IE61520B1 - Arrangement for capactive filling level measurement - Google Patents

Abstract

The arrangement for capacitive fluid level measurement in a container made of electrically non-conductive material has two capacitive probes, which are arranged at a distance from one another in the container in such a way that the capacitance between the two probes depends upon the fluid level. Arranged in the vicinity of the container is a measuring transducer which is connected to an evaluation device separated therefrom via a connecting line and converts a high-frequency alternating current that is dependent upon the probe capacitance into an electrical measured value signal that is suitable for transmission via the connecting line, from which signal the fluid level to be measured is determined in the evaluation device. Inserted into the connecting line between the measuring transducer and the evaluation device is a filter arrangement which prevents the transmission of the high-frequency alternating current and allows the transmission of the measured value signal.

Description

Arrangement for capacitive filling level measurement The invention relates to an arrangement for capacitive filling level measurement in a container of electrically non-conductive material comprising two capacitive probes which are arranged in the container spaced from each other in such a manner that the capacitance between the two probes depends on the filling level, and a measuring converter which is arranged in the vicinity of the container and which via a connecting line is connected to an evaluating unit separated therefrom and converts a high-frequency measuring alternating current dependent on the probe capacitance to an electrical measured value signal which is suitable for transmission via the connecting line and from which in the evaluating unit the filling level to be measured is determined.

Arrangements of this type can be used both for continuous filling level measurement and for monitoring limit levels.

In continuous filling level measurement the capacitive probes extend over the entire height of the container and are so formed that the capacitance varies as linearly as possible with the filling level. For limit level monitoring capacitive probes of small length are arranged at the height of the filling level to be monitored in such a manner that the capacitance changes abruptly when its value exceeds or drops below the limit level to be monitored.

In the capacitive filling measurement in metal containers generally the capacitance dependent on the filling level is measured between a probe arranged in the container and the container wall, the container wall forming a grounded reference electrode simultaneously serving as shield. In containers of electrically non-conductive material, such as plastic tanks, no such natural reference electrode is present. For this reason in the container two parallel probes are arranged and the capacitance between the two parallel probes is determined as a measure of the filling level. However, because there is no grounded reference electrode interference and stray capacitances affect the measurement. Plastic tanks are used in particular for chemically aggressive materials such as acids or liquors which would attack metal containers. Since such material contents have a high electrical conductivity the capacitance of the filling material itself with respect to ground is in particular also among the interference and stray capacitances which affect the filling level measurement. The consequence of this is considerable measuring errors due to changes in the ground capacitance due to external influences, a contact sensitivity in the case of uncovered probes, which affects the zero point balance, and a non-linearity of the relationship between the measured capacitance and the filling state in continuous filling level measurement.

The problem underlying the invention is to provide an arrangement of the type set forth at the beginning which with low circuit expenditure eliminates the effects of interference and stray capacitances on the filling level measurement in containers of material which is not electrically conductive.

According to the invention this problem is solved in that into the connecting line between the measuring converter and the evaluating unit a filter arrangement is inserted which prevents the transmission of the high-frequency measuring alternating current and permits the transmission of the measured value signal.

In the arrangement according to the invention the filter arrangement is disposed in the circuit which is formed by the series circuit, connected via ground, of the ground capacitance of the filling material and the interference and stray capacitances of the connecting line and the circuits connected thereto. The filter arrangement thus prevents a high-frequency measuring alternating current flowing via said circuit and influencing the measuring result.

As a result both the non-linearity and the influence of the grounding conditions on the measurement are eliminated. The contact sensitivity in the case of uncovered probes is also no longer present because due to the filter arrangement i:_ the circuit is ground-free for high frequencies and may be grounded at any desired point.

These advantageous effects can be achieved with very low circuit expenditure. In the simplest case the filter arrangement may consist of a blocking circuit which is inserted into the connecting line and is formed by a parallel resonant circuit tuned to the frequency of the measuring J:, alternating current. If the connecting line consists of a lead with a plurality of conductors such a blocking circuit is inserted into each conductor of the connecting line. In every case the filter arrangement is preferably arranged directly at the output of the measuring converter or transformer .

Further features and advantages of the invention will be apparent from the following description of examples of embodiment which are illustrated in the drawings, wherein: Fig. 1 shows an arrangement according to the invention for the capacitive filling level measurement in a container of electrically non-conductive material for the case of continuous filling measurement, Fig. 2 is a schematic illustration of the capacitances which are present in the arrangement of Fig. 1 and affect the filling level measurement, Fig. 3 shows the electrical equivalent circuit diagram of the arrangement of Fig. 1 without the filter arrangement, Fig. 4 shows the electrical equivalent circuit diagram of the arrangement of Fig. 1 with uncovered probes, Fig. 5 shows the electrical equivalent circuit diagram of Fig. 3 with the filter arrangement present and Fig. 6 is an illustration corresponding to Fig. 1 for the case of a two-wire lead as example of a multiconductor connecting line.

Fig. 1 shows schematically a container 10 of electrically non-conductive material, for example plastic, which contains a filling material 11 of which the filling level is to be measured. For measuring the filling level in the container two rod-shaped probes 12, 13 are arranged spaced from each other and extend in height over the entire filling level range to be covered. The probe 12 consists of a metallic probe rod 14 which is surrounded by an insulating sheath 15 and in the same manner the probe 13 consists of a metallic probe rod 16 surrounded by an insulating sheath 17. The insulating sheaths 15, 17 insulate the probe rods from the filling material 11 if the latter is electrically conductive and protect the probe rods against a corrosive or aggressive effect of the filling material.

The filling level measurement is based on the fact that the capacitance measured between the two probe rods depends on the degree of covering by the filling material. With insulating filling materials the two probe rods 14 and 16 form the electrodes of a capacitor, the dielectric of which in the covered region is formed by the filling material and in the uncovered region by air so that due to the different dielectric constants of these two media the capacitance measured is dependent on the filling level. In the case assumed in Fig. 1 of an electrically conductive filling material however each probe rod 14 or 16 forms in the covered region with the conductive filling material a capacitor of which the dielectric is formed by the material of the insulating sheath 15 and 17. The series circuit of these two individual capacitances may be measured between the two probe rods and the magnitude of said individual capacitances and thus the measured total capacitance depends on the degree of covering.

In filling measurement with a container of non-conductive material it is necessary to use two rod-shaped probes because, in contrast to filling measurement in a metal container the container wall cannot be utilized as counter electrode. The two rod-like probes are shown a considerable distance apart in Fig. 1 only for clarity,- in practice they may be combined mechanically to form a constructional unit, for example in the form of a double rod probe. Of course, in known manner instead of rod probes cable probes may also be employed.

For measuring the filling-level-dependent probe capacitance between the two rod-shaped probes 12 and 13, in the vicinity of the container 10, generally in the probe head at the top side of the container, a measuring converter 20 is disposed which is connected by a connecting line 22 to an evaluating unit 21 arranged at a remote point. In Fig. 1 it is assumed that the connecting line 22 is formed asymmetrically as coaxial cable, and for simplification only the inner conductor is shown.

The measuring converter 20 contains an oscillator 23 which generates a high-frequency AC voltage of which the frequency for example is of the order of magnitude of 1 MHz. The AC voltage is coupled via a transformer 24 into the circuit which connects the two probes 12 and 13 via two coupling capacitors 25, 26 to the two inputs of a current amplifier 27. Thus, in said circuit a high-frequency measuring value current I flows and the magnitude thereof depends on the capacitance between the two probes 12 and 13. The measuring alternating current is amplified by the current amplifier 27.

Connected to the output of the current amplifier 27 is a signal converter 28 which converts the amplified measuring alternating current to a measured value signal suitable for transmission via the connecting line 22. The measured value signal may have any form usual and known per se in measuring technology. For example, according to a widely used standard it may be a direct current variable between 4 and 20 mA or also a DC voltage variable between two limit values. The measured value signal generated by the converter 28 may however also be a pulse train which transmits the information on the magnitude of the measuring alternating current in one of the usual pulse modulation types (pulse amplitude modulation, pulse duration modulation, pulse frequency modulation, pulse phase modulation, pulse code modulation). Finally, the measured value signal may also be an alternating current or an AC voltage of lower frequency than the measuring alternating current IM, the measured value information again being transmitted by a suitable modulation of the alternating current or the AC voltage. In all these cases the measured value signal on the transmission line can be superimposed on a supply direct current or a supply DC voltage if in accordance with a likewise known and usual technique the transmission line 22 is simultaneously employed for supplying the measuring converter 20 from the evaluating unit 21 with the energy necessary for its operation.

Containers of electrically non-conductive material, such as plastic tanks, are usually employed when the material is chemically aggressive and would attack metal containers.

Such chemically aggressive filling materials, such as acids or liquors, generally have a high electrical conductivity. This presents problems as outlined above in capacitive filling level measurement.

Fig. 2 shows likewise schematically the previously described measuring arrangement and additionally various capacitances are represented which can affect the filling level measurement and Fig. 3 shows the corresponding electrical equivalent circuit diagram for the case of a filling material with high electrical conductivity.

In Fig. 2 is the capacitance between the transmission line 22 and ground, C2 the capacitance of the probe leadthrough and the capacity between the filling material and ground. The measuring capacitance between the two probes consists of the two individual capacitances and connected in series by the filling material. Cg denotes all the capacitances between the inputs and the outputs of the measuring converter 20 and Cy represents the capacitance of the evaluating unit 21 and the devices connected thereto with respect to ground.

The capacitances of Fig, 2 give the electrical equivalent circuit diagram of Fig. 3 when the capacitances C^, Cg and Cy of Fig. 2 are combined to a single capacitance Cg which is transformed to the input side of the measuring converter 20. The capacitances and Οθ depend on the filling level to be measured whilst the capacitances Οθ and Οθ depend on the particular local conditions. This gives a number of problems in the filling level measurement.

A first problem concerns the non-linearity of the relationship between the measured capacitance and the filling level. Although the capacitances 0^ and Οθ vary linearly with the filling level the total capacitance measured does not vary linearly with the filling level because the series circuit of the capacitances Οθ and Οθ lies in parallel with Οθ. The capacitance Οθ thus changes with the filling level relatively less than the capacitance 0^. In practice non-linearities up to 6 % have been measured resulting therefrom.

A second problem is the measuring accuracy. As mentioned above, the capacitances Οθ and Οθ depend on the local conditions. In particular, the ground capacitance Οθ of the filling material can also change greatly during the measurement, for example due to persons standing close to the container, the influence of variable filling levels of adjacent containers, changing ambient influences through transport systems near the container, sudden ground contacts through conductive media in incoming and outgoing conduits when filling or emptying the container, etc. In practice, considerable measured value deviations have been measured due to such influences.

Finally, a third problem is the contact sensitivity with uncovered probes, i.e. in an empty container. In this case the equivalent circuit diagram of Fig. 4 applies, the capacitance Cg representing the capacitance of the uncovered probe 12 with respect to ground. For this condition the measuring arrangement is balanced to the filling level zero. If for example a hand is moved up towards the probe 12 the capacitS ance Cg and thus the balanced zero point changes. The same of course applies when other foreign bodies are moved up to the probe 12.

In the measuring arrangement of Fig. 1 all these disadvantageous phenomena are rendered ineffective in that at the output of the measuring converter 20 into the connecting line 22 a filter arrangement 30 is inserted which blocks the measuring alternating current IM but allows the measured value to pass. In the simplest case the filter arrangement consists as shown in Fig. 1 of a blocking circuit 31 which is formed by a parallel resonant circuit having a coil 32 and a capacitor 33 and tuned to the frequency of the measuring alternating current IM„ Fig. 5 shows how the equivalent circuit diagram of Fig. 3 is changed by insertion of the blocking or rejection circuit 31. For the measuring alternating current the series circuit of the two capacitances and lying in parallel with the capacitance is interrupted. This interruption eliminates both the non-linearity caused by said series circuit and the influence on the measurement of the ground conditions caused by changes in the ground capacitance.

The contact sensitivity is also eliminated because the circuit is now ground-free for high frequencies and can be grounded at any desired point.

Fig. 6 shows an illustration corresponding to Fig. 1 for the case where the connecting line between the measuring converter 20 and the evaluating unit 21 is formed by a twowire lead 22. In this case the filter arrangement 30' is so configured that it prevents flowing of the high-frequency measuring alternating current via each of the two conductors of the two-wire lead 22’,, this being done in the example illustrated in that into each of the two conductors at the output of the measuring converter 20 a blocking circuit 31’ and 31s3 respectively is inserted which is formed by a parallel resonant circuit tuned to the frequency of the measuring alternating current. The remaining parts of the arrangement of Fig. 6 correspond to the parts of the arrangement of Fig. 1 denoted by the same reference numerals and will therefore not be described again.

Claims (5)

1. Arrangement for capacitive filling level measurement in a container of electically non-conductive material comprising two capacitive probes which are arranged in the container spaced from each other in such a manner that the capacitance between the two probes depends on the filling level, and a measuring converter which is arranged in the vicinity of the container and which via a connecting line is connected to an evaluating unit separated therefrom and converts a high-frequency measuring alternating current dependent on the probe capacitance to an electrical measured value signal which is suitable for transmission via the connecting line and from which in the evaluating unit the filling level to be measured is determined, characterized in that into the connecting line between the measuring converter and the evaluating unit a filter arrangement is inserted which prevents the transmission of the high-frequency measuring alternating current and permits the transmission of the measured value signal.

2. Arrangement according to claim 1, characterized in that the filter arrangement comprises at least one parallel resonant circuit which is tuned to the frequency of the measuring alternating current and which is inserted as blocking circuit in series into a conductor of the connecting line.

3. Arrangement according to claim 2, characterized in that with a multi-conductor connecting line a blocking circuit is inserted into each conductor of the connecting line.

4. Arrangement according to any one of the preceding claims, characterized in that the filter arrangement is disposed directly at the output of the measuring converter.

5. An arrangement for capacitive filling level measurement in a container according to any preceding claim substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.