METHOD AND APPARATUS FOR DETECTING THE LEVEL OF A MEDIUM

Abstract

Apparatus is disclosed for detecting a first medium such as sludge having a relatively low dielectric constant wherein the first medium is located below a second medium such as water having a relatively high dielectric constant. The apparatus comprises a probe adapted to launch a pulse signal at a lower extremity thereof such that the pulse signal enters the first medium before being transferred or transmitted to the second medium. The first medium may be located at or near a bottom of a vessel and the second medium may be located above the first medium. A method for detecting the first medium located below the second medium is also disclosed.

Claims

1. Apparatus for detecting a first medium having a relatively low dielectric constant wherein said first medium is located below a second medium having a relatively high dielectric constant, said apparatus comprising a probe adapted to launch a pulse signal at a lower extremity thereof such that said pulse signal enters said first medium before being transferred or transmitted to said second medium.

2. Apparatus according to claim 1 wherein said first medium is located at or near a bottom of a vessel and said second medium is located above said first medium.

3. Apparatus according to claim 2 wherein said probe is adapted to be mounted through a top of said vessel.

4. Apparatus according to claim 1, wherein said first medium is relatively dense and includes sludge and said second medium is less dense and includes water and/or a gas.

5. Apparatus according to claim 1, wherein said probe includes a sensing element and a signal feed line for interfacing said sensing element at or near a lower extremity thereof.

6. Apparatus according to claim 5 wherein said sensing element includes a stainless steel rod and a conducting shield and said feed line includes a coaxial cable.

7. Apparatus according to claim 5 wherein said probe includes a non-conducting core and said shield includes a geometry in cross-section adapted to eliminate or at least reduce build-up of foreign material between said rod and said shield.

8. Apparatus according to claim 6 wherein said probe includes an impedance matching circuit between said stainless steel rod and said coaxial cable.

9. Apparatus according to claim 1 further including plural feed lines connected to a bottom extremity of said probe for measuring multiple interface levels.

10. Apparatus according to claim 1 further including one or more additional feed lines connected to a top extremity of said probe for performing a measurement of a low dielectric to high dielectric interface.

11. Apparatus according to claim 1 wherein said apparatus is adapted to employ one or more of Time Domain Reflectometry (TDR), Frequency Modulated Continuous Wave (FMCW) and/or Stepped Frequency Continuous Wave (SFCW) techniques.

12. Apparatus according to claim 5 further including a transmitter/receiver in combination with a controllable switch matched to said signal feed line for launching said pulse signal.

13. Apparatus according to claim 12 and adapted for measuring single or multiple levels/interfaces and for outputting measures of single or multiple levels/interfaces respectively.

14. A method for detecting a first medium having a relatively low dielectric constant wherein said first medium is located below a second medium having a relatively high dielectric constant, said method comprising providing a probe; and arranging said probe to launch a pulse signal at a lower extremity thereof such that said pulse signal enters said first medium before being transferred or transmitted to said second medium.

15. A method according to claim 14 wherein said first medium is located at or near a bottom of a vessel and said second medium is located above said first medium.

16. Method according to claim 15 including mounting said sensing element through a top of said vessel.

17. A method according to claim 14, wherein said first medium is relatively dense and includes sludge and said second medium is less dense and includes water and/or a gas.

18. A method according to claim 14 wherein said probe includes a sensing element and a signal feed line for interfacing said sensing element at or near a lower extremity thereof.

19. A method according to claim 18 wherein said sensing element includes a stainless steel rod and a conducting shield and said feed line includes a coaxial cable.

20. A method according to claim 18 wherein said probe includes a non-conductive core and said shield includes a geometry in cross-section adapted to eliminate or at least reduce build-up of foreign material between said rod and said shield.

21. A method according to claim 19 wherein said probe includes an impedance matching circuit between said stainless steel rod and said coaxial cable.

22. A method according to any one of claim 14 further including connecting plural feed lines to a bottom extremity of said probe for measuring multiple interface levels.

23. A method according to any one of claim 14 further including connecting one or more additional feed lines to a top extremity of said probe for performing a conventional measurement of a low dielectric to high dielectric interface.

24. A method according to claim 14 further including employing one or more of Time Domain Reflectometry (TDR), Frequency Modulated Continuous Wave (FMCW) and/or Stepped Frequency Continuous Wave (SFCW) techniques.

25. A method according to claim 14 further including using a transmitter/receiver in combination with a controllable switch matched to said signal feed line for launching said pulse signal.

26. A method according to claim 25 and adapted for measuring single or multiple levels/interfaces and for outputting measures of single or multiple levels/interfaces respectively.

Description

DESCRIPTION OF THE DRAWINGS

[0038] FIGS. 1(a) to 1 (c) show TDR signal guidance along a sensing element associated with a probe;

[0039] FIG. 2 shows characteristic impedances associated with different media (Gas, Water, Sludge);

[0040] FIGS. 3 (a) to 3 (c) show signal reflections at interfaces between different media (Z1, Z2, Z3);

[0041] FIGS. 4 (a) and 4 (b) show a conventional (top-down) probe model for TDR application;

[0042] FIG. 5 shows a conventional coaxial TDR probe;

[0043] FIGS. 6 (a) and 6 (b) show a bottom-up probe model for TDR application according to an embodiment of the present invention;

[0044] FIGS. 7 (a) to 7 (c) show a TDR probe mounting, a probe with a single sensing element, and a probe with multiple sensing element respectively; and

[0045] FIG. 8 shows an instrument for launching pulses to multiple sensing elements.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0046] The present invention may provide an alternative approach to the conventional TDR probe illustrated in FIG. 4 (a). In particular, the present invention may make use of a “bottom-up” feed arrangement as shown in FIG. 6. In the “bottom-up” arrangement shown in FIG. 6 (a), an impedance matched first signal feed 61 from TDR instrument 67 is extended to the bottom of sensing element 62 via shielded coaxial cable 63, and is launched from the bottom of vessel 60 towards the top. A suitable impedance matching circuit (not shown) may be provided between coaxial cable 63 and sensing element 62. Sensing element 62 may be provided in any suitable form such as a stainless steel rod or the like. As described with reference to FIG. 4(b) sensing element 62 may be modelled as a series of transmission lines 62a, 62b, 62c with different characteristic impendences Z.sub.1, Z.sub.2, Z.sub.3 as shown in FIG. 6b. An impedance matched second signal feed 65 from TDR instrument 62 may be connected to the top of sensing element 67 via coaxial cable 66.

[0047] FIG. 7(a) shows one form of mounting for instrument 68 and associated TDR probe 69 atop a storage tank including a top and bottom and including gas, high dielectric medium and low dielectric medium below the high dielectric medium. Probe 69 includes a sensing element and a signal feed line for launching a TDR signal at or near a lower extremity 72 thereof. This configuration may be adapted to detect the low dielectric medium below the high dielectric medium.

[0048] The sensing element may comprise a single sensing element and associated feed line as shown in FIG. 7b. The sensing element may include an elongated stainless steel rod 74 and outer shield 75 and the feed line may include coaxial cable 70 for bottom up sensing and coaxial cable 76 for top down sensing. The sensing element may include an impedance matching circuit between stainless steel rod 74 and coaxial cable 70 (not shown).

[0049] Alternatively there may be provision to attach to top extremity 79 of probe 69, multiple sensing elements and associated feed lines SE1 to SEn as shown in FIG. 7c to measure multiple interface levels. This configuration may facilitate measurement of a Gas/Water (low dielectric to high dielectric) interface via a conventional TDR technique.

[0050] As described above the probe of the present invention may alleviate a tendency for foreign material to build-up or form a bridge in the empty space 52 between inner sensing element 51 and outer conductor/shield 50 associated with the conventional TDR probe shown in FIG. 5.

[0051] Bridging may be reduced or at least alleviated by partly filling the empty space 52 and/or by adopting a partly open geometry for outer conductor/shield 50. To this end and referring to the cross sectional view in FIG. 7(b), probe 69 may include a partly open or arcuate outer conductor/shield 75. In one form conductor/shield 75 may be semi-annular or half-annular in cross-section.

[0052] Probe 69 may include a substantially cylindrical core 73 formed from a non-conducting, low dielectric material such as Teflon. Core 73 may be positioned between conductor/shield 75 and stainless steel rod 74, such that rod 74 is at least partly or substantially exposed to the medium.

[0053] It is desirable to maintain at least a 10 mm distance between rod 74 and shield 75. Core 73 includes a longitudinal slot 78 for receiving coaxial cable 70 therein. Core 73 includes a longitudinal recess 71 for receiving part of stainless steel rod 74 such that rod 74 remains substantially exposed to the medium. FIG. 7(b) includes a perspective view 77 of the geometry of the sensing element.

[0054] FIG. 8 shows electronics 80 associated with instrument 68 for launching pulses to multiple sensing elements SE1 to SEn. Electronics 80 includes transmitter 81 for generating pulses and receiver 82 for receiving echoes of the pulses.

[0055] The present invention may provide a modified TDR feeding arrangement including a shielded line to launch a TDR pulse signal from the bottom or top of one or more sensing elements. In particular electronics 80 may be used in conjunction with an electronically controllable switch 83 matched to the or each shielded line to launch the pulse signal from the bottom or top of sensing elements SE1-SEn. This technique may provide an advantage in that a signal launched from the bottom may minimise attenuation of reflected signal from a low to high dielectric interface (eg. Sludge/Water interface) while a signal launched from the top may allow detection of a Gas/Water interface.

[0056] Alternatively by using a single sensing element and switching the launch signal from the bottom to the top as described above, the probe of the present invention may avoid a need for two separate instruments to measure level of a medium.

[0057] Reflected signals resulting from a bottom-up feed arrangement may detect a low dielectric/high dielectric (Sludge/Water) interface as follows: [0058] i. Since the signal is launched into a low dielectric medium, it may have a closely matched feed impedance. Since probe dimensions may be controlled, the resulting impedance may be better matched, and may be independent from tank dimensions; [0059] ii. Since the signal travels a minimal distance within the medium, it may not be subjected to heavy attenuation; [0060] iii. Therefore a higher signal may be reflected from the interface of low to high dielectric compared to a conventional method and, furthermore since this reflection is the first to be received by electronics 80, it may not be interfered with by multiple reflections.

[0061] From the above analysis it may be seen that a bottom-up feed arrangement may provide several advantages including: [0062] i. Reflected signals from Sludge/Water interface may arrive first and hence may not be interfered by other signal reflections; [0063] ii. Attenuation of the reflected signal from Sludge/Water interface may be relatively minimal; [0064] iii. Reflected signals from Sludge/Water interface may have negative polarity while the Water/Gas interface may generate a reflected signal with positive polarity. Hence both interfaces may be more distinctive.

[0065] Although a preferred embodiment the present invention may make use of a TDR technique for detecting a low dielectric medium below a high dielectric medium, the present invention is not thereby limited to such techniques and is equally capable of using techniques other than TDR techniques including but not limited to Frequency Modulated Continuous Wave (FMCW) and Stepped Frequency Continuous Wave (SFCW) techniques.

[0066] Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.