GB1603793A

GB1603793A - Apparatus and method for measuring the level of electrically conducting liquids

Info

Publication number: GB1603793A
Application number: GB2260978A
Authority: GB
Original assignee: Kernforschungsanlage Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 1977-05-27
Filing date: 1978-05-25
Publication date: 1981-11-25
Also published as: DE2723999A1; DE2723999C2; BE867530A

Description

(54) APPARATUS AND METHOD FOR MEASURING THE LEVEL OF ELECTRICALLY CONDUCTING LIQUIDS (71) We, KERNFORSCHUNGS ANLAGE JULICH GESELL SCHAFT MIT BESCHRXNK- TER HAFTUNG of Postfach 1913, 5170 Julich Federal Republic of Germany, a Body Corporate organised according to the Laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to an apparatus and method for measuring the level of electrically conducting liquids, using an electrode forming an electrical resistance, which is immersed in the liquid and is electrically connected thereto.

Measuring apparatus for indirectly reading the level of a liquid is required, for example, when a direct measurement of the level is not possible because of high pressure or inaccessibility of the measuring point. In addition to mechanical, acoustic, optical and pneumatic methods, there are electrical methods based on the variation of a capacitance, of a resistance or of the ratio of two resistances. Thus for example, there is disclosed in German Patent Specification 842,554 a measuring arrangement which consists of three electrodes, of which two are connected in series and which are immersed in liquid, and wherein the measurand is formed of the ratio of the contact resistances between the seriesconnected electrodes. However, the measurand ascertained in this known measuring arrangement exhibits very considerable non-linearity in regard to the liquid level to be ascertained.Attempts have been made to achieve a linearisation by means of a particular geometrical design of the electrode surfaces, but such a step is not applicable in all cases. Also, German Patent Specification 2,215,950 proposes a method aimed at eliminating the nonlinearity of the measurand which arises with the apparatus of the specification 842 554.

However, the result obtained is to derive from the measurement of an absolute quantity a ratio measurement which is independent of the supply voltage. In this case, the non-linearity of the measurand is not completely eliminated.

German Patent Specification 1 798 084 proposes an apparatus in which there is measured the stray potential which is set up at an elongate electrode immersed in the liquid and situated at a small distance from and parallel to a resistance electrode which is present in the liquid and through which a direct current is passed. This arrangement, however, has the disadvantage that the stray current accompanying the resistance electrode only partially flows through the immersed electrode acting as a potential sensor, so that the measurand again exhibits a non-linearity in regard to the level of the liquid. In addition it is not possible to avoid a characteristic non-linearity which results from the streamline displacement on the surface of the liquid and from the shunting effect of the base of the container and which depends upon the ratio of the spacing to the length of the two electrodes.

Moreover owing to the use of direct current in this known arrangement, the potential threshold between the electrode surfaces and the liquid has unknown values which are not stable over a period of time.

A further apparatus for measuring the level of a liquid is disclosed in German Auslegeschrift 1 908 921. It consists of two electrodes disposed coaxially one within the other one of which consists of a lowresistance metal and the other of a highresistance metal. This arrangement is based upon the idea of making the resistance of one electrode so high that the liquid acts as a short-circuiting slider. However, inaccuracies due to wetting of the part of the high-resistance electrode which is not immersed are substantially unavoidable.

According to one aspect of this invention there is provided apparatus for measuring the level of an electrically conducting liquid, having an electrode with an inherent electrical resistance, to be immersed in the liquid and electrically connected thereto by way of the surface of its immersed part, the end portions of which electrode are connected by means of respective supply conductors electrically insulated from the liquid, to an alternating-current source, there being no substantial current path between the said source and the said liquid except by way of the surface of the immersed part of the electrode, the electrode, the supply conductors and the alternating-current source forming an electrical circuit, and there being provided means for measuring the potential difference, serving as a measure of the liquid, between the potential of the liquid in a region spaced from the electrode and the potential at a point in the circuit outside the electrode. In order that there should be no substantial current path between the said source and the said liquid they should, in particular, not both be connected to a common earth.

In another aspect the invention provides a method of measuring the level of an electrically conducting liquid by use of apparatus according to the first aspect of the invention with the electrode immersed in the liquid, the shape of the electrode and the spacing between it and the said region of the liquid being such that the potential of the liquid at that region is substantially equal to the potential of the mid-point of the immersed part of the electrode, while the resistance of the electrode and the frequency of the alternating voltage supplied by the alternating current source are such that on the one hand the resistance of the electrode and on the other hand the capacitive reactance of any insulating separating layer produced on the surface of the electrode, for example by a gas or oxide film, are negligibly low in relation to the resistance between the electrode and the liquid in the said region.

In order that the potential at the said region should be substantially equal to the potential of the mid-point of the immersed part of the electrode, the region should be sufficiently far from the electrode as to be outside the region currents flowing from the electrode and back to it through the liquid in the vicinity of the electrode. These will be referred to hereinafter as shunt currents.

The invention is based upon the observation that the potential at the centre of the immersed part of the electrode corresponds to the potential of the liquid outside the region of the shunt currents which flow between different regions of the electrode through the liquid in the vicinity of the immersed part of the electrode, because with a sufficiently slender electrode, these - currents extend exactly symmetrically about the centre of the immersed part of the electrode. The potential difference between the potential of the liquid - and hence the potential of the centre of the immersed part of the electrode -- and the potential of a point in the circuit outside the electrode consequently exhibits a linear dependence upon the depth of immersion of the electrode.That point in the circuit at which the potential difference in relation to the potential of the liquid is picked up, for example when a generator is used as the alternating-current source, may be one of the terminals of the generator. In practice, the alternating-current source will often consist of the secondary winding of a lowcapacitance transformer from which the potential is tapped. For picking up the potential of the liquid, use may be made of the wall of the container, or an auxiliary electrode. If the liquid is conductively electrically connected to earth, the earth serves as reference potential.

Generally it may be desirable that the electrode is of substantially uniform resistivity and cross sectional area along at least that part of its length which is use can be either above or below the liquid surface as the liquid level changes and/or shaped so that the shunt currents in the liquid are essentially symmetrical about the part of the electrode in electrical contact with the liquid.

By virtue of the fact that the frequency of the alternating current is made such that the capacitive reactance of the separating layer which may be present on the electrode is negligibly low, the separating layer acts as a dielectric short-circuit and hence does not influence the measured value - in contrast to the known methods in which direct current is employed. This is generally the case with a frequency of the alternating voltage above about 5000 Hz.

There may be employed as the arrangement for measuring the potential difference an indicating amplifier tuned to the frequency of the alternating current and consisting of an alternating-voltage amplifier, a measuring rectifier and an indicating instrument or a synchronous rectifier. In this case, it has been found desirable to pick up the potential difference by means of a resistance connected in parallel with the alternating-current source and having an adjustable centre tap. Then, the quantity which is measured -- as an indication of the level of the liquid - can be a voltage which differs from the potential difference by an amount which is preset by adjustment of the centre tap. In this way, the zero point can be shifted to the beginning of the scale of the measuring instrument.

In an advantageous further development of the apparatus according to the invention, there is provided an arrangement for the automatic null point adjustment of the potential difference, the centre tap being shifted by means of a servo-motor and the adjustment distance being a measure of the potential difference and whence of the level.

For this purpose, there is desirably employed a self adjusting bridge arrangement which adjusts itself to the null point.

The electrode used in the apparatus of the invention may have different forms of construction. Thus, it may be advantageous for the portion of the supply conductor which, together with the electrode, is immersed in the liquid to be surrounded by a tube consisting of electrically insulating material, along the outside of which there extends a resistance wire forming the electrode. In a further very desirable form of construction of the electrode, for example, the electrode may consist of a tube whose electrical resistance is high in relation to the resistance of the supply conductors and low in relation to the resistance of the liquid and inside which there is disposed a metal wire or metal rod to form one of the supply conductors, the said wire or rod being electrically insulated in relation to the tube.The material for the tube may be, for example, graphite or, with an appropriately thin-walled tube, a metal resistant to attack by the media into which it is to be immersed.

In a further particularly advantageous form of construction of the electrode, the latter is surrounded by a protective tube formed with apertures for the passage of the liquid, the spacing between the protective tube and the electrode being such that the protective tube is situated outside the region of the shunt currents in the vicinity of the electrode. The protective tube may here consist of electrically conducting material and, in addition to protecting the electrode from damage, it may also serve to earth the liquid.

In cases where the apparatus according to the invention is to be used to measure the level of poorly conducting liquids, it is further desirable to provide a trimming capacitor (or trimmer) which is connected to one of the poles of the alternating-voltage source and to earth. In this way, good null point definition is ensured even with poorly conducting liquids. In addition, the permissible length of the supply conductors, which is up to 20 m depending upon the conductivity of the liquid, can thereby be extended. In this case, the supply conductor is preferably constructed in accordance with the double-screen technique, the inner screen being connected to a bootstrap voltage, i.e. a voltage sup lied by the amplifier and corresponding to the amplifier input voltage.The potential and current supply conductors must then be made separate as is customary in the remote measurement of resistances. On the other hand, in the case of liquids having very good conductivity, it must be ensured that the resistance value of the electrode is low in relation to the resistance value of the liquid.

In this way, it is possible to employ the apparatus according to the invention for liquids whose electrolytic conductivity is in a range extending from the conductivity of concentrated acids to the conductivity of technically pure water.

It has been found that, if the electrode is sufficiently slender, adequate linearity is obtained, but it is alternatively possible to make the electrode thick if an insulating disc is provided at that end of the electrode which extends into the liquid, because in this way the geometrical distortion occurring at the surface of the liquid is balanced out by an exactly opposed distortion at the opposite end and the symmetry of the shunt currents in the vicinity of the electrode, is restored.

The apparatus according to the invention may also be employed in cases where it is desired to determine the level of a liquid above which there is present another liquid, and the separating face of the two liquids is at the same time the surface of the lower liquid. It is also possible for a number of electrodes disposed at a relatively small distance apart to be operated simultaneously. For this it is merely necessary to ensure that the electrodes are each situated outside the respective regions of the shunt currents of flowing in the vicinities of other electrodes.

Instead of the electrode being immersed in the liquid in a vertical position -- which is preferred -- it may be immersed in the liquid in an inclined position. Also, the electrode may be introduced into the liquid from below through the base of the container.

In a further advantageous development of the invention, the electrode is of strip form, in which case it is preferable, if the vessel in which the liquid is kept consists of electrically conducting material, for the electrode to be disposed in the form of a metal strip at a short distance from the wall of the vessel. However, if the vessel is made of electrically non-conducting material, the strip-form electrode may alternatively be adhesively bonded to, or let into, the inside wall of the vessel.

In addition, a development of the invention resides in that the electrode is of spiral form. In this way, with equal metal cross-section, as in the case of an elongate wire, for example, a higher electrical resistance is obtained, so that a lower measuring current is required. The spiralform electrode may then consist of a wire wound on a bar of ceramic material, or of a spiral metal coating applied to a ceramic carrier member.

The electrode may make conductive contact with the liquid. However, in a further advantageous development of the invention, a separating layer of a material which does not conduct direct current is deliberately applied to the electrode. This development is based upon the observation that an intentionally applied electrically insulating layer also acts as a dielectric short-circuit for high frequency alternating current - up to about 1 MHz -- and hence also does not influence the measured value provided only that its capacitive reactance is sufficiently low in relation to the capacitive reactance of the insulation of the electrically insulated supply conductors immersed in the liquid.

In this case, it is desirable for the material of the separating layer to be a thermosetting or thermoplastic synthetic material, for example such as polyperfluoroethylene propy ene, phenolformaldehyde resin furan resin, melamineformaldehyde resin, aniline formaldehyde resin, polystyrene, polymonochlorotrifluoroethylene, polytetrafluoroethylene, polyvinylvinylidene, chloride, polyvinyl chloride, polyethylene, polyamide and synthetic substances produced from natural substances.

However, it is of course alternatively possible to use other electrically insulating plastics or other materials, for example such as ceramic materials (Al203), glazes (SiO2) and enamels for the production of the separating layer. It has been found desirable for the thickness of the separating layer to be 0.01 to 1 mm. Generally speaking, however, account should be taken of the increase in the electrically insulating layer and the resultant increase of the internal resistance in the indicating circuit by increasing the input resistance of the indicating amplifier and/or reducing the capacitance of the supply conductors. In order also to maintain linearity of the level indication when an electrode having an intentionally applied separating layer is used, this separating layer should have a uniform thickness.

Examples of the construction of apparatus embodying to the invention and the manner of operation thereof will be more particularly described with reference to the accompanying diagrammatic drawings, in which: Figure 1 illustrates the arrangement of an electrode with means for measuring the potential difference, Figure 2 shows a similar arrangement to that of Figure 1 with a bridge arrangement for the null point adjustment, Figure 3 shows a similar arrangement to that of Figure 1 with means for measuring the potential difference with automatic null point adjustment, Figure 4 is the electrical equivalent circuit diagram of the circuit of Figure 1 consisting of an electrode, supply conductors and an alternating-current source, Figure 5 shows a simple form of construction of an electrode, Figure 6 shows a like electrode, but with a protective tube, Figure 7 shows a further form of construction of an electrode, Figure 8 shows an electrode with an insulating disc, Figure 9 illustrates the form of construction of an electrode with an insulating layer, Figure 10 shows an electrode of stripform fitted to the side of a liquid vessel, and Figure 11 shows a spiral-form electrode.

As is apparent from Figure 1, the arrangement for measuring the level of a liquid consists of an electrode 1 which is connected by means of supply conductors 2 to an alternating-current source 3, i.e. an alternating-current generator. The electrode 1 is dipped into the liquid 4 situated in a container 5 consisting of electrically conducting material. In the region of the liquid 4, the supply conductors run within a body 6 consisting of electrically insulating material. For the measurement of the potential difference between the potential of the wall of the container -- and hence, as will be explained more fully hereafter, the potential of the point P (Fig.

4) at the centre of the immersed part of the electrode -- and the potential of one of the terminals of the alternating-current generator 3, there is further provided a measuring device 7 consisting of an alternating-voltage amplifier, a measuring rectifier and an indicating instrument. In Figure 2, there is additionally provided for picking up the potential difference a resistor RN connected in parallel with the alternating-current generator 3 and having a centre tap, which serves for the null point adjustment. In the arrangements shown in Figures 1 and 2, the potential difference, which serves as a measure of the level of the liquid, is measured by means of the indicating instrument.

In the case of the apparatus shown in Figure 3, a servo motor 8 is connected on to the output side of the measuring device 7, from which the indicating instrument may optionally be omitted. The servo-motor 8 operates automatically to set the centre tap of the bridge resistor RN to the null point position. Consequently, the regulating travel of the centre tap of the resistor R varies in proportion to the potential difference and hence is a measure of the level of the liquid.

In the equivalent circuit diagram shown in Figure 4, which corresponds to the arrangement illustrated in Figure 1, the resistance of the electrode 1 is shown as divided into component resistances R, to R5 of which Rl is the resistance of that section of the electrode which is situated above the liquid, while that section of the electrode 1 which is situated above the point P and is immersed in the liquid is subdivided into the resistances R2 and R3, and that section of the electrode which is situated below the point P is divided into the resistances R4 and R5.

The resistances of the individual sections with respect to the liquid at a point X of the liquid outside the region of the shunt currents flowing in the vicinity of the electrode, is shown in the form of resistances R. to Re. The resistances of the supply conductors 2 are Rh and Rs. If the resistances Rl to R5 are negligibly small in relation to the resistances R. to R,, the following applies R4+R5+Rr UZP=UZX=UO Rh+R1+R2+R3+R4+Rs+Rr where UG is the voltage present across the terminals of the alternating-current generator UPX is the voltage between the points P and X, Uzx is the voltage between the points Z and X, and Uzp is the voltage between the points Z and P.

Since, with symmetrical distribution of the shunt currents surrounding the electrode, for example with a slender cylindrical electrode whose diameter is negligibly small in relation to its length, Rye is substantially equal to Ra and Rb is substantially equal to Rd, Rc becomes currentless and hence we have upx=o and Upz=Uzx, respectively.

The voltage measured by means of the measuring device 6 therefore has linear dependence upon the depth of immersion.

In the case of the form of construction of the electrode 1 as illustrated in Figure 5, the latter consists of a simple resistance wire which serves at the same time as a supply conductor 2 and for this purpose is passed centrally through an insulating tube 6. A portion of the wire extends along the outside of the tube. It is conductively connected to the liquid and serves as the electrode 1. An electrode constructed in the form illustrated in Figure 5 and additionally having a protective tube 9 surrounding the electrode 1 is illustrated in Figure 6. The protective tube formed with apertures 10 is here mounted on the insulating tube 6 by means of a connecting body 11.

The spacing between the protective tube and the electrode is such that the tube lies outside the region of shunt currents surrounding the electrode. The tube can be of conductive material and used to earth the liquid.

The electrode 1 shown in Figure 7 consists of a poorly conducting tube made of graphite or of a thin-walled high-grade steel tube, for example of austenitic chromium-nickel steel, in which there is disposed as the supply conductor 2 a metal rod or bar, from which the graphite tube is separated by insulation 12.

In Figure 8, there is illustrated an electrode 1 whose form of construction corresponds substantially to that of the electrode illustrated in Figure 7, but having in addition an insulating disc 13 provided at its end extending into the liquid. This form of construction of the electrode is preferable when if the disc were absent, Re would be less than R. owing to the fact that the diameter of the electrode is large in relation to its length. Since the symmetry of the course of the shunt currents is restored by the insulating disc 13, a relatively thick electrode can consequently be used when such an insulating disc is provided.

There is illustrated in Figure 9 an electrode 1 whose form of construction also corresponds substantially to that of the electrode illustrated in Figure 7, but which.

is coated by a thin separating layer 14 consisting of electrically insulating material.

The electrode 1 illustrated in Figure 10 is in the form of a straight strip disposed parallel to the well of the container 5 and slightly spaced away therefrom. If the wall consists of electrically conducting material, insulation will be provided where the electrode passes through the wall.

The electrode 1 illustrated in Figure 11 is of spiral form. Such an electrode can be produced, for example, by first applying a metal coating to a bar of ceramic material and thereafter spirally milling away a part of the metal coating.

The forms of apparatus described above are of simple construction and provide substantial linearity in the relationship between the depth of immersion of the measuring sensor, of the level of the liquid and of the ascertained measurand, without any complex electrode configuration being required.

WHAT WE CLAIM IS: 1. Apparatus for measuring the level of an electrically conducting liquid, having an electrode with an inherent electrical resistance, to be immersed in the liquid and electrically connected thereto by way of the surface of its immersed part, the end portions of which electrode are connected by means of respective supply conductors electrically insulated from the liquid, to an alternating-current source, there being no substantial current path between the said source and the said liquid except by way of the surface of the immersed part of the electrode; the electrode, the supply conductors and the alternating-current source forming an electrical circuit, and there being provided means for measuring the potential difference, serving as a measure of the liquid level, between the potential of the liquid in a region spaced from the electrode and the potential at a point in the circuit outside the electrode.

2. Apparatus according to claim 1 in which the means for measuring the potential difference includes a resistor connected in parallel with the alternatingcurrent source and having an adjustable centre tap, there being ascertained as the measure of the level of the liquid a voltage which differs from the potential difference by an amount preset by the adjustment of the centre tap.

3. Apparatus according to claim 2, in which the means for measuring the potential difference includes a device for the automatic null point adjustment of the potential difference, the centre tap being adjusted by means of a servo-motor and the amount of the adjustment being a measure of the potential difference and hence of the liquid level.

4. Apparatus according to any one of claims 1 to 3, in which a portion of one of the supply conductors is surrounded by a tube consisting of electrically insulating material, along the outside of which there extends a resistance wire forming the electrode.

5. Apparatus according to any one of the preceding claims in which the electrode is of strip form.

6. Apparatus according to any one of the preceding claims in which the electrode is of spiral form.

7. Apparatus according to any one of claims 1 to 3, in which the electrode consists of a tube whose resistance is high in relation to the resistance of the supply conductors, and within which there is disposed a metal wire or metal rod acting as one of the supply conductors and electrically insulated from the tube.

8. Apparatus according to any one of the preceding claims, in which the electrode is surrounded by a protective tube spaced therefrom and having apertures therein for the passage of the liquid.

9. Apparatus according to claim 8, in which the protective tube consists of an electrically conducting material.

10. Apparatus according to claim 9, in which the protective tube is connected to earth.

11. Apparatus according to any one of the preceding claims, in which there is provided a trimming capacitor which is connected to one of the poles of the alternating-voltage source and to earth.

12. Apparatus according to any one of the preceding claims, in which an insulating disc is provided at that end of the electrode which extends into the liquid.

13. Apparatus according to any one of the preceding claims in which the electrode is such as to make conductive contact with the liquid.

14. Apparatus according to any one of claims 1 to 12 in which the electrode is coated with a layer of a material which does not conduct electrical direct current.

15. Apparatus according to claim 14 in which the material of the said layer > is a thermosetting or thermoplastic synthetic material.

16. Apparatus according to claim 14, in which the material of the said layer is a ceramic material such as Also3.

17. Apparatus according to claim 14, in which the material of the said layer is a glaze such as SiO2.

18. Apparatus according to claim 14, in which the material of the said layer is an enamel.

19. Apparatus according to any one of claims 14 to 18, in which the thickness of the said layer is 0.01 to 1 mm.

20. Apparatus for measuring the level of an electrically conducting liquid substantially as herein described with reference to any of Figures 1 to 8 of the accompanying drawings.

21. Apparatus for measuring the level of an electrically conducting liquid substantially as herein described with reference to either Figure 10 or Figure 11 of the accompanying drawings.

22. Apparatus for measuring the level of electrically conducting liquids substantially as herein described with reference to Figure 9 of the accompanying drawings.

23. A method of measuring the level of an electrically conducting liquid by use of the apparatus of any one of the preceding claims with the electrode immersed in the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. measuring sensor, of the level of the liquid and of the ascertained measurand, without any complex electrode configuration being required. WHAT WE CLAIM IS:

1. Apparatus for measuring the level of an electrically conducting liquid, having an electrode with an inherent electrical resistance, to be immersed in the liquid and electrically connected thereto by way of the surface of its immersed part, the end portions of which electrode are connected by means of respective supply conductors electrically insulated from the liquid, to an alternating-current source, there being no substantial current path between the said source and the said liquid except by way of the surface of the immersed part of the electrode; the electrode, the supply conductors and the alternating-current source forming an electrical circuit, and there being provided means for measuring the potential difference, serving as a measure of the liquid level, between the potential of the liquid in a region spaced from the electrode and the potential at a point in the circuit outside the electrode.

liquid, the shape of the electrode and the spacing between it and the said region of the liquid being such that the potential of the liquid at that region is substantially equal to the potential of the mid-point of the immersed part of the electrode, while the resistance of the electrode and the frequency of the alternating voltage supplied by the alternating current source are such that on the one hand the resistance of the electrode and on the other hand the capacitive reactance of any insulating separating layer produced on the surface of the electrode, for example by a gas or oxide film, are negligibly low in relation to the resistance between the electrode and the liquid in the said region.

24. A method for measuring the level of an electrically conducting liquid, substantially as herein described with reference to any Figures of the accompanying drawings.