GB2108667A

GB2108667A - Filling level limit indicator

Info

Publication number: GB2108667A
Application number: GB08225937A
Authority: GB
Inventors: Heino Zimmermann
Original assignee: Gustav F Gerdts KG
Current assignee: Gustav F Gerdts KG
Priority date: 1981-11-03
Filing date: 1982-09-10
Publication date: 1983-05-18
Also published as: DE3143515C2; DE3143515A1; GB2108667B; FR2515817A1

Abstract

A filling level limit indicator comprises an oscillating rod which extends through a partition (3) into a space receiving the material under detection. At least one transmitting piezoelectric element (12, 13) and at least one receiving piezoelectric element (9, 10) are disposed on the oscillating rod. The receiving piezoelectric element (9, 10) is located in an oscillating node between a first portion (4) of the rod which is connected to the partition (3) and a following portion (8) of the oscillating rod, whilst the transmitting piezoelectric element (12, 13) is located in an oscillation node between the second portion (8) and a third portion (11) of the oscillating rod. The indicator may be used with materials at high temperatures. <IMAGE>

Description

SPECIFICATION Filling level limit indicator This invention relates to a filling level limit indicator of the type having a piezoelectric transmitter-receiver means and a partition arranged between the latter and the space receiving the filled material.

A filling level limit indicator of this type is known from German Auslegeschrift No. 2512060 in which a piezoelectric element is fixed to the partition. The resonant impedance of the piezoelectric element changes as a function of whether the partition is immersed in air or the filled material. The impedance change serves as a filling level signal. However, unless the resonant frequency is very accurately fixed, the impedance change is so small that an unambiguous, undisturbed filling level signal is not always ensured. Owing to the parametric dispersion of the piezoelectric element, it is extremely difficult to fix the resonant frequency in an accurate manner.

At present, the temperature limit for the use of piezoelectric elements is approximately 1500C, 1 50 C, so that this limit indicator cannot be used for higher filled material temperatures.

The present invention aims to provide a filling level limit indicator of the aforementioned type, which can also be used for high temperatures and which provides a considerable signal deviation between the immersed and non-immersed state.

According to the invention, there is provided a filling level limit indicator with piezoelectric transmitter-receiver means and a partition arranged between the latter and a space receiving the filled material, wherein the piezoelectric transmitter-receiver means comprises at least one transmitter piezoelectric element and at least one receiver piezoelectric element, an oscillating rod comprising three portions is connected to the partition, the receiver piezoelectric element is located between the portion connected to the partition and the following second portion of the oscillating rod, the transmitter piezoelectric element is located between the second portion and the third portion of the oscillating rod, and the length of the rod portions are matched in such a way that both the transmitter piezoelectric element and the receiver piezoelectric element are located in the oscillation nodes of the resonant oscillation of the oscillating rod when not oscillation damped by filled material.

Through the provision of an oscillating rod, the piezoelectric elements are not directly exposed to the thermal action of the filled material. Owing to the natural temperature gradient in the oscillating rod, in the case of a corresponding length of the first oscillating rod portion, the temperature prevailing at the receiver piezoelectric element is well below the filled level temperature. Thus, the latter can exceed the permitted temperature of use of the piezoelectric elements.

When not immersed in the filled material, the oscillating rod oscillates at its natural frequency, in the case of resonance excitation by the transmitter piezoelectric element.

As the proposed probe is physically a coupled circuit oscillator in which the piezoelectric elements are only oscillation exciters, the dimensioning of the oscillating rod defining the resonant frequency, the latter is independent of the parametric dispersions of the piezoelectric elements. In addition, an increase in the band width of the resonance curve can be obtained through the choice of the oscillating rod materials, e.g. annealed wrought iron, so that variations from the desired resonance do not have great effects on the amplitude and phase position of the receiver voltage.

As both transmitter and receiver are in oscillation nodes, the receiver piezoelectric element, in a given phase angle to the transmitter voltage, supplies a signal voltage corresponding thereto. By immersion in the filled material, the oscillating characteristics of the oscillating rod undergo changes, so that the oscillation nodes are displaced therein. Thus, compared with the emerged state, there is a considerable phase displacement, as well as amplitude change between the transmitter and receiver a.c. voltage and this can be used for a clear and unambiguous signalling.

According to a preferred embodiment of the invention, the length of the third oscillating rod portion is approximately a quarter of the wavelength of the resonant frequency. Preferably, the length of the second oscillating rod portion is approximately one half of the wavelength of the resonant frequency. These arrangements give the shortest construction of the second and third oscillating rod portions permitting an optimum signal formation.

It is also preferred that the length of the first oscillating rod portion is a multiple of the wavelength of the resonant frequency, plus a quarter wavelength. This arrangement gives the optimum length formula for the first oscillating rod portion.

In order to make it possible to provide a partition able to withstand high pressures, without impairing the measuring sensitivity of the limit indicator, a part of the first oscillating rod portion remote from the receiver piezoelectric element desirably projects into the filled material space and the partition is desirably located in an oscillation node of the first oscillating rod portion.

Preferably, the length of the part of the first oscillating rod portion projecting into the filled material space is approximately a quarter of the wavelength of the resonant frequency.

Increased damping of the oscillating rod on immersion in the filled material is obtainable if a coupling plate is secured to the said part of the first oscillating rod portion, said coupling plate extending transversely to the axis of the oscillating rod. The coupling plate is preferably arranged in a laterally closed measuring chamber connected to the filled material space.

Desirably, the said part of the first oscillating rod portion has such a length that it corresponds to the wavelength in the filled material when said material is water and the diameter of the coupling plate is twice this wavelength. A lateral ring gap may be provided between the wall of the measuring chamber and the coupling plate, said gap having a width which is smaller than the sound wavelength of the filled material when said material is water. These arrangements permit a particularly great energy removal or a considerable change to the resonant frequency, in that the measuring chamber acts between the partition and the coupling plate like a Helmholtz resonator.

The three oscillating rod portions can be connected particularly simply by a central tie rod.

It is advantageous if all three portions are at the same electric potential, so that there is no need for the portions to be electrically insulated from one another. Such an equipotential can be obtained if the piezoelectric transmitter-receiver means has two superimposed transmitter piezoelectric elements which are connected in parallel and two superimposed receiver piezoelectric elements which are electrically connected in parallel.

The invention will now be further described, by way of example, with reference to the drawings, in which Fig. 1 is a part-sectional side elevation of one embodiment of a filling level limiter indicator according to the invention; and Fig. 2 is an oscillation diagram of the limit indicator shown in Fig. 1 not immersed in the filled material.

Referring to the drawings, the limit indicator according to the invention has a retaining flange 1 for connection to a container, of which only a cutout of its sidewall 2 is shown. This container can be e.g. a boiler in which the feed water filling level is to be monitored.

The retaining flange 1 centrally forms a pressure-tight partition 3, which carries a first oscillating rod portion 4, one part 5 of which projects e.g. horizontally into the container level with the desired feed water level and is transversely provided at the free end with a circular coupling plate 6. A connecting piece 7 on the retaining flange 1 surrounds the oscillating rod portion 5 and coupling plate 6 in the container.

A second oscillating rod portion 8 faces the outer end of the oscillating rod portion 4.

Between these two portions are located two mechanically series-arranged receiver piezoelectric elements 9, 10, which are electrically connected in parallel. Finally, there is a third oscillating rod portion 11 and between it and the second oscillating rod portion 9 are located two mechanically series-arranged transmitter piezoelectric elements 12, 13, which are also electrically connected in parallel. The piezoelectric elements 9, 10, 12, 1 3 can be attached in a simple manner, e.g. by a central tie rod (not shown) which mechanically and electrically interconnects the three oscillating rod portions 4, 8 and 11.

The transmitter piezoelectric elements 12, 13 with the oscillating rod portions 8, 11 form a first coupled circuit oscillator and the receiver piezoelectric elements 9, 10 with the oscillating rod portions 4, 8 form a second coupled circuit oscillator, which is fixed to the first. The third oscillating rod portion 11, plus the piezoelectric element 12 has a length ;1/4, in which A is the wavelength of the resonant frequency of the oscillating rod 4, 8, 11.The second oscillating rod portion 8 with the two piezoelectric elements 9, 1 3 has a total length of j1/2, whilst the first oscillating rod portion 4, together with the piezoelectric element 10 has a length of nxA+A/4. The factor n is selected in such a way that at the piezoelectric elements 9, 10, the temperature is so far below the filled material temperature owing to the natural temperature gradient in the oscillating rod portion 4 that it is below the permitted operating temperature for the said piezoelectric elements 9, 10.

The diagram of Fig. 2 shows the oscillation course in the oscillating rod 4, 8, 11 when the latter is excited in resonance by the transmitter piezoelectric elements 12, 13 and the coupling plate 6 is surrounded by air or steam. Thus, the coupling plate 6 is located in the area of maximum longitudinal oscillation amplitude A.

Both the transmitter and the receiver piezoelectric elements 9, 10, 12, 13 are in two oscillation nodes. The signal voltage measurable at the receiver piezoelectric elements 9, 10 is displaced in phase by 1 80C relative to the feed voltage for the transmitter piezoelectric elements 12,13.

The partition 3 is also located in an oscillation node, which obviates the need for a very flexible and elastic construction of the partition 3 and a high compressive strength thereof is obtained. In addition, possible disturbing influences are essentially avoided by supporting the oscillating rod portion 4 by means of the partition 3.

If the filling level rises to such an extent that a measuring chamber 1 4 formed in the connecting piece 7 is filled with the filled material, the mass of the filled material coupled to the coupling plate 6 changes the oscillating characteristics of the oscillating rod 4, 8, 11. Owing to the resultant change to the resonant frequency, the oscillating nodes in the oscillating rod 4, 8, 11 are displaced and consequently there is a change to the phase displacement between the feed voltage of the transmitter piezoelectric elements 12, 13 and the signal voltage of the receiver piezoelectric elements 9, 10. There is also an energy removal by ultrasonic radiation in the liquid and this leads to an additional damping of the signal voltage amplitude if the feed voltage remains constant.

These changes can be used for producing the signals Immersed and Emerged.

The change in the phase displacement between the feed voltage and the signal voltage can be determined in known manner for this purpose in the case of a forced transmitter frequency. Additionally or alternatively, it is also possible to measure the damping of the signal voltage amplitude.

Another possibility is to build up a selfoscillating system with the transmitter piezoelectric elements 12, 13, the excitation of the oscillations taking place by the feedback by means of the signal voltage of the receiver piezoelectric elements 9, 10. The amount of feedback is selected in such a way that there is no oscillation on immersing the coupling plate 6 in the feed water as a result of damping or phase displacement.

As a result of the special construction of the part 5 immersed in the filled material, when the latter is water there is an additional damping action and consequently an increase in the signal difference in the receiver piezoelectric elements 9, 10. The velocity of sound propagation in water is approximately a quarter of that in iron.

As a result of the construction of the part 5 immersed in the water with a quarter the wavelength of the oscillating rod iron, the water volume enclosed by the coupling plate 6 and partition 3 in the immersed state is excited to a full oscillation. The diameter of the coupling plate 6 is a quarter of the wavelength of the oscillating rod iron. The measuring chamber 14 acts in the manner of a Helmholtz resonator, which ensures a particularly great energy removal and consequently further increased the signal difference compared with the emerged state.

Claims

1. A filling level limit indicator with a piezo electric transmitter-receiver means and a partition rranged between the latter and a space receiving the filled material, wherein the piezoelectric transmitter-receiver means comprises at least one transmitter piezoelectric element and at least one receiver piezoelectric element, and oscillating rod comprising three portions is connected to the partition, the receiver piezoelectric element is located between the portion connected to the partition and the following second portion of the oscillating rod, the transmitter piezoelectric element is located between the second portion and the third portion of the oscillating rod, and the length of the rod portions are matched in such a way that both the transmitter piezoelectric element and the receiver piezoelectric element are located in the oscillation nodes of the resonant oscillation of the oscillating rod when not oscillation damped by filled material.

2. A filling level limit indicator according to claim 1, wherein the length of the third oscillating rod portion is approximately a quarter of the wavelength of the resonant frequency.

3. A filling level limit indicator according to claim 1 or claim 2, wherein the length of the second oscillating rod portion is approximately one half of the wavelength of the resonant frequency.

4. A filling level indicator according to any preceding claim, wherein the length of the first oscillating rod portion is a multiple of the wavelength of the resonant frequency, plus a quarter wavelength.

5. A filling level limit indicator according to any preceding claim, wherein a part of the first oscillating rod portion remote from the receiver piezoelectric element projects into the filled material space and wherein the partition is located in an oscillation node of the first oscillating rod portion.

6. A filling level limit indicator according to claim 5, wherein the length of the part of the first oscillating rod portion projecting into the filled material space is approximately a quarter of the wavelength of the resonant frequency.

7. A filling level limit indicator according to claim 5 or claim 6, wherein a coupling plate is secured to the said part of the first oscillating rod portion, said coupling plate extending transversely to the axis of the oscillating rod.

8. A filling level limit indicator according to claim 7, wherein the coupling plate is arranged in a laterally closed measuring chamber connected to the filled material space.

9. A filling level limit indicator according to claim 8, wherein the said part of the first oscillating rod portion has such a length that it corresponds to the wavelength in the filled material when said material is water and wherein the diameter of the coupling plate is twice this wavelength.

1 0. A filling level limit indicator according to claim 9, wherein a lateral ring gap is provided between the wall of the measuring chamber and the coupling plate, said gap having a width which is smaller than the sound wavelength of the filled material when said material is water.

11. A filling level limit indicator according to any preceding claim, wherein the piezoelectric transmitter-receiver means has two superimposed transmitter piezoelectric elements which are electrically connected in parallel.

1 2. A filling level limit indicator according to any preceding claim, wherein the piezoelectric transmitter-receiver means has two superimposed receiver piezoelectric elements which are electrically connected in parallel.

1 3. A filling level limit indicator substantially as described herein with reference to the drawings.