Note: Descriptions are shown in the official language in which they were submitted.
CA 02556126 2006-08-15
POROUS MEDIUM TENSIOMETER
Field of the Invention
The invention relates to tensiometers for measuring soil water potential in
porous
media and, more particularly, it relates to a liquid filling design for
tensiometers and to
self-priming tensiometers.
Description of the Prior Art
Tensiometers for monitoring matric water potential LP,,, (or soil moisture
tension, in
soil are known. Matric water potential is an indirect measure of soil water
content.
Tensiometers are used in irrigation scheduling to help farmers and other
irrigation
managers to determine when to water. In conjunction with a water retention
curve,
tensiometers can be used to determine how much to water. Tensiometers can also
be
used in the scientific study of soils and plants.
To reduce farmer's workload such as the tensiometer maintenance and the
reading
adjustements in accordance with the water level in the tensiometer, there is a
need for
tensiometers which are easier to use.
BRIEF SUMMARY OF THE INVENTION
It is therefore an aim of the present invention to address the above mentioned
issues.
According to an aspect, there is provided a porous medium tensiometer. The
porous
medium tensiometer comprises: a housing defining a fluid chamber therein and
having
a fluid channel extending therethrough, the fluid channel having a first port
in fluid
communication with the fluid chamber and a second port in fluid communication
with the
atmosphere, the housing being adapted to be at least partially inserted in a
porous
medium; and a liquid injector insertable in the fluid channel through the
second port of
the fluid channel for injecting a liquid into the fluid chamber through the
first port of the
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fluid channel and simultaneously ejecting gas contained in the fluid chamber
in the
atmosphere through the second port.
According to another aspect, there is provided a porous medium tensiometer
comprising: a housing defining a fluid chamber therein and having an inner
plug
provided above the fluid chamber, the inner plug having a fluid channel and a
gas exit
channel extending therethrough, the fluid channel having a first port in fluid
communication with the fluid chamber and a second port in fluid communication
with the
atmosphere, the gas exit channel having a first port in fluid communication
with the fluid
chamber and a second port in fluid communication with the fluid channel and
merging
therewith, the second port of the gas exit channel remaining unobstructed when
filling
the fluid chamber with liquid through the fluid channel and allowing gas
contained in the
fluid chamber to exit therethrough.
According to another aspect, there is provided a self-priming tensiometer
comprising: a porous material tip having a first section surrounded by a fluid-
impermeable membrane, a second section, a threshold suction range and pores,
the
pores auto-filling with liquid, through the second section, when in fluid
communication
with a porous medium having a liquid potential being at least equal, in
absolute value, to
the threshold suction range, the liquid contained in the pores having a liquid
pressure
representative of the liquid potential; a one-way fluid control device in
fluid
communication with the porous material tip, through the first section, and
allowing fluid
contained in pores of the porous material to exit therethrough when the porous
material
tip auto-fills with liquid; and a pressure transducer in liquid communication
with the
porous material tip and measuring the liquid pressure therein when the pores
are filled
with liquid.
According to a further aspect, there is provided a self-priming tensiometer
insertable
in a porous medium, the self-priming tensiometer comprising: a housing; a
porous
material tip mounted to the housing and having pores and a threshold suction
range, the
pores being in fluid communication with the porous medium when inserted
therein and
auto-filling with liquid when the porous medium has a liquid potential at
least equal, in
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absolute value, to the threshold suction range; a pressure transducer in fluid
communication with the porous material tip; and a gas exit valve extending
between
the housing and the porous material tip and movable between a closed position
preventing fluid communication between the porous material tip and the housing
and
an open position allowing gas contained in the pores of the porous material
tip to exit
therethrough.
According to still another general aspect, there is provided a porous medium
tensiometer comprising: a housing defining a fluid chamber therein, having a
fluid
channel and a gas exit channel extending therethrough, the fluid channel
having a
first fluid port in fluid communication with the fluid chamber and a second
fluid port in
fluid communication with the atmosphere, and the gas exit channel having a
first gas
port in fluid communication with the fluid chamber and a second gas port in
fluid
communication with the fluid channel, the gas exit channel merging with the
fluid
channel at the second gas port and being in fluid communication with the
atmosphere through the second fluid port, the housing being adapted to be at
least
partially inserted in a porous medium; and a liquid injector insertable in the
fluid
channel through the second fluid port of the fluid channel for the injection
of a liquid
into the fluid chamber through the first fluid port of the fluid channel and
simultaneously ejecting gas contained in the fluid chamber in the atmosphere
through the gas exit channel and the second fluid port.
According to a further general aspect, there is provided a porous medium
tensiometer comprising: a housing defining a fluid chamber therein and having
an
inner plug provided above the fluid chamber, the inner plug having a fluid
channel
and a gas exit channel extending therethrough, the fluid channel having a
first fluid
port in fluid communication with the fluid chamber and a second fluid port in
fluid
communication with the atmosphere, the gas exit channel having a first gas
port in
fluid communication with the fluid chamber and a second gas port in fluid
communication with the fluid channel and merging therewith at the second gas
port,
the second gas port of the gas exit channel remaining unobstructed when
filling the
fluid chamber with liquid through the fluid channel and allowing gas contained
in the
fluid chamber to exit therethrough.
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In this specification, the term "porous medium" is intended to mean the soil
of a
field in agriculture, or the soil of pots for growing plants in a greenhouse
or in a
nursery, and any porous medium which fills with liquid. It can also be called
a
substrate, a mixture, a medium, or a soilless medium.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a perspective view of a tensiometer apparatus in accordance with an
embodiment of the invention;
Fig. 2 is side elevation view of the tensiometer apparatus shown in Fig. 1,
showing, in dashed lines, internal components of the tensiometer apparatus;
Fig. 3 is side elevation view of the tensiometer apparatus shown in Fig. 1;
Fig. 4 is a sectional view taken along lines 4-4 of Fig. 3, wherein the
tensiometer
apparatus is inserted in a porous medium;
Fig. 5 is a front elevation view of a tensiometer apparatus in accordance with
another embodiment of the invention;
Fig. 6 is side elevation view of the tensiometer apparatus shown in Fig. 5,
showing, in dashed lines, internal components of the tensiometer apparatus;
Fig. 7 is front elevation view of the tensiometer apparatus shown in Fig. 5,
showing, in dashed lines, internal components of the tensiometer apparatus;
Fig. 8 is side elevation view of the tensiometer apparatus shown in Fig. 5;
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Fig. 9 is a sectional view taken along lines 9-9 of Fig. 8 and Fig. 9a is a
detailed
view, enlarged, of a section of the tensiometer apparatus shown in Fig. 9;
Fig. 10 is a sectional view of a tensiometer apparatus in accordance with
another
embodiment of the invention, wherein the tensiometer includes pressure
transducer in
fluid communication with a gas exit valve and Fig. 10a is a detailed view,
enlarged, of a
section of the tensiometer apparatus shown in Fig. 10;
Fig. 11 is a schematic graph showing the quantity of liquid contained in a
porous tip
of the tensiometer apparatus as a function of the suction measured; and
Fig. 12 is a schematic graph showing the soil moisture tension as a function
of the
pressure measured in millivolts.
It will be noted that throughout the appended drawings, like features are
identified by
like reference numerals.
DETAILED DESCRIPTION
Referring to the drawings and, more particularly, to Fig. 1, it will be seen a
tensiometer apparatus 20 (or water potential sensor) in accordance with an
embodiment. The tensiometer 20 is designed to monitor matrix water potential
in a
porous medium such as, for instance, earthen soil or greenhouse soil.
The tensiometer apparatus 20 has a body 22 which includes a tubular housing 24
with a lower end 26 and an upper end 28, a porous material tip 30, a head 32,
and an
antenna 34. The porous material tip 30 is mounted to the lower end 26 of the
tubular
housing 24. The porous material tip 30 has a first section which extends in
the tubular
housing 24 and a second section which is in direct contact with the porous
medium
when inserted therein, as will be described in more details below.
The head 32 of the tensiometer 20 is mounted to the upper end 28 of the
tubular
housing 24. The antenna 34 is mounted to the head 32 of the tensiometer
apparatus 20,
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the purpose of which will be described below. The head 32 and the antenna 34
extend
above the porous medium when the tensiometer 20 is inserted therein.
The tubular housing 24 has a liquid inlet aperture 36 (or fluid aperture) and
a gas
inlet aperture 38 which extends therethrough. These apertures 36 and 38 are
proximate
to the upper end 28 of the tubular housing 24. In the embodiment shown, the
gas inlet
aperture 38 is located below the liquid inlet aperture 36 and is
longitudinally in line
therewith. However, in an alternative embodiment, these apertures 36 and 38
could be
positioned differently. The liquid inlet aperture 36 and the gas inlet
aperture 38 extend
above the porous medium when the tensiometer 20 is inserted therein.
The lower face 39 of the head 32 has two connectors 40, 42 (Figs. 2-4)
extending
downwardly therefrom. The purpose of these connectors 40, 42 will be described
in
more details below.
The upper face 43 of the head 32 includes an electronic dial 44. The
electronic dial
44 can display, amongst others, the matrix water potential measured by the
tensiometer
20.
Referring now to Figs. 2 to 4 simultaneously, the internal structure of the
tensiometer
will be described in more detail. The housing 24 has a peripheral wall 50
which
defines a fluid chamber 52 therein. The fluid chamber 52 extends from the
first end 26
to the second end 28 of the tubular housing 24. The liquid inlet aperture 36
and the gas
20 inlet aperture 38 extend throughout the peripheral wall 50.
As shown in Fig. 4, the lower portion of the housing 24 is divided in two
sections. A
seal 54 is inserted between these sections to prevent fluid communication
between the
fluid chamber 52 and the porous medium, in which the tensiometer 20 is
inserted.
A pressure transducer 56 is inserted in the fluid chamber 52. The pressure
transducer 56 is located above and proximate to the porous tip 30 without
being
inserted therein. The pressure transducer 56 has a liquid port 58 in fluid
communication
with the fluid chamber 52. As will be described in more details below, the
fluid chamber
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52 is designed to be filled with a liquid, typically water, the pressure in
the liquid being
representative of the matrix water potential sampled through the porous
material tip 30.
The pressure of the liquid in the fluid chamber 52, sampled through the liquid
port
58, is compared by the pressure transducer 56 to the atmospheric pressure.
Therefore,
the pressure transducer 56 is in fluid communication with the atmosphere
through an
atmospheric gas channel 60 which extends longitudinally in the fluid chamber
52. The
atmospheric gas channel 60 has a first port 62 connected to a reference port
63 of the
pressure transducer 56 and a second port 64 connected to a plug 74 as will be
described in more details below.
An electric wire channel 66 also extends longitudinally in the fluid chamber
52. As in
the atmospheric gas channel 60, the electric wire channel 66 has a first end
68
connected to the pressure transducer 56 and a second end 70 connected to an
electric
circuit board 72 located in the head 32 of the tensiometer apparatus 20. The
electric
wire channel 66 contains electric wires (not shown) in which the data acquired
or
monitored by the pressure transducer 56 are transferred to the electronic
circuit board
72.
The plug 74 (or obstruction member) is inserted in the upper portion of the
fluid
chamber 52 proximate to the upper end 28 of the tubular housing 24. The plug
74 can
be either built in with the housing 24, can be secured to the peripheral wall
50 or can be
inserted in the fluid chamber 52 without being mounted to the peripheral wall
50. A fluid
channel 76 is defined in the plug 74. The fluid channel 76 has a first section
which
extends transversally in the plug 74 and a second section which extends
longitudinally
therein.
The fluid channel 76 has a first port 78, which is in fluid communication with
the
liquid inlet aperture 36 of the housing 24 when the plug 74 is inserted in the
fluid
chamber 52, and a second port 80, which is in fluid communication with the
fluid
chamber 52 and, more particularly, with the section of the fluid chamber 52
extending
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below the plug 74. The fluid channel 76 is designed to receive a liquid
therein to fill the
fluid chamber 52, as will be described in more details below.
The peripheral wall defining the transversal section of the fluid channel 76
in the plug
74 can include threads (not shown) for fastening a fluid injector 98 therein,
as will be
described in more details below.
A gas outlet channel 84 is also defined in the plug 74. The gas outlet channel
84 has
a first port 86 in fluid communication, or merging, with the transversal
section of the fluid
channel 76 proximate to the first port 78. It also has a second port 88 which
is in fluid
communication with the fluid chamber 52 and, more particularly, with the
section of the
fluid chamber 52 extending below the plug 74. As will be described in more
details
below, the gas outlet channel 84 is designed to allow the gas contained in the
fluid
chamber 52 to exit therethrough while simultaneously filling the fluid chamber
52 with
liquid.
Finally, an atmospheric gas channel 90 is also defined in the plug 74. The
atmospheric gas channel 90 has a transversal section and a longitudinal
section. The
atmospheric gas channel 90 has a first port 92 in the transversal section
which is in fluid
communication with the gas aperture 38 when the plug 74 is inserted in the
fluid
chamber 52. The atmospheric gas channel 90 has a second port 94 which extends
in
the fluid chamber 52 and is in fluid communication therewith. As for the ports
80 and 88
of the previously described channels 76 and 84, the second port 94 extends in
the
section of the fluid chamber 52 which is below the plug 74. The second port 94
is
connected to the second port 64 of the atmospheric gas channel 60 and is in
fluid
communication therewith. The second ports 64, 94 are sealed together to
prevent fluid
infiltration therebetween.
Referring to Figs. 2 and 3, it will be seen that the tensiometer 20 can
include a fluid
injector 98 which can be inserted in the fluid channel 76 for injecting a
fluid therein and
into the fluid chamber 52 for filling the latter. When inserted in the fluid
channel 76, the
injector 98 does not obstruct the second port 88 of the gas outlet channel 84
thereby
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allowing simultaneously gas, previously contained in the fluid chamber 52, to
exit
therethrough while filling the fluid chamber 52 with liquid.
The injector 98 defines a channel (not shown) in which the liquid circulates.
The
channel has a first port 102 insertable in the fluid channel 76 through which
liquid flows
from the channel into the fluid channel 76, and a second port 104 opposed to
the first
port 102 which is connectable to a fluid supply (not shown). The outer face of
the
injector 98 can include treads 106 proximate to the first port 102 for
connection with the
treads defined in the peripheral wall of the fluid channel 76.
In an embodiment, the data acquired by the pressure sensor or pressure
transducer
56 are transmitted through electric wires located in the electric wire channel
66 to the
electronic circuit board 72 located in the head 32. The data transferred are
typically
tension data provided in milivolts. These tension data are converted in
pressure
measures by the electronic circuit board 72. The pressure monitored by the
transducer
56 can be displayed on the electronic dial 44 and can also be transmitted to a
data
logger (not shown) which records the data transmitted from the tensiometer 20.
The
data can be transferred with wireless technology through the antenna 34 or the
tensiometer can be physically connected to a data logger through the connector
42
The other connector 40 is used to connect the tensiometer 20 to a power supply
(not
shown). The power supply provides power to the tensiometer 20 and, more
particularly,
to the electric circuitry including the electronic circuit board 72.
To measure matrix water potential in a porous medium, the tensiometer 20 is
first
inserted in the porous medium. The fluid chamber 52 is filled or refilled with
water to
ensure that the pressure sensor 56 is immerged. The fluid chamber 52 has, from
time to
time, to be refilled if the tension, in the porous medium, reaches a critical
air
breakthrough point. For refilling or filling the tensiometer 20, the injector
98 is inserted in
the fluid channel 76 and connected to a fluid supply. A liquid, typically
water, is injected
into the channel of the injector 98 and flows into the fluid channel 76 to
reach the fluid
chamber 52.
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Simultaneously while the fluid chamber 52 is being filled with liquid, gas
previously
contained therein exits through the gas outlet channel 84 and the fluid
channel 76 which
are not obstructed by the insertion of the injector 98. Once the fluid chamber
52 is filled
with liquid, the injector 98 is disconnected and removed from the fluid
channel 76 and
the fluid channel 76 can be closed with a plug (not shown).
Once the tensiometer 20 is connected to a power supply (not shown), the
pressure
transducer 56, monitors the matrix water potential in the porous medium where
the
tensiometer 20 is inserted. In accordance with the matrix water potential,
liquid is either
drawn into or rejected from the porous tip 30 and the pressure in the pores of
the
porous tip 30 varies accordingly. The pressure in the fluid chamber 52, which
is in fluid
communication with the porous material tip 30 also varies simultaneously and
accordingly. Therefore, the pressure transducer 56, located proximate to the
porous tip
30, compares the pressure of the liquid contained in the fluid chamber 52 to
the
atmospheric pressure, i.e. the transducer 56 measures the pressure caused by
the
water within the fluid chamber 52 as a function of atmospheric pressure.
The data monitored are transferred to the electronic circuit board 72 which
displays,
transmits and/or records the data.
The fluid chamber 52 can include a sensor (not shown) to indicate when the
fluid
chamber 52 needs to be refilled. The tensiometer 20 can send a signal, either
through
the antenna 34 or through the data logger, connected to the tensiometer 20, to
indicate
that the tensiometer 20 needs to be refilled. The head 32 can include a GPS or
any
other positioning system known to one skilled in the art to facilitate the
user/farmer to
localize the tensiometer 20, which needs to be refilled, in the field. It is
also appreciated
that the tensiometer 20 can include other warning functions, such as a battery
charge
level, as described in more details below.
It will be appreciated that the filling / re-filling mechanism can be used
with
tensiometers 20 having the pressure transducer 56 located externally of the
fluid
chamber 52 or located higher in the fluid chamber 52. However, it is
advantageous to
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position the transducer 56 proximate to the porous tip 30 since the transducer
56 is kept
below the liquid level in the fluid chamber 52 and in fluid contact with both
the liquid and
the ambient atmosphere. The tensiometer 56 therefore does not require pressure
measurement corrections since liquid level variations within the fluid chamber
52 does
not affect the monitored pressure measurement. The tensiometer 56 monitors
pressure
changes relative to atmospheric pressure and is independent of changes in
liquid level
within the tensiometer 20. Typically, the fluid chamber 52 needs to be
refilled when the
liquid level is at the level of the upper end of the tensiometer 20.
In the tensiometer 20, the transducer 56 is below the various water levels
such that
the pressure measurement side of the immersed transducer 56 is open to the
water
contained with the fluid chamber 52 while the other end of the transducer 56,
i.e. the
reference port 63, is vented to the atmosphere. An absolute pressure
transducer may
also be substituted for the transducer described above.
Referring now to Figs. 5 to 9, another embodiment of the tensiometer apparatus
20
will be described wherein the features are numbered with reference numerals in
the 100
series which correspond with the reference numerals of the previous
embodiment. As
shown in Fig. 5, the tensiometer 120 is self-priming and is similar to the
tensiometer 20.
The tubular housing 124 has two apertures extending therethrough, i.e. an
atmospheric
gas aperture 136 and a gas outlet aperture 138. Contrary to the tensiometer
20, it does
not include a liquid inlet aperture 36 since the chamber 152 is not filled
with a liquid, as
will be described in more details below.
Referring simultaneously to Figs. 6 to 9, it will be seen the internal
components of
the tensiometer 120. The porous material tip 130 is mounted to the lower end
126 of the
tubular housing 124. The porous material tip 130 can be divided in two
sections: a lower
section 130a (Fig. 9) in contact with the porous medium when the tensiometer
120 is
inserted therein and an upper section 130b (Fig. 9) which is covered by the
housing
124. The lower end 126 of the housing 124 is sealed to the outer surface of
the porous
tip 130 to prevent liquid contained in the porous medium from infiltrating the
porous tip
130 therebetween. Therefore, liquid can infiltrate the porous tip 130 through
the lower
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section 130a and flow upwardly into the upper section 130b but cannot
infiltrate the
porous tip 130 directly through the upper section 130b.
The housing 174 has a peripheral wall 150 which defines a chamber 152. The
chamber 152 is isolated from the porous tip 130 with an inner wall 148.
Therefore, the
chamber 152 is not in fluid communication with the porous tip 130 as in the
previously
described embodiment. The inner wall 148 has two apertures 155, 157 therein.
The first
aperture 155 is designed to insert therein a portion of the pressure
transducer 156, i.e.
an insertion member 159, while the second aperture 157 is designed to insert
therein a
gas exit valve 158, the purpose of which will be described in more details
below.
The insertion member 159 of the pressure transducer 156 extends through the
aperture 155 defined in the inner wall 148 and into the porous tip 130.
Therefore, the
insertion member 159 of the transducer 156 is in fluid communication with the
porous tip
130. The upper portion of the transducer 156 extends in the chamber 152.
The pressure of the liquid contained in the pores of the porous material tip
130 and
sampled through the insertion member 159 is compared by the pressure
transducer 156
to the atmospheric pressure. Therefore, the pressure transducer 156 is in
fluid
communication with the atmosphere through an atmospheric gas channel 160 which
extends in the chamber 152. The atmospheric gas channel 160 has a first port
162
connected to the reference port 163 of the pressure transducer 156 and a
second port
164 connected to a plug 174 as will be described in more details below.
An electric wire channel 166 also extends longitudinally in the chamber 152.
As in
the atmospheric gas channel 160, the electric wire channel 166 has a first end
168
connected to the pressure transducer 156 and a second end 170 connected to an
electric circuit board 172 located in the head 132 of the tensiometer
apparatus 120. The
electric wire channel 166 contains electric wires (not shown) in which the
data acquired
or monitored by the pressure transducer 156 are transferred to the electronic
circuit
board 172.
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The gas exit valve 158 is in fluid communication with the atmosphere through a
gas
outlet channel 184 which extends in the chamber 152. The gas outlet channel
184 has a
first port 186 connected to the gas exit valve 158 and a second port 188
connected to
the plug 174 as will be described in more details below. The gas exit valve
158 is
movable between a closed position preventing fluid communication between the
porous
tip material 130 and the gas outlet channel 184, and an open position allowing
gas
contained in the pores of the porous material tip 130 to exit therethrough and
into the
gas outlet channel 184, as will be described in more details below.
A wire 185 extends longitudinally in the chamber 152, between the gas exit
valve
158 and the head 132, to provide power to the valve 158.
The plug 174 is inserted in the upper portion of the chamber 152, proximate to
the
upper end 128 of the tubular housing 124. As for the plug 74, the plug 174 can
be either
built in with the housing 124, can be secured to the peripheral wall 150 or
can be
inserted in the chamber 152 without being mounted to the peripheral wall 150.
An atmospheric gas channel 190 is defined in the plug 174. The atmospheric gas
channel 190 has a first port 192 which is in fluid communication with the
atmospheric
gas aperture 136, defined in the peripheral wall 150, when the plug 174 is
inserted in
the chamber 152. The atmospheric gas channel 190 has a second port 194 which
extends in the chamber 152 and is connected to the second port 164 of the
atmospheric
gas channel 160. These two ports 162, 194 can be sealed together to prevent
gas
exchanges between the continuous channel defined by the atmospheric gas
channels
160, 190.
A gas outlet channel 196 is also defined in the plug 174. The gas outlet
channel 196
has a first port 198 which is in fluid communication with the gas outlet
aperture 138,
defined in the peripheral wall 150, when the plug 174 is inserted in the
chamber 152.
The gas outlet channel 190 has a second port (not shown) which extends in the
chamber 152 and is connected to the second port 188 of the gas outlet channel
184.
These second ports 188, (not shown) of the gas outlet channel 184 and the gas
outlet
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channel 190 can be sealed together to prevent gas exchanges between the
continuous
channel defined by the gas outlet channels 184, 196.
As for the previously described embodiment, the data acquired by the pressure
sensor or pressure transducer 156 are transmitted through electric wires
located in the
electric wire channel 166 to the electronic circuit board 172 located in the
head 132. The
pressure monitored by the transducer 156 is displayed on the electronic dial
and/or
transmitted to a data logger which records the data transmitted from the
tensiometer
120. The data can be transferred with wireless technology with the antenna 134
or the
tensiometer 120 can be physically connected to a data logger through the
connector
140.
To measure matrix water potential in a porous medium, the tensiometer 120 is
first
inserted in the porous medium. The tensiometer 120 is self-priming. Therefore,
it does
need to be filled or refilled with water to measure matrix water potential.
Once connected to a power supply, the transducer 156 monitors the matrix water
potential in the porous medium where the tensiometer 120 is inserted. When
inserted in
the porous medium, the pores of the porous material tip 130 are filled with
gas. Above a
threshold value (or critical suction) of matrix water potential, the pores of
the porous
material tip 130 fill with water, drawn from the porous medium. When the pores
draw
water from the porous medium, the gas exit valve 158 opens to allow the gas,
previously contained therein, to exit therethough and flow outwardly of the
tensiometer
120. The threshold value of matrix water potential is characteristic of the
properties of
the porous material and, more particularly, the pore size. Once the pores
filled with
water, the gas exit valve 158 closes.
When the matrix water potential in the porous medium increases, the pressure
in the
pores varies accordingly. Therefore, the transducer 156 compares the pressure
in the
pores, sampled through the insertion member 159, to the atmospheric gas
pressure,
provided by the atmospheric gas channels 160, 190 in fluid communication with
the
atmospheric gas aperture 136.
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If the matrix water potential falls below the threshold value, water contained
in the
pores flows into the porous medium and gas, also provided by the porous
medium, fills
the pores. Once again, if the matrix water potential of the porous medium
rises above
the threshold value, the pores re-fill with water, drawing the latter from the
porous
medium and the valve 158 opens to allow gas exit.
Therefore, when the matrix water potential rises above the threshold value,
gas,
previously contained in the pores, must escape to be replaced by water. The
gas
escapes through the gas exit valve 158 which moves between the closed position
into
the open position. In the open position of the gas exit valve 158, the gas
flows upwardly
into the valve 158, the gas outlet channels 184, 196, through the gas outlet
aperture
138 and into the atmosphere.
Referring now to Fig. 11, another embodiment of the self-priming tensiometer
apparatus 120 will be described wherein the features are numbered with
reference
numerals in the 200 series which correspond with the reference numerals of the
previous embodiment. Instead of having both the gas exit valve 158 and the
pressure
transducer 156 directly in fluid communication with the porous tip 130, in the
tensiometer 220, the pressure transducer 256 is in fluid communication with
the porous
tip 230 through the gas exit valve 258. Moveover, the chamber 252 does not
include a
plug 174 inserted therein. The chamber 252 is free of atmospheric gas channel
160 and
gas outlet channel 184. The peripheral wall 250 can include an an atmospheric
gas
aperture 236 extending therethrough and in fluid communication with the
chamber 252.
Therefore, the gas exit valve 258 is directly in fluid communication with the
chamber
252 for releasing gas therein and the reference port 263 of the pressure
transducer 256
is directly in fluid communication with the chamber 252 to measure the
atmospheric
pressure.
The chamber 252 is isolated from the porous tip 230 with the inner wall 248.
The
inner wall 248 has one apertures 257 therein which is designed to insert
therein a
portion of the gas exit valve 258, i.e. an insertion member 259.
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The insertion member 259 of the gas exit valve 258 extends through the
aperture
257 defined in the inner wall 248 up to the porous tip 230. Therefore, the
insertion
member 259 of the valve 258 is in fluid communication with the porous tip 230.
The
upper portion of the valve 258 extends in the chamber 252.
The liquid sampled in the pores of the porous material tip 230 flows into the
valve
258 towards the transducer 256. The pressure transducer 256 and the gas exit
valve
258 are in fluid communication through a liquid port 276 of the valve 258 and
a liquid
port 258 of the transducer 256.
As for the above described tensiometer 120, the pressure of the liquid
contained in
the pores of the porous material tip 230 and sampled through the insertion
member 259
is compared by the pressure transducer 256 to the atmospheric pressure.
Therefore,
the pressure transducer 256 is in fluid communication with the atmosphere
through the
reference port 263. The reference port 263 is in fluid communication with the
chamber
252 which is in fluid communication with the atmosphere through the
atmospheric gas
aperture 236 defined in the peripheral wall 250.
An electric wire channel 266 also extends longitudinally in the chamber 252.
The
electric wire channel 266 has a first end 268 connected to the pressure
transducer 256
and a second end 270 connected to an electric circuit board 272 located in the
head
232 of the tensiometer apparatus 220. The electric wire channel 266 contains
electric
wires (not shown) in which the data acquired or monitored by the pressure
transducer
256 are transferred to the electronic circuit board 272.
The gas exit valve 258 is in fluid communication with the atmosphere through a
gas
outlet port 286 which extends in the chamber 252 and is in fluid communication
therewith. As mentioned above, the fluid chamber 252 is in fluid communication
with the
atmosphere through the atmospheric gas aperture 236 defined in the peripheral
wall
250. As for the valve 158, the gas exit valve 258 is movable between a closed
position
preventing gas contained in the porous tip material 130 to flow into the
chamber 252,
and an open position allowing gas contained in the pores of the porous
material tip 130
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CA 02556126 2006-08-15
to exit therethrough and into the chamber 252. A wire 285 extends
longitudinally in the
chamber 252, between the gas exit valve 258 and the head 232, to provide power
to the
valve 258.
As for the previously described embodiments, the data acquired by the pressure
sensor or pressure transducer 256 are transmitted through electric wires
located in the
electric wire channel 266 to the electronic circuit board 272 located in the
head 232.
To measure matrix water potential in a porous medium, the tensiometer 220 is
first
inserted in the porous medium. The tensiometer 220 is self-priming. Therefore,
it does
need to be filled or refilled with water to measure matrix water potential and
its operation
is similar to tensiometer 120.
As mentioned above, the critical suction of the porous material tip 130
depends on
the characteristics of the porous material constituting the tip 130 and, more
particularly,
the pore size. Referring to Fig. 11, it will be seen two typical curves
representing the
suction (or the matrix water potential) as a function of the water contained
in the porous
material tip 130. The pores are substantially empty above a critical suction,
or threshold
value. Once the critical value reached, the pores of the porous material tip
130 fills with
water until saturation is reached. Even for lower suction value, the water
contained in
the pores does not increase since water is hardly compressible. In an
embodiment, the
curve has a step shape (full line), i.e. the pores fill at the critical
suction value while, in
another embodiment, the pores fill over a threshold suction range surrounding
the
threshold suction value (dashed line).
Therefore, the pores of the porous material tip 130 auto-fills with liquid
when inserted
in the porous medium and the porous medium is characterized by a liquid
potential at
least equal to the threshold suction range (in absolute value). Once the pores
filled with
liquid, the pressure of the liquid inside the pores is representative of the
liquid potential
of the porous medium.
It is appreciated that the valve can be replaced by any one-way fluid control
device
adapted to be in fluid communication with the porous material tip and allowing
fluid
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CA 02556126 2006-08-15
contained in pores of the porous material and, more particularly, gases to
exit
therethrough when the porous material tip auto-fills with liquid.
As for the previously described embodiment, the data monitored by the
transducer
156 are transferred to the electronic circuit board 172 which displays,
transmits and/or
records the data.
The tensiometers 20, 120 can be calibrated at the factory, i.e. the readings,
in
milivolts, obtained from the pressure transducer 56, 156 are automatically
converted
into cbar or kPa readings by electronics, which prevents the need to run
conversion.
As shown in Fig. 12, when the tensiometer 20, 120 drifts (line b), the
calibration
curve between the milivolt output and the water potential moves parallel to
the
calibrated curve, upward or downward. To recalibrate the tensiometers 20, 120,
the
tensiometers 20, 120 are removed from the porous medium and the porous
material tip
30, 130 is immersed in water. When immersed in water, the matrix water
potential or
pressure read by the tensiometer 20, 120 should be void (line a). If the
measured value
is non-void, the electronic circuit board 72, 172 is recalibrated to obtain a
void value.
The porous material tip 30, 130 can have an hollow space, or a depression,
therein
to obtain faster kinetics, or faster time constant.
The embodiments of the invention described above are intended to be exemplary
only.
It will be appreciated that a plurality of tensiometers 20, 120 can be
distributed all
over the field, the greenhouse or the nursery and are connected to a central
station (not
shown). For example, the tensiometers can be connected to the central station
using
radio frequency but they could also be connected by wire or using any other
wireless
technology such as cell phone technology, satellite telecommunications or an
Internet
connection using, for example, a cell phone or a device such as a BlackBerry
to
connect to the Internet. Each tensiometer 20, 120 can be adapted to repeatedly
transmit
the sensed data and to also transmit self-check data, such as the battery
charge level.
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CA 02556126 2006-08-15
The self-check data can be simply Boolean data stating a low battery charge or
a low
water level. The water level can be not directly monitored but can be inferred
from the
sensed data received at the central station. When the water tension reaches a
given
value that is out of the range of the tensiometer 20, the central station
determines that
the tensiometer 20 needs to be refilled. Alternatively, the water level could
be directly
read by a sensor and transmitted to the central station. The remote check of
the
tensiometers 20, 120 is thus provided either using a self-check data
transmission, by
signal processing of the sensed data at the central station or using a
combination of the
latter two. Thanks to this feature, the grower is not required to routinely do
a round
check of the tensiometers 20, 120 in the field but can rely on data received
at the
central station to plan the maintenance of the tensiometers 20, 120. He will
thus only
have to go in the field to look for the tensiometers 20, 120 when maintenance
is actually
required and he will only have to look for the specific tensiometers 20, 120
that requires
maintenance.
In order to assist the grower in locating the tensiometers 20, 120 in the
field or in the
greenhouse, each tensiometers 20, 120 can additionally include a global
positioning
system (GPS), or any other appropriate positioning system, that provides the
position of
the tensiometers 20, 120 in real time. The tensiometer coordinates are
transmitted to
central station where the grower can read the exact position of each
tensiometer 20,
120 in the field. In one aspect, it allows him to easily locate the
tensiometers 20, 120 to
be maintained. In a second aspect, the exact position of each tensiometer 20,
120 is
used by the field monitoring software to provide a very accurate map of the
soil
condition which is very useful in water management and hydrozoning, i.e.
providing
specific irrigation for each group of plant. Installation of the tensiometer
system in the
field is also facilitated as the installer does not need to note the position
of each
tensiometer 20, 120 as it is installed in the field in order to allocate a
field zone to each
tensiometer 20, 120.
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CA 02556126 2006-08-15
In alternative embodiments, it will be appreciated that the peripheral wall
50, 150 can
either be made of a transparent or opaque material. Opaque tubes provide the
advantage of having no algae growth within the tensiometer 20.
The transducer 56, or a portion of the transducer 156, reference ports 63,
163,
sections of the atmospheric gas channels 60, 160 and the electric wire channel
66, 166,
the valve 158, and/or the gas outlet channel 184 can be embedded (or
encapsulated) in
a substantially solidified material, such as epoxy, (or rigid tubing) to
stabilize and protect
these components within the tensiometer 20, 120.
The atmospheric gas channels 60, 160, the electric wire channel 66, 166, and
the
gas outlet channel 184 can be made either of flexible, semi-rigid or rigid
tubing.
It will be appreciated that the shape of the tensiometer 20, 120 can vary,
that the
power supply can be integrated within the tensiometer 20, 120. The fluid
chamber 52,
and the chamber 152 can be shorter or longer in length than that of the
tubular housing
24, 124 respectively. The position of the apertures 36, 38, 136 and 138 in the
peripheral
wall 50, 150 can be located elsewhere in the tensiometer body 22, 122. The
electric
wires can extend directly from the pressure transducer 56, 156 to the
electronic circuit
board 72, 172, i.e. not in the wire channel 66, 166.
It will also be appreciated that the electronic circuit board 72, 172 can be
located
elsewhere in the housing 24, 124. Several types of pressure transducers 56,
156, which
can or cannot compensate for temperature variations, can be used. For example,
a
piezoresistive pressure transducer can be used. Moreover, it is not necessary
for the
pressure transducer to be a comparative pressure transducer, i.e. wherein the
pressure
transducer is in fluid communication with atmospheric gas. In this embodiment,
the
design of the tensiometer 20, 120 can be modified accordingly.
The electronic circuit board 72, 172 can be replaced by any electronic or
mechanical
device which can process pressure data.
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CA 02556126 2006-08-15
The plug 74, 174 can be one piece with the housing 24, 124 and/or the head 32,
132. The shape of the fluid channel 76, the gas outlet channel 84, 184 and the
atmospheric gas channel 90, 190 can vary from the ones shown in the above-
described
embodiment.
The first port 86 of the gas outlet channel 84 can merge at a different
location with
the fluid channel 76 provided that gas can escape without being obstructed
when filling
the fluid chamber 52 through the fluid channel 76. For example, in an
alternative
embodiment, the plug 74 cannot include a gas outlet channel 84 which is
distinct from
the fluid channel 76 to allow simultaneously escape of the gas contained in
the fluid
chamber 52 while filling the latter with liquid. The diameter of the fluid
channel 76 can
be larger than the diameter of the injector used to fill the fluid chamber 52
with liquid.
For filling the fluid chamber 52, the tensiometer 20 can be inserted in the
porous
medium or withdrawn therefrom.
It will also be appreciated that, in the tensiometer 120, the housing 124
cannot
include a chamber 152 and the gas outlet channel 184, connected to the valve
158, can
communicate directly with the atmosphere.
The scope of the invention is therefore intended to be limited solely by the
scope of
the appended claims.
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