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An improved soil water flux sensor
Agricultural Meteorology- Elsevier Publishing Company, Amsterdam- Printed in The Netherlands
A N I M P R O V E D SOIL W A T E R F L U X S E N S O R G. F. B Y R N E
Division of Land Research, C.S.1.R.O., Canberra, ,4.C.T. ( ,4ustralia)
Received December 31, 1970) ABSTRACT BYR~E, G. F., 1971. An improved soil water flux sensor. ,4gric. Meteorol., 9: 101-104.
The design, construction, and calibration of a sensor which can be used to measure water flow rates through soil of 10-3 cm/sec is described. The instrument is free of some disadvantages associated with previous soil water flux sensors. INTRODUCTION
The flow rate of water through soil is a parameter of great significance in many agricultural and engineering studies. It may be measured by determining the distortion, which the flow produces, in the temperature field about a point or line source of heat (BVRNE et al., 1967, 1968). The "point source" instrument previously described has the advantage of a monotonic calibration curve. However, it requires a stable source of bias current for the two temperature-measuring resistance elements and also, the making of these elements is fairly demanding in skill and time. The thermocouple version of the "line source" instrument (BYRNE et al., 1968) does not have these drawbacks, but is less conveniently shaped for positioning in soil in the field and also has the disadvantage of a double-valued calibration curve. In order to combine the best features of both the point and line source instruments mentioned above, a series thermocouple version of the point source instrument has been devised. This instrument has the additional advantage of being considerably smaller than any of the previous instruments, without any significant sacrifice in output signal. THE INSTRUMENT
The instrument (Fig.l) consists of a hollow cylindrical body having a number of turns of fine constantan wire wound in an axial direction on the wall. The sections of wire on the outer surface of the wall are copper plated thus forming a series circuit of copper-constantan thermocouples. Around the circumference, at the centre of the body, a constantan wire heating element is wound. Connections to the thermocouple circuit and to the heater are brought out through one end of the tubular body. The whole instrument, save the leads, is encapsulated in a waterproof resin.
Agric. Meteorol., 9 (1971/1972) 101-104
102
0. F RYRNE 0 i
10 !
20 i
30 i
40 i
mm
A
B
C
I ThermocOup'e / circuit
l
Heater
/
Fig.1. Stages in the construction o f the soil water flux sensor.
With the instrument in place in the soil and no water movement, the power dissipated in the heater produces the same rise in temperature at each end of the tube apart from the zero offset that may be produced by any lack in symmetry. Thus the thermocouple circuit produces no output. When water moves past the instrument in an axial direction, the temperature field is distorted and the thermocouple circuit produces an output which varies with the rate of water flow. METHOD OF CONSTRUCTION
The body of the instrument is cut from 5 m m I.D. by 6 mm O.D. bakelized fibre tubing. The tube is slit longitudinally to facilitate the subsequent wire winding operation and the four holes are drilled as shown in Fig. IA. 42 S.W.G. constantan Agric. Meteorol., 9 (1971/1972) 101-104
103
AN IMPROVED SOIL WATER FLUX SENSOR
wire to form the thermocouple circuit is then wound on (17 turns in the case of the prototype) (Fig.lB). The end plugs, through one of which pass the four leads, are placed in position (Fig. 1C). The ends of the thermocouple winding are soldered to two of the lead wires and the interior of the instrument is sealed off from the exterior with a waterproof two-part resin encapsulating material (Fig.lD). Any exposed solder and the ends of the two so far unused lead wires are temporarily protected with the same resin. Copper is then electrodeposited on the exposed constantan wire. The instrument is then thoroughly washed, dried and coated with a thin layer of encapsulating resin. Several layers of Teflon tape, such as that used as a pipe thread sealant, are wound on the centre portion of the instrument. The 20 heater winding, again of 42 S.W.G. constantan wire, is wound on top of this. The ends of the heater winding are soldered to the two central, and so far unused, lead wires (Fig.lE). The whole instrument is then given a second coating of the same encapsulating resin. It will be noticed (Fig.lA) that the fibre tubing is chamfered slightly at each end to assist in the production of an even coating of resin when the resin is allowed to drain off one end and set. To further assist this, the second coating of resin is allowed to drain off the opposite end of the tube from the first coating. PERFORMANCE
The instrument was calibrated in saturated sand (Fig.2), using procedures similar to those described by BYRNE et al. (1967, 1968). Two levels of power input 2.0
2.46 W
1.5
Q.
0 1.o
i'i -0.5
,
0.00
0.01
0.02
0.03
Flow rate (cm/sec)
Fig.2. Calibration curves of the prototype at two levels of heater power input.
Agric. Meteorol., 9
(1971/1972) 101-104
104
G . F . BYRNE
to the heater (1.22 W and 2.46 W) were used. The time constant for response to a step change in water flow rate was less than ten minutes. At the higher power input, a greater output is obtained, as might be expected, but there is a danger of the instrument destroying itself when it is used in situations in which the soil occasionally dries out. A copper-constantan thermocouple was attached to the heater surface with encapsulating resin and with the instrument in saturated sand under zero flow the resin surface temperature was found to be 23°C and 10°C above ambient at the higher and lower power input levels respectively. To investigate the possibility of self destruction the instrument was placed in dry sand. At the lower power the instrument surface temperature rose to a maximum of 34°C above ambient. DISCUSSION
The theory of the point source instrument indicates that with a power input of 2.46 W and a water flow rate of 0.01 cm/sec in sand the difference in temperature between the hot and cold junctions should be of the order of l 8 °C. With 17 hot and cold junction copper-constantan pairs, the output would then be 12 mV, compared with the 2.5 mV obtained in the calibration, representing a substantial departure from the behaviour postulated in the theory. However, it appears to become increasingly difficult to achieve a one-to-one correspondence between theory and practice as the size of the instrument is reduced, possibly because wire diameters, resin coating thickness, etc., cannot be scaled down correspondingly. Other considerations involved are, firstly, the heat source cannot be made very small without generating excessively high temperatures and, secondly, the thermal conductivity of the body of the instrument must be significantly greater than that of the surrounding medium if mechanical strength is to be retained. An appreciable part of the heat flux then takes place in the instrument itself rather than in the medium and temperature gradients are therefore smaller than might be expected. Notwithstanding these considerations, the instrument described gave a single valued calibration curve over the range from zero to well above 0.01 cm/sec, a flow rate that might reasonably be described as very rapid. Also, the shape of the instrument is such that it may be inserted neatly in a vertical hole 8 mm in diameter. REFERENCES
BYRNE,G. F., ROSE, C. W. and DRUMMOND,J. E., 1967. A sensor for water flux in soil. 1. "Point source" instrument. Water Resources Res., 3: 1073-1078. BYRNE, G. F., ROSE, C. VV'.and DRUMMOND,J. ~., 1968. A sensor for water flux in soil. 2. "'kine source" instrument. Water ResoHrces Res., 4:607 611.
Agric. Meteorol., 9 (1971/1972) 101-104
A N I M P R O V E D SOIL W A T E R F L U X S E N S O R G. F. B Y R N E
Division of Land Research, C.S.1.R.O., Canberra, ,4.C.T. ( ,4ustralia)
Received December 31, 1970) ABSTRACT BYR~E, G. F., 1971. An improved soil water flux sensor. ,4gric. Meteorol., 9: 101-104.
The design, construction, and calibration of a sensor which can be used to measure water flow rates through soil of 10-3 cm/sec is described. The instrument is free of some disadvantages associated with previous soil water flux sensors. INTRODUCTION
The flow rate of water through soil is a parameter of great significance in many agricultural and engineering studies. It may be measured by determining the distortion, which the flow produces, in the temperature field about a point or line source of heat (BVRNE et al., 1967, 1968). The "point source" instrument previously described has the advantage of a monotonic calibration curve. However, it requires a stable source of bias current for the two temperature-measuring resistance elements and also, the making of these elements is fairly demanding in skill and time. The thermocouple version of the "line source" instrument (BYRNE et al., 1968) does not have these drawbacks, but is less conveniently shaped for positioning in soil in the field and also has the disadvantage of a double-valued calibration curve. In order to combine the best features of both the point and line source instruments mentioned above, a series thermocouple version of the point source instrument has been devised. This instrument has the additional advantage of being considerably smaller than any of the previous instruments, without any significant sacrifice in output signal. THE INSTRUMENT
The instrument (Fig.l) consists of a hollow cylindrical body having a number of turns of fine constantan wire wound in an axial direction on the wall. The sections of wire on the outer surface of the wall are copper plated thus forming a series circuit of copper-constantan thermocouples. Around the circumference, at the centre of the body, a constantan wire heating element is wound. Connections to the thermocouple circuit and to the heater are brought out through one end of the tubular body. The whole instrument, save the leads, is encapsulated in a waterproof resin.
Agric. Meteorol., 9 (1971/1972) 101-104
102
0. F RYRNE 0 i
10 !
20 i
30 i
40 i
mm
A
B
C
I ThermocOup'e / circuit
l
Heater
/
Fig.1. Stages in the construction o f the soil water flux sensor.
With the instrument in place in the soil and no water movement, the power dissipated in the heater produces the same rise in temperature at each end of the tube apart from the zero offset that may be produced by any lack in symmetry. Thus the thermocouple circuit produces no output. When water moves past the instrument in an axial direction, the temperature field is distorted and the thermocouple circuit produces an output which varies with the rate of water flow. METHOD OF CONSTRUCTION
The body of the instrument is cut from 5 m m I.D. by 6 mm O.D. bakelized fibre tubing. The tube is slit longitudinally to facilitate the subsequent wire winding operation and the four holes are drilled as shown in Fig. IA. 42 S.W.G. constantan Agric. Meteorol., 9 (1971/1972) 101-104
103
AN IMPROVED SOIL WATER FLUX SENSOR
wire to form the thermocouple circuit is then wound on (17 turns in the case of the prototype) (Fig.lB). The end plugs, through one of which pass the four leads, are placed in position (Fig. 1C). The ends of the thermocouple winding are soldered to two of the lead wires and the interior of the instrument is sealed off from the exterior with a waterproof two-part resin encapsulating material (Fig.lD). Any exposed solder and the ends of the two so far unused lead wires are temporarily protected with the same resin. Copper is then electrodeposited on the exposed constantan wire. The instrument is then thoroughly washed, dried and coated with a thin layer of encapsulating resin. Several layers of Teflon tape, such as that used as a pipe thread sealant, are wound on the centre portion of the instrument. The 20 heater winding, again of 42 S.W.G. constantan wire, is wound on top of this. The ends of the heater winding are soldered to the two central, and so far unused, lead wires (Fig.lE). The whole instrument is then given a second coating of the same encapsulating resin. It will be noticed (Fig.lA) that the fibre tubing is chamfered slightly at each end to assist in the production of an even coating of resin when the resin is allowed to drain off one end and set. To further assist this, the second coating of resin is allowed to drain off the opposite end of the tube from the first coating. PERFORMANCE
The instrument was calibrated in saturated sand (Fig.2), using procedures similar to those described by BYRNE et al. (1967, 1968). Two levels of power input 2.0
2.46 W
1.5
Q.
0 1.o
i'i -0.5
,
0.00
0.01
0.02
0.03
Flow rate (cm/sec)
Fig.2. Calibration curves of the prototype at two levels of heater power input.
Agric. Meteorol., 9
(1971/1972) 101-104
104
G . F . BYRNE
to the heater (1.22 W and 2.46 W) were used. The time constant for response to a step change in water flow rate was less than ten minutes. At the higher power input, a greater output is obtained, as might be expected, but there is a danger of the instrument destroying itself when it is used in situations in which the soil occasionally dries out. A copper-constantan thermocouple was attached to the heater surface with encapsulating resin and with the instrument in saturated sand under zero flow the resin surface temperature was found to be 23°C and 10°C above ambient at the higher and lower power input levels respectively. To investigate the possibility of self destruction the instrument was placed in dry sand. At the lower power the instrument surface temperature rose to a maximum of 34°C above ambient. DISCUSSION
The theory of the point source instrument indicates that with a power input of 2.46 W and a water flow rate of 0.01 cm/sec in sand the difference in temperature between the hot and cold junctions should be of the order of l 8 °C. With 17 hot and cold junction copper-constantan pairs, the output would then be 12 mV, compared with the 2.5 mV obtained in the calibration, representing a substantial departure from the behaviour postulated in the theory. However, it appears to become increasingly difficult to achieve a one-to-one correspondence between theory and practice as the size of the instrument is reduced, possibly because wire diameters, resin coating thickness, etc., cannot be scaled down correspondingly. Other considerations involved are, firstly, the heat source cannot be made very small without generating excessively high temperatures and, secondly, the thermal conductivity of the body of the instrument must be significantly greater than that of the surrounding medium if mechanical strength is to be retained. An appreciable part of the heat flux then takes place in the instrument itself rather than in the medium and temperature gradients are therefore smaller than might be expected. Notwithstanding these considerations, the instrument described gave a single valued calibration curve over the range from zero to well above 0.01 cm/sec, a flow rate that might reasonably be described as very rapid. Also, the shape of the instrument is such that it may be inserted neatly in a vertical hole 8 mm in diameter. REFERENCES
BYRNE,G. F., ROSE, C. W. and DRUMMOND,J. E., 1967. A sensor for water flux in soil. 1. "Point source" instrument. Water Resources Res., 3: 1073-1078. BYRNE, G. F., ROSE, C. VV'.and DRUMMOND,J. ~., 1968. A sensor for water flux in soil. 2. "'kine source" instrument. Water ResoHrces Res., 4:607 611.
Agric. Meteorol., 9 (1971/1972) 101-104