Electricity: conductors and insulators – Conduits – cables or conductors – Single duct conduits
Patent
1993-02-04
1995-02-21
Picard, Leo P.
Electricity: conductors and insulators
Conduits, cables or conductors
Single duct conduits
73304C, 324448, G01F 2326
Patent
active
053918398
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a device for the electrically insulated and pressure-sealed attachment of a metallic probe electrode in the opening of a housing, preferably in the opening of a probe head housing of a measuring probe for measuring the filling status (level) inside a container or also the level height in a channel. In order to determine the content, i.e., the height of the contents in a container or the level height in a channel, frequently capacitive or conductive measuring probes are used. Such capacitive probes are suited for contents. Since the dielectric constant is different from that of the air. For this purpose, the electrode of the measuring probe represents the one electrode, the metallic container wall of the container, the filling height of which is to be measured, represents the second electrode and the filling material represents the dielectric of a precision capacitor.
Depending on the level, the measuring probe is more or less covered by the filling material. Depending on the overlap, the capacitance of the capacitor changes. With an electronic circuit, frequently arranged at the measuring site in the interior of the probe head housing, this capacitance change is determined. Advantageously, this is brought about in that an alternating current is applied to the measuring electrodes of the precision capacitor and the capacitive alternating current flowing through the precision capacitor is measured, converted into a measuring signal and transferred to a switching station which is located, in most cases, away from the measuring probe. In the switching station, indicator devices then show the existing level, or switching elements are operated which intervene with actuators in the course of a process in a controlling manner.
By means of suitable means already known to the expert it can be achieved that the electrical field distribution of the precision capacitor remains the same across the entire length of the measuring probe, so that the capacitance change is directly proportional to the level change.
However, very often conditions exist in the containers, due to the contents to be measured, which can influence the measurement or can destroy the measuring probe itself. Thus, for example, increased temperature, high pressure or corrosive measuring media may exist. Thus, thermal, mechanical or chemical influences may change the capacitance, that condensation deposits on the probe or the capacitance of the probe is increased due to an accumulation of filling material in the area of attachment, i.e., near the passage of the probe through the container wall. Thus, the total capacitance of such measuring probes is composed of the probe capacitance, i.e., the capacitance along the actual measuring probe and the capacitance in the attachment area, i.e., near the passage through the container wall. In order to eliminate these influences which may bring about a falsification of the measuring results, it is usually customary to arrange a metallic protection electrode (guard electrode) between the container wall and the measuring electrode. It is its task to influence the field distribution of the measuring capacitor such that these negative influences cannot falsify the measuring signal. Since, very frequently an increased pressure is also part of the negative influences, it is additionally necessary to seal off the interior of the probe head housing such that, with respect to the interior of the container, no pressure can escape and thus no aggressive medium can come into contact with the electrical circuit arranged in the interior of the probe head housing and destroy the same. Naturally, however, the sealing must take place such that the measuring electrode, acting as an electrode of the measuring capacitor, remains in electrical connection with the electronic circuit. This is also necessary for the metallic guard electrode.
From the German Patent No. 27 44 864 (Offenlegungsschrift), a device for attaching a probe in the opening of a container is known. Here, for achieving a pressure-sealed att
REFERENCES:
patent: 3477290 (1969-11-01), Lerner
Alznauer Miroslaw
Lang Hugo
Endress & Hauser GmbH & Co.
Picard Leo P.
Tone David
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