This application is a division of application Ser. No. 08/376,165, filed Jan. 19, 1995 now U.S. Pat. No. 5,644,947.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention disclosed under contract number DE-AC07-76ID01570 between the U.S. Department of Energy and EG&G Idaho, Inc., now contract number DE-AC07-94ID13223 with Lockheed Idaho Technologies Company.

TECHNICAL FIELD

This invention relates to tensiometers and to techniques for measuring soil moisture potential using tensiometers.

BACKGROUND OF THE INVENTION

If moisture potential of soil can be accurately monitored, irrigation can be controlled to optimize the rate of plant growth. One type of instrument for measuring soil moisture potential is a tensiometer. A conventional tensiometer comprises a sealed tube defining a chamber which is normally completely filled with water, a hollow porous tip on one end of the tube, and a vacuum gauge connected to the water chamber. The porous tip is inserted in the soil, and establishes liquid contact between the water in the tube and moisture in the soil surrounding the tip. Relatively dry soil tends to pull water from the tube through the porous tip. However since the tube is sealed, only a minute amount of water is actually withdrawn. Accordingly, the water in the tube is placed under tension by the pulling effect of the dry soil, thus creating a measurable subatmospheric pressure in the tube. Higher moisture contents in the soil produce correspondingly less vacuum in the tube, and completely saturated soil registers substantially zero vacuum or atmospheric pressure.

Typical tensiometer constructions provide a tube or column of water which extends from the porous tip to above grade. It will be apparent that the deeper the porous tip is buried, the longer the column of liquid above it will become.

Air presence in the water reservoir during tensiometric measurement is undesirable. Air can enter the reservoir by diffusing through the porous tip. More commonly, dissolved air present in the water that enters the vessel comes out of solution in the reduced pressure environment of the tensiometer. Eventually, the entire tensiometer would become filled with air. This air will increase the time required to reach pressure equilibrium because large volumes of water must move through the porous tip to effect the mass transfer of air through the tip. Thus in order to obtain accurate readings, the water and air are desirably purged periodically from the tensiometer reservoir and replaced with degassed water.

To facilitate purging of air from the tensiometer reservoir, a conventional tensiometer is typically provided with a column of water connecting a surface located pressure measuring device to the soil-embedded porous tip. However, there is a physical limit to the length of a column of water which can be supported by atmospheric pressure (about 1000 cm at sea level), and the useful measurement range of the tensiometer is reduced as the column of water above the porous tip is lengthened. The pressure exerted by the column of water increases the pressure in the porous tip, which in turn increases the apparent soil moisture tension recorded by the above-surface pressure measuring device.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below with reference to the following accompanying drawings.

FIG. 1

is a diagrammatic side elevational view of a sidewall tensiometer in accordance with the invention.

FIG. 2

is a side elevational view corresponding to that of

FIG. 1

, but for a 90° rotation of the tensiometer.

FIG. 3

is a longitudinal sectional view of an alternate embodiment of a sidewall tensiometer device in accordance with the invention as positioned within a borehole for tensiometric measurement.

FIG. 4

is a sectional view taken through line

4

—

4

of FIG.

3

.

FIG. 5

is a longitudinal sectional view of an alternate embodiment sidewall tensiometer apparatus in accordance with the invention.

FIG. 6

is a longitudinal sectional view of another alternate embodiment sidewall tensiometer apparatus in accordance with the invention.

FIG. 7

is a diagrammatic side elevational view of another alternate embodiment sidewall tensiometer apparatus in accordance with the invention.

FIG. 8

is a side sectional view the

FIG. 6

tensiometer apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).

In accordance with one aspect of the invention, a sidewall tensiometer to in situ determine below-grade soil moisture potential of earthen soil comprises:

a body adapted for insertion into an opening in earthen soil below grade, the body having lateral sidewalls;

a laterally oriented porous material provided relative to the body lateral sidewalls, the laterally oriented porous material at least in part defining a fluid chamber within the body;

a pressure sensor in fluid communication with the fluid chamber; and

sidewall engaging means for engaging a portion of a sidewall of an earth opening to laterally urge the porous material into hydraulic communication with earthen soil of another portion of the opening sidewall.

In accordance with another aspect of the invention, a method of monitoring soil moisture potential in below-grade earthen soil comprises the following steps:

inserting a tensiometer into an earthen opening below grade in earthen soil; the tensiometer having a porous material, a fluid chamber in fluid communication with the porous material, and a degassed liquid within the fluid chamber

laterally urging the porous material against a sidewall of the earthen opening to effectively establish hydraulic communication between the fluid chamber and the earthen material;

permitting the degassed liquid to permeate the porous material to cause a change in pressure in the fluid chamber; and

determining the change in pressure within the chamber.

The sidewall tensiometer and method to in situ determine below-grade soil moisture potential is described with reference to

FIGS. 1 and 2

. Tensiometer apparatus

10

includes a body

12

which is adapted for insertion into a bore in earthen soil below grade. Body

12

is elongated and has surrounding lateral sidewalls

13

. Body

12

is substantially hollow defining an internal fluid chamber

26

. An arcuate or curved laterally-oriented, porous material

24

is provided relative to one of body lateral sidewalls

13

, and in part defines a boundary of fluid chamber

26

within body

12

. A pressure transducer

30

is provided externally of housing

12

, and communicates with chamber

26

via a conduit

59

. Accordingly in the illustrated embodiment, pressure transducer

30

is mounted externally of housing or body

12

. An example transducer is Model ST2P15G1, having a range of from +15 to −15 psig, sold by SenSym of Milpitas, Calif. It could of course also be directly connected to sidewalls

13

, or retained internally relative to housing

26

. A fill tube

61

extends outwardly of housing

12

, and communicates with fluid chamber

26

. A sealing cap

62

is provided to seal fluid chamber

26

. An electric lead

34

would extend from transducer

30

to the surface. Component

65

diagrammatically illustrates either a flexible line or a rigid rod for utilization in raising or lowering housing

12

relative to a borehole in which the apparatus will be utilized for tensiometric measurements. Alternately, tensiometer apparatus

10

can be raised and lowered using the electrical leads.

To utilize such a device, fluid chamber

26

would be filled with a degassed liquid via fill tube

61

. Thereafter, body

12

would be inserted into a bore or trench below grade in earthen soil. Porous member

24

would be laterally urged against a sidewall of the earthen bore to effectively establish hydraulic communication between fluid chamber

26

and earthen material. Degassed liquid would permeate the porous material to cause a change in pressure in fluid chamber

26

, which would be monitored by pressure transducer

30

. Such a construction method provides an advantage of obtaining tensiometric measurements via sidewall bore access as opposed to hydraulic access within the bore at the base.

FIGS. 3 and 4

illustrate a reduction-to-practice tensiometer apparatus

10

a

in accordance with an aspect of the invention. Like numerals form

FIGS. 1 and 2

are utilized with distinctions and construction being indicated by different numerals or a suffix “a”. Body

12

a

is configured with a bottom opening

67

(

FIG. 4

) which is sealable by means of a rubber stopper

69

. Such opening is utilizable to fill fluid chamber

26

a

with degassed fluid. Porous material

24

a

in one of body sidewalls

13

a

preferably has an arcuate periphery, as shown, corresponding in male size and shape to a female arcuate periphery size and shape of the size of bore

16

for which the apparatus is primarily adapted.

An inflatable bladder

70

is provided laterally of porous material

24

a

against one of lateral sidewalls

13

a

of housing

12

a

. Such is preferably adhered by an adhesive or other means to the outer portion of sidewall

13

a

. An inflation/deflation hose

71

extends outwardly of bladder

70

to an above-grade location.

When positioning apparatus

10

a

within bore

16

, bladder

70

would be initially deflated and the apparatus then lowered to a desired depth within the soil. Thereafter, bladder

70

would be inflated with a fluid (either liquid or gas, or a combination thereof). Such causes the bladder to engage a portion of a sidewall of bore

16

to laterally urge porous material

26

a

into hydraulic communication with earthen soil of another portion of the bore sidewall, as shown. Tensiometric measurements are then determined after equilibrium is reached, as described above.

Alternate mechanisms might also be utilized for urging porous material

24

a

against the sidewall of a bore.

FIG. 5

, by way of example only, illustrates one such alternate construction

10

b

. Such illustrates in diagrammatic form a piston and cylinder assembly

72

which is positioned laterally of porous tip

24

b

for expanding the lateral expanse of the apparatus for urging porous material

24

b

against the sidewall of the bore.

Another alternate embodiment

10

c

is diagrammatically shown in FIG.

6

. Such diagrammatically comprises a plurality of sensors adhered to one side of a common inflatable bladder

70

c

at different elevations. This would enable moisture potential to be measured at different elevations within the soil. The sensors could be adhered to bladder

70

c

by velcro.

The above sidewall tensiometric measuring apparatus might of course also be constructed without an attached pressure sensing member, and instead use an accessible septum. For example, cap

62

(

FIGS. 1 and 2

) could comprise a septum.

Yet another alternate embodiment tensiometer apparatus

10

d

is illustrated in

FIGS. 7 and 8

. In this embodiment, a thin porous plastic sheet

24

d

is combined with a non-porous acrylic or PVC backing sheet

77

. An example and preferred material for sheet

24

d

is wettable porous plastic (A-20 or A-40) manufactured by FMC. Backing sheet

77

is configured to provide a void

78

within the device, which is completely filled with a porous material

83

. An example and preferred material is a plastic or fiberglass screen, or a scrim material. Sheets

24

d

and

78

are adhered to one another in fluid-tight communication via perimeter adhesive

79

. A passageway

80

extends from the rear of non-pervious backing

77

to void

78

, which is filled with screen

83

. A PVC fitting

82

having opening

80

extending therethrough is provided against backing plate

77

, and communicates with a conduit

84

. Conduit

84

branches to fluid communicate with a transducer

30

d

, and a fill conduit

86

.

Suitable bore sidewall engaging means such as an inflatable bladder would also be associated with the device, as described above. Such would be inflated once the device were inserted within a borehole to urge or push material

24

d

outwardly against the bore sidewall. The described materials are sufficiently flexible to enable the apparatus to curve or bend to conform to the general arcuate sidewall shape to provide intimate contact with the sidewall. Thus, this embodiment provides flexible, lateral sidewalls having porous material received therein which in this embodiment substantially fills the void, and is capable of flexibly conforming to the internal sidewalls surfaces.

In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.