WO1993000573A1

WO1993000573A1 - Interface level detector

Info

Publication number: WO1993000573A1
Application number: PCT/EP1991/001185
Authority: WO
Inventors: Falk Alexander Schakowski
Original assignee: Endress & Hauser Gmbh & Co.
Priority date: 1991-06-25
Filing date: 1991-06-25
Publication date: 1993-01-07

Abstract

An interface level detector is provided for detecting the vertical position of an interface between two different media which may be immiscible liquids, a liquid (8) and a froth (9), or a liquid, or froth and a gas. The detector comprises a probe (1) having a series of detectors (2), generally in the form of electrodes (3), spaced apart along the length of the probe and arranged to be temporarily or permanently paired with a nearby electrode to determine the conductivity of a surrounding medium therebetween relative to a reference detector (2a). Control circuitry (10, 11, 12, 13) determines such conductivity between paired electrodes at a series of positions along the length of the probe sequentially and indicates a position at which the conductivity changes which corresponds to the position of the interface.

Description

"INTERFACE LEVEL DETECTOR"

FIELD OF THE INVENTION

This invention relates to an interface level detector by which term is meant a detector for detecting the position of a fluctuating interface level between two media which may be immiscible liquids, a liquid and a froth, and either a liquid or froth and a gas thereabove, such as air. BACKGROUND TO THE INVENTION

The proper control of many processes or the like is often dependent upon the vertical position of the interface level between two different adjacent media as indicated above. One such process is a froth flotation process in which the interface between the liquid and froth is of importance, although numerous other processes also require that any fluctuations in an interface level be detected.

As far as applicant is aware detectors for automatically detecting an interface level have, in general terms, not been adequately reliable for employment as part of an automatic process control arrangement, particularly in instances where a froth and pulp interface is to be controlled, and most particularly where such froth is deep and of high density.

Prior art interface level detectors have, as far as applicant is aware, operated on the basis of detecting changes in density, electrical capacitance, electrical conductivity, and also electrical resistance. In the case of the latter, one arrangement which has been employed with reasonable, but inadequately reliable results, is one in which the different resistivity of two media on each side of an interface has been employed, in combination with potentiometric resistance measurements to give an analogue output which is proportional to the interface level. Such an arrangement has, in particular in the case where the conductivity of the froth is almost equal to, or greater than, about half the conductivity of the liquid, been inadequately accurate as a measurement technique.

In the case of prior art interface detectors which operate on the basis of detecting changes in electrical conductivity using a series of individual electrodes spaced apart along the length of a probe, each electrode has been sequentially paired with a single, fixed, reference electrode which is usually located either at the top or the bottom of the probe. Such an arrangement has been found to be inadequately accurate as a measurement technique possibly due to stray electrical currents between electrodes; the different distances between the various electrodes and the reference electrode; and the depβndance of the conductivity measurements between the individual electrodes and the reference electrode on the conductance of the intervening media.

It is the object of this invention to provide an interface level detector which is particularly well suited to inclusion in an automatic process control arrangement and which will give sufficiently accurate results in all cases where the electrical conductivities of the two media meeting at the relatively interface are different to any reasonable extent.

SUMMARY OF THE INVENTION

In accordance with one aspect of this invention, there is provided a method of detecting an interface level between two media including the detection of the conductivity at a series of different levels relevant to the interface to be detected and determining between which two of the series a change in conductivity takes place.

Further features of this aspect of the invention provide for detection of the conductivity of the media at each level to take place sequentially and for each measurement to be compared with a reference measurement taken at substantially the same time and at a level permanently within one of the media which meet at the relevant interface.

In accordance with a second aspect of this invention there is provided an interface level detector comprising a probe adapted for upstanding location in a container in which an interface level is to be detected, said probe being provided with a series of detectors spaced apart along at least a portion of the length of the probe and adapted for detecting the conductivity of a surrounding medium between two operatively, and either temporarily or permanently paired detectors, at a series of positions along said portion of the length of the probe; and control circuitry for applying a voltage between the paired detectors to determine the conductivity of the surrounding medium at said series of positions, said control circuitry also having means for identifying the two adjacent paired detectors between which the conductivity changes.

Further features of the invention provide for the probe to have a reference detector pair at an operatively lower region thereof; for the detectors to be either arranged in pairs thereof at selected positions along said portion of the length of the probe, or for the detectors to be arranged to be intermittently paired with an axially adjacent detector and, in particular, for the detectors of each temporary or permanent pair thereof to be located at opposite sides of the probe; for the means for applying the voltage across the members of a detector pair to provide an alternating current output, for such output to be applied sequentially to the detector pairs of the series thereof; and for a comparator to compare each conductivity measurement of any detector pair with that of the reference detector pair. In order that the invention may be more fully understood, one embodiment, and a variation thereof, will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings :-

FIG. 1 is a schematic illustration of a probe and associated circuitry according to this invention;

FIG. 2 illustrates a section of the length of a probe having one detector pair associated therewith; and,

FIG. 3 illustrates a section of the length of an alternative probe.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In this embodiment of the invention an interface level detector probe, generally indicated by numeral 1, is provided with a series of temporarily paired detectors the pairs being indicated in Fig. 1, for illustrative purposes, by numeral 2. The series extends along the length of the probe and 5 the pairs are effectively located at a series of equally spaced positions.

In this arrangement, and as illustrated in Fig. 2, the individual detectors or electrodes 3, are arranged in two rows extending along the length of ^■jO the probe, with one row being located on each diammetrically opposite side of the probe. The detectors of each row thereof are equally spaced but are located centrally between two detectors of the other row thereof. These detectors are 5 arranged to be intermittently paired with a detector of one row being temporarily paired with the next lower electrode of the other row thereof, followed by the pairing of the latter with the next lower of the first row.

0 In this way, as indicated by dotted lines 4, the detectors are temporarily paired sequentially with two different detectors of the other row. This clearly cuts down enormously on the number of wires that have to be employed in the probe, as each detector serves as a member of two different pairs thereof intermittently, when compared to the case where electrodes are permanently paired together.-

The individual detectors are carried on a tubular support 5 and the electrical leads 6 to the detector members are passed centrally through the tube and are encased in insulating resinous material 7.

Spacing between the positions of location of the detectors is* chosen such that proper control can be exercised on a process or the like once it is known that the interface level is located between any particular two adjacent, temporarily paired, electrodes.

The lowermost detector pair 2a (see Fig. 1) serves as a reference detector pair and is arranged such that it will always be immersed in the lower of the two media, the interface between which is to be detected. In this embodiment of the invention, the interface level detector is particularly adapted for use in detecting the interface level between a liquid 8, and a froth 9, in a froth flotation process.

The control circuitry for use in conjunction with the detector probe described above includes a sinewave generator 10 for generating an alternating current voltage and a multiplexing circuit 11 adapted to intermittently pair the detectors and connect each of such detector pairs (other than the lowermost reference electrode pair) sequentially to the sinewave generator so that the conductivity of the medium present between the members of each intermittently formed detector pair can be measured sequentially.

It will be understood that the sequential detection of the conductivity at the position at which each detector pair is located avoids any possibility that electrical current strays from one detector pair to another. Also, it is to be noted, that each time the conductivity of a detector pair is measured, simultaneously the conductivity between the members of the reference electrode pair 2a is measured.

^ The two measurements are then compared by a comparator 12, the output of which is connected to a "sample and hold" circuit 13, which in turn provides an output to a level indicator 14. The

. fact that the conductivity at a level is compared 0 with that of the reference electrode, which is always in the lower medium, means that any fluctuations or variations in the conductivity of the lower medium are automatically compensated.

It has been found that a most reliable level 5 detection arrangement is provided by the invention.

In one practical implementation of the invention thirty electrode pairs were employed. In order to provide a conductivity profile a sample and hold and multiplexing circuits were adapted to scan the 0 electrode pairs fourteen or fifteen times and then average the reading to provide an output reading related to conductivity at each electrode pair position which could then be recorded permanently for subsequent analysis, if required. Such a set of conductivity readings can be employed, where required, to construct a conductivity profile through either or both media.

Whilst the arrangement of detectors and intermittent formation of pairs thereof is preferred (as it halves the number of conductors required) the detectors could be permanently paired. In this case, and as shown in Fig. 3, the individual detectors 15 of a permanent pair could be located at the same axial position and each pair would then acquire two conductors to service it.

It will be understood that numerous variations may be made to the embodiment of the invention described without departing from the scope hereof which is limited only to the detection of conductivity (ie. resistance) at various positions up the height of a range of possible locations of an interface level and determining between which two positions a change in conductivity takes place. In particular two vertically spaced interfaces may be detected by a single probe with obvious modifications to the circuitry and the possible inclusion of a second reference pair of detectors and also a second comparator circuit.

Claims

C A I M S

1. A method of detecting an interface level between two media including the detection of the conductivity at a series of different levels relevant to the interface to be detected and determining between which two of the series a change in conductivity takes place.

2. A method as claimed in claim 1 in which the detection of the conductivity of the media at each level is effected sequentially.

3. A method as claimed in claim 2 in which each measurement is compared with a reference conductivity measurement taken at substantially the same time at a level permanently within one of the media which meet at the relevant interface.

4. An interface level detector comprising a probe (1) adapted for upstanding location in a container in which an interface level is to be detected, said probe being provided with a series of detectors (2) spaced apart along at least a portion of the length of the probe and adapted for detecting the conductivity of a surrounding medium between two operatively, and either temporarily or permanently paired detectors, at a series of positions along said portion of the length of the probe; and control circuitry (10, 11, 12, 13) for applying a voltage between the paired detectors to determine the conductivity of the surrounding medium (8, 9) at said series of positions, said control circuitry also having means for identifying the two adjacent paired detectors between which the conductivity changes.

5. An interface level detector as claimed in claim 4 in which a reference pair of detectors (2a) is provided at an operatively lower region of the probe.

6. An interface level detector as claimed in claim 5 in which a comparator (12) is provided for comparing each conductivity measurement with that of the reference detector pair (2a) .

7. An interface level detector as claimed in any one of claims 4, 5, or 6 in which the detectors (15) are arranged in pairs thereof at a series of positions along the said portion of the length of the probe.

8. An interface level detector as claimed in any one of claims 4, 5, or 6 in which the detectors (2) are arranged to be intermittently paired with an axially adjacent detector to form temporary detector pairs at a series of positions along the said portion of the length of the probe.

9. An interface level detector as claimed in claim 8 in which the detectors are arranged in two angularly spaced, axially extending rows thereof with the detectors of one row being axially centrally located between two adjacent detectors of the other row thereof.

10. An interface level detector as claimed in any one of claims 4 to 9 in which the paired detectors are located on opposite sides of the probe.

11. An interface level detector as claimed in claim 10 in which the paired detectors are located diammetrically opposite each other and the probe is of circular shape in cross-section.

12. An interface level detector as claimed in any one of claims 4 to ll in which the control circuitry (10) is adapted to apply an alternating voltage to the paired detectors.

13. An inter ace level detector as claimed in any one of claims 4 to 12 in which the control circuitry is adapted to apply said voltage to paired detectors at said series of positions sequentially.

14. An interface level detector as claimed in any one of claims 4 to 13 in which the control circuitry includes means (13) for scanning the electrode pairs and retaining a reading for each electrode pair position in order to. provide a conductivity profile up the height of either or both media meeting at an interface.