Measuring and testing – Liquid level or depth gauge
Patent
1995-09-25
1998-04-28
Oda, Christine K.
Measuring and testing
Liquid level or depth gauge
73 1DV, G01F 2328
Patent
active
057431345
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for determining and/or monitoring a predetermined level of material in a container with the help of a level sensor having a mechanical oscillatory structure, an electromechanical excitation transducer and an electromechanical reception transducer, the mechanical oscillatory structure being mounted at the height of the predetermined level of material in such a way that it comes into contact with this material when said material reaches the predetermined level, and the reception transducer being connected to the input of an amplifier circuit, whose output is connected to the excitation transducer and which is designed in such a way that the mechanical oscillatory structure is excited to oscillate at its self-resonant frequency, an evaluation circuit being connected to the output of the amplifier circuit for causing display and/or switching operations depending on the frequency of the output signal of the amplifier circuit, as well as to an arrangement for implementing the method.
A method of this kind is disclosed in the published German patent application 33 36 991, with the mechanical oscillatory structure having two oscillatory rods which are attached to a diaphragm and which are placed in a state of opposite vibrations. When the oscillatory structure oscillates in air, it has a higher self-resonant frequency than when it is in contact with the material. This phenomenon is exploited to indicate and/or monitor the predetermined level of material. The evaluation circuit compares the instantaneous frequency of the output signal of the amplifier circuit, which is always identical with the current self-resonant frequency of the mechanical oscillatory structure, with a nominal frequency, which is identical with the self-resonant frequency which the oscillatory structure has when it oscillates in air, and which is recorded, for example, during manufacture or installation of the level indicator sensor. If this instantaneous frequency is lower than the nominal frequency by more than a predetermined difference, it is assumed that the mechanical oscillatory structure is in contact with the material, that is, that the level to be monitored in the container has been reached or exceeded, whereas otherwise it is assumed that the mechanical oscillatory structure is oscillating in air, that is, that the level to be monitored in the container has not been reached.
The evaluation circuit can also detect certain faults in the level monitoring arrangement from the frequency determined by it. If the frequency measured is zero, this means a complete failure of the device. If the frequency measured is significantly higher than the nominal frequency, then it can be suspected that the oscillatory rods have been shortened through corrosion or breakage. In such cases, the evaluation circuit can give an alarm signal.
However, there are also faults that cannot be detected in this way, because they can lead to frequency values that are within the range of possible frequency values. Such faults can be caused, for example, by long-term changes (degradation) and ageing phenomena in the components of the sensor and in the circuit components of the electronic circuit arrangement. If the oscillation frequency of the sensor is reduced by such a fault, it is possible for the evaluation circuit erroneously to indicate that the oscillatory rods are covered by the material, that is, that the predetermined level has been reached, even though the oscillatory rods are in fact oscillating in air. If, on the other hand, the oscillation frequency of the sensor is increased by such a fault, it is possible for the evaluation circuit erroneously to indicate that the oscillatory rods are oscillating in air, even though the oscillatory rods are in fact covered by the material.
The object of the invention is to provide a method of the type mentioned above, with which faults which cannot be detected from the value of the frequency measured can still be detected, as well as an arrangement for imple
REFERENCES:
patent: 4594584 (1986-06-01), Pfeiffer
patent: 4896535 (1990-01-01), Duckart
patent: 5269188 (1993-12-01), Esin
Endress & Hauser GmbH & Co.
Oda Christine K.
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