The invention relates to a device for determining and/or monitoring the level of a medium in a container or for ascertaining the density of a medium in the container.
The device is embodied as a vibration detector having a housing, a diaphragm, an osciliatable unit, a drive/receiving unit, and a regulating/evaluating unit. The diaphragm closes one end face of the housing, the oscillatable unit is secured to the diaphragm, and the drive/receiving unit is disposed in the interior of the housing such that it causes the diaphragm and the oscillatable unit to oscillate, and detect the oscillations, and the regulating/evaluating unit, from the detected oscillations, detects the attainment of the predetermined level or ascertains the density of the medium.
Such vibration detectors are known in different versions. The oscillatable unit is at least one oscillatory rod, whichxe2x80x94as already notedxe2x80x94is secured directly to a diaphragm. The diaphragm is induced to oscillate via an electromechanical converter, such as a piezoelectric element. Because of the oscillations of the diaphragm, the oscillatable unit secured to the diaphragm also executes oscillations.
Vibration detectors embodied as fill level measuring instruments exploit the effect that the oscillation frequency and oscillation amplitude are dependent on the particular degree of coverage of the oscillatable unit: While the oscillatable unit in air can execute its oscillations freely and without damping, it does undergo a change of frequency and amplitude as soon as it dips partway or all the way into the product filling the container. From a predetermined frequency change (typically, the frequency rather than the amplitude is measured), an unambiguous conclusion can accordingly be drawn as to whether a predetermined fill level of a product in the container has been attained. Fill level measuring instruments of this type are moreover primarily used as means to secure against overfilling or for the sake of preventing a pump from running empty.
Moreover, the damping of the oscillation of the oscillating element is affected by the density of the particular product as well. For a constant degree of coverage of the at least one oscillating element, there is a functional relationship with the density of the product, making vibration detectors extremely well suited for detecting a predetermined limit state as well as for detecting density.
In practice, to monitor and detect the level or density of the product in the container, the oscillations of the diaphragm are picked up and converted into electrical reception signals. A piezoelectric element is typically used for this. The electrical reception signals are then evaluated by an electronic evaluator. In the case of level determination, the electronic evaluator monitors the oscillation frequency and/or the oscillation amplitude of the oscillatable unit and signals that the state is either xe2x80x9csensor coveredxe2x80x9d or xe2x80x9csensor uncoveredxe2x80x9d, as soon as the measured values undershoot or overshoot a predetermined reference value. This can be reported accordingly to the operating staff visually and/or acoustically. Alternatively or in addition, a switching operation is tripped; an inflow or outflow valve, for instance, on the container is then opened or closed.
From European Patent Disclosure EP 0 810 423 A2, a vibration-fill level limit switch has been disclosed in which the piezoelectric element is prestressed in the interior of the housing between a pressure plate and the center of the diaphragm. The pressure plate is braced on a securing ring that is disposed in an annular groove on the inside of the housing. A comparable embodiment of a vibration limit switch is described in German Patent Disclosure DE 198 14 669 A1: Once again, the drive elementxe2x80x94a piezoelectric stackxe2x80x94is prestressed between the diaphragm and a yoke part. Here as well, the yoke part is braced directly on the housing. For that purpose, stop parts are provided on the inner wall of the housing.
In the vibration detectors that have been disclosed, functional problems arise if the process conditions abruptly change, and especially if the process temperature suddenly rises. In the high-temperature range, process temperatures up to about 300xc2x0 C. can furthermore occur. If the process temperature increases suddenlyxe2x80x94that is, if a temperature shock occursxe2x80x94then the housing or so-called sensor tube expands much faster than the parts located inside the housing, which means the piezoelectric element or piezoelectric stack and the pressure plate or yoke part. As a consequence, the extremely undesirable situation ensues that the piezoelectric drive is no longer in mechanical contact with the diaphragm, or that the set prestressing between the piezoelectric drive and the diaphragm is lost. The vibration detector then functions only incorrectly or not at all. Only once the parts located in the interior of the housing have also been heated to the process temperature is the specified prestressing between the piezoelectric drive and the diaphragm restored, and the vibration detector is then fully functional again. Detector failures caused by a temperature shock can last on the order of magnitude of several minutes. This length of time is naturally entirely unacceptable for a limit state detector that is used to prevent against overfilling or against running empty.
The object of the invention is to provide a device that functions reliably even upon abrupt changes in the process temperature.
This object is attained in that a tubular inner part is provided, which is dimensioned such that it is positioned between the housing wall and the drive/receiving unit, and that the tubular inner part is connected to the housing or to the diaphragm.
Depending on the embodiment of the device of the invention, the parts disposed inside the housing, in particular the prestressed drive/receiving unit, are completely or partly decoupled thermally from the housing by the tubular inner part. If a temperature shock occurs, the housing heats to the process temperature relatively fast. However, it takes a relatively long time until the tubular inner part and the drive/receiving unit assume the altered process temperature. This is due to the poor thermal conductivity of the air in the air gap between the tubular inner part and the housing wall and of the tubular inner part and the drive/receiving unit, which in the high-temperature range is preferably made from piezoceramic materials. To guarantee high corrosion resistance, the housing and the tubular inner part of the vibration detector are moreover both made from stainless, acid-resistant steel. Because of the thermal decoupling of the housing and the drive/receiving unit provided according to the invention, the different thermal expansion of the individual parts does not impair the proper function of the vibration detector.
In a preferred feature of the device of the invention, the tubular inner part is connected to the housing or to the diaphragm in a defined region. The fastening of the tubular inner part to the diaphragm or the housingxe2x80x94in the latter case, the connection is preferably made in the vicinity of the diaphragmxe2x80x94is optimal, since in the event of a sudden temperature increase, the heat can spread evenly over the tubular inner part and the drive/receiving unit.
In an advantageous refinement of the device of the invention, the tubular inner part, in the region of the connection of the tubular inner part and the housing, has a beadlike ring, whose outer edge in the installed state is located essentially in the same plane as the outer wall of the housing. The connection between the tubular inner part and the housing, or between the tubular inner part and the diaphragm, is preferably done by welding. It is understood, however, that a screw connection or any alternative way of connecting the individual parts to one another can also be employed.
In a preferred embodiment of the device of the invention, the drive/receiving unit is a piezoelectric stack composed of at least two piezoceramic disklike elements; one disklike element functions as a drive unit, while the other disklike element functions as a receiving unit. The most various drive/receiving units are already known from the prior art. It is self-evident that in conjunction with the present invention, all types of drive/receiving unit can be used. As especially advantageous features, drive/receiving units that are used in vibration detectors offered for sale and distributed by the present Applicant under the tradename xe2x80x9cLiquiphantxe2x80x9d can be named.
One favorable feature of the device of the invention is that the drive/receiving unit is positioned between the diaphragm and a fastening part, the latter being braced on the inner wall of the housing or on the tubular inner part. The fastening part is preferably a pressure screw, with which the desired prestressing between the drive/receiving unit and the diaphragm can be preset.
In the case where the fastening part is braced on the tubular inner part, care must be taken that the material comprising the tubular inner part have a coefficient of thermal expansion that is as similar as possible to the coefficient of thermal expansion of the piezoelectric drive and the fastening part. However, if a defined, sufficiently great prestressing is selected, then it is entirely possible to select materials for the individual parts that are not so well adapted to one another in terms of their coefficients of thermal expansion. Preferably, however, the housing, tubular inner part and fastening part, for instance the pressure screw, are all of the same material, such as stainless and acid-resistant steel. In this connection, it can be noted that the coefficient of thermal expansion of a piezoelectric material and the coefficient of thermal expansion of stainless and acid-resistant steel are approximately the same.
In the event that the fastening part is braced directly on the housing, the vibration detector will function reliably well after a temperature shock only if the housing comprises a material that does not expand, or expands only minimally, as a function of the temperature. Materials such as Inwar and Kowar have this property. The disadvantage of these materials is without doubt their limited applicability: Inwar and Kowar materials are known to be less corrosion-resistant.
If the fastening part is braced on the tubular inner part, then an advantageous refinement proposes that the tubular inner part, on the end region remote from the diaphragm, has a female thread, and that the fastening part is a pressure screw that has a male thread corresponding to the female thread of the tubular inner part. It is understood that for locking the fastening part to the tubular inner part, the types of fastening known from the prior art for prestressing the drive/receiving unit can also be used analogously.