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A dielectric tensiometer
Agricultural Water Management, 13 (1988) 411-415 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands Technical
411
Note
A Dielectric Tensiometer HILHORST~and J.J. DE JONG 2 1Technical and Physical Engineering Research Service for Agriculture, P.O. Box 356, 6700 AJ Wageningen (The Netherlands) 2Department of Physical Geography and Soil Science, University o/Amsterdam, Dapperstraat 115, 1093 BS Amsterdam (The Netherlands) M.A.
ABSTRACT Hilhorst, M.A. and De Jong, J.J., 1988. A dielectric tensiometer. Agric. Water Manage., 13: 411415. A new technique is pesented for measuring soil water tension, using the relation between the dielectric behaviour and the water tension of an absorbent placed in the soil. A practical realisation of the dielectric tensiometer is described, consisting of a porous metal cylinder as an outer conductor, containing an inner conductor, and filled with glass beads which serve as an absorbent. Together they form a coaxial condenser system. The electrical capacitance between the two electrodes is a measure for the soil water tension. The dielectric tensiometer is usable from zero to over 10 bar, without suffering from most of the common disadvantages of tensiometer pressure transducer systems. The dielectric tensiometer is ideally suited for use in conjunction with a capacitive soil water content meter for determining the water-retention characteristics of soil material. Patent is pending for the dielectric tensiometer.
INTRODUCTION It is desirable to be able to m e a s u r e soil w a t e r tension, because p l a n t res p o n s e to m o i s t u r e is closely r e l a t e d to it. I n t h e p l a n n i n g of irrigation projects it is f u r t h e r n e c e s s a r y to a s c e r t a i n t h e w a t e r c o n t e n t o f t h e soil. T h e relation b e t w e e n w a t e r t e n s i o n a n d w a t e r c o n t e n t is called t h e waterretention characteristic. T h e m o s t c o m m o n m e t h o d for m e a s u r i n g soil w a t e r t e n s i o n uses a tensiometer. A p o r o u s c e r a m i c cup, w h i c h allows w a t e r to pass b u t is i m p e r m e a b l e to air, is p l a c e d in t h e soil. T h i s cup is c o n n e c t e d to a p r e s s u r e t a n s d u c e r via a hydraulic s y s t e m filled with degassed water. W h e r e t h e r e is no t e m p e r a t u r e difference b e t w e e n t h e inside a n d t h e outside o f t h e t e n s i o m e t e r , a n d w h e r e soil c o m p o s i t i o n v o l u m e c h a n g e s are negligibly small, this h a s been, up to now, t h e m o s t a c c u r a t e a n d r e p r o d u c i b l e m e t h o d for m e a s u r i n g soil w a t e r tension. U n f o r t u n a t e l y this t e n s i o m e t e r still suffers f r o m such d i s a d v a n t a g e s as: - p r e p a r a t i o n , i n c l u d i n g filling t h e s y s t e m with degassed water, is necessary;
0378-3774/88/$03.50
© 1988 Elsevier Science Publishers B.V.
412
the range of measurement is restricted to about 800 mbar, because above this pressure the air entry value is exceeded (air will pass through the membrane ); air tends to infiltrate the hydraulic system, especially when soil water tension is high; - checking the hydraulic system is required on a regular basis; - the system may not be used in sub-zero temperatures because the water in the hydraulics will freeze; tensiometers are fragile instruments which need to be handled with care; most tensiometers exhibit hysteresis; and - if the ceramic cup is suddenly pulled out from the soil, the pressure transducer may be damaged by pressure shocks. The aim of this paper is to present a new type of tensiometer which eliminates most of the above-mentioned short-comings and, in addition, allows water retention curves of soils to be determined with one single read-out instrument: "the capacitive soil water content meter" (Hilhorst, 1984). Water has a relatively high dielectric constant ( ~ 80) in comparison with dry soils ( < 5), so the capacitance of a condensor, the electrodes, with soil as dielectricum, will be dominated by the water-volume fraction (Hasted, 1973; Grand et al., 1978). This is the case for frequencies higher than about 15 MHz. The capacitive soil water-content meter is based on this principle. Soil water content cannot be measured using a tensiometer system because of the difference between the wetting and drying curves of the water-retention characteristic.
-
-
-
-
PRINCIPLE OF OPERATION
The volume fraction of water in absorptive materials dominates their complex relative permittivity e -- c' - j e " (dielectric behaviour) so permittivity is a function of the water content. The water retention characteristic of absorptive materials depends strongly on the texture of the material. Combining both characteristics, permittivity and water tension as function of water content, gives permittivity as a function of water tension. The measurement of permittivity is possible by means of a condenser, with the subject material as a dielectric, connected to special electronic circuitry (Hilhorst, 1984) for sufficient separation of the real and imaginary parts of the complex permittivity. A micro-processor may then be programmed to translate the measured water content into terms of water tension.
413
CONSTRUCTION OF THE DIELECTRIC TENSIOMETER
The water tension versus permittivity for glass beads of known size distribution proves to be accurately reproducible. Figure 1 gives the permittivity versus water tension for glass beads of 60 pm with a standard deviation of about 10 ~m. Hysteresis and accuracy for the given curve are within + 1.5%. A further advantage of the properties of glass beads is their relatively high hydraulic conductivity compared to soil, so they rapidly follow changes in soil water content. The glass beads are (in effect) integrated as part of the soil system and form the dielectricum of a coaxial condenser system (Fig. 2). The outer conductor is a porous metal cylinder. Both ends are closed with rubber plugs and sealed
3o.
20
10
!
,
i
,
101
102
103
104
Soil water tension (mbar)
Fig. 1. Permittivity versus water tension for glass beads of 60 #m. inner conductor
_epoxy~'eSi n/ ~
~'~
glass beads
rubber p]ug
' ~
C
~'roul outer iorlductor
cable to electroniLs
Fig. 2. A practical rea|isation of the dielectric tensiometer.
414
with epoxy resin. The given shape has been chosen only for experimental reasons; the tensiometer can be almost any shape required. MEASURINGFREQUENCIESAND ELECTRONICS For measuring soil water content we are forced to use radio frequencies of more than 15 MHz. At lower frequencies problems may occur due to the Maxwell-Wagner effect, which leads to an apparent increase in the permittivity of soil in the low-frequency range, depending on the conductivity of the soil and its small colloidal particles. Extensive tests on the dielectric behaviour of glass beads in our laboratory at between 1 MHz and 1 GHz showed an almost flat response of e' with the water content at between 1 and 5 MHz, without showing the Maxwell-Wagner effect. The bound water fraction in the lower water content region (less than 10 m3/m 3) is comparatively high. Due to the low relaxation frequency for bound water in glass beads, e' will decrease at frequencies higher than about 5 MHz. This leads to a lower sensitivity for the high-tension region. The unknown ionic conductivity of the tensiometer requires an instrument containing special electronic circuitry which is insensitivie to ionic conductivity, as is the case for soil water content measurements; such an instrument has been developed at the Technical and Physical Engineering Research Service for Agriculture (TFDL), Wageningen, The Netherlands. The instrument is ideally suited for use in conjunction with the dielectric tensiometer, making it possible to measure the complete water-retention curve of soil in one operation. The measuring frequency for soil is 20 MHz. Electronics are in development for the 1-5 MHz region, in order to bring about a more competitive price for the dielectric tensiometer compared with hydraulic models. CONCLUSION A new method for measuring soil water tension has been developed: the dielectric tensiometer, which shows great promise for care-free continuous measurement of soil water tension in the field and in the laboratory. In many cases it will successfully replace the tensiometer pressure transducer system. The measuring range is from 0 to 10 bar and beyond. It is possible to measure the complete water-retention curve with a single readout instrument. With relatively cheap, low-frequency read-out electronics the dielectric tensiometer can easily compete in price with conventional tensiometers.
415 REFERENCES Grand, E.H., Shappard, R.J. and South, G.P., 1978. Dielectric Behaviour of Biological Molecules in Solution. Oxford University Press, Oxford. Hasted, J.B., 1973. Aqueous Dielectrics. Chapman and Hall, London. Hilhorst, M.A., 1984. A sensor for the determination of the complex permittivity of materials as a measure for the moisture content. In: P. Bergveld (Editor), Sensors and Actuators. Kluwer, Deventer, The Netherlands, pp. 79-84.
411
Note
A Dielectric Tensiometer HILHORST~and J.J. DE JONG 2 1Technical and Physical Engineering Research Service for Agriculture, P.O. Box 356, 6700 AJ Wageningen (The Netherlands) 2Department of Physical Geography and Soil Science, University o/Amsterdam, Dapperstraat 115, 1093 BS Amsterdam (The Netherlands) M.A.
ABSTRACT Hilhorst, M.A. and De Jong, J.J., 1988. A dielectric tensiometer. Agric. Water Manage., 13: 411415. A new technique is pesented for measuring soil water tension, using the relation between the dielectric behaviour and the water tension of an absorbent placed in the soil. A practical realisation of the dielectric tensiometer is described, consisting of a porous metal cylinder as an outer conductor, containing an inner conductor, and filled with glass beads which serve as an absorbent. Together they form a coaxial condenser system. The electrical capacitance between the two electrodes is a measure for the soil water tension. The dielectric tensiometer is usable from zero to over 10 bar, without suffering from most of the common disadvantages of tensiometer pressure transducer systems. The dielectric tensiometer is ideally suited for use in conjunction with a capacitive soil water content meter for determining the water-retention characteristics of soil material. Patent is pending for the dielectric tensiometer.
INTRODUCTION It is desirable to be able to m e a s u r e soil w a t e r tension, because p l a n t res p o n s e to m o i s t u r e is closely r e l a t e d to it. I n t h e p l a n n i n g of irrigation projects it is f u r t h e r n e c e s s a r y to a s c e r t a i n t h e w a t e r c o n t e n t o f t h e soil. T h e relation b e t w e e n w a t e r t e n s i o n a n d w a t e r c o n t e n t is called t h e waterretention characteristic. T h e m o s t c o m m o n m e t h o d for m e a s u r i n g soil w a t e r t e n s i o n uses a tensiometer. A p o r o u s c e r a m i c cup, w h i c h allows w a t e r to pass b u t is i m p e r m e a b l e to air, is p l a c e d in t h e soil. T h i s cup is c o n n e c t e d to a p r e s s u r e t a n s d u c e r via a hydraulic s y s t e m filled with degassed water. W h e r e t h e r e is no t e m p e r a t u r e difference b e t w e e n t h e inside a n d t h e outside o f t h e t e n s i o m e t e r , a n d w h e r e soil c o m p o s i t i o n v o l u m e c h a n g e s are negligibly small, this h a s been, up to now, t h e m o s t a c c u r a t e a n d r e p r o d u c i b l e m e t h o d for m e a s u r i n g soil w a t e r tension. U n f o r t u n a t e l y this t e n s i o m e t e r still suffers f r o m such d i s a d v a n t a g e s as: - p r e p a r a t i o n , i n c l u d i n g filling t h e s y s t e m with degassed water, is necessary;
0378-3774/88/$03.50
© 1988 Elsevier Science Publishers B.V.
412
the range of measurement is restricted to about 800 mbar, because above this pressure the air entry value is exceeded (air will pass through the membrane ); air tends to infiltrate the hydraulic system, especially when soil water tension is high; - checking the hydraulic system is required on a regular basis; - the system may not be used in sub-zero temperatures because the water in the hydraulics will freeze; tensiometers are fragile instruments which need to be handled with care; most tensiometers exhibit hysteresis; and - if the ceramic cup is suddenly pulled out from the soil, the pressure transducer may be damaged by pressure shocks. The aim of this paper is to present a new type of tensiometer which eliminates most of the above-mentioned short-comings and, in addition, allows water retention curves of soils to be determined with one single read-out instrument: "the capacitive soil water content meter" (Hilhorst, 1984). Water has a relatively high dielectric constant ( ~ 80) in comparison with dry soils ( < 5), so the capacitance of a condensor, the electrodes, with soil as dielectricum, will be dominated by the water-volume fraction (Hasted, 1973; Grand et al., 1978). This is the case for frequencies higher than about 15 MHz. The capacitive soil water-content meter is based on this principle. Soil water content cannot be measured using a tensiometer system because of the difference between the wetting and drying curves of the water-retention characteristic.
-
-
-
-
PRINCIPLE OF OPERATION
The volume fraction of water in absorptive materials dominates their complex relative permittivity e -- c' - j e " (dielectric behaviour) so permittivity is a function of the water content. The water retention characteristic of absorptive materials depends strongly on the texture of the material. Combining both characteristics, permittivity and water tension as function of water content, gives permittivity as a function of water tension. The measurement of permittivity is possible by means of a condenser, with the subject material as a dielectric, connected to special electronic circuitry (Hilhorst, 1984) for sufficient separation of the real and imaginary parts of the complex permittivity. A micro-processor may then be programmed to translate the measured water content into terms of water tension.
413
CONSTRUCTION OF THE DIELECTRIC TENSIOMETER
The water tension versus permittivity for glass beads of known size distribution proves to be accurately reproducible. Figure 1 gives the permittivity versus water tension for glass beads of 60 pm with a standard deviation of about 10 ~m. Hysteresis and accuracy for the given curve are within + 1.5%. A further advantage of the properties of glass beads is their relatively high hydraulic conductivity compared to soil, so they rapidly follow changes in soil water content. The glass beads are (in effect) integrated as part of the soil system and form the dielectricum of a coaxial condenser system (Fig. 2). The outer conductor is a porous metal cylinder. Both ends are closed with rubber plugs and sealed
3o.
20
10
!
,
i
,
101
102
103
104
Soil water tension (mbar)
Fig. 1. Permittivity versus water tension for glass beads of 60 #m. inner conductor
_epoxy~'eSi n/ ~
~'~
glass beads
rubber p]ug
' ~
C
~'roul outer iorlductor
cable to electroniLs
Fig. 2. A practical rea|isation of the dielectric tensiometer.
414
with epoxy resin. The given shape has been chosen only for experimental reasons; the tensiometer can be almost any shape required. MEASURINGFREQUENCIESAND ELECTRONICS For measuring soil water content we are forced to use radio frequencies of more than 15 MHz. At lower frequencies problems may occur due to the Maxwell-Wagner effect, which leads to an apparent increase in the permittivity of soil in the low-frequency range, depending on the conductivity of the soil and its small colloidal particles. Extensive tests on the dielectric behaviour of glass beads in our laboratory at between 1 MHz and 1 GHz showed an almost flat response of e' with the water content at between 1 and 5 MHz, without showing the Maxwell-Wagner effect. The bound water fraction in the lower water content region (less than 10 m3/m 3) is comparatively high. Due to the low relaxation frequency for bound water in glass beads, e' will decrease at frequencies higher than about 5 MHz. This leads to a lower sensitivity for the high-tension region. The unknown ionic conductivity of the tensiometer requires an instrument containing special electronic circuitry which is insensitivie to ionic conductivity, as is the case for soil water content measurements; such an instrument has been developed at the Technical and Physical Engineering Research Service for Agriculture (TFDL), Wageningen, The Netherlands. The instrument is ideally suited for use in conjunction with the dielectric tensiometer, making it possible to measure the complete water-retention curve of soil in one operation. The measuring frequency for soil is 20 MHz. Electronics are in development for the 1-5 MHz region, in order to bring about a more competitive price for the dielectric tensiometer compared with hydraulic models. CONCLUSION A new method for measuring soil water tension has been developed: the dielectric tensiometer, which shows great promise for care-free continuous measurement of soil water tension in the field and in the laboratory. In many cases it will successfully replace the tensiometer pressure transducer system. The measuring range is from 0 to 10 bar and beyond. It is possible to measure the complete water-retention curve with a single readout instrument. With relatively cheap, low-frequency read-out electronics the dielectric tensiometer can easily compete in price with conventional tensiometers.
415 REFERENCES Grand, E.H., Shappard, R.J. and South, G.P., 1978. Dielectric Behaviour of Biological Molecules in Solution. Oxford University Press, Oxford. Hasted, J.B., 1973. Aqueous Dielectrics. Chapman and Hall, London. Hilhorst, M.A., 1984. A sensor for the determination of the complex permittivity of materials as a measure for the moisture content. In: P. Bergveld (Editor), Sensors and Actuators. Kluwer, Deventer, The Netherlands, pp. 79-84.