A recording field tensiometer with rapid response characteristics

Journal of Hydrology 5 (1967) 33-39; © North-Holland Publishing Co., Amsterdam Not to be reproduced by photo print or microfilm without written permission from the publisher

A RECORDING FIELD TENSIOMETER WITH

RAPID RESPONSE CHARACTERISTICS* K. K. WATSON** U.S. Water Conservation Laboratory, Phoenix, Arizona, U.S.A.

Abstract: The advantages of using a tensiometer-pressure transducer system for the field measurement of capillary pressure are discussed. The design of an accurate and rapidly responsive field unit using a commercial pressure transducer is then described and relevant drawings of the equipment presented. The performance of the equipment, in particular its rapid response, is illustrated by an experiment in which the capillary pressure changes in a sand profile are measured during intermittent surface inundations of short period. The field calibration of the unit using a compressed air source is also described.

1. Introduction

Recent studies (Watson 1), Klute and Peters 2)) have illustrated the decided advantages of using a tensiometer-pressure transducer system in the laboratory measurement of capillary pressure. Such a system permits the capillary pressure to be measured with rapid response and provides facility for recorder output. The rapid advance in recent years in transducer technology has resulted in the commercial availability of pressure transducers which are small in size, yet highly accurate and stable. This development affords the opportunity of designing, for field use, a system possessing equal sensitivity and response with that achieved in the laboratory. It is usual in the laboratory to use one transducer in conjunction with several tensiometers. The hydraulic leads from the tensiometer are connected to a selector valve which in turn is connected to the transducer. Except in those soil-water systems where the capillary pressures change magnitude extremely rapidly, the cyclic selection of the tensiometers provides sufficient data for the continuous record to be accurately interpolated. A similar arrangement, using either manual or automatic means of tensiometer selec-

*

Contribution from the Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture. ** During 1966, the author was on Sabbatical Leave from the School of Civil Engineering, The University of New South Wales, Kensington, N.S.W., Australia.

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K. K. WATSON

tion, is possible in field installations when the tensiometers are positioned relatively near the soil surface. However, for many field experiments, particularly where measurements of capillary pressure at depth are required, as in recharge basin research, a much more compact, simple and efficient system is possible by designing a self-contained tensiometer-transducer unit for placement at each measuring point. This is more expensive but not prohibitive when compared with the cost of instrumenting and maintaining many field experiments. In addition, the self-contained arrangement allows simple electronic means to be used for transducer selection and thus enables one recording unit to monitor very rapidly the pressures at several points. This type of approach is particularly useful for the automatic recording of capillary pressure at remote field stations. The advantages of a self-contained tensiometer-pressure transducer for field use can therefore be summarized as follows: (a) Rapid response of system due to small volume displacement of transducer. (b) Convenience of obtaining a chart record of the pressure changes with time. (c) Avoidance of diurnal temperature effects such as occur in tensiometermanometric systems with above-ground components. (d) Applicability of the method to the measurement of the capillary pressure at depth. (e) Relative ease with which the system can be instrumented for automatic control. (f) Elimination of the response problems which occur in soils of low conductivity when using a selector valve to switch from one tensiometer to another. Two descriptions of developments in this type of instrumentation should be noted. Bianchi 3) has discussed the characteristics and construction of a strain gauge tensiometer cell in which the capillary pressure is measured by the deflection of a stainless steel diaphragm to which strain gauges are bonded. Thiel et al. 4 ) have also described an instrument which employs a stainless steel sensing diaphragm and a linearly variable differential transformer to record the diaphragm deflection. Since the performance of the entire system depends on the accuracy and reliability of the pressure sensor, it is preferable, in the opinion of the present writer, to use commercially available pressure transducers for this purpose. Although more costly, these instruments offer the overriding advantages of small size, excellent stability, high linearity and hysteresis accuracy, wide range temperature compensation, and choice of pressure range. This paper

A RECORDING FIELD TENSIOMETER

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describes the design and performance of a recording field tensiometer assembly which incorporates one such commercial pressure transducer.

2. Design The instrument selected for this study on the basis of size and performance was the miniature bi-directional differential pressure transducer, type PM 131 'I'C, manufactured by Statham Instruments, Inc. * This instrument has a sensing diaphragm which is attached to a resistive unbonded strain gauge. The transduction unit is arranged in a conventional, balanced four-activeelement bridge circuit. The detailed specification of the instrument is included in the Appendix, where the hysteresis and linearity accuracy is listed as ± 0.25 ~~.~ full scale, which is a very satisfactory figure. The bi-directional differential feature was selected to permit measurement of positive hydraulic pressures if these occur and to allow all measurements to be made relative to atmospheric pressure. In tensiometry work it is usual to select a pressure transducer with a ± 15 psi range so that the entire tensiometer range can be monitored. However, for some experiments (e.g., pressure measurements in sand profiles during intermittent inundations) the ± 5 psi instrument may be preferable. A transducer of this range was used in obtaining the data presented in the following section on performance. The mounting of the transducer in a 7/8 in. dia cylindrical housing of stainless steel is shown in Fig. I. In effect, the arrangement converts the flush diaphragm transducer to a cavity-type instrument. The transducer is held firmly in position against the lock washer by screwing the lower unit, which houses the ceramic, against the shoulder of the transducer. The fibre washer, inset into the leading edge of the lower unit, isolates the pressuresensing volume from the remainder of the instrument. The ceramic is cemented in position with epoxy resin. Included in the design of the lower unit is a small screwed bleeder outlet which prevents excessive pressures being induced on the transducer diaphragm during assembly. Screwed stainless steel extension rods of various lengths were machined so that the tensiometer could be set at any depth up to 180 em. Additional lengths of tubing would allow deeper siting if required. The diameter of the tubing (7/8") is a reasonable compromise between undue bulk and sufficient flexural strength to permit deep installation. In addition, the tubing houses

*

Trade names and company names are included for the benefit of the reader and do not infer endorsement or preferential treatment of the product listed by the Ll.S, Department of Agriculture.

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K. K. WATSON

Air pressure connection 1/8" std. flare fitting

steel tubing 3/16" wall thickness

~GaSket

Electrical connection Reference Port

Statham Pressure Transducer Gasket

PM 131 TC

Fibre gasket Lock washer Ceramic

Sensing diaphragm

Fig. 1.

The tensiometer - pressure transducer assembly.

A RECORDING FIELD TENSIOMETER

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the electrical leads, allows atmospheric pressure to be maintained at the transducer reference port and facilitates retrieval of the instrument. Although the stainless steel tubing could be positioned in the profile so that it protruded through the surface, there are advantages in terminating it 15-20 em below the soil surface. With such an arrangement, temperature effects are minimized and leakage down the walls of the tubing is prevented. The termination of the tubing below the surface requires the use of the screwed fitting shown in Fig. 1. This provides suitable leakproof outlets for the electrical and air pressure connections. The access hole is backfilled around these connections. Nylon tubing ot 1/8 in. dia bore has been found satisfactory for the air pressure lead. The electronic measuring requirements are those normally associated with strain gauge instrumentation, namely a regulated DC power supply and an accurate means of measuring the voltage output. If line voltage is available at the field site, a zener diode power supply and a potentiometric recorder are satisfactory. Battery-operated units are also obtainable for these purposes. 3. Performance The hydraulic response of the tensiometer-transducer system depends on the transducer sensitivity S(cm 3mB - 1) and the tensiometer conductance K(cm 3min- 1mB- 1 ) . The ratio SIK is the time constant of the system which, for a transducer of ± 5 psi pressure range and a ceramic of 600 mB bubbling pressure and proportions shown in Fig. I, is approximately 0.2 sec. In effect, this gives an essentially instantaneous response to an imposed pressure change. In the laboratory a tensiometer-transducer system is calibrated by applying known hydraulic pressures to the positive port. However, in a field installation this is not possible, and the calibration must be made via the reference port. This is achieved by connecting the nylon air pressure lead to a portable and well-regulated compressed air supply to which is attached an accurate pressure gauge capable of reading to 0.2 mB. By increasing the air pressure in increments up to the limit of reading of the transducer, a full calibration relationship is obtainable. This procedure must be carried out when the capillary pressure at the tensiometer face is in equilibrium or is changing slowly. Upon the sudden removal of the air pressure, the resulting exponential response curve can be used to determine the time constant magnitude. Electronic drift in the equipment has been found to be negligible. This can be checked by the application of a specified air pressure at regular time intervals. To limit the accumulation of moisture in the stainless steel tubing, it is recommended that the air pressure lead outlet be set in a container in which is placed a drying agent.

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K. K. WATSON

Special care must be exercised in removing the equipment from the soil profile, particularly when it is sited at depth in a heavy soil. If the steel tubing is suddenly pulled out, the pressures developed at the ceramic face by adhesion forces may exceed the differential overload of the transducer. The effect can be minimized by applying a vacuum to the air pressure lead during removal or by using a jetting process to assist in loosening the tubing from the surrounding soil. Fig. 2 illustrates the behaviour of the unit in measuring the capillary pressure at a depth of 20 em in a coarse sand profile subject to periodic INUNDATION SCHEDULE

t

PZ72::J ON

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VZZZ/1 OFF

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OFF

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OFF

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OFF

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a. >-

~-IO

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0 0

10

15

20

25

30

35

40

45

TIME (min.)

Fig. 2.

Variation of capillary pressure with time during intermittent inundations.

inundations as indicated in the figure. The rapid response of the equipment during the wetting and draining cycles is immediately apparent. Pressure changes of 0.2 mB are detectable. With a time constant of 0.2 sec, errors due to slowness of response are negligible and the record as reproduced should represent the capillary pressure changes with time at the elevation in question. Research into the movement of water in unsaturated soil in field situations has reached the stage where future progress is largely dependent upon increased measuring accuracy and response. The equipment described above offers one method of achieving this with a minimum of development effort and fabrication complexity. Appendix Specification of Statham Differential Pressure Transducer, PM 131 TC Rated Excitation Nominal Output Nominal Bridge Resistance

5 volts DC or AC 4mV/V 350 ohms

A RECORDING FIELD TENSIOMETER

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0.75 % (normal) 0.25 % (special) - 65° to + 250°F 0.01 %1°F 0.01 %1 of Flush diaphragm, stainless steel Dry gases (non-corrosive) 1/4 oz. ± 2.5, ± 5, ± 10, ± 15, ± 25 200 % of range 5.9 x 10- 5 cc/mB

Hysteresis and Linearity (% FS) Compensated Temperature Range Thermal Shift: Sensitivity Zero Positive Port Reference Port Weight Differential Pressure Ranges (psi) Differential Overload Hydraulic Sensitivity

References 1) K. K. WATSON, Some operating characteristics of a rapid response tensiometer system.

Water Resources Res. 1(1965)577-586 2) A. KLUTE and D. B. PETERS, Hydraulic and pressure head measurements with strain gauge pressure transducers. Symposium on Water in the Unsaturated Zone, Wageningen ( 1966) 3) W. B. BIANCHI, Measuring soil moisture tension changes. Agric. Eng. 43(1962)398-399 4) T. J. THIEL, J. L. Fauss and A. P. LEECH, Electrical water pressure transducers fer field and laboratory use. Soil Sci. Soc. Am. Proc. 27(1963)601-602