Sensor for measuring the electrical conductivity of a liquid medium

The invention relates to a transducer for detecting the electrical conductivity of a liquid medium.

Such transducers are in various Known versions. Thus, EP-A-0386660 a Transducers measuring the conductivity of a fluid is known in which a plurality of electrodes in the form of Metal rings on one end surface of a cylindrical Probe body are arranged.

DE-A-2802 182 describes a measuring instrument for measuring the electrical resistance of a fluid with several plate-shaped electrodes in a common Enveloping cylinder are arranged. From EP-A-0193015 it is known, with a sensor for measuring the electrical Conductivity for the monitoring of combustion processes at least two electrodes with leads in the form of Conductor paths on an insulating, heat- and to arrange corrosion resistant substrate and the Supply lines, with the exception of the contact points with a heat and / or corrosion-resistant dielectric cover.

EP-B-0113427 describes an electrical probe for Detecting the electrical conductivity of a liquid Material to be measured with electrodes to form an in Probe longitudinally provided test section, the material to be measured by the is crossed, abgechirmten together with the material to be measured electrical leads a mechanically stabilizing and electrically conductive conductor framework of the form of forming probe determining casing body consisting of thermoplastic injection molding is prepared. A similar probe is also in DE 30 06 877..

Other transducers are from US4496906, US4808931 and DE4040333 known.

The invention is based on the object, a Measuring sensors for measuring the electrical to provide conductivity of a liquid medium, wherein in a relatively simple, but nevertheless stable Construction of the apparatus, the stray field of the measurement path through external influences as well as by the type of installation and Location in a medium receiving container or in the Flow of the medium is affected as little as possible without that an additional shielding must be provided.

According to the invention this object is achieved with the features of Patent claim 1.

Further developments of the invention result from the Subclaims.

The Meßwertäufnehmer invention allows a high, by external influences largely independent Accuracy in detecting the conductivity of a liquid medium. It is inexpensive to produce and also holds a relatively little gentle treatment in Operating stand permanently.

Preferred embodiments of the invention are described below with reference to the drawings.

In the drawings, schematically in each case view <dl tsize="7"><dt>Fig.1</dt><dd>Parts of an embodiment of the present invention Transducer in perspective and</dd><dt>FIG. 2</dt><dd>the embodiment of FIG. 1 in longitudinal section.</dd></dl>

The embodiment of the shown in Fig. 1 and 2 Transducer according to the invention consists of a Probe body 1, the electrically by a block of insulating plastic is formed and a cylindrical or rod-like elongated shape with a circular or having an oval cross section. In the illustration in Fig. 1 2 and it has in a lower region to an Longitudinal axis of the probe body 1 radially extending Recess 2, which is open at both ends, by a the Probe body 1 through extensive channel forms.

In the embodiment illustrated in the drawings the transducer is at the upper end of the cylindrical Probe body 1 a radially projecting end 8 formed, which is shown only in Fig. 2 and to can serve, the transducer in a potentially foreseen major measuring device or in a store stocker or fasten. From the arrangement of a derarigen, radially projecting End piece may possibly also be waived. It is also possible instead of radially projecting end piece 8 into not provide illustrated handle the drawings, with the transducer by hand in liquid medium inserted and held therein during the duration of the measurement can be.

In Innneren the probe body 1 extends in the axial Direction almost over the entire length thereof from a formed of a flexible material printed circuit board 3, the U-shaped is bent and with the free ends of its two legs to the edge of the upper boundary of the probe body 1 enough.

On opposing inside surfaces of the two legs of the U-shaped circuit board 3 in the amount the recess 2 are each a pair of electrodes 4a, 4b in a mutual distance parallel to each other arranged, that the electrodes 4a and 4b of the two Electrode pairs in the recess 2 each face. Each of the two pairs of electrodes 4a, 4b consists of an annular outer Elekrode 4a and a disk-shaped inner electrode 4b of the annular outer electrode 4a, while maintaining a intervening distance is surrounded.

Preferably, the two lateral surfaces of the Recess which face each other at a distance, at least in the direction of the central longitudinal axis of the Recess such that they protrude beyond the area which the electrodes 4a, 4b of the two electrode pairs on these lateral surfaces of the recess 2 occupy. This ensures that the outer ring electrode 4a of the two pairs of electrodes at its edges a certain minimum distance to an inner wall surface of a the liquid medium receiving container, or Guide channel or other interior surface in this Vessel or duct true, even if the Transducer is inserted into the medium such that Special the body 1, this inner wall surface or other Inner surface of the container or guide channel touches. By maintaining this minimum distance of the outer Edges of the electrode of any areas in which the liquid medium receiving container or duct it is achieved that an existing during measurement electric field between the outer edges of two of opposing electrodes 4a along curved outwardly Lines of force runs through inner surfaces of the container or will guide channel no way disturbed and distorted the measurement result are avoided.

The electrodes of each Elekrodenpaares 4a, 4b are each connected to conductor tracks 5, which on the surfaces PCB 3 and arranged in the printed circuit board are embedded so as to 3 of the printed circuit board are surrounded on all sides. The contact paths 5 result in the Representation of FIGS. 1 and 2 towards the top and are attached there a connector 6 is connected, at the upper end the probe body 1 is arranged and shown only in FIG. 2 is. The interconnects 5 are on the one hand to the respective Electrodes 4a and 4b, and on the other hand, to the Connector 6 is connected such that when terminal a not-shown external voltage source and a likewise not shown external electrical Meter to the connector 6, the annular outer Electrodes 4a of the two pairs of electrodes with the aforementioned voltage source are connected, while the discoid inner electrodes 4b of the two Electrode pairs are at the aforementioned instrument. Thus are the annular outer electrodes 4a than the Electrodes with the larger cross section than current electrodes connected, while the disc-shaped inner electrodes than the electrodes with smaller cross section than derm serve voltage electrodes to which the voltage drop practically in the acting as resistance medium no load measured and thus the conductivity of the Medium is determined.

To reduce the risk of damage to the to the circuit board 3 arranged, preferably gold coated electrodes 4a, 4b at Biegebeanspruch the flexible circuit board 3 to avoid the PCB is in the field of electrodes his back covered with rigid plates, which have approximately the size of each electrode pair.

The instrument used a conductivity meter is preferably used.

The electrodes 4a, 4b of the two electrode pairs are each provided with an electrically conductive coating, which has a higher electric potential Normal, ie nobler than the conductor material of the conductor tracks. 5 A applied to the circuit board 3 insulation coating covers the interconnects 5 except in the area of Connector 6 formed from contact points and leaves the electrodes 4a, 4b of Elekrodenpaare free.

Further, located on the circuit board 3 is also in Area of ​​the recess 2 as a resistance thermometer or temperature sensor acting temperature-dependent Resistor 7, the illustrated only in FIG. 2 and not separately shown, arranged on the circuit board 3 Interconnects also to the connector 6 connected.

The printed circuit board 3 and the thereon electric Components are from the plastic material of the probe body 1 coated, but the area of ​​the electrodes 4a, 4b of the two pairs of electrodes and the area of temperature-dependent resistor 7 with the interior of the 2 recess facing surfaces exposed. Preferably the probe body 1 is formed by injection molding, wherein the circuit board 3 arranged with her to electrical components as well as the connector 6 are injected into the probe body first This is unnecessary a separate mounting and electrical connection of PCB 3 and the connector 6 with the Probe body. 1

To capture the electrolytic conductivity of a liquid medium is the above-described Primary head with the recess 2 containing lower region in the medium such arranged such that it by the recess 2 of the probe body flow 1, while the electrodes 4a, 4b of each Electrode pair can Provide flow. Simultaneously, the annular outer electrodes 4a an external of the Voltage source supplied AC voltage or other uneven voltage applied. The current flow through the Medium is measured. At the inner electrodes of the Voltage drop measured on the medium. From the determined Values ​​for current and voltage, the conductivity calculated.

Illustrative is this still noted that according to changes in the applied voltage an electric field changes that between the opposite disk-shaped internal electrodes 4b is produced.

The stray field between the two pairs of electrodes lying test section is within the transducer, so that an adverse influence of external disturbance on Field is low. Additional shielding is therefore dispensable.

A possible function of the measurement result of the Temperature of the medium at the Conductivity detection is the temperature-dependent Resistance 7 compensated.

In a simpler embodiment of the invention is also a two-wire probe possible.

On the other hand, if necessary, also more four electrodes be provided.

The free end portions of the U-shaped circuit board can be superimposed on each other in parallel as This is shown in Fig. 2, or also of the upward facing surface of the probe body 1 protrude (not shown) to contacts with the connector 6 to be connected.

Instead of the U-shaped circuit board 13 can in manner not shown, two separate, mutually parallel printed circuit boards provided are to be arranged similar to the two Legs of the illustrated in Fig. 1 and 2 U-shaped PCB. 3

Such an arrangement also allows a division manner the probe body 1 into two symmetrical partial bodies along axially parallel with axis-parallel surfaces of a Plane, preferably the plane of symmetry, aneinandersetzbar (not shown in the drawings). In this case the connector can be 6 to the composite Partial body of the probe body 1 releasably attached and with the conductor tracks be 5 connected via detachable contacts. Such construction permits a particularly simple Maintenance of the electrodes by temporarily releasing the two subsidiary bodies of Sondenlkörpers 1 from each other, thereby unobstructed access to the electrodes for is cleaning purposes etc. possible.

In particular, when using a greater number of Elektoden several circuit boards can be provided be (inoe the drawings not shown).

On the circuit board 3 or on multiple boards of Transducer may provide additional active or are arranged passive circuit elements, eg. B. Preamplifier and / or circuit elements for the Signal separation or other signal processing (in the Drawings not shown).