DE2902471C2 - Arrangement for determining a liquid level - Google Patents

Description

characterized by the union of the following Characteristics:

e) using an alternating voltage source (14) which generates an alternating voltage, whose Frequency is dimensioned so that the capacitive component of the current between the one electrode plate (12) and the other electrode plate (13) is large compared to the ohmic component of this current,

f) the changing electrode plate (13) is connected to the Amigeeinrichtung (19, 20) connected, and

g) the electrode plates (Ί2,13- are on the wall of the tank (11) aligned parallel and at a distance from one another in a vertical position.

2. Arrangement for determining the liquid level according to claim 1, characterized in that the areas of the electrode (12, 13) are dimensioned proportionally to the respective volume of the tank (11) are.

3. Arrangement for determining the liquid level according to claim 1 or 2, characterized in that that each individual electrode (12, 13) consists of at least two on opposite surfaces of the tank (11) arranged partial electrodes consists.

4. Arrangement according to one of claims 1 to 3, characterized in that the alternating voltage is a high frequency sine wave.

5. Arrangement according to one of claims 1 to 3, characterized in that the alternating voltage is a pulse signal.

6. Arrangement according to claim 5, characterized in that the alternating voltage is a square wave is.

7. Arrangement according to claim 5, characterized in that the alternating voltage is a square wave is.

The present invention relates to an arrangement for determining the liquid level in one of one non-metallic material existing tank according to the preamble of claim 1.

Such arrangements are known. For example, from DL-PS 41 579 and from »Grave Electric Measurement of non-electrical quantities "(1962), pp. 260 to 266, in particular Figure 216, arrangements known, in which the liquid-contacting measuring sensors are arranged inside a tank. A disadvantage Such arrangements consists in the fact that the measuring sensors arranged in the liquid are highly corrosive Subject to influences and, in the case of a waste water tank, a catcher for solid or semi-solid materials

ίο form, which lead to a malfunction of the sensor can. When depositing solid or semi-solid materials on the in tanks for waste water storage arranged measuring sensors can also result in hygienic problems and unpleasant odors.

• i5 be.;. Another disadvantage is that it is used for Arrangement of such sensors in tanks is required. To provide holes in the tank walls.

From DE-PS 8 91 459 and DE-AS 2i 51 078 capacitive transducers for level measurements of liquid, solid and highly viscous media in electrically insulating containers are known. In the arrangement described in DE-PS 8 91 459, electrode pads are provided on the outer wall of the tank. In contrast, in the capacitive level measuring device known from DE-AS 21 51 078, the measuring electrodes of the measuring capacitor are provided on the inner wall of an open channel. In the case of the two last-mentioned known arrangements, the change in capacitance between the respective electrode arrangements is used to display the liquid level. The display of the liquid level can, however, be impaired in such arrangements in that, for example, in the case of waste water tanks, human waste and other solid or semi-solid materials contained in the waste water are deposited on the walls of the tank. As a result of the high conductivity of these waste substances or materials, the strength of the signal current flowing between the respective electrodes increases above the value that would only result from the pure liquid level in the tank. In particular, such deposits arise on the tank walls when the tank is moved or filled and then emptied.
The object of the present invention is therefore to provide an arrangement for determining the liquid level in a tank, by means of which measurements which are disturbed by the effects of pollution, for example as a result of leakage currents, can be avoided.
This task is carried out by one as mentioned at the beginning

so-called arrangement solved by the in the characterizing Part of claim 1 listed features is characterized.

A major advantage of the present invention is that due to the special arrangement of the Both electrodes, in contrast to the known arrangements, provide an accurate and linear level display.

Another advantage of the invention is that no sensors are inserted into the interior of a tank The arrangement according to the invention is therefore advantageous in particular for waste water tanks suitable as they are used, for example, in camping vehicles.

Another advantage of the invention is that the measuring device for level display at a central Place can be arranged, and that several tanks can be connected to the measuring device. In the following the invention and its aspect are

Events explained in more detail in connection with the figures It shows

F i g. 1 is a block diagram of a preferred embodiment of the invention;

F i g. 2A is a simplified block diagram of part of FIG Arrangement of the F i g. 1;

FIG. 2B shows an equivalent circuit diagram of the arrangement of FIG. 2A; and

F i g. 3 different waveforms to explain the operation of the invention.

F i g. 1 shows an arrangement 10 according to the invention for determining the liquid level in one of one non-metallic material existing tank ti. The tank 11 is provided with two parallel, spaced apart attached metal or electrode plates 12, 13 provided on a wall in a vertical Location are appropriate. When the liquid in the tank 11 rises, there are ever larger sections of the Electrode plates 12, 13 next to the liquid. The electrode plates i2, i3 can be at any vertical Wall of the tank 11, even be attached to the same on different walls. As in the following Will be described in detail, takes the plates on the electrode 12, 13 defined Capacity increases linearly with a linear increase in the liquid level. This effect can be used to to generate an output signal characterizing the liquid level in the tank.

The arrangement 10 includes an oscillator or an alternating voltage source 14, which is on a constant Value held alternating voltage generated. The output of the AC voltage source 14 is with a of the electrode plates 12 or 13, for example with the Electrode plate 12 connected. The other of the electrode plates, namely the electrode plate 13, is over a shielded cable 15 connected to one of the inputs of opposite polarity of an amplifier 16, which is part of a display device of the level display. The other input of amplifier 16 is connected together with the shield 17 of the cable 15 to the earth connection 18.

The output signal of the amplifier 16 is an alternating current signal, which indicates the liquid level in the tank. The signal is fed to the input of an AC-DC converter 19 is applied, which converts the alternating current signal into a direct current signal, with which a known display instrument 20 is operated. The gauge 20 and the transducer 19 are also part of the display device.

The working principle of the arrangement 10 is based on the fact that the water and other liquids are much larger Have dielectric constant than air. For example, water has a dielectric constant that is approximately 70 times the dielectric constant of air. The AC voltage source 14 supplies a AC voltage signal, which will be described in more detail below. This signal becomes the electrode plate 12, which is attached to one of the edges of the tank 11. If there is liquid in the Tank 11 is located, is that of the electrode plates 12, 13 defined capacitance is relatively small and the AC resistance is correspondingly high, whereby the current flowing between the electrode plates 12, 13 is limited. When the liquid level rises the capacitance between the electrode sheets 12. 13 increases. since the dielectric constant of the The gap between the electrode plates 12, 13 increases, so the AC resistance is reduced between the electrode plates 12,13 and es a larger current can flow between the electrode plates 12, 13.

This current is fed via the shielded cable 15 to the amplifier 16, the output voltage amplitude of which the current flowing between the electrode plates 12, 13 characterizes the AC voltage output signal is converted by the converter 19 into a DC voltage signal, which then is applied to the display instrument 20. The gauge 20 provides a linear indication of the liquid level in the tank 11.

When measuring the liquid level in the waste water tank of a camping vehicle or any other, In tanks containing human waste, there are certain difficulties. In the waste water tank The liquid contained is highly conductive and also contains solid and semi-solid substances? ;, which are attached to the tank walls be liable. If the Tar.k is moved or filled and then emptied, the Tank walls with a film of these highly conductive substances. The signal current between the electrode plates 12.13 can flow is therefore stronger than in the case in which there is only liquid without substances in the Tank 11 is located. A value that is too high is therefore displayed. The material adhering to the tank walls is often only dried after hours or days, whereupon the display returns to the correct value

The resistance of the material adhering to the tank wall is ohmic and therefore largely from independent of frequency. The conductivity of the liquid contained in the tank 11, on the other hand, is both ohmic as well as capacitive.

The capacitive conductivity is a function of the frequency of the AC voltage source 14. If according to the invention the capacitive component of the current is made large compared to the ohmic component egg:. Value can be obtained which corresponds to the actual liquid level in the tank 11.

F i g. 2A and 2B show a simplified representation of the arrangement 10 or its equivalent circuit diagram 30. In FIG. 2B, the capacity of the walls of the tank 11 is denoted by Cw. Cl denotes the capacity of the liquid 31. Rl denotes the resistance of the liquid 31. Ro denotes a small resistance which is used to measure the current.

As the current is given through a capacitor

'< ■ ^c "ir ·

while the current passes through a resistor

h = - = r

is given, the current through the capacitor can be increased by increasing dvlat , ie by increasing the frequency. This can be achieved by applying a very high frequency sine wave to the electrode plate 12. Furthermore, it is possible to achieve this by applying a pulse, preferably a square or square wave, to the electrode plate 12 because of such a waveform

a derivative can easily be obtained. Very high dv / df values are achieved through very short rise and fall times. so that there is a high momentum current through T _L. On the other hand, the current through the resistor Ri. Is initially equal to zero and only increases as quickly as the voltage applied to the capacitance Cl increases. So initially only current flows through the capacitor. A little later, current also begins to flow through the resistor Rl . After reaching a certain peak value of the current, the current decreases again when the voltage builds up at Cw. The voltage appearing at the resistor Ro is therefore the sum of these two currents.

The processes are carried out on the in F i g. 3 explained waveform shown. A square wave 32 assumed as an input signal has finite rise and fall times. The current through Ci. is shown as waveform 33, while the current through Ri. is shown as waveform 34. The current through Ro, shown as waveform 35, corresponds to the sum of these latter two currents.

If dv / dt can be made large, the peak value Ip of the current through R _{0 is given} exclusively by the capacitances C _L and Cw . The current //> is thus only a function of the path of the liquid 31 in the tank 11. The capacitive component of the liquid adhering to the walls of the tank 11 is very small compared to the capacity of the total amount of the liquid 11 and leads to an insignificant error. Similarly, the portion of the current resulting from R _{L gives a negligible error.}

These considerations are only correct if R _{0 is} small compared to the capacitive capacitor Cw and the series-connected capacitor Cl . The voltage occurring at A ₀ thus corresponds to the sum of the currents that flow through the circuit shown in FIG. 2A network shown

pi »IUn F? ¹¹ ' ^D * «ι> · ~ - ~ n: -. .-: .. j :. c: 11- ο

on and the circuit does not work. In other words, the voltage appearing at Ro must be proportional to dv / df, which means that the circuit must be a differentiating stage.

It has been empirically established that if R ₀ is sufficiently small, ie R ₀ values of 20 to 30 ohms in the case of tanks in camping vehicles, the display takes place practically without delay. With much larger Ao values, there are long, time delays.

It is known to those skilled in the art that there are numerous circuits that can derive an input signal produce. In the present invention, all of these circuits can be used as there are is essential. that the capacitive component of the signal current is measured, the resistance component is eliminated. In this case there is then an instant indication of the liquid level in one Tank made of a non-metallic material containing human waste, including the outside Conductive electrode plates attached to the tank can be used.

By arranging a pair of parallel electrode plates 12, 13 on the outer surface of the tank with an essentially vertical orientation, an accurate, linear output display, which indicates the level of the liquid 31 in the tank, where de · - increase in the liquid ins Tank the area of the electrode plates 12, 13 located next to the liquid increases As that of the electrode plates 12, 13 is proportional to the areas covered by the liquid 31 are Measurements are also possible on tanks with irregular shapes. The electrode plates 12, 13 then become so shaped so that the surfaces covered by the liquid during the rise correspond to the increasing volume of liquid correspond. This can be achieved very easily because the electrode plates 12, 13 made of aluminum foil with a sharp knife or scissors.

It can also be achieved that the operation with the electrode plates 12, 13 does not start from the flat position of the tank Il depends. When the plates are attached to one end of the tank or in a corner of the same the display of the display instrument 20 changes greatly when the tank 11 is tilted. On the other hand, if electrode plates are attached to all four corners of the tank 11, and two electrode plates at a time to the AC voltage source 14 and the other two electrode plates to the amplifier 16 are connected, there is an automatic correction when tilting the tank, since the If an electrode plate is tilted, an area that is no longer covered is automatically replaced by a corresponding one Enlargement of the cover of the other electrode plate is compensated. Electrode plates can also be attached to either side of the center of the tank 11 will. If the tank 11 is tilted in one direction, the area of one electrode plate becomes larger released, while a larger area of the other plate is covered by the liquid in the tank will. In the other direction, the level remains unchanged.

For this purpose 2 sheets of drawings

Claims (1)

Patent claims: 1. Arrangement for determining the liquid level a) in a tank (tl) _{r made of a non-metallic material} b) with two electrode plates (12, 13) attached to the Outer surface of the wall of the tank (11) are arranged, wherein when the liquid rises in the tank the areas of the plates next to the liquid and the capacity between to increase the plates, c) with an alternating voltage source connected to one of the two electrode plates (12) (14), and d) with a display device (16, 19, 20) for level display,