Level measuring device

(42) Date of commencement of the patent: 15. 7.1996 (45) Date of issue: 25. 3.1997 (12) (56) Documentation:

US 4879902A

The invention relates to a measuring device for detecting the height (6, 7) of a free liquid level of a liquid (13) with a measuring electronics (1). their power supply (2), a line (3) for the signal outputs and the actual Meßwertaufnehmem, consisting of at least two electrodes (4, 5) and optionally a reference electrode (9) which are electrically connected to the measuring electronics (1).

According to the invention is provided

a) that the measuring electronics (1) is accommodated with at least one of the electrodes (4) in a housing (11),

b) that a known potting compound (12), which is inert to the liquid (13), the measuring electronics (1) together with their connections to the power supply (2), the signal line (3) and to the electrodes (4, 5 ) and wrapped

c) that the measuring electronics (1) is inseparably connected to a further electrode (5).

(73) Patent owner:

KONECNY FRANZ ING. A-1100 VIENNA (AT).

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AT 402 237 Β

The invention relates to a measuring device for determining the altitude of a free liquid level.

In such measuring instruments is to distinguish between those that determine in a given range, how high the liquid level is and those that determine whether the liquid level is above or below a predetermined mark or outside a predetermined range. The invention relates to the last mentioned devices.

Prior art devices of this type consist of an electronic control device, its power supply, the line for the signal outputs and the actual transducers, consisting of two electrodes, which are arranged at a predetermined vertical distance from each other and which are connected to the electronics via cable.

The structure of the device is the following: From an external power supply, the energy required for the device is supplied via a cable of the measuring electronics, which is located in a housing at a sheltered as possible position above the highest possible liquid level. From this housing also goes off the signal line, with which the results of the measurement are transmitted to the respective observer or user. Furthermore, connections for the two measuring electrodes are provided, wherein the lower electrode also carries a reference electrode or mitausgebildet as such.

The functioning of such systems has the basis that, depending on the position of the liquid level, the electrical resistance, usually an alternating current resistance, changes and so the height of the free liquid level can be determined based on a given physical model.

The operation of such a device is as follows:

As soon as the upper electrode is above the liquid level, the associated measuring signal which is obtained, for example by resistance measurement, between it and the lower or the reference electrode changes. Even though the lower electrode is no longer in the liquid, its measuring signal (when measured against a reference electrode) also changes. In this way, one obtains the basis for a two-step control of a pump or the like. with full value, blank value and intermediate value. Of course, only one level can be monitored, but more than two levels are not monitored because of the complex structure.

Areas of application for such measuring instruments are everywhere, where either the level of fluid must be kept between two heights prescribed by the position of the electrodes, so that when leaving this area by the changing signals in the signal line an inflow or outflow is changed, until the liquid level has again reached the predetermined level range or in devices, in which certain operating conditions must be maintained or avoided depending on the measured level of the liquid level.

Examples of the first application are intermediate tanks in process engineering, examples of the second application are protective devices for pumps, which may only pump water from wells, if a certain minimum water level is given. There are of course other applications, such as the use as alarm devices for a threatening overflow of a tank or emptying a gas tank of a motor vehicle, possible.

Although the prior art devices are proven in operation, but have a number of disadvantages: it is difficult, for example, when using with domestic water works to find a suitable mounting location for the electronics and it is also in the presence of such a place an unpleasant work correctly install various cables. The passageways of these wires through the electronics housing are inherent weak points of the devices, which are specifically designed for environments that are hostile to electronics.

In addition, in the well shaft or in the tank or the like. the two electrodes must be drained, with an as accurate as possible vertical and superimposed layer is that, without there being a special mounting option, even over the entire period of use, usually several years, is complied with. That this is difficult to guarantee is readily apparent.

The invention aims to avoid these disadvantages and to provide a measuring device of the type mentioned, which is easy to assemble and does not have the disadvantages of the prior art solutions.

According to the invention it is provided that the measuring electronics firmly connected to at least one electrode housed together with her in a housing and with a known potting compound, which is inert to the liquid whose level height is inert, is welded and that the measuring electronics is inseparably connected to the second electrode, wherein the electrical connection points are also enveloped by the potting compound.

The measures according to the invention make it possible to dispense with any electrical connection in the area of ​​the measuring point to be produced on site since the entire measuring arrangement is fixed

AT 402 237 Β and lead out only the electrical conductors for the power supply and data transmission from the device. The electrical conductor between the measuring electronics and the second electrode determines the distance between the two measuring electrodes in a reliable manner, since there is no other electrical conductor in this area, so that entanglements or the like. excluded are.

In a preferred embodiment, it is provided that both electrodes are rigidly housed with the measuring electronics in a single housing. By this measure, the accuracy of the distance between the two electrodes is further improved.

In another embodiment, it is provided that the electrical conductors for the power supply and the data transmission are guided in a single cable, which further increases the reliability and the accuracy of the lowering of the device according to the invention.

A device according to the prior art and a device according to the invention are illustrated by way of example in the accompanying drawings, to which the following description refers. 1 shows a device according to the prior art, Fig. 2a shows a device according to the invention with an elastic connection between the two electrodes and Fig. 2b shows a device according to the invention, in which both electrodes are attached to a common housing.

In Fig. 1 a measuring device according to the prior art is shown. An above the highest expected liquid level to be mounted control electronics 1 is powered by a cable 2 with energy. The measuring signals are passed via a cable 3 to the receiver, not shown. Furthermore, leads from the control electronics 1, a cable to an upper electrode 4 and another cable to a lower electrode 5, which also serves as a reference electrode or carries such a reference electrode, but which is not specifically shown.

An alternating voltage is present at the two electrodes 4, 5 or between them and the reference electrode 9 (FIG. 2), and the resistance changes as a function of the height of the liquid level, as explained above. Depending on the current fluid level, this will result in the full value or blank value being retained.

The achievement of the two aforementioned values ​​is detected by the control electronics 1 and converted into signals, which are passed through the line 3.

Thus, for example, when the liquid level drops below the level 6 in the case of a domestic waterworks, the power supply to a pump, not shown, are interrupted in order to prevent their dry running. It may be the signal line 3 directly the power supply line of the pump when the power consumption is not too large. But it can also be the signal line 3 is a typical signal line, namely a low-voltage line, which is designed only for low power.

A situation similar to that shown in Fig. 1 is shown both in Fig. 2a and in Fig. 2b in each case embodiments of the invention. In this case, comparable components with the same reference numerals as in Fig. 1 are provided.

The control electronics 1 carries erfindungsgmäß an electrode, preferably the upper electrode 4 and is located together with this in a housing 11, preferably a pipe section made of stainless steel, aluminum or plastic. The control electronics 1 and the upper electrode 4 are molded solid and liquid-tight in the housing 11 by means of a potting compound 12.

In the embodiment shown, which represents a preferred variant of the invention, the cable 2 for the power supply and the signal line 3 of FIG. 1 connected to a cable 8, which is led out centrally from the housing 11 upwards. For this purpose, and also to protect the electrode 4 from damage, the housing 11 is extended upward beyond the electrode 4 also and preferably also completed by potting compound 12, wherein the combined cable 8 emerges substantially centrally from this lid-like closure 12.

In order to allow the access of the measuring liquid 13 to the electrode, at least one opening 14 in the housing 11 is provided. Preferably, as shown, a plurality of such openings, offset in height and along the circumference, arranged to ensure escape of air and unhindered inflow and outflow of the liquid.

According to a first embodiment of the invention, which is shown in Fig. 2a and which is particularly preferred when there is a large vertical distance between the two to be monitored liquid levels 6 and 7, it is provided that of the control electronics 1 from a connecting cable 10 to lower electrode 5 leads, whose connection points are also firmly embedded in the control electronics 1 by the potting compound 12.

AT 402 237 Β

At the other end of the cable 10, an electrode housing 11 a, preferably also a pipe section, by means of potting compound 12 is mounted at a predetermined distance. In the potting compound, the electrical connection of the cable 10 is housed protected both with the lower electrode 5 and with the reference or reference electrode 9 drawn extra in this embodiment.

The housing 11a preferably protrudes downwards over the reference electrode 9, which is longer than the lower electrode 5, in order to protect both electrodes against mechanical damage. Again, openings 14 are provided for the reasons mentioned above.

The distance between the upper electrode 4 and the lower electrode 5 is given by the height of the two serving as electrode carrier housing 11 and 11 a and the length of the connecting cable 10. A reduction of this distance can be achieved by looping in the cable, but it is preferred that the cable is cast during manufacture in a predetermined length.

An embodiment which is preferably used when the vertical spacing of the electrodes is not too large is shown in FIG. 2b: Both electrodes 4, 5 and the reference electrode 9 are accommodated in a single housing 11, which further facilitates the assembly.

Also, the mechanical resistance is increased, since the connection cable of Fig. 2a and thus eliminates the risk of breakage and the risk of changing the distance between the electrodes.

The invention is not limited to the illustrated embodiments and the described applications.

Thus, it is possible to provide a plurality of electrodes and thus come within the range of the initially mentioned in the description of the first measuring instruments. In this case, the electrodes can protrude, similar to the tines of a comb, either outwards or preferably into an axial cavity of the housing.

The electronics can also record or process other measured values, for example water hardness, temperature or the like. Here, too, the housing advantageously serves as a carrier for the actual transducers.

The housing itself can have a wide variety of cross sections and dimensions, depending on the purpose and the available space. The material must be able to withstand the environmental conditions and must not affect the measurement. Predictable influences of the housing can be compensated by the measuring electronics. Examples of materials in water pipe construction are common plastics, aluminum and stainless steel.

The electrodes are subject to the same conditions as the housing, stainless steel (austenitic alloys), copper or other, adapted to the particular conditions materials are easily selectable for the skilled person in the given field of application.

As a potting compound, the most diverse, already used in the field of electronics materials can be used. As casting resin for water as the measuring liquid, only WEPURAN casting resin VT 3407 of Lackwerke Peters in the Federal Republic of Germany, further examples are in Stamm u. Hanella: Electrical Casting Resins, Berlin; Verl. Technology 1968 or under the VDE-standardized cable potting compounds to find.