Abstract:
A pressure self-compensating tensiometer and method to in situ determine below grade soil moisture potential of earthen soil independent of changes in the volume of water contained within the tensiometer chamber, comprising a body having first and second ends, a porous material defining the first body end, a liquid within the body, a transducer housing submerged in the liquid such that a transducer sensor within the housing is kept below the working fluid level in the tensiometer and in fluid contact with the liquid and the ambient atmosphere.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     This invention was made with United States Government support under Contract No. DE-AC07-99ID13727 awarded by the United States Department of Energy. The United States Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to tensiometers for measuring soil water potential and more specifically to a tensiometer that self-compensates to changing water levels and pressures in the tensiometer reservoir, thereby permitting longer useful operating periods. 
     The use of tensiometers for monitoring moisture potential in soil is known. For example, our invention claimed in U.S. Pat. Nos. 5,644,947 and 5,915,476 (which are hereby incorporated by reference) describe portable tensiometers for monitoring moisture potential in soil. As can be seen in FIG. 2 of the &#39;947, the tensiometer has a transducer  30  mounted externally to, and in fluid communication with, a fluid chamber  26 . Because the transducer of &#39;947 is mounted externally, a disadvantage of the &#39;947 patent is that the pressure measured by the transducer is subject to changes in both the water level within the chamber as well as the changing air pressure of the headspace above the water level within the chamber. Also, the amount of water that can be contained within the chamber of the &#39;947 invention is limited by the length of the column of water in the water chamber thereby restricting the operating period of the tensiometer. The invention disclosed &#39;947 has the pressure sensor on the top of the instrument to measure the soil water potential in the adjacent sediment. The pressure sensor is located on the top of the instrument for ease of connection and to reduce the overall size (diameter) since it is vertically oriented to be placed in a borehole. 
     It is an object of the present invention to provide a tensiometer that automatically corrects the pressure measurement so that decreasing water levels within the tensiometer do not affect the recorded pressure measurements. 
     It is another object of the present invention to provide a tensiometer capable of longer operation periods as a result of its increased water volume capacity. 
     It is still a further object of the present invention to provide a tensiometer that will record pressure changes relative to atmospheric pressure and is independent of changes in water level within the tensiometer. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and other objects, the present invention provides a self-compensating tensiometer that measures the total head in the system (i.e., the pressure head and the water level elevation head). In one preferred embodiment the self-compensating tensiometer comprises a body having a porous cap at one end and a chamber filled with liquid within the body. A pressure transducer housing contains a transducer sensor, the housing being immersed within the liquid. A first side of the transducer housing being in fluid communication with the liquid contained within the tensiometer housing. A second side of the transducer housing is vented through a reference port and vent tube to the atmosphere (either at the earth surface or the ambient atmosphere within the bore hole). In another embodiment of the invention, the transducer housing, reference port and portion of the reference vent tube are embedded in substantially solidified material (for example epoxy) to stabilize these components within the tensiometer. In this embodiment, a portion of the transducer housing may extend beyond the stabilizing material to provide fluid communication with the liquid within the tensiometer, or alternatively fluid communication means could be provided through an opening provided in the stabilizing material. A method for utilizing the self-compensating tensiometer is also claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated in the accompanying drawings where: 
     FIG. 1 is a diagrammatic representation of a tensiometer apparatus positioned within a borehole for measurement in accordance with the present invention. 
     FIG. 2 is a simplified diagrammatic view of a prior art tensiometer. 
     FIG. 3 is a simplified diagrammatic view of the tensiometer in accordance with the present invention. 
     FIG. 4 is an enlarged side sectional view of a portion of the tensiometer of the present inventions showing the transducer housing immersed below of the water level contained with the transducer chamber. 
     FIG. 5 is an enlarged side sectional view of a portion of the tensiometer of the present invention showing the transducer, wiring and vent tube encapsulated in solid material such as an epoxy. 
     FIG. 6 is a table showing water pressure as a function of time for the self-compensating tensiometer of the present invention, a conventional tensiometer and a control tensiometer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 a diagrammatic representation of a tensiometer apparatus  10  is shown positioned in a borehole. Tensiometer apparatus  10  comprises a body  12  and flexible suspension line  14 . Line  14  is utilized to raise and lower body  12  relative to a bore hole  16  provided within earthen soil  18 . A data logger  35  which records the data transmitted from the tensiometer apparatus is shown on the surface of the earth. The pressure sensor and wire that connects to the data logger is not shown in this figure. The artisan will recognize utility of the devices and methods disclosed herein with other earthen openings, such as trenches and exposed earthen faces. 
     A diagrammatic drawing of a conventional tensiometer, such as the tensiometer of &#39;947, is shown in FIG  2 . As can be seen in FIG. 2, the transducer  30  is mounted externally to the sealed body  12 . Air tight connectors  20  and  21  provide the means for sealing the body  12 . The tensiometer has a porous cup  24  and one end of the body and in physical contact with the soil. Various water levels are shown in FIG. 2 represented as  28   a ,  28   b , and  28   c . Headspace  25  is shown as the volume between the water level and within the sealed body. The pressure sensing portion of the instrument shown in FIG. 2 has a column of water in the device and over time air accumulates above the water in the chamber (trapped in the top of the device). The instrument is originally filled completely with water but air accumulates in the device in normal operation and the water level declines in this chamber over time. This decline in the water level affects the measurements obtained by a pressure sensor located above the water column. As can be seen from FIG. 2, the pressure measured by transducer sensor (not shown) contained within transducer housing  30  is a function of both the air pressure within the headspace  25  and the pressure due to the changing water level. An alternative tensiometer disclosed in &#39;947 contemplates mounting the transducer internally within the headspace of the body. (See &#39;947 column 4, lines 39-41). Such alternative mounting within the headspace would likewise suffer the disadvantage of measuring pressure as a function of both the air pressure and pressure changes due to changes in the water level within the reservoir. 
     Referring now to FIG. 3 a diagrammatic view of the self-compensating tensiometer of the present invention is shown. The self-compensating transducer comprises a body  12  having first and second ends, the first body end having a porous cup  24 . Various water levels are also shown in FIG. 3 represented as  28   a ,  28   b  and  28   c . In the self-compensating transducer the transducer housing is below the various water levels such that the pressure measurement side of the immersed transducer housing is open to the water contained with the body  12  while, as more fully described below, the other end of the transducer housing (reference port) is vented to the atmosphere or to the soil gas a short distance above the measurement depth (porous cup  24 ) of the tensiometer (See FIG.  4 ). A absolute pressure transducer may also be substituted for the transducer described above. By immersing the transducer housing in the water, the transducer measures the pressure within the body  12  independent of changes in the water level. To describe how this lowering of the water level effects measurements recorded by a pressure sensor it is assumed for the following example, that the soil water potential in the surrounding sediment does not change over time. In the instruments original water filled situation, and once the pressure in the instrument has equilibrated with the soil water potential in the device, the sensor is sensing the combination of the negative pressure from the hanging water column in the device and the water potential as reflected as a negative pressure, relative to atmospheric pressure. As the water level declines inside the instrument, allowing more air to enter, the total water potential pressure is the same but the hanging water column has declined. If the water level in the tensiometer declines, for example, the equivalent pressure of 20 cm of water the sensor would provide a measurement that is equivalent to the pressure in the soil with the addition of the equivalent pressure of 20 cm of water. Thus, if the water potential was −100 cm pressure, the apparent pressure from the sensor would be −80 cm. Since the rate of accumulation of air into the device cannot be predicted in advance of operation of the instrument, the change in measurements from actual readings cannot be predicted. It then follows that increasing the water filled length of the device and allowing the instrument to operate (accumulating air) for extended time period will introduce more error into the measurements. 
     The pressure sensor in the self-compensating tensiometer is located beneath the operational water level of the tensiometer. The sensor then senses the negative pressure from the hanging water column (which does not change), and the combination of the pressure of the water and the pressure of the air above sensor. If the water potential is constant, as the water level in the device changes the air pressure changes to reflect this pressure change. As long as the measurement depth of the sensor is covered by water, the sensor will obtain measurements reflective of the true water potential in the adjacent material. 
     FIG. 4 shows an enlarged sectional view of the self-compensating transducer of the present invention. The self-compensating transducer comprises a body  12  having first and second ends, the end being a porous cup  24 . Within the body  12  and proximate to the first body end is the transducer housing  32 . The transducer housing  32  is contained within a sealed nose cone  38  which is positioned below the water level  28 . Preferably the entire nose cone  38  is filled with epoxy to encapsulate and protect the transducer housing and connections. Alternately as described below with reference to FIG. 5, the nose cone can be omitted, the transducer apparatus  32 ,  34 ,  36 , and  32  placed in the rigid tubing  40  and the transducer apparatus encapsulated with epoxy with the port  33  in fluid communication with the fluid of the tensiometer. The transducer sensor (not shown) is contained within the transducer housing  32 . The nose cone is attached to a rigid tubing  40 . Preferably rigid tubing  40  is PVC tubing, however other types of impermeable tubing could also be utilized (i.e. stainless steel, brass, plastic). The transducer housing  32  has a first end  33  that is in fluid contact with the degassed water  26 . Wire leads  42  which extend from the transducer sensor/housing to the data logger  35  (in FIG. 1) are also shown. 
     Attached to the transducer housing  32  second end is a reference port  34  that further connects to a reference vent tube  36 . Reference vent tube  36  is a small diameter tubing that can extend beyond the rigid tubing  40  and into the tubing that extends to land surface  46 . Tubing  46  can be flexible semi-rigid tubing or rigid tubing and attached to rigid tube  40  by several methods such as band  48  shown in FIG.  4 . Its purpose is to raise and lower the pressure sensor and allow downward pressure to be applied to the sensor to seal the device. Reference vent tube  36  is attached to the reference port  34  via heat shrink tubing or other sealable means. The reference vent tube  36  vents the second end of the transducer sensor to the atmosphere. 
     Accordingly the transducer measures the pressure caused by the water within the body as a function of atmospheric pressure. Since the self-compensating tensiometer measurements are not affected by the height (length) of the water reservoir, the reservoir can be lengthened to allow more water to be contained within the tensiometer reservoir. In other words, the volume of water used can be increased without increasing the diameter of the tensiometer. Increasing the diameter of the tensiometer is to be avoided because of limited bore hole diameter and the potential of having other instruments in the bore hole. A larger water volume correlates to a longer operation time between refilling with water, thereby significantly increasing the time between maintenance. It is estimated the self-compensating tensiometer can hold enough water to permit an operational time period of over one year. 
     FIG. 5 shows an enlarged sectional view of the self-compensating transducer of the present invention. In this figure the transducer apparatus  32 ,  34 , and  36  are placed in the rigid tubing  40  and the transducer apparatus encapsulated within a solidified material, such as epoxy, with the port  33  in fluid communication with the fluid of the tensiometer. 
     FIG. 6 shows the water level response from three portable advanced tensiometers where one is a control, the second is a standard tensiometer and the third is a self-compensating portable tensiometer. The instruments have been placed in a beaker with a known depth of water and all three of the instruments referenced to read about 8 cm of water pressure in the data logger. Instrument (A) is a control with the water level in the instrument staying steady over the entire test. This instrument shows the normal variations of measurements over time for the pressure sensors used in the test. The self compensating tensiometer (B), shows the response from opening the tensiometer at time zero and lowering the water level inside the instrument about 15 cm, sealing the chamber (resulting in an increase in pressure in the apparent reading) and placing the tensiometer back in the beaker of water with the same depth of water. The sensor returns to the original reading plus or minus 1 cm of water, within 20 minutes of the perturbation of pressure, showing that the measurement is not affected by changes in the water level inside of the tensiometer. This indicates that the device works as intended and that the measurement is not affected by the water level inside the tensiometer. The third instrument (C) shows the response of a standard portable tensiometer with the pressure sensor located above the water reservoir (as shown in FIG.  2 ). As with the self-compensating tensiometer, the instrument is opened, the water level in the instrument lowered about 15 cm and the instrument resealed and placed in the beaker of water at the original depth. The water pressure inside the tensiometer builds up due to the sealing of the tensiometer and then the pressure decreases. The pressure does not approach the original measurement (8 cm of water) but rather is 15 cm higher when the measurements equilibrate. This is because the length of the hanging water column in the tensiometer is 15 cm lower and so this hanging water column does not exert this downward (negative) pressure on the sensor. Thus, a change in the water level within a standard tensiometer with at transducer above the water level will change the readings from the pressure sensor in the tensiometer. Since tensiometers lose water over time so that the water level in the instrument change, this affects the overall accuracy of the instrument. Removing this source of error in the measurement improves the accuracy of the instrument. 
     As can be seen in FIG. 6, after water is removed from the self-compensating tensiometer reservoir the water pressure of the self-compensating tensiometer return to the control level of an tensiometer having constant water pressure. The upward spike in the data is a response of sealing the instrument, thereby temporarily increasing the pressure in the chamber. However, it has been observed that a conventional tensiometer would remain at an elevated water pressure after water has been removed from the reservoir. Accordingly, for a conventional tensiometer the reading would have to be corrected as the water level declines in the tensiometer during normal use and since the water level decline may not be predictable or linear this detrimentally affects the accuracy of the tensiometer pressure readings.