calibration and use of pressure transducers in soil hydrology

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IIYDROLOGICAL PROCESSES, VOL. 3, 43-49 (1989) CALIBRATION AND USE OF PRESSURE TRANSDUCERS IN SOIL HYDROLOGY J. F. DOWD School of Forest Resources, University of Georgia, Athens, GA 30602, U.S.A. AND A. G. WILLIAMS Department of Geographical Sciences, Plymouth Polytechnic, Plymouth, PL4 8AA, U. K. ABSTRACT Laboratory and field experiments demonstrated that solid state pressure transducers are accurate and reliable devices for frequent measurements of soil suction. However, each transducer had to be individually calibrated before use and a hanging column procedure designed for this purpose is described. Analysis showed that each transducer had a linear response and that environmental conditions such as temperature had minimal influence. Twenty four tensiometers with pressure transducers were intalled in a forest soil to test their operation and their output was monitored by a data logger. An example of soil suction results measured during four storms is given to demonstrate their stability and their rapid response. The transducers were found to perform accurately and were only affected by temperatures below 0°C. KEY WORDS Pressure transducer Soil suction Tensiometer INTRODUCTION Calculation of soil moisture flux requires frequent measurement of soil suction during a storm. This can be achieved using tensiometers monitored by individual transducers recorded on a data logger. In the past this technique has not been utilized to its full potential because the sensors were susceptible to environmental effects, particularly changes in temperature. The object of the paper is to describe the calibration of a pressure transducer system and to show how the transducers operated in the field in conjunction with tensiometers. An example of field results is provided. The scope of the study is limited to a discussion of the performance of the transducers and does not consider the adequacy of ceramic cup tensiometers, which is dealt with elsewhere (Cassell and Klute, 1986). Several researchers have recognized the merits of pressure transducers in soil hydrology, while others have used data loggers to good effect, but there are few examples where they have been used successfully together. Long (1982) described the use of transducers in the laboratory and claimed that they were more accurate than mercury manometers. Errors in reading the suction in tensiometers with pressure transducers are discussed by Trotter (1984) and unlike Long (1982) he concluded that transducers were not capable of matching the measurement accuracy of mercury manometers. A field system for measuring soil suction was reported by Long (1984) and Lowery et al. (1986) presented details of a field experiment which showed that the sensors performed well in the field. They also showed that they are thermally stable but do not indicate the size of their measurement errors. Van Grinsven et al. (1987) used tensiometers with pressure transducers, but the results were not data logged and measurements were made at weekly or bi-weekly intervals. Excellent data logging facilities are described by Essery et al. (1987) and Durham et al. (1986), but neither exploited the capabilities of pressure transducers. Essery monitored soil moisture with probes 08854087/89/01OO4347$05 .OO 0 1989 by John Wiley & Sons, Ltd. Received 26 January 1988 Revised 3 June 1988

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Page 1: Calibration and use of pressure transducers in soil hydrology

IIYDROLOGICAL PROCESSES, VOL. 3, 43-49 (1989)

CALIBRATION AND USE OF PRESSURE TRANSDUCERS IN SOIL HYDROLOGY

J. F. DOWD School of Forest Resources, University of Georgia, Athens, GA 30602, U.S.A.

AND A. G . WILLIAMS

Department of Geographical Sciences, Plymouth Polytechnic, Plymouth, PL4 8AA, U. K .

ABSTRACT

Laboratory and field experiments demonstrated that solid state pressure transducers are accurate and reliable devices for frequent measurements of soil suction. However, each transducer had to be individually calibrated before use and a hanging column procedure designed for this purpose is described. Analysis showed that each transducer had a linear response and that environmental conditions such as temperature had minimal influence. Twenty four tensiometers with pressure transducers were intalled in a forest soil to test their operation and their output was monitored by a data logger. An example of soil suction results measured during four storms is given to demonstrate their stability and their rapid response. The transducers were found to perform accurately and were only affected by temperatures below 0°C.

KEY WORDS Pressure transducer Soil suction Tensiometer

INTRODUCTION

Calculation of soil moisture flux requires frequent measurement of soil suction during a storm. This can be achieved using tensiometers monitored by individual transducers recorded on a data logger. In the past this technique has not been utilized to its full potential because the sensors were susceptible to environmental effects, particularly changes in temperature. The object of the paper is to describe the calibration of a pressure transducer system and to show how the transducers operated in the field in conjunction with tensiometers. An example of field results is provided. The scope of the study is limited to a discussion of the performance of the transducers and does not consider the adequacy of ceramic cup tensiometers, which is dealt with elsewhere (Cassell and Klute, 1986).

Several researchers have recognized the merits of pressure transducers in soil hydrology, while others have used data loggers to good effect, but there are few examples where they have been used successfully together. Long (1982) described the use of transducers in the laboratory and claimed that they were more accurate than mercury manometers. Errors in reading the suction in tensiometers with pressure transducers are discussed by Trotter (1984) and unlike Long (1982) he concluded that transducers were not capable of matching the measurement accuracy of mercury manometers. A field system for measuring soil suction was reported by Long (1984) and Lowery et al. (1986) presented details of a field experiment which showed that the sensors performed well in the field. They also showed that they are thermally stable but do not indicate the size of their measurement errors. Van Grinsven et al. (1987) used tensiometers with pressure transducers, but the results were not data logged and measurements were made at weekly or bi-weekly intervals.

Excellent data logging facilities are described by Essery et al. (1987) and Durham et al. (1986), but neither exploited the capabilities of pressure transducers. Essery monitored soil moisture with probes

08854087/89/01OO4347$05 .OO 0 1989 by John Wiley & Sons, Ltd.

Received 26 January 1988 Revised 3 June 1988

Page 2: Calibration and use of pressure transducers in soil hydrology

44 J. F. DOWD AND A. G. WILLIAMS

of a design similar to Bouyoucos blocks but they were not very accurate and the authors state that ‘attempts to produce absolute measurements are fraught with experimental problems’. A complex installation to monitor water table height using piezometers was described by Durham et uf. (1986) with a system of floats, pulleys and potentiometers to provide a data logged pulse-width modulated signal. Pressure transducers offer a straightforward yet accurate alternative to the potentiometer method.

A number of hydrological studies have monitored soil suction using a system of 24 tensiometers connected to a Scanivalve fluid switch and a single transducer (Anderson and Burt, 1978; Burt, 1978; Williams, 1978; Wheater et al., 1987). This scheme has provided useful information on hillslope flow processes. However, significant operational problems with the method include equilibration time and air entrapment. The Scanivalve switches the transducer to each tensiometer in turn causing pressure fluctuations in the system. The minimum time required for the system to stabilize with each tensiometer is about two minutes. The Scanivalve has 24 inputs and therefore each tensiometer can be scanned about once an hour. During rapidly changing soil moisture conditions, this may cause difficulties in interpretation. In addition, the Scanivalve requires positive head, limiting its location, and problems can arise due to air being trapped at the valve.

PRESSURE TRANSDUCERS

Our investigations have shown that pressure transducers provide accurate, reproducible results under varying environmental conditions. In addition, rapidly changing soil moisture conditions can be monitored in great detail using a data logger. Furthermore, transducers are commercially available and relatively inexpensive. A hillslope or plot can be intensively instrumented at modest cost using equipment that requires no special modifications.

1. Data can be logged electronically for efficient processing. 2. Recording frequency is limited only by equilibration of the ceramic cup, as the transducers respond in

less than one second. 3. They are extremely accurate. Soil water potential can be measured to an accuracy better than 5cm HzO, whereas conventional mercury manometers are accurate only to within 20 cm H20.

4. Environmental factors have limited effect. 5. Transducers measure gas or water pressure and operate until the ceramic cup dries. 6. Both positive and negative pressures can be measured.

Transducers can also be used to measure the saturated zone with a piezometer or mini-piezometer. Different pressure ranges are available to monitor either large fluctuations of a permanent water table or small fluctuations of a variable source area.

Advantages of using pressure transducers for monitoring soil water potential are:

OPERATION AND TYPES OF TRANSDUCERS

Solid state pressure transducers use a small diaphragm connected to piezoresistors. When pressure or suction is applied, the diaphragm flexes and changes the resistance, causing an output voltage proportional to pressure. Transducers are available with different full scale voltage output, temperature compensation, and linear response. Choice of transducer model will depend on its application. For example, to measure soil suction a range of one bar is required, the air entry value of the ceramic cup of a tensiometer.

Field experience has shown that Honeywell Micro Switch (use of trade names does not imply endorsement of the product by the authors) pressure sensors operate satisfactorily over a wide range of environmental conditions. Table I lists the transducer models used, their applications and costs. Similar transducers are produced by Sensym. In all cases, pressure was measured with respect to atmospheric pressure. All transducers used were temperature compensated and had a linear calibration curve.

Page 3: Calibration and use of pressure transducers in soil hydrology

PRESSURE TRANSDUCERS IN SOIL HYDROLOGY 45

Table I. Pressure transducer details ~ ~~

Application Instrument Pressure (cm H20) Voltage output Model cost ($1

Soil suction tensiometer -1000 to +loo0 WOmV 156PC15GWL 50

Water table elevation piezometer -20 to +120 0-5v 163PCOlD48 100

Water table elevation piezometer -13 to +13 0-5v 163PCOlD36 100

CALIBRATION

Before use in the field, each transducer was calibrated individually because of their slightly different response characteristics. Failure to use individual calibrations will result in large measurement errors. Transducers (type 156PC15GWL, Table I) for use in conjunction with tensiometers were calibrated using the apparatus shown in Figure 1.

A hanging column provided a known head for the transducer in the range of 0 to 5 m of water, with the millivolt response recorded by a data logger. The data logger used was a Campbell 21X, which records the differential voltage output from each transducer. It is capable of accurately measuring voltages between 0-50 mV, the range of response of the transducer. Some loggers may not be capable of measuring voltage in the appropriate range. In addition, the Campbell 21X is capable of transmitting a programmed 5V signal. This signal was used to trigger a relay that switched on a battery, exciting the transducers at 10 volts (Figure 1). Using the system presented in Figure 1, 10 transducers were calibrated at one time. The

Data Logging System Constant Head Appentus with Two Transducers illustrated (Tiand Tz)

I

I Data c.mzr' Logger P I I I

& I I-1 Campbell Yultipluur

J 7

> > 1 A J I AM32

I I I I I

1 Relay

I

I

I I

I I

Figure 1 . Laboratory set-up to calibrate transducers. Two of ten transducers shown

Page 4: Calibration and use of pressure transducers in soil hydrology

46 J . F. DOWD AND A. G. WILLIAMS

multiplexer facilitates the recording of the output from many transducers via individual leads, and allows them to be switched to one input channel to the data logger. In this study a Campbell AM32 multiplexer was used.

An example calibration curve of suction versus millivolt sensor response is presented in Figure 2. All transducers calibrated were linear with a correlation coefficient of 1.000. In order to test the accuracy of the calibration curve, a replicate calibration run was performed. Suction predicted by the calibration curve from the transducer output was plotted against the observed suction in the hanging column (Figure 3). For convenience the 1 : 1 curve is also plotted. Nineteen distinct suctions were measured at least 10 times in the validation illustrated to check for drift. All 211 values measured are plotted. For most of the transducers, predicted results deviated from observed by a maximum of three to five centimetres of water. These results are in good agreement with Trotter (1984) who determined the maximum offset repeatability to be 4-lcm of water. Experimental errors in this study were attributed to the practical difficulties in reading the head as well as the accuracy of the transducer response. They are within suction limits imposed by field equipment.

C 0 'S 200

100

. . . . . . . . . . . . . . . . . . . . . . -20 -16 -12 -8 - 4 0

Transducer Response (mv)

Figure 2. Transducer calibration curve

500 - n

E 2 400- C 0 * 300- 0 3 rn

a, n = 211

g 100- !?

.-

200- * u .-

a 0 I I I I I

0 100 200 300 400 500

500 - n

E 2 400- C 0 * 300- 0 3 rn

a, u

.-

200- * .- g 100- !? a

0 I I I I I

0 100 200 300 400 500

Observed Suction (cm)

Figure 3. Predicted versus observed suction

Page 5: Calibration and use of pressure transducers in soil hydrology

PRESSURE TRANSDUCERS IN SOIL HYDROLOGY 47

FIELD EXAMPLE

A field trial to test the tensiometer/transducer instrumentation was established at a site in the Piedmont of northeast Georgia. The instrumentation consisted of. 24 tensiometers/transducers and a data logging system similar to that shown in Figure 1. Tensiometers of the type normally used with mercury manometers (Soil Moisture Equipment Corporation Cup Tube Kit, model 2300) were installed in the conventional manner with diatomaceous earth to ensure good contact between the ceramic cup and soil. The transducers were fitted immediately above the tensiometers using a short length of glass tubing (about 5cm long). Problems which can arise due to differential heating in systems with long tubes are therefore minimized. Each transducer was fitted in a water tight container with desiccant to minimize problems with moisture in the plugs or transducers.

After initial installation, atmospheric pressure was measured to determine the zero offset for each transducer. Using this offset, the transducer calibration curves were adjusted so that they predicted approximately zero for atmospheric pressure. Variations of about 0.1 millivolts were typical , which represents about eight centimetres of suction. For operational measurements, tensiometers were monitored at 15 minute intervals.

A representative data set of soil suctions was examined to evaluate the performance of the transducers in the field. Changes in the soil suction for three depths at one site for the period 14 December to 4 January are plotted in Figure 4. The main characteristics of the transducers demonstrated by the graphs are:

1, Stability of response: The smoothness of the curves suggest that the results are very repeatable. This is particularly well illustrated by the stable but abrupt change when the 90cm tensiometer failed. No oscillations in transducer response were recorded.

2. Rapidity of response: The sudden drop in the deep tensiometer (90cm depth) when it ran dry on 17 December and its steep rise on servicing the next day is clearly visible in Figure 4. Similarly there is a sharp decline in suction from 700 to 50cm suction on 14 December.

3. Great precision: Four storms occurred during the period and their effect on the soil suction at 30 cm is clearly visible. Deeper in the profile there was only a gradual wetting up of the soil profile occurred as exemplified by the decline in suction at 60cm.

4. Minimal effect of temperature: There is a pattern of minor diurnal variations in suction at 30 cm depth due to temperature fluctuations. The problem is related to its effect on the tensiometer rather than the transducer because the tensiometers nearest the surface undergo the greatest fluctuations. Heating and cooling of the tensiometer causes the air inside the tensiometer to expand and contract, and these changes in pressure are monitored by the transducer. Evapotranspiration, which was considered to be the cause of such fluctuations by Lowery et al. (1986) may be of limited significance.

1 C A n

0 1 1 l i t ' , 1 0 ' I " I " I " I " I 26 29 1 4 14 17 20 23

December, 1987

Figure 4. Soil suction for adjacent tensiometers

Page 6: Calibration and use of pressure transducers in soil hydrology

48 J. F. DOWD AND A. G. WILLIAMS

200-

160- 0, I E 120- 0 v - Temperature 80- .- @ 0

40-

To 10 y

3 CI L a, Q

E o a , r t-

0:J -10 15 16 17 18 19 20 21 22 23

December, 1987

Figure 5. Reference suction and temperature

A reference pressure, consisting of a hanging column containing antifreeze, was used to test the accuracy, reliability, the repeatability of the transducer system under field conditions. In addition to providing a control on data quality, it provided information on the effectiveness of the transducer temperature compensation. Variations in the hanging column suction, as recorded by the transducer, were minimal, even though the air temperature dropped below 0°C several times (Figure 5). Fluctuations of about 5cm per day took place from 16-19 December when temperatures fell below WC, the stated lower limit for temperature compensation. When air temperatures were higher, the variation was reduced to about 1 cm. These deviations represent an upper bound on the environmental effects for the transducer because environmental factors also affect the hanging column. The efficiency of the internal temperature compensation for the transducer was also confirmed during the calibration exercise. Although the temperature varied typically between 10-15"C due to the length of time required, the suction could be predicted from millivolt output alone, and could not be improved with the addition of temperature as a variable.

CONCLUSIONS

Pressure transducers are able to measure suction to a high level of accuracy and are less susceptible to environmental influences than other methods. They are reliable; the transducer and data logger system described in this paper has successfully operated in the field for about one year. This approach has many other advantages such as being able to record data electronically for efficient management and processing. In addition, soil suction can be monitored at frequent time intervals to determine more readily the nature of water pathways through unsaturated soils. Such results will be of great benefit to the academic study of soil water fluxes as well as to applied studies of the movement of nutrients and pollutants.

ACKNOWLEDGEMENTS

This study was funded in part by the National Agricultural Impact Assessment Program, U.S.D.A. Forest Service, Region 8, Atlanta, Georgia. Thanks are due to Dr. P. Bush for his contribution. A. G. W. wishes to acknowledge financial support from British Council and Plymouth Polytechnic. We also wish to thank Joan Miller and Joe Garcia for assistance in the field.

Page 7: Calibration and use of pressure transducers in soil hydrology

PRESSURE TRANSDUCERS IN SOIL HYDROLOGY 49

REFERENCES

Anderson, M. G. and Burt, T. P. 1978. ‘Automatic monitoring of soil moisture conditions in a hillslope spur and hollow’, J.

Burt, T. P. 1978. ‘An automatic fluid-scanning switch tensiometer system’, British Geomorph. Res. Group Tech. Bull. N o . 21,

Cassell, D. K. and Klute, A. 1986. ‘Water potential: Tensiometry’, in Methods of Soil Analysis Parf I: Physical and Mineralogical

Durham, 1. H., O’Loughlin, E. M., and Moore, I. D. 1986. ‘Electronic acquisition of hydrologic data from intensively instrumented

Essery, C. I., Wilcock, D. N., and McClean, W. 1987. ‘A computer-based data logging system used to investigate the infiltration

Long, F. L. 1982. ‘A new solid-state device for reading tensiometers’, Soil Sci., 133, 131-132. Long, F. L. 1984. ‘A field system for automatically measuring soil water potential’, Soil Sci., 137, 227-230. Lowery, B. , Datiri, B. C., and Andraski, B. J. 1986. ‘An electrical readout system for tensiometers’, SoilSci. Am. J., 50,494-496. Trotter, C. M. 1984. ‘Errors in reading tensiometer vacua with pressure transducers’, Soil Sci., 138, 314-316. Van Grinsven, .I. J. M., van Breemen, N., and Mulder, J. 1987. ‘Impacts of acid deposition on woodland soils in the Netherlands: 1.

Calculation of hydrologic and chemical budgets’, Soil Sci. SOC. Am. J . , 51, 1629-1634. Wheater, H. S . , Langan, S. J., Miller, J. D., and Ferrier, R. C. 1987. ‘The determination of hydrological flow paths and associated

hydrochemistry in forested catchments in central Scotland’, Forest Hydrology and Watershed Management (Proc. of the Vancouver Symposium, August 1987). IAHS Publ. No. 167, 433-449.

Hydrol., 33, 27-36.

Geo-Abstracts Ltd., University of East Anglia, Norwich, Norfolk.

Methodr, Second Addition, Agronomy No. 9, Part 1 , Am. SOC. Agr., Soil Sci. SOC. Am., Madison, Wi, 563-596.

hillslopes’, Hydrol. Procs., 1, 79-97.

process under natural rainfall conditions’, Hydrol. Procs., 1, 283-292.

Williams, T. H. 1978. ‘An automatic scanning and recording tensiometer system’, J . Hydrol., 39, 175-183.