DE102014109366A1 - Inductive conductivity sensor for measuring the specific electrical conductivity of a medium - Google Patents

Abstract

The invention relates to an inductive conductivity sensor (1) for measuring the specific electrical conductivity of a medium (6), comprising a transmitting coil (2) fed by means of an input signal, a receiving coil (3) coupled to the transmitting coil (2) via the medium Output signal which is a measure of the conductivity of the medium (6), and a housing (7) surrounding the transmitting coil (2) and the receiving coil (3), which has at least one housing section intended for immersion in the medium (6). whose housing surrounds the transmitter coil (2) and the receiver coil (3). The conductivity sensor (1) is characterized in that the housing section comprises at least one first electrically conductive contact (4) intended for contact with the medium (6) and a second electrically conductive contact (5) intended for contact with the medium (6), wherein the conductivity sensor (1) comprises an electrically conductive conductor (8) and the first contact (4) is connected to the second contact (5) via the conductor (8), the conductor (8) and the electrically conductive contacts (4 , 5) are configured such that they form a closed current path about the transmitting coil (2) and the receiving coil (3) via an ionic line (9) of the medium (6). The invention further relates to a method comprising a conductivity sensor (1).

Description

The invention relates to an inductive conductivity sensor for measuring the specific electrical conductivity of a medium and to a method for measuring the specific electrical conductivity with such a conductivity sensor.

Inductive conductivity sensors are used in a variety of applications in the laboratory and process measurement technology to detect the specific electrical conductivity of a liquid medium. They are preferably used where large measuring ranges and high chemical or thermal loads occur. This is the case, for example, in a large number of industrial chemical processes, but also in superheated steam sterilization processes, which are frequently used because of the high hygienic requirements in the field of food technology.

An inductive conductivity sensor comprises a transmitting coil and a receiving coil, which are generally designed as toroidal coils, also called toroidal coils. Such a conductivity sensor functions in the manner of a double transformer, wherein the transmitting and the receiving coil are inserted so far into the medium, that a running through the medium, the transmitting and the receiving coil enforcing, closed current path can form. When the transmitting coil is excited with an AC signal as the input signal, it generates a magnetic field which, in the self-contained medium path passing through the coils, induces a current whose magnitude depends on the electrical conductivity of the medium. Since this alternating electrical current in the medium in turn causes a variable magnetic field surrounding it, an alternating current is induced in the receiving coil as an output signal. This supplied by the receiving coil as an output AC or a corresponding AC voltage is a measure of the electrical conductivity of the medium.

For feeding the transmitting coil with an alternating voltage, an inductive conductivity sensor comprises a driver circuit connected to the transmitting coil. For detecting the output signal of the receiving coil, the conductivity sensor further comprises a receiving circuit electrically connected to the receiving circuit, which is configured to output the detected and optionally processed by the receiving circuit measurement signal to a sensor electronics, which serves to further process the measurement signal and optionally to digitize. Conductivity sensors are often designed as measuring probes which can be submerged in the medium at least in sections. Such a measuring probe has a housing in which the transmitting and receiving coil, possibly the driver circuit and the receiving circuit and further, combined with the transmitting and receiving circuit in a sensor circuit circuit parts are housed. In such an embodiment, the measuring probe is connected to a remote superordinate unit, for example a display unit, a measuring transducer, a computer or a control system. The higher-level unit can be configured both for supplying energy to the measuring probe and for data communication with the measuring probe. The optionally included in the probe sensor circuit may be configured to pass the further processed, possibly digitized measurement signal to the parent unit. The corresponding measured value can be displayed via the higher-level unit by means of a display device or output via a data interface.

Inductive conductivity sensors have a lower detection limit in the range of about 10-200 μS / cm. As a result, these sensors can not be used in pure and ultrapure water.

The reason for the lower measuring range limit for inductive conductivity sensors is that the measuring current induced in the analyte becomes very small in small medium conductivities and the existing capacitive coupling of the transmitting coil to the receiving coil generates a fundamental signal which is then greater than or equal to the measuring signal. In other words, the signal-to-noise ratio gets worse.

In addition to the already inductive conductivity sensors, there are also conductive conductivity sensors. In this case, the determination of the conductivity in the medium with a measuring arrangement, in which, as in a capacitor, two electrodes are opposite each other. According to Ohm's law, the electrical resistance or its reciprocal value, the conductance, is measured. With the cell constant determined by the sensor geometry, the specific conductivity is determined from the conductance.

In the region of high conductivities, polarization phenomena occur at conductive conductivity sensors at the electrodes which contact the analyte. Due to the principle, this effect does not exist with inductive conductivity sensors. Therefore, inductive conductivity sensors in the range of high conductivities and conductive conductivity sensors are used at low conductivities. Today, processes in which both very high and very low conductivities have to be measured are equipped either with two conductivity sensors, namely an inductive and a conductive conductivity sensor. or with conductive quadrupole sensors. However, the latter still have a lower upper measuring range than an inductive conductivity sensor.

The object of the invention is to provide a conductivity sensor, which has the ability to measure in pure and pure water, and it should be in the range of high conductivities no disturbing polarization effects.

The object is achieved by a conductivity sensor having a transmission coil fed by means of an input signal, a reception coil coupled to the transmission coil via the medium, which delivers an output signal which is a measure of the conductivity of the medium, and a housing surrounding the transmission coil and the reception coil, which has at least one housing section intended for immersion in the medium, the housing wall of which surrounds the transmitter coil and the receiver coil. The conductivity sensor is characterized in that the housing section comprises at least one first electrically conductive contact intended for contact with the medium and a second electrically conductive contact intended for contact with the medium, wherein the conductivity sensor comprises an electrically conductive conductor and the first contact via the conductor is connected to the second contact, wherein the conductor and the electrically conductive contacts are configured so that they form a closed current path about the transmitting coil and the receiving coil via an ion line of the medium.

It is thus possible to determine the conductivity by means of an inductive conductivity sensor even at low conductivities. At the same time, the possibility remains to determine larger conductivities with the same conductivity sensor. This is achieved by shortening the path length of the induced current in the medium, which increases the measuring current and thus extends the measuring range.

In a preferred embodiment, the first contact and the second contact is a metal contact, and the conductor is designed as a metallic conductor.

In an advantageous development of the first contact and the second contact are made in thin-film technology. In a variant, the first contact and the second contact are mounted on the surface of the housing section intended for immersion in the medium. The contacts can therefore also be retrofitted.

To prevent ingress of water, the first contact and the second contact are sealed to the interior of the housing.

In an advantageous embodiment, the first contact and the second contact are guided into the interior of the housing, and the conductor is at least partially guided in the housing.

Alternatively, the first contact and the second contact are guided outside the housing, and the conductor is at least partially out of the housing.

In a preferred embodiment, the conductor is designed such that it is guided, in particular wound, several times around the transmitting coil or the receiving coil or the transmitting coil and the receiving coil. Thus, the resulting measuring current can be increased by a multiple.

In an advantageous embodiment, the conductor comprises at least one switch which opens and closes the current path. The resulting extended measurement range can thus be adjusted as needed, i. depending on conductivity, on or off.

In this case, the switch switches individual guides, in particular turns, of the conductor, depending on the conductivity. With a low conductivity, more turns are switched on than with an increased conductivity.

The object is further achieved by a method comprising a conductivity sensor as described above, comprising the steps of: transmitting the input signal from the transmitting coil into the medium; Converting the output signal of the receiving coil into a value for the conductivity; and connecting the conductor to reduce the effective path length of the induced current in the medium when the value of the conductivity falls below a certain value.

The invention will be explained in more detail with reference to the following figures. Show it

1 the conductivity sensor according to the invention in a three-dimensional representation,

2 the transmitting coil and the receiving coil of a conductivity sensor,

3 a conductivity sensor in cross-section,

4 a conductivity sensor according to the invention in cross-section,

5 a conductivity sensor according to the invention in cross section in an embodiment, and

6 a conductivity sensor according to the invention in cross section in a further embodiment.

In the figures, the same features are identified by the same reference numerals.

The conductivity sensor according to the invention in its entirety has the reference numeral 1 and is in 1 shown. First, let's briefly look at the measuring principle of a conductivity sensor using the 1 . 2 and 3 To be received.

The cited figures show a sensor module of an inductive conductivity sensor 1 with a transmitting coil 2 and a receiving coil 3 in a housing 7 are housed. The transmitting coil 2 and the receiver coil 3 are arranged opposite each other on opposite sides of a printed circuit board (not shown). The transmitter and receiver coils designed as rotationally symmetrical toroids ("toroids") 2 respectively. 3 are arranged coaxially one behind the other in this way. The printed circuit board comprises the coils contacting conductor tracks, which are the transmitting coil 2 with a driver circuit and the receiver coil 3 connect to a receiving circuit. The driver circuit and the receiving circuit may be components of a sensor circuit arranged on the printed circuit board.

The housing 7 forms one the transmitting coil 2 and the receiver coil 3 along its axes of rotation passing through channel 10 , Will the housing 7 in an electrically conductive medium 6 immersed, this surrounds the housing 7 and penetrates the channel 10 one, so that in the medium a two coils 2 . 3 penetrating, closed current path 9 can form when the transmitting coil 2 is excited with an input signal, that is an AC voltage.

The conductivity sensor operates in the manner of a double transformer, wherein the transmitting and the receiving coil 2 . 3 as mentioned so far in the medium 6 be introduced that one through the medium 6 extending, the transmitting and the receiving coil 2 . 3 penetrating, closed current path 9 can train. When the transmission coil 2 is excited with an AC signal as an input signal, it generates a magnetic field, the one the coils 2 respectively. 3 passing through the current path 9 whose strength depends on the electrical conductivity of the medium 6 depends. This results in a current path with an ionic line in the medium 6 , Since this alternating electrical current in the medium again causes a variable magnetic field surrounding it, an alternating current in the receiving coil 3 induced as an output signal. This one from the receiving coil 3 supplied as an output signal AC or a corresponding AC voltage is a measure of the electrical conductivity of the medium 6 ,

However, the size of the induced measurement current also depends on the path length through the medium. By shortening the path length 9 of the induced current in the medium, the measuring current can be increased, and the measuring range can be extended.

4 shows an embodiment. There will be a contact with the medium 6 certain first electrically conductive contact 4 and one for contact with the medium 6 certain second electrically conductive contact 5 appropriate. The electrically conductive contacts 4 . 5 are preferably designed as metal contacts. In one embodiment, the contacts 4 . 5 sealed to the housing interior.

The contacts 4 . 5 are inside the case with a conductor 8th connected, the a closed current path around the transmitting and receiving coil 2 . 3 generated. In one embodiment, the conductor 8th a metallic conductor.

The contacts 4 . 5 and the leader 8th form over an ionic line 9 of the medium 6 a closed current path around the transmitter coil 2 and the receiver coil 3 out. In one embodiment, the conductor around the transmitting and receiving coil 2 . 3 wound.

In the area of high medium conductivities, this measure is not necessary and can be achieved by a switch 11 to be interrupted. At low medium conductivities, the switch can 11 be closed, the measuring current increased and the lower measuring range limit extended to lower conductivities. The connection of the conductor 8th to reduce the effective path length of the induced current in the medium 6 happens when the value of the conductivity falls below a certain value, eg at about 100 μS / cm.

5 shows a further embodiment. In it, the inside of the housing ladder 8th several times, about twice, three times, or generally N times, around the transmitting or receiving coil 2 . 3 be guided. As a result, the measuring current increases to a multiple value, for example to twice, three times or generally to an N-fold value. In one embodiment, the conductor becomes 8th several times, about twice, three times, generally N times around the transmitting and receiving coil 2 . 3 wound. The leader 8th can thus each time several times to the transmitting coil 2 or the receiver coil 3 or the transmitting coil 2 and the receiver coil 3 be guided.

In a further embodiment, the in 6 shown are the contacts 2 . 3 mounted in thin-film technology on the surface of the housing, and via a process seal 12 contacted outside the medium to the housing interior. These contacts are mounted approximately on the surface of the intended for immersion in the medium housing portion.

In one embodiment, the conductor 8th both in the cable and the thin-film variant at least partially outside the housing 7 be guided.

LIST OF REFERENCE NUMBERS

1

 Inductive conductivity sensor

2

 transmitting coil

3

 receiving coil

4

 First electrically conductive contact

5

 Second electrically conductive contact

6

 medium

7

 casing

8th

 ladder

9

Ion conduction in the medium 6

10

 channel

11

 switch

12

 process seal

Claims (11)

Inductive conductivity sensor ( 1 ) for measuring the specific electrical conductivity of a medium ( 6 ), with a fed by an input signal transmitter coil ( 2 ), one via the medium with the transmitting coil ( 2 ) coupled receiving coil ( 3 ), which provides an output signal which is a measure of the conductivity of the medium ( 6 ), and one the transmitting coil ( 2 ) and the receiving coil ( 3 ) surrounding housing ( 7 ), which contains at least one for immersion in the medium ( 6 ) has certain housing section whose housing wall, the transmitting coil ( 2 ) and the receiving coil ( 3 ), characterized in that the housing section at least one for contact with the medium ( 6 ) certain first electrically conductive contact ( 4 ) and one for contact with the medium ( 6 ) certain second electrically conductive contact ( 5 ), wherein the conductivity sensor ( 1 ) an electrically conductive conductor ( 8th ) and the first contact ( 4 ) over the ladder ( 8th ) with the second contact ( 5 ), the conductor ( 8th ) and the electrically conductive contacts ( 4 . 5 ) are designed so that they are connected via an ionic line ( 9 ) of the medium ( 6 ) a closed current path around the transmitting coil ( 2 ) and the receiving coil ( 3 ) train. Inductive conductivity sensor ( 1 ) according to claim 1, wherein at the first contact ( 4 ) and at the second contact ( 5 ) is a metal contact and the conductor ( 8th ) is designed as a metallic conductor. Inductive conductivity sensor ( 1 ) according to claim 1 or 2, wherein the first contact ( 4 ) and the second contact ( 5 ) are manufactured in thin-film technology. Inductive conductivity sensor ( 1 ) according to claim 3, wherein the first contact ( 4 ) and the second contact ( 5 ) on the surface of the medium for immersion ( 6 ) are mounted on certain housing section. Inductive conductivity sensor ( 1 ) according to at least one of claims 1 to 4, wherein the first contact ( 4 ) and the second contact ( 5 ) to the interior of the housing ( 7 ) are sealed. Inductive conductivity sensor ( 1 ) according to claim 5, wherein the first contact ( 4 ) and the second contact ( 5 ) inside the case ( 7 ) and the leader ( 8th ) is performed at least in sections in the housing. Inductive conductivity sensor ( 1 ) according to at least one of claims 1 to 4, wherein the first contact ( 4 ) and the second contact ( 5 ) outside the housing ( 7 ) and the leader ( 8th ) at least in sections outside the housing ( 7 ) is guided. Inductive conductivity sensor ( 1 ) according to at least one of claims 1 to 7, wherein the conductor ( 8th ) is designed such that it in each case several times around the transmitting coil ( 2 ) or the receiving coil ( 3 ) or the transmitting coil ( 2 ) and the receiving coil ( 3 ), in particular wound, is. Inductive conductivity sensor ( 1 ) according to at least one of claims 1 to 8, wherein the conductor ( 8th ) at least one switch ( 11 ) which opens and closes the current path. Inductive conductivity sensor ( 1 ) according to claim 8 and 9, wherein the switch ( 11 ) individual guides, in particular turns, of the conductor ( 8th ). Method for measuring the specific electrical conductivity of a medium ( 6 ) with an inductive conductivity sensor ( 1 ) according to claim 9 or 10, comprising the steps: Sending the input signal from the transmitting coil ( 2 ) in the medium, - converting the output signal of the receiving coil ( 3 ) into a value for the conductivity, and - connecting the conductor ( 8th ) for reducing the effective path length of the induced current in the medium when the value of the conductivity falls below a certain value.

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