passed. Precipitation rate was highest during two distinct periods, separated by about 15h. All piezometers, except no. 8 responded to the initial peak; response lag times varied from 2 to 4 h. Following a slight decline in water depth during the middle part of the storm, when the rainfall rate dropped, groundwater levels rose again, in response to the second storm peak. Wells no. 3 and 11, located along the central axis of the catchment, exhibited artesian conditions for short periods. Water-level decline following the storm varied from well to well, reflecting I~

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localized differences in hydraulic conductivity and subsurface drainage. Wells no. 5 and 12 show slower declines, perhaps revealing contributions from adjacent areas. Well no. 3 is most responsive, suggesting both high conductivity and rapid transport in the subsurface along the axis of the catchment. This example illustrates the kind of temporal and spatial detail resolvable with our multitransducer system. Specific analyses and their interpretation would, of course, depend on the objectives of the user. For example, because our system can provide accurate and coincident time-series from many sampled locations, we foresee being able to infer hydraulic properties not estimable from conventional data bases.

SUMMARY

The system described here has proven reliable and cost-effective for continuous monitoring of water level in wells. With only minor changes, it could be adapted for measuring rainfall and streamflow. The simplicity of the recording technique and the unique advantages of the capacitive transducer design make the system highly applicable to remote field installations; it has performed reliably in the field during several storm seasons in coastal Alaska. The multitransducer design offers researchers and land managers the capability to acquire data from an array of wells at much lower cost than that of commercial water-level recorders with single transducers.

ACKNOWLEDGMENT

This work was sponsored by cooperative research agreement PNW 80-254 with the USDA Forest Service, Pacific Northwest Forest and Range Experiment Station, and by the Forest Engineering Department, Oregon State University. This is paper 1716, Forest Research Laboratory, Oregon State University, Corvallis 97331 (U.S.A.).

REFERENCES Dash, I.G. and Boorse, H.A., 1951. Transport rates of the Helium II film over various surfaces. Phys. Rev., 82: 851. Hinson, W.H., 1971. A simple method of obtaining analogue output from capacitance as a transducer. J. Phys. Eng., 4: 778--779. Holbo, H.R., Harr, R.D. and Hyde, J.D., 1975. A multiple-well, water-level measuring and recording system. J. Hydrol., 27: 199--206. Revesq, G., 1958. Process instrumentation for the measurement and control of level. IRE Trans. Ind. Electron., PGIE-7, pp. 11--16. Sidle, R.C., 1985. Shallow groundwater fluctuations in unstable hillslopes of coastal Alaska. z. Gletscherkd. Glazialgeot. 20(2 ), in press.