Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
1997-11-19
2001-01-30
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
C073S29000R, C073S30400R, C324S637000, C324S639000, C324S640000, C324S642000
Reexamination Certificate
active
06178817
ABSTRACT:
The present invention is directed to detection of level of material in a storage vessel, and more particularly to a system and method that employ time domain reflectometry techniques for either point-level or continuous measurement of material level in the storage vessel.
BACKGROUND AND OBJECTS OF THE INVENTION
It has heretofore been proposed to employ so-called time domain reflectometry techniques to measure or detect the level of material in a storage vessel. In general, this technique involves placement of a conductive transmission line probe in the vessel at an orientation to be contacted by material in the vessel. Microwave pulses of short duration are periodically launched onto the transmission line probe, typically in a downward orientation through air toward the material surface. When the pulses encounter an electrical discontinuity, such as the change in dielectric constant at the interface between the air and the material, a portion of each energy pulse is reflected back along the transmission line probe to detection electronics. Time-delayed gating techniques are employed in a process referred to as equivalent time sampling to locate the position of the reflecting discontinuity along the transmission line probe, and thereby to determine the level of the material surface with respect to the probe.
Although the material level sensing technique so described has overcome problems and difficulties theretofore extant in the art, further improvements remain desirable. For example, systems employing this technology have been implemented for continuous level measurement—i.e., measurement of material level as a continuous function throughout a desired range. However, there remains a need for a point-level measurement system employing this technology that is less expensive to manufacture and easier to implement than use of continuous-level technology coupled with a point-level threshold detection. It is one object of the present invention to provide a material point-level detection system and method that address this need in the art.
Another deficiency in the art surrounds the mounting of the transmission line probe to the vessel. In typical situations, the electronics are mounted within a housing, and the housing is mounted to the wall of the vessel such that the transmission line probe extends into the vessel for contact with the material. The transmission line probe typically takes the form of a coaxial transmission line, in which the probe rod forms the center conductor and the vessel wall forms the outer conductor. It is important that the characteristic impedance of the transmission line probe match that of the interconnection to the electronics until the probe rod is within the vessel, so that the vessel wall can form a launch plate for the probe rod without undesirably bleeding energy from the probe rod or causing spurious reflections due to impedance mismatch. It is therefore another object of the present invention to provide an electronics/probe interconnection arrangement that efficiently transfers pulsed microwave energy from the launching electronics to the probe without excessive energy loss or spurious reflections. A further object of the present invention is to provide a system for continuous measurement of material level in a vessel in which the upper and lower level limits for the continuous measurement may be readily reprogrammed by an operator. Yet another object of the present invention is to provide an inexpensive and reliable method of making coaxial connection to a transmission line probe in a point or continuous level measurement system of the described character.
SUMMARY OF THE INVENTION
A system for measuring level of material in a vessel includes a transmission line probe adapted to be mounted to a wall of the vessel so as to extend within the vessel for contact with material, and electronics operatively coupled to one end of the probe for launching microwave energy along the probe. The electronics is responsive to energy reflected from the probe, employing time domain reflectometry techniques, for obtaining a measurement of the level of the material in the vessel. In accordance with one aspect of the present invention, the transmission line probe includes coaxial connection means having a center conductor connected to the system electronics, an outer conductor and a characteristic impedance between the center and outer conductors. A cylindrical shell of electrically conductive composition has the coaxial connection means coupled to one end and is electrically connected to the outer conductor of the coaxial connection means. An electrically conductive rod extends through the shell coaxially with the shell and is electrically connected at a first end to the center conductor of the coaxial connection means. Insulation is disposed between the shell and the rod. The dielectric properties of this insulation, and the dimensions of the rod, shell and insulation, are such that the combination of the shell, the rod and the electrical insulation has a characteristic impedance that matches that of the coaxial connection means. The conductive rod has a second end with threads or other suitable means for electrical and mechanical coupling to a probe rod that extends into the vessel. The shell preferably has external threads at opposed ends for mounting to an electronics enclosure and for removable mounting to a gland affixed to a vessel wall.
At least a portion of the insulation preferably is cured in situ within the shell surrounding the rod, with the rod including external threads or other suitable means for interengagement with the insulation to prevent axial removal of the rod. For high-pressure or other heavy duty applications, a portion of the insulation is separately formed and assembled within the shell surrounding the conductor rod. Elastomeric O-rings or other suitable sealing rings are disposed in grooves on this separately-formed insulation element for sealing engagement with the rod and the shell. The shell may extend into the vessel and surround a portion of the probe rod to help the vessel wall function as a launch plate for the probe rod.
In one embodiment of the invention, the coaxial connection means that couples the electronics to the probe rod comprises a standard coaxial connector. In another embodiment, the coaxial connection means comprises a coaxial cable having an outer or shield conductor that is slidably received into a socket on the end of the shell for making electrical grounding contact with the shell, and a center conductor slidably received in a socket at the end of the conductive rod. This second embodiment is preferred because the expense of the coaxial connector and the time associated with making electrical connection thereto are eliminated. Thus, in accordance with another aspect of the invention, there is provided an improved technique for making electrical connection between the measurement electronics and the probe, in which a coaxial cable extends from the electronic to the probe for matching characteristic impedances, and the need for an intervening coax connector is eliminated.
In accordance with a further aspect of the present invention that finds particular application in connection with continuous level measurement systems, the system electronics includes means for selectively setting upper and lower limits of continuous level measurement within the vessel. This limit setting may be accomplished either at the system electronics, or from a remote location. The electronics preferably are disposed within a housing having a removable cover and a base mounted to the vessel, and the limit adjustment may be accomplished at the electronics either with or without removing the cover from the base.
A system for point-level detection of material within a vessel in accordance with a fourth aspect of the present invention includes a transmission line probe adapted to be mounted to a wall of the vessel so as to extend within the vessel for contact with material when the material reaches a preselected level within the vessel. Elect
Hewelt Scott M.
Marsh Norman F.
Torzewski Michael C.
Loo Dennis
Reising, Ethington, Barnes, Kisselle, Learman & McCullock, P.C.
Venture Measurement Company LLC
Williams Hezron
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