Soil Science Society of America Journal 67:737-739 (2003)
© 2003 Soil Science Society of America
DIVISION S-1—NOTES
Tensiometer modification for diminishing errors due to the fluctuating inner water column
Martin Thalheimer*
Research Centre for Agriculture and Forestry, Laimburg 39040 Auer/Ora (Bz), Italy
* Corresponding author (martin.thalheimer{at}provinz.bz.it)
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ABSTRACT
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The fluctuation of the height of the water column inside tensiometers constitutes a source of systematic error in the measurement of soil water potential. The influence of the fluctuating water column can be strongly reduced by measuring pressure in a confined air space close to the tensiometer tip. A simple technique of inserting a lower air space connected to the pressure sensor is presented. Successful laboratory tests were performed on a tensiometer of 1-m length with an applied pressure range from 0 to -80 kPa.
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INTRODUCTION
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TENSIOMETERS ARE relatively simple devices for monitoring the water status of soils. They are often preferred to other types of soil moisture sensors because of their low cost, simplicity of use, high accuracy of measurement, and the possibility of electronic data acquisition through differential pressure transducers (Marthaler et al., 1983; Nyhan and Drennon, 1990), rendering them suitable also for automated irrigation control.
Tensiometer readings may, however, be affected by various factors, most importantly by temperature changes (Butters and Cardon, 1998; Warrick et al., 1998). Significant errors may arise also from the changing hydraulic head of the water column inside the tensiometer tube during wetting and redrying cycles. The water column influences the head space air pressure at a rate of 
0.1 kPa cm-1. With increasing tensiometer length, this leads to considerable deviation of the measurements from the values of actual soil water pressure. In the case of manual data acquisition, a simple formula allows for easy adjustment of the readings, provided the height of the water column can be recorded. Such corrections cannot be performed when water columns descend below ground level and in the case of automated data acquisition systems, where the height of the water column is not accounted for.
Various attempts have been made to minimize this inconvenience, such as placing the differential pressure transducer inside the tensiometer tube close to the ceramic cup (Hubbell and Sisson, 1998) or estimating the air volume inside a tensiometer on the basis of the ideal gas law (Villa Nova et al., 1989). Introducing a lower air space near the ceramic cup and connecting it to the pressure-measuring device constitutes an effective method of minimizing the influence of fluctuating water columns. A two-cell tensiometer based on this principle was proposed by Faybishenko (2000). This device was conceived for measurements at depths also >5 m and involves a relatively complex construction design, which renders its extensive use for water potential measurements in surface soils unlikely.
The objective of this paper is to outline a simple and effective technique designed to improve measurement precision of tensiometers of limited length, such as those most commonly used in agriculture or for environmental studies in upper soil layers. It consists in shifting the measurement location lower into the tensiometer tube so that the effect of the hanging water column is automatically compensated for.
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Tensiometer Design
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The proposed tensiometer modification consists essentially of an inserted air-filled capillary tube extending from the vacuum sensor at the top of the tensiometer down to the level of the ceramic cup (Fig. 1)
. A bulb with a small hole at the bottom is attached to the lower end of the capillary tube. This bulb serves as a chamber into which the air contained in the capillary tube expands on decompression inside the tensiometer and which prevents water from entering the capillary tube when decompression is reversed.
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Fig. 1. Modified tensiometer design and parameters. zT is the height of the water column in the tensiometer; zB is the water level in the air-expansion bulb; he is the external pressure.
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With this design, the pressure developing at the level of the transducer is given by the sum of the external pressure (he) acting on the ceramic cup and the hydraulic head of the water column in the tensiometer (zT), which is counteracted at the same time by the hydrostatic pressure of the water column above the water level in the bulb (zT – zB). The pressure at the transducer is therefore given by
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which simplifies to
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This formula reveals that at zero flux conditions the pressure at the transducer is determined only by the external pressure (he) acting on the porous cup and by the water level inside the air-expansion bulb (zB). Therefore, the effect of fluctuating water levels inside the tensiometer is restricted to the minor changes of zB.
The size of the air expansion bulb must be sufficient to accommodate the whole volume to which the air in capillary tube and pressure sensor may expand within the presumed operating range. If the inner volume of the capillary tube is known, the necessary volume of the bulb can easily be derived from the ideal gas law. Assuming an operating range of the tensiometer of 0 to 80 kPa external pressure, the volume of the expansion bulb needs to equal at least five times the inner volume of the capillary tube. A certain increase of the calculated volume provides a safety margin for the influence of temperature variations and for the transient positive pressure, which builds up at the level of the bulb when the tensiometer is opened for refilling.
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Materials and Methods
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Tensiometers were self-assembled from commercially available components. Ceramic cups (Weninger Kunststoff-Keramikwerk, Telfs, Austria) were fitted to transparent 20-mm-o.d., 18-mm-i.d. polyvinyl chloride pipes with hot-melt adhesive. The upper end of the tensiometers was closed with a perforated silicone stopper into which an electronic differential pressure transducer was inserted. The sensors used were Motorola MPX5100DP piezoresistive differential pressure transducers (Nippon Motorola Ltd., Tokyo, Japan) with integrated temperature compensation and signal amplification circuits, providing a linear voltage output for a pressure differential range from 0 to 100 kPa. The pressure transducers were connected to a programmable microcontroller (C-control, Conrad Electronic, Hirschau, Germany) provided with an 8-bit A/D converter and 8 KB memory, which served both for data logging and for the automation of the vacuum device. The achieved measurement resolution was of 0.44 kPa, corresponding to an equivalent water column of 4.5 cm, which was sufficient for assessing the efficacy of the proposed technique.
The capillary tube had an inner diameter of 1 mm. The expansion bulb was made of a piece of 14-mm-o.d., 13-mm-i.d. plastic pipe, with the opening at the lower end reduced to an orifice of 1 mm. The reported data refer to a tensiometer of 1-m length with an air expansion bulb of 8.5 cm3 volume.
The device for tensiometer testing (Fig. 2)
consisted of a vacuum tank connected to a water-jet suction pump and to an Erlenmeyer filter flask. An electric valve actuated by the microcontroller via a relay switch controlled the suction pump. The tensiometer was inserted in a perforated rubber stopper and plugged onto the filter flask as described by Puckett and Dane (1981). The water level in the filter flask partially covered the tensiometer's ceramic cup. The tensiometer's pressure transducer as well as a second transducer fitted to the vacuum tank was connected to the A/D inputs of the microcontroller. The microcontroller was programmed to check and maintain the vacuum inside the tank by operating the electric valve according to a scheduled decrease of the applied pressure (up to a final level of -80 kPa) and to record the readings of both pressure transducers at a time interval of 4 min. This configuration allowed a direct comparison between the applied pressure outside the ceramic cup and tensiometer readings. Before the measurement cycle, the water in the tensiometer was degassed by a preliminary vacuum application.
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Results and Discussion
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The results of a measurement cycle with a progressive reduction of the external pressure and a final return to atmospheric pressure are shown in Fig. 3
. The graph shows that readings of the modified tensiometer match very closely with the applied external pressure across the whole measurement range once equilibrium is reached, putting into evidence that the error due to the fluctuating hanging water column has largely been eliminated. The observed shift of the height of the tensiometers water column during this cycle was of 42 cm. With conventional tensiometers such variations can, if not taken into account, lead to misinterpretation of irrigation needs of water-sensitive crops or of soil water fluxes in environmental studies.
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Fig. 3. Comparison of laboratory readings from the modified tensiometer and external pressure during a cycle of progressive pressure reduction and final return to atmospheric pressure.
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At the beginning of the measurement cycle, readings differed from the external pressure by 0.87 kPa, in accordance to the initial height of zB. At an external pressure of -80 kPa, the difference was reduced to 0.44 kPa as a result of the concomitant lowering of zB. The effect of the variation of zB within the range of 0 to -80 kPa of applied external pressure on tensiometer readings amounted therefore to 0.44 kPa or 4.5 cm water column. Such an effect of the variations of zB on tensiometer readings can be regarded as negligible for most purposes of agricultural water management. However, as outlined by Faybishenko (2000), the water level fluctuations in the lower air pocket can be minimized by adopting the lowest possible ratio between height and base of the lower air pocket. The practical limit to this postulate is in this case given by the inner diameter of the tensiometer tube, which should be somewhat larger then the outer diameter of the air expansion bulb to guarantee free water movement without hindrance from air plugs developing between the tubes. At the same time, the narrowest possible capillary tube extending from the air-expansion bulb to the pressure-recording device will minimize the total bulb volume required.
A good estimation of zB could nevertheless be achieved on the basis of the ideal gas law, since zB does not consistently shift across time, in contrast to the water level in the tensiometer tube. In fact, any buildup of air inside the expansion bulb will periodically be released into the tensiometer tube under conditions of reducing pressure. Also, temperature variations are generally much more restricted below ground level. Alternatively, a preliminary tensiometer calibration could provide an empirical equation for the adjustment of the readings for variations of zB.
By inserting a capillary tube and air expansion bulb, a small volume of air is introduced into the tensiometer. It should be noted that this amounts only to the inner volume of the capillary tube plus the mentioned safety margin. The air expansion bulb itself is almost entirely water-filled when no pressure is applied. In this present case, the air volume in the capillary tube was <1 cm3. The adverse effect of this modification on tensiometer time response is therefore rather limited. Ideally, a tensiometer tube should be completely water-filled. With progressive cycles of drying and rewetting, however, a head air space will inevitably build up also with this modified design, through the release of air dissolved in soil water filtering into the tensiometer. Increasing volumes of air in the tensiometer lead to delayed response times (Tokunaga and Salve, 1994). Within certain limits, this may not necessarily reduce the tensiometer's effectiveness, as the degradation in response occurs notably in the dry range of soil moisture where water movement is relatively slow.
Various types of commercially available tensiometers can be modified with the proposed technique. The simplicity of the design allows also the easy removal of parts for inspection or servicing. Furthermore, the modification can be fitted not only to electronic pressure transducers but also to Bourdon type pressure gauges. It is also conceivable to apply the proposed technique to tensiometers for portable, syringe-type pressure transducers (Marthaler et al., 1983); for example, by sealing the upper end of the capillary tube extending from the silicone stopper with a minute septum, through which the needle of the handheld meter would be inserted. The fitting of a capillary tube with an air expansion bulb to tensiometers constitutes therefore a widely applicable, inexpensive, and effective method of improving the accuracy of water potential measurements in upper soil layers. This system is less suited for exceedingly long tensiometers such as those used for vadose zone monitoring, since the augmenting total air volume inside the capillary tube would determine increasingly delayed response times and because the residual error due to fluctuations of zB increases proportionally with tensiometer length, thus diminishing the main benefit of the proposed system.
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ACKNOWLEDGMENTS
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The implementation of the vacuum device by Mr. Paul Cazzanelli is gratefully acknowledged.
Received for publication May 21, 2002.
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REFERENCES
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- Butters, G.L., and G.E. Cardon. 1998. Temperature effects on air-pocket tensiometers. Soil Sci. 163:677–685.
- Faybishenko, B. 2000. Tensiometer for shallow and deep measurements of water pressure in vadose zone and groundwater. Soil Sci. 165:473–482.
- Hubbell, J.M., and J.B. Sisson. 1998. Advanced tensiometer for shallow or deep soil water potential measurements. Soil Sci. 163:271–277.
- Marthaler, H.P., W. Vogelsanger, F. Richard, and P.J. Wierenga. 1983. A pressure transducer for field tensiometers. Soil Sci. Soc. Am. J. 47:624–627.[Abstract/Free Full Text]
- Nyhan, J.W., and B.J. Drennon. 1990. Tensiometer data acquisition system for hydrologic studies requiring high temporal resolution. Soil Sci. Soc. Am. J. 54:293–296.[Abstract/Free Full Text]
- Puckett, W.E., and J.H. Dane. 1981. Testing tensiometers by a vacuum method. Soil Sci. 132:444–445.
- Tokunaga, T., and R. Salve. 1994. Gauge sensitivity optimization in air pocket tensiometry: Implications for deep vadose zone monitoring. Soil Sci. 158:389–397.
- Villa Nova, N.A., K. Reichardt, P.L. Libardi, and S.O. Moraes. 1989. Direct reading "air-pocket" tensiometer. Soil Technol. 2:403–407.
- Warrick, A.W., P.J. Wierenga, M.H. Young, and S.A. Musil. 1998. Diurnal fluctuations of tensiometric readings due to surface temperature changes. Water Resour. Res. 34:2863–2869.
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J. M. Hubbell and J. B. Sisson
Comments on "Tensiometer modification for diminishing errors due to the fluctuating inner water column"
Soil Sci. Soc. Am. J.,
March 1, 2004;
68(2):
709 - 710.
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ABSTRACT
INTRODUCTION
