Probe for in situ monitoring the electrical conductivity of soil solutions

Abstract

A probe for monitoring electrical conductivity of ground water in soil is provided. The probe includes a hollow tube that includes (a) an upper tube section with an apertured stopper, (b) a lower tube section, aligned with the upper tube section and having a porous ceramic cap for inserting into soil, and (c) a T-fitting connecting between the tube sections and having an apertured T-fitting stopper. A vacuum tube is inserted through the T-fitting stopper aperture; and a conductivity sensor electrode is inserted through the upper tube section stopper aperture. The conductivity sensor electrode includes a pair of electrode poles that are spaced apart at a distance of 3-5 mm and the distance between the conductivity sensor electrode and the hollow tube is greater than 3 mm, whereby interference by air bubbles in the ground water is mitigated.

Claims

1. A probe for monitoring electrical conductivity of ground water in soil comprising: a hollow tube comprising: an upper tube section stoppered at one end with a stopper having an upper tube section stopper aperture, a lower tube section, aligned with the upper tube section and capped at its bottom with a porous ceramic cap configured to be insertable into soil and having an inner wall, a T-fitting connecting between the upper and the lower tube sections and having a T-fitting stopper with a T-fitting stopper aperture; a vacuum tube inserted through the T-fitting stopper aperture; and a conductivity sensor electrode inserted through the upper tube section stopper aperture and having an inside diameter and an outside diameter, wherein the conductivity sensor electrode is configured to be used in-situ and comprises a pair of electrode poles that are spaced apart at a distance of about 3-5 mm and the distance between an outer wall of the conductivity sensor electrode and the inner wall of the lower tube section is greater than about 3 mm, whereby interference by air bubbles in the ground water is mitigated.

2. The probe of claim 1, wherein the T-fitting stopper aperture is perpendicular to the upper and the lower tube sections.

3. The probe of claim 1, wherein the T-fitting stopper aperture is at an angle to the upper and the lower tube sections.

4. The probe of claim 1, wherein the hollow tube of the probe has an outside diameter of about 18 to 25 mm and an inside diameter range of about 14 to 21 mm.

5. The probe of claim 1, wherein the probe is between 15 cm and 20 cm long.

6. The probe of claim 1, wherein the conductivity sensor electrode extends down adjacent to the ceramic cap of the lower tube section.

7. The probe of claim 1, wherein the vacuum tube extends down adjacent to the ceramic cap of the lower tube section.

8. The probe of claim 1, wherein the vacuum tube in the hollow tube is parallel to the electric conductivity sensor electrode.

9. The probe of claim 1, wherein the outside diameter of the electric conductivity sensor electrode is in the range of about 6-7 mm.

10. The probe of claim 1 wherein the vacuum tube is composed of a semi-rigid section inserted into the probe and connected to a flexible section attached to a vacuum generator.

11. The probe of claim 10, wherein the semi-rigid section of the vacuum tube has an outside diameter of about 3-4 mm and the flexible section has an inside diameter of about 3-4 mm.

12. The probe of claim 1, wherein the porous ceramic cap is configured to allow free movement of all ions in the ground solution into the tube.

13. The probe of claim 1, wherein the vacuum tube is connected to a syringe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1-3 illustrate embodiments of a probe for direct monitoring of soil solution properties in accordance with the present invention.

(2) FIG. 4 is an enlarged view of the probe's electro-conductivity sensor in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

(3) FIG. 1 shows a probe 10 including a hollow tube with a lower tube section 12 and an upper tube section 14. Tube sections 12 and 14 are connected in series through a T-fitting 16 for example by hermetically gluing the sections into the T-fitting. Lower tube section 12 is capped at the bottom with a porous ceramic tip or cap 18 for inserting into soil. Upper tube section 14 is sealed at the top with a stopper 19, which has an opening through which an EC sensor probe 24 can be inserted. Sensor probe 24 has a sensor electrode 26 at its distal end, which is typically inserted down toward the bottom of lower tube section 12.

(4) T-fitting 16 has a side aperture from which extends a tube 28 sealed (e.g. 10 glued) therein hermetically. Tube 28 is sealed with a stopper 30. A flexible vacuum tube 32 is inserted through stopper 30 and is connected hermetically to a semi-rigid inner vacuum tube 34 that extends into lower tube section 12. The other end 35 of vacuum tube 32 is connected to a valve 36 that is connected to a syringe 38 through conical tube section 40.

(5) Semi-rigid inner vacuum tube 34 preferably has an outside diameter of about 3 mm-4 mm, and flexible outer vacuum tube 32 preferably has an inside diameter of about 3 mm to 4 mm.

(6) Such a tubing arrangement is suitable since inner vacuum tube 34 can neither be too flexible nor too rigid in order to maneuver the angle through T-fitting 16 if inner vacuum tube 34 is to be inserted substantially down into lower tube section 12. Outer vacuum tube 32 is preferably flexible for easy manipulating and positioning of syringe 38.

(7) A plunger 42 in syringe 38 can pull a vacuum in probe 10. It is preferable that inner vacuum tube 34 be inserted close to the bottom of lower tube section 12. This will ensure that the entire ground solution drawn into probe 10 can be withdrawn and that fresh ground solution can be drawn in.

(8) Sensor probe 24 has the tip of sensor electrode 26 located parallel to inner vacuum tube 34 in lower tube section 12 and preferably extends its entire length (generally 12 cm) down into lower tube section 12, but may be shorter as long as it can be immersed in the soil solution. In this embodiment, probe 10 is relatively short, about 15 cm, and sensor electrode 26 extends close to the top of ceramic cap 18.

(9) Sensor electrode 26 is an electrical conductivity (EC) sensor. Electrical conductivity measurement is very sensitive, and unlike measurements such as pH, chlorides, oxygen, nitrates, phosphorus, ammonium, water hardness, potassium, magnesium, or other soil solution properties that are not sensitive to the presence of air bubbles in the liquid media. EC measurements can not be carried out if the liquid media contains air bubbles that may accumulate around the sensor. Such air bubbles are introduced upon drawing in the ground water through ceramic cap 18. Therefore, to provide a soil solution free of air bubbles, lower tube section 12 preferably has an inner diameter wide enough to enable bubbles to rise to the top surface of the solution and not adhere to the inner walls thereof, vacuum inner tube 34 and around or between sensor electrode 26 itself, which would interfere with the EC measurement. Moreover, lower tube section 12 should be sufficiently wide to accommodate both the sensor and the vacuum tube. Therefore, in accordance with embodiments of the present invention, tube sections 12 and 14 preferably have an outside diameter of between 18-25 mm and an inside diameter of between 14-21 mm in order to accommodate sensor probe 24 and inner vacuum tube 34 side by side. In a preferred embodiment sensor probe 24 is connected to a controller (not shown) via cable 44.

(10) Porous ceramic cap 18 is made of inert material, which is not affected by soil components such as fertilizers and allows free transfer of the ions present in the ground solution into probe 10.

(11) Stoppers 19 and 30 can be made of rubber, plastic, cork, glass or other material able to form a tight seal. T-fitting 16 should be made from a tight fitting material to match hermetic sealing (gluing) with tube sections 12, 14 and tube 28. Syringe 38 should preferably be of sufficient volume (not less than 60 ml.) in order be able to create a vacuum of up to 0.9 bars easily.

(12) The role of syringe 38 is two-fold: (1) to create a vacuum and draw soil solution into probe 10 to at least cover sensor electrode 26 by opening valve 36 and pulling plunger 42 outward and then closing valve 36, at which time the electrical conductivity (EC) of the solution can be measured; and (2) to transfer the soil solution from lower tube section 12 to syringe 38 by opening valve 36 and drawing the solution from the tube into syringe 38 while drawing in new fresh soil solution into probe 10 for the next measurement, then closing valve 36 and expelling the solution from syringe 38. Thus, filling and emptying of probe 10 with a refreshed ground solution can be achieved quickly and without requiring a significant period of time for the soil solution to come to equilibrium.

(13) It should be noted that sensor probe 24 is inserted through stopper 19 of upper tube section 14 into lower tube section 12 sufficiently to have sensor electrode 26 completely immersed in the soil solution drawn into probe 10. It is desired to keep the diameter of lower tube section 12 within the limits as defined, in order to draw in soil solution and avoid air bubbles that interfere with EC measurements.

(14) FIG. 2 illustrates another embodiment of probe 10. In this case the electric conductivity (EC) sensor probe 24 does not go to the bottom of lower tube section 12, but still can measure the electric conductivity of the soil solution since the vacuum pulled via inner vacuum tube 34 will bring the solution up in lower tube section 12 and cover sensor electrode 26, enabling correct measurement. The vacuum created by plunger 42 via syringe 38 is preferably up to about 0.9 bar as probe 10 is long and the soil solution is to be drawn up high.

(15) FIG. 3 shows another embodiment of probe 10, wherein both inner vacuum tube 34 and electric conductivity (EC) sensor probe 24 are introduced through a stopper 54 into a tube 56. As can be seen in FIG. 3, probe 10 can vary in length depending on the particular soil to be measured, but since (EC) sensor probe 24 is generally only about 12 cm long, the soil solution is drawn up high enough in the probe to completely cover sensor electrode 26. Inner vacuum tube 34, however, can be as long as desired; the closer it is to the bottom of the tube the easier to remove all of the soil solution from tube 56 by vacuum.

(16) FIG. 4 shows an enlarged view of the end of sensor electrode 26 whereby a pair of electrode poles 50 is illustrated. Poles 50 are disposed adjacent the inner walls of electrode 26 and are spaced apart at a distance dl of about 3-5 mm, whereby interference by air bubbles in the ground water is mitigated. Without limitation to theory, it is believed that due to the aforementioned spacing, any bubbles in the ground water that enter lower tube section 12 will either not enter the distal opening of electrode 26, for example a grouping of adjacent bubbles; or if a small grouping of singular bubble, those bubbles will readily exit without inappropriate interference between poles 50, whereby conductivity measurement issues are mitigated.

(17) With respect to the annular area between conductivity sensor electrode 26 and lower tube section 12 of probe 10, the distance d3 between the outer wall of the conductivity sensor electrode and the inner wall of the lower tube section is greater than about 3 mm, whereby any bubbles that are drawn into lower tube section 12 of probe 10 can be readily evacuated from the probe via the annular area.

(18) Operating Procedures

(19) The operation will be discussed with reference to FIG. 1. Probe 10 operates as follows: the probe, having a suitable length for the specific soil depth is inserted into the soil to the depth desired. Initially, syringe 38 is connected to valve 36 at its connector 40 while plunger 42 is pushed in fully to the end. Valve 36 is then opened to provide a free air path from syringe 38 through vacuum tubes 32, 34 to the inner space of probe 10. Plunger 42 is then fully withdrawn to create a vacuum via vacuum tubes 32, 34 in probe 10 and valve 36 is then closed to block the path between syringe 38 and vacuum tube 32, in order to maintain the vacuum in probe 10. Syringe 38 may be disconnected from valve 36 at this time. The soil solution is thus drawn into lower tube section 12 through porous ceramic cap 18.

(20) Since the soil solution is drawn up into lower tube section 12, the solution covers sensor electrode 26 and measurements can be made continuously.

(21) It should be noted that syringe 38 as such, is not part of probe 10, rather an exemplary and practical means of creating a vacuum, and can be replaced by other vacuum generators.

(22) After rain or irrigation, the relative concentrations of nutrients, fertilizers, etc. in the ground solution may vary significantly. Therefore one way of measuring electrical conductivity (EC) manually on site is as follows:

(23) (a) Syringe 38 is connected to valve 36 with plunger 42 pushed in fully to the end.

(24) (b) Valve 36 is set to an open position between syringe 38 and vacuum tubes 32, 34.

(25) (c) Plunger 42 is pulled back enough to create a suitable vacuum, drawing in soil solution into lower tube section 12.

(26) (d) Valve 36 is then closed to block the path between syringe 38 and vacuum 15 tube 32, in order to maintain the vacuum in probe 10.

(27) (e) Syringe 38 may be disconnected from valve 36 at this time, and plunger 42 is pushed back into syringe 38.

(28) (f) The above procedure may be repeated in order to increase the vacuum in probe 10 up to about 0.9 bars.

(29) The solution drawn into lower tube section 12 is a representative sample of the ground solution and the first measurement may be conducted once the ground solution reaches above sensor electrode 26.

(30) As time passes, the amount of nutrients, fertilizers and amounts of water in the ground vary and the ions will flow in and out of lower tube section 12 until equilibrium is established between the ions in the ground water (solution) and the ions in lower tube section 12. Continuous monitoring of the solution in probe 10 will provide a curve of the actual fluctuation of ion concentration in the ground solution over time.

(31) In order to improve the equilibrium process, it is recommended to refresh the solution in probe 10 by repeating the above manual procedure, preferably once a week.

(32) The in-situ measurements may be transmitted continuously from the EC sensor probe 24, via a cellular data-logger to an internet database server for a continuous viewing of the data which may be presented at the website in the form of a graph or table, and for generating analyses of the obtained data based on which the farmer may regulate the delivery and supply of fertilizers, nutrients and irrigation.