Sensing water and moisture using a delay line

A phase shift oscillator circuit having a delay line in a feedback loop is employed to determine soil moisture content and water level. The delay line is connected to the oscillator such that the dielectric value of the medium inside the delay line influences the frequency of the oscillator circuit. The delay line is placed in the medium to be studied. Soil moisture content and water level are calculated as a function of the frequency of the oscillator output, and changes in soil moisture content or water level are calculated as a function of changes in oscillator frequency over time. An elongated delay line is employed to measure water level.

CROSS REFERENCE TO RELATED APPLICATIONS 
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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
Not applicable 
BACKGROUND OF THE INVENTION 
The present invention is generally related to sensors, and more 
particularly to an improved sensor for measuring soil moisture content and 
water level. 
Apparatus and sensors for providing quantitative measurement of soil 
moisture and water level are known. Pressure actuated sensors known as 
"tensiometers" provide an indication of soil moisture content by employing 
a column of liquid within a closed generally cylindrically shaped 
container that is separated from the soil by a membrane. Relatively moist 
soil exerts little or no force on the tensiometer liquid. Relatively dry 
soil causes a reduction in pressure within the container, i.e., the 
pressure decreases within the container as the soil develops a greater 
capacity to soak up the tensiometer liquid. Soil moisture content is 
determined from the pressure exerted on the tensiometer liquid. However, 
data from tensiometers can be inaccurate and must be manually gathered and 
stored. 
Relatively accurate electromechanical soil moisture sensors are also known. 
Soil exhibits a dielectric property which varies in proportion to moisture 
content. If the dielectric value of a region of soil can be measured, the 
moisture content of that region of soil can be calculated as a function of 
the measured dielectric value. Both time domain reflectomotry and 
capacitance measurement have been employed to determine the dielectric 
value of a region of soil to calculate moisture content. 
In time domain reflectomotry a voltage pulse is transmitted through a 
length of transmission cable that terminates in the soil. A portion of the 
pulse is reflected at the cable/soil interface. The magnitude of the 
reflected portion of the pulse is indicative of the dielectric value of 
the soil. However, devices that employ time domain reflectomotry are 
relatively complex to implement and consequently tend to be costly to 
manufacture. 
A capacitance based measurement may also be performed in which a soil 
"capacitor" is formed between two electrodes that are inserted into the 
soil. An excitation signal is applied to a circuit such as a Wein Bridge 
that includes the soil "capacitor" as the only component of unknown value. 
An output signal is produced in response to the excitation signal, and 
analysis of the output signal allows calculation of the capacitance value 
of the soil "capacitor." However, the apparant capacitance between the 
electrodes is affected by soil salinity which tends to introduce errors 
into soil moisture calculations when using a soil "capacitor." 
Similar sensors are known for measuring water level, such as the level of 
water in a well. For example, it is known to calculate water level based 
on pressure. In a column of water, pressure increases as depth increases. 
Water pressure can be measured with a pressure sensor and employed to 
calculate water depth. However, variations in atmospheric pressure also 
affect pressure sensors. To compensate for variations in atmospheric 
pressure a separate sensor may be employed to measure atmospheric pressure 
and calculations may be based on differential pressure measurements. 
However, the equipment required for compensation increases the complexity 
of the apparatus and also increases the cost to manufacture such devices. 
It is also known to calculate water level by placing electrodes at 
opposite ends of a water column and determining the capacitance between 
the electrodes. However, the apparant capacitance between the electrodes 
is adversely influenced by the salinity of the water. Further, both types 
of water level measurement devices must be individually calculated to 
compensate for variations in construction. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the present invention, an oscillator circuit with a 
feedback loop delay line is employed to determine soil moisture content or 
water level. The feedback loop is connected to the oscillator such that 
the dielectric value of the medium inside the feedback loop directly 
influences the operating frequency of the oscillator circuit. When the 
medium inside the loop is soil, the amount of moisture in the soil 
influences a delay in the propagation of a feedback signal through the 
feedback loop. The feedback delay influences the operating frequency of 
the oscillator circuit. Soil moisture content values are calculated from 
the operating frequency of the oscillator circuit. When the medium inside 
the loop includes both air and water, the proportion of water to air 
influences the feedback delay because air and water have different 
dielectric values. Water level values may also calculated from the 
operating frequency of the oscillator circuit. 
The oscillator circuit includes an inverting amplifier and a feedback loop. 
The feedback loop is placed in the soil to be analyzed and the amplifier 
output frequency is monitored. In one embodiment a transistor is employed 
as the amplifier to form a phase shift oscillator with the feedback loop 
coupled between the collector and base of the transistor. Transistor 
operating frequency is influenced by feedback loop propagation delay. The 
propagation delay is related to the dielectric value of the medium inside 
the feedback loop. Since soil moisture content is related to soil 
dielectric value, soil moisture content may be ascertained from the 
operating frequency of the oscillator. Changes in soil moisture content 
are calculated as a function of changes in oscillator frequency over time. 
The oscillator circuit and feedback loop allow soil moisture measurement to 
be averaged over a relatively large area. The soil moisture measurement 
produced with the feedback loop is approximately an average of the soil 
moisture content of the soil inside the feedback loop. The area over which 
the soil moisture content is averaged can be modified by adjusting the 
size and shape of the feedback loop. Measurement averaging over a 
relatively large area generally produces more useful measurement data that 
is less susceptible to local variation in soil constitution. 
The oscillator circuit and feedback loop provide accurate data without 
complex circuitry or the need for sophisticated calculations. Because the 
output of the oscillator circuit is electronic, data monitoring can be 
automated. In particular, the oscillator circuit and feedback loop can be 
implemented as a remote sensing device or as a stand-alone data logger. 
Simple ancillary circuitry may be employed since output is in the form of 
frequency. In particular, a scaling circuit and standard computer 
interface may be employed to obtain soil moisture measurements. 
A delay line in the form of a pair of transmission wires of constant 
diameter that are maintained at a constant separation distance relative to 
one another is employed in the feedback path of a water level monitor. A 
first length of the transmission wire pair extending from the oscillator 
to the lower end of the loop is maintained in a parallel orientation 
relative to a second length of the transmission wire pair extending from 
the lower end of the loop to the oscillator. The elongated feedback loop 
is positioned vertically in an area where water level is to be measured, 
such as from the top of a well to the bottom of the well.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a moisture and liquid level measurement device 8 
includes a phase shift oscillator circuit 10 and a converter circuit 20. 
The oscillator circuit 10 includes an inverting amplifier 11 and a 
feedback loop delay line 12. The feedback loop 12 is connected between an 
amplifier input node 14 and an amplifier output node 16. An output signal 
18 is produced by the oscillator circuit 10. The oscillator output signal 
18 is applied to feedback loop 12 and the frequency to moisture content 
converter circuit 20. The dielectric value of the medium 22 inside the 
feedback loop 12 affects the propagation delay in the feedback signal 
applied from the output node 16 to the input node 14. The propagation 
delay directly affects the operation of the oscillator circuit. In 
particular, the propagation delay influences the operating frequency of 
the oscillator output signal 18. Hence, the moisture content or liquid 
level of medium 22 inside the feedback loop 12 can be ascertained from the 
frequency of the output signal 18. 
The frequency to moisture content/liquid level converter circuit 20 
provides an indication of moisture content or water level in response to 
the frequency of the output signal 18. In the illustrated embodiment the 
converter circuit 20 scales the frequency of the output signal 18 for 
simplified frequency measurement. The scaled frequency is measured and 
employed as an index into a translation table or other suitable device to 
produce a measurement in desired units. 
FIG. 2 is a schematic diagram of the soil moisture measuring device of FIG. 
1. The illustrated embodiment provides a substantially 
temperature-independent output signal 18. A transistor oscillator circuit 
24 is employed to provide the output signal 18. A DC voltage 26 combined 
with a feedback signal 28 from the transistor collector is applied to the 
base of the transistor 29. Propagation delay of the feedback signal 28 
through the feedback loop 12 is dependent, at least in part, upon the 
dielectric value of the medium proximate with the feedback loop 12. In 
general, the greater the dielectric value of the medium, the greater the 
feedback propagation delay. Feedback delay influences oscillation of the 
transistor 29. Hence, the dielectric value of the medium inside the 
feedback loop can be ascertained from the frequency of the oscillator 
output. 
In the illustrated embodiment the feedback loop 12 is approximately two 
feet of 300 .OMEGA. twin lead transmission wire, such as the type that is 
typically used for television and FM radio antenna leads. Transmission 
wire with a thick insulating coating is less sensitive to conditions in 
the surrounding medium than transmission wire with a thin coating. In the 
illustrated embodiment the coating provides approximately a factor of two 
change in frequency between air and water (f.sub.air =2*f.sub.water), as 
measured at the collector of the transistor 29. The operating frequency of 
the illustrated 35 oscillator in an air medium is about 150 Mhz. In the 
illustrated configuration the transistor oscillator 24 output signal 
generally oscillates at a frequency between 100 MHz to 200 MHz depending 
upon the moisture content of the soil it is placed in. The transistor's 
phase shift is slightly dependent on temperature and contributes a delay 
equivalent to about one foot of feedback loop in air in the illustrated 
embodiment. The temperature dependence becomes negligible for many 
applications when the feedback loop is buried in soil. 
The output of the transistor oscillator 24 is applied to the converter 
circuit 20 which reduces the frequency to one that is easily measured. The 
illustrated converter circuit 20 includes a high frequency prescaler 
circuit 32 that divides the frequency of the transistor oscillator 24 
output signal 18 down to about 1 Mhz. A second divider circuit 34 operates 
on the output of the prescaler circuit 32 to further lower the frequency. 
The output of the divider circuit 34 is of sufficiently low frequency to 
be easily and accurately measured using a computer 36. In response to the 
output of the divider circuit 34 the computer 36 provides measurement data 
in units of soil moisture or water level. A frequency to measurement-unit 
conversion can be made by a conversion equation or via a predefined 
translation table. The table may include a plurality of frequency entries 
across the range of the measurement device, and a soil moisture value 
corresponding to each frequency entry. If the measured frequency is 
between two entries in the table, the nearest entry may be employed or an 
interpolated value may be calculated. 
Referring to FIG. 3, when the medium being examined is soil 38, the 
illustrated measuring device is buried beneath the surface 40 of the soil 
38. A protective housing or potted enclosure material 42 such as epoxy is 
employed to protect the internal electronics of the measurement device 8 
from moisture in the soil 38. In the illustrated embodiment the 
measurement device 8 includes an interface that allows the feedback loop 
12 to emerge from the sides of the protective housing 42. 
In alternative embodiments the soil moisture measuring device 8 can be 
implemented as a remote sensor or as a stand-alone data logger. In the 
data logger configuration the measurement device is battery powered and 
moisture content measurements are automatically taken and stored in 
internal memory. A power-up reset circuit on the divider 34 (FIG. 2) 
allows the measurement device to be powered up only when needed, and 
minimizes the time needed to measure the oscillator circuit 10 (FIG. 1) 
frequency, thereby conserving battery power. The measurements are 
periodically downloaded for analysis. The download may be accomplished via 
a data transfer wire 44. If the data transfer wire is not employed, the 
download may be accomplished by removing the measurement device 8 from the 
soil 38. In the remote sensor configuration, soil moisture measurements 
are downloaded to a computer in real time via the data transfer wire 44. 
Further, power is supplied via a power-in wire 46, thereby eliminating 
reliance on battery power. 
The area of soil 38 that is sampled to produce moisture content data is 
determined by the position, size and shape of the feedback loop 12 in the 
soil. The delay attributable to the feedback loop 12 is substantially 
determined by the dielectric value of the soil around the transmission 
line feedback loop 12. To measure average soil moisture content at a 
specific depth D below the surface 40, the feedback loop 12 is situated 
horizontally in a plane at a desired depth D. 
Referring now to FIG. 4, soil moisture measurements can be taken across a 
range of soil depths. To measure soil moisture content across a range of 
depths below the surface 40, e.g., from D.sub.0 to D.sub.1, the feedback 
loop 12 is situated generally vertically in the soil 38 so that it extends 
between depth D.sub.0 and depth D.sub.1. It should be noted that erroneous 
data may be obtained if the feedback loop 12 extends above the surface 40 
of the soil 38. 
In an alternative embodiment illustrated in FIGS. 5, 6 and 7 the oscillator 
and converter circuits are employed with an elongated feedback loop 48 to 
measure the level of a medium such as water 50 in an area such as a well 
52. The elongated feedback loop 48 includes a pair of transmission wires 
that are maintained at a constant separation distance 56 relative to one 
another. In the illustrated embodiment the elongated feedback loop 48 is a 
20 foot length (in two 10 foot portions) of 300 .OMEGA., 30 gauge 
transmission wire having a separation distance 56 of 0.1 inch. A first 
portion 58 of the transmission wire pair that extends from the oscillator 
housing 42 to a lower loop end 62 is substantially parallel to a second 
portion 60 of the transmission wire pair extending from the lower loop end 
62 to the oscillator housing 42. As shown in the cross-section view of 
FIG. 7, a constant separation distance 64 is maintained between the first 
and second loop portions 58, 60. In the illustrated embodiment the 
separation distance 64 is 0.2 inch. Protective strips 66 on each side of 
the elongated feedback loop 48 insulate the transmission wires from the 
walls of the well 52. The operating frequency of the illustrated 
oscillator/feedback loop combination in an all-air medium is approximately 
10-20 MHz. 
The elongated feedback loop 48 is positioned generally vertically in an 
area such as a well where water level is to be studied. A feedback delay 
that is dependent, at least in part, upon the level of water relative to 
the elongated feedback loop 48 is employed to calculate water level. The 
feedback delay is dependent upon the proportion of water to air within the 
elongated feedback loop 48. The operating frequency of the oscillator is 
dependent on the feedback delay. The dielectric value of water is 
different from the dielectric value of air. Consequently, water level is 
calculated from the operating frequency of the oscillator, with higher 
operating frequencies indicating higher water level. A table or other 
suitable device can be employed to convert the frequency measurement to a 
measurement in units of level and/or depth. Changes in water level are 
calculated as a function of changes in oscillator frequency over time. 
Having described the preferred embodiments of the invention, other 
embodiments which incorporate concepts of the invention will now become 
apparent to those skilled in the art. Therefore, the invention should not 
be viewed as limited to the disclosed embodiments but rather should be 
viewed as limited only by the spirit and scope of the appended claims.