Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
1999-03-18
2002-05-28
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
C073S29000R, C333S252000
Reexamination Certificate
active
06393909
ABSTRACT:
FIELD OF THE INVENTION
The invention deals with the touch less measuring of material levels in containers by use of microwave measuring technology, and, in particular, with a microwave level measuring device for performing such measurements under extreme conditions, like at high temperatures, and/or at high pressures, and/or in the presence of chemically aggressive substances.
BACKGROUND OF THE INVENTION
A number of different measuring principles and devices derived from them are used for measuring the filling levels of materials in containers. Until now, for containers with extreme measuring conditions, like unusually high temperatures, and/or unusually high pressures, or chemically aggressive substances, direct material contact techniques were used exclusively for the measuring of levels, such as capacitive and hydrostatic measuring, as well as the combined measuring of in- and outflows.
In capacitive measuring systems, material, container, and measuring probe form an electrical condenser. Here, the filling level is determined by measuring the condenser capacitance. Generally, such capacitive probes cannot be used universally, but must be exclusively designed for a special application. For electrically conductive materials, depending on the material being measured, and the pressure, and temperature at which it is being stored, special insulating materials must be chosen for the probe. Moreover, if the measured materials stick to the probe, the latter must be coated with substances which neutralize adhesion.
In contrast, hydrostatic measuring methods determine material levels by measuring their hydrostatic pressures. However, this approach is only suitable for materials with fluid to paste consistency. Solid or highly viscous materials are not suitable for hydrostatic measuring.
Combined in- and outflow measurements generally permit no more but coarse estimates of the level of flowable materials by determining the quantity of the material in the container by measuring the difference between the quantity of material poured in and the quantity drained out. Capacitive measuring probes, hydrostatic pressure measurement converters, and flow measuring devices are all manufactured and distributed by the assignee of the present application.
Level measuring devices corresponding to the state-of-the-art often have the disadvantage of requiring contact of the measuring probe with the material to be measured. This results in a number of difficult technical problems, which leads to relatively high and, therefore, expensive manufacturing costs, even for devices that are used for less demanding conditions.
SUMMARY OF THE INVENTION
To avoid the disadvantages of limited application to certain filling materials and the corresponding high cost, it would be highly desirable to have a no-contact level measuring probe based on a measuring method that is practically suitable for all fluent filling materials as well as for operation in extreme measuring conditions.
The task of the invention is to create a measuring device based on a measuring method where there is no direct contact with the material where the device can be practically used determine the material level in containers of any material, even under extreme measuring conditions, like high temperatures, and/or high pressures, and/or the presence of aggressive chemical substances.
The present invention solves the problem by providing a level measuring device that determines the material level with no direct contact of the material, by timing the interval between an emitted and, after reflection from the material surface, a returned microwave signal, using suitable devices, for operation at high temperatures, and/or high pressures, and in presence of aggressive chemical substances.
An especially advantageous design of the invention provides for the operation of the level measuring device at temperatures above 300° C., and at pressures above 35 bar.
Commonly available level measuring devices based on interval timing of a microwave signal are not suitable for operation at such extreme conditions. The fundamental design and mode of functionality of such a known, state-of-the art measuring device, is approximately the following:
Control of the sending/receiving process, as well as the evaluation of the returned signal, occurs in a central control and measuring unit, generally implemented, in practice, by an electronic unit contained in the housing. The sent and returned signals are reciprocally transmitted by an electrical line, like a coaxial cable, that connects the sending/receiving unit with the electronic unit. The sending/receiving unit for sending and receiving the microwave signal consists of a horn antenna with an antenna feeder and a horn. The antenna feeder consists of a filled waveguide coupled with the microwave signal by an exciter pin. The transition from the filled waveguide to the horn is electrically adapted by a linear taper element. For determination of the material level, the microwave signal reflected from the surface of the filling material is subsequently received by the antenna and transmitted to the central control and measuring unit by the electrical cable. The device is installed on the container by a mounting unit, in most cases with a mounting flange that is rigidly connected to the container. Generally, the part of the mounting unit or assembly comprising horn, taper, and waveguide, is freely exposed to the conditions in the container. In the customary design, in which e.g. the waveguide filling material is sealed to the waveguide by an elastomer seal, there is no pressure sealing or pressure proofing against high container pressures. Moreover, the measuring device, especially its sensitive measuring and electronic control unit, is exposed to high container temperatures by the mounting unit. Without special provisions for protection against damaging influences, known microwave level measuring devices cannot be used for extreme container conditions.
To operate a common microwave level measuring device in extreme measuring conditions, the invention provides special devices and design modifications that insulate the sensitive device components against potentially destructive container conditions. These devices provide for pressure sealing and pressure proofing against high container pressures and protection against high container temperatures, as well as protection against chemically aggressive substances. This especially protects the sensitive measuring and electronic control, as well as the sending and receiving unit against damaging influences.
The devices for pressure proofing and pressure sealing enable the container pressure to act on the taper element, and is absorbed by the filling material of the waveguide. The waveguide is tightly connected with the mounting unit, especially with the mounting flange, preferably welded to it, so that the pressure force acting on the taper element is transmitted by the waveguide to the mounting flange.
Moreover, the devices for pressure proofing and pressure scaling provide protection against chemically aggressive substances.
Significant performance losses, possibly in connection with disturbances in the microwave transmission, like undesirable reflections, also referred to as “ringing”, through strong changes in impedance, and similar symptoms do not occur with the devices manufactured in accordance with the present invention.
In a preferred design version of the invention, the housing connection between electronic unit and mounting flange is extended into a spacer tube, separating both components for thermal shielding against high container temperatures. The temperature gradient along the spacer tube and the spatial separation both achieve thermal shielding of the components. As material for the spacer tube, a metal, such as stainless steel, preferably, is provided.
In an advantageous design of the invention, at least one transverse separating wall is provided for thermal protection by dividing the spacer tube into separate, thermally isolated sections. A material with espec
Dietmeier Jürgen
Fahrenbach Josef
Motzar Jürgen
Rapp Günter
Schwegman Lundberg Woessner & Kluth P.A.
VEGA Grieshaber XG
Williams Hezron
Wilson Katina
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