DE102011087188A1 - Method for determining process variable of medium by electrochemical sensor, involves assigning reference half cell with temperature sensor, so that process variable is determined using temperatures detected by sensors - Google Patents

Abstract

The method involves arranging reference half cell (18), active component (15) and temperature sensor (8), such that the reference half cell is assigned with temperature sensor (7) arranged in electronic circuit. The process variable is determined by higher-level unit using temperatures detected by temperature sensors. A sensor plug-in head (17) is provided at distal end of media of electrochemical sensor (1). The active component is selected from pH glass membrane, ion sensitive FET (ISFET), metal or non-metallic or metal oxide or carbon electrode. An independent claim is included for an electrochemical sensor electrode.

Description

The invention relates to a method for determining at least one process variable of a medium by means of an electrochemical sensor, an electrochemical sensor and a combination electrode, for example for determining an electrochemical potential. Electrochemical sensors are used, for example, in pH determination and are used in a variety of applications, for example in chemistry, medicine, industry, environmental or water analysis.

The problem underlying the invention will be explained with reference to potentiometric sensors. However, the invention is not limited to potentiometric sensors, but generally relates to sensors for use in electrochemical measurement systems, e.g. As well as sensors for amperometric method.

Potentiometric sensors have at least one measuring electrode and a reference electrode, between which a potential difference is determined. The potential difference is dependent on the concentration or the activity of the substance to be measured, in the case of a pH measurement, for example, by the activity of the hydrogen ions H ⁺ .

To measure the concentration or the activity, potentiometric sensors have a sensorically active component, for example an ion-selective membrane, in the case of a pH measurement, for example a glass membrane, or a semiconductor-insulator layer stack.

In addition to systems of measuring electrode and reference electrode, which are introduced separately into a medium, so-called single-rod measuring chains are also known in particular in the field of pH measurement, which combine measuring electrode and reference electrode in a probe. Without limitation, the idea according to the invention will be explained in more detail below with reference to such a combination electrode.

The measuring electrode has a mostly wire-shaped discharge element, which usually consists of silver / silver chloride and is immersed in a buffer solution of known pH and known chloride activity (often a phosphate buffer solution). Discharge element and buffer solution are usually introduced into a glass tube, which is closed on the media side with a glass membrane (the sensor-active component). The diverting element and the buffer solution form the measuring half cell.

The reference electrode also has a diverting element and may also consist of silver / silver chloride. The diverter element is immersed in a reference solution, for example potassium chloride. About a implementation, for. As a diaphragm, the reference solution is in electrochemical exchange with the medium. The potential is also by means of an electrical conductor, for. B. in the form of a metal wire, connected to a corresponding interface at the medium-remote end of the electrode. Since the metal wire itself is not intended to contribute to the electrochemical potential and must be electrochemically stable, it usually consists of a noble material, usually a noble metal such as silver or platinum. The diverting element and the reference solution form the reference half-cell.

The potential difference between the reference electrode and the measuring electrode depends on the pH of the medium to be measured. The individual potentials are by means of electrical conductors, for. In the form of metal wires, which are connected to a corresponding interface at the medium-distal end of the electrode.

The electrical conductors conduct the potentials of the diverting elements via a sealing element to a corresponding interface at the medium-remote end of the measuring system. Via the interface, for example a plug-in system with cable, the measuring signal is sent to a higher-level system, for example a measuring transducer. In the transmitter, the potentials are further processed and, for example, charged to a pH value. Recently, digital interfaces with galvanic decoupling have become established. For this purpose, the measurement signals in the sensor plug head are partially offset by means of an electronic circuit, further processed and / or digitized.

There are embodiments with over 40 cm long sensors. Thus, the electrical conductor for deriving the potential must also correspond to at least this length. When using silver or platinum, considerable material costs are incurred for the electrical conductor, which is usually made of solid material.

The potentials of electrochemical sensors are strongly temperature-dependent according to the Nernst equation, therefore a temperature sensor is necessary for the exact determination of the measured value. This is usually integrated directly into the half cell. Even small temperature differences between the measuring and reference electrodes lead to a falsification of the measurement result. A temperature profile is found mainly along the longitudinal axis of the sensor, which is why typically the Ableitelemente the reference and measuring electrode are at the same height, such as from DE 10 2008 055 082 A1 known.

It is problematic that due to temperature differences over the length of the sensor, solubility and complexing equilibria differ within the electrode space. These location- and temperature-dependent concentration ratios lead to transport processes within the electrode space, to corrosion and under unfavorable circumstances to dissolve the derivative and thus to the failure of the sensor.

The invention has for its object a method to provide an electrochemical sensor and a combination electrode, which permanently ensure an accurate measurement and yet are inexpensive to produce.

The object is achieved by a method comprising
at least one reference half cell,
at least one sensory component,
at least a first temperature sensor,
characterized in that
the reference half cell is assigned at least one second temperature sensor,
the process variable is determined with the electrochemical sensor, and
wherein at least two temperatures determined by different temperature sensors are taken into account in the determination of the process variable.

The use of at least one second temperature sensor is to be regarded as advantageous. Thus, the temperature necessary for a correct determination of the process variable of at least two different temperature sensors can be taken into account. It can be ensured that the process variable is permanently determined correctly.

The object is further achieved by an electrochemical sensor for determining at least one process variable of a medium comprising
at least one reference half cell,
at least one sensory component,
at least a first temperature sensor,
characterized in that
the reference half cell is assigned at least one second temperature sensor,
wherein at least one higher-level unit is provided, which is designed such that it takes into account for determining the process variable at least two temperatures determined by different temperature sensors.

The assignment to the reference half cell of at least one second temperature sensor is to be regarded as advantageous. Thus, the temperature of at least two different temperature sensors necessary for a correct determination of the process variable can be taken into account by the at least one higher-order unit. It can be ensured that the process variable is permanently determined correctly.

In an advantageous embodiment, the second temperature sensor is arranged in or on the reference half-cell. Thus, the temperature can be determined directly at the point of interest, namely at the reference half-cell.

In an alternative advantageous embodiment, a sensor plug head is provided at the media-remote end of the electrochemical sensor, and the second temperature sensor is arranged in the sensor plug head. This is advantageous because the arrangement of the second temperature sensor in the sensor plug head no further lines, etc. must be used and that measurement data of the second temperature sensor can be forwarded directly through the sensor plug head, since there is usually an interface for communication on or in the sensor plug head. Another advantage is that the reference half-cell can now be made shorter in relation to the sensor-active component and thus material costs are saved.

In a further alternative embodiment, a sensor plug head is provided at the media-distal end of the electrochemical sensor,
wherein an electronic circuit is provided, which is arranged in the sensor plug head, and
the second temperature sensor is arranged in the electronic circuit. This is advantageous because the arrangement of the second temperature sensor in the electronic circuit no further lines, etc. must be used and the measurement data of the second temperature sensor can be forwarded directly through the sensor plug head, since there is usually an interface for communication on or in the sensor plug head.

Preferably, the at least one higher-level unit is part of the electronic circuit. This is advantageous, since thus components and thus costs are saved.

wobei u(x, t) die räumlich und zeitlich abhängige Wärmeverteilung, α die Temperaturleitfähigkeit, x den Ort und t die Zeit darstellt. Dies ist vorteilhaft: aus den Messdaten der zumindest zwei Temperatursensoren und der bekannten Geometrie des Sensors berechnet die zumindest eine übergeordnete Einheit die Temperatur entlang der Referenzhalbzelle. Der der Referenzhalbzelle zugeordnete Temperatursensor kann somit relativ frei positioniert werden, und die zumindest eine übergeordnete Einheit berechnet anhand der bekannten Position des Sensors und der Messdaten die Temperatur über den Verlauf der Referenzhalbzelle.In a preferred embodiment, the at least one higher-order unit calculates the temperature along the reference half-cell, in particular at the media-side end of the reference half-cell, with the aid of the at least two temperature sensors and the heat conduction equation. The heat equation in a space dimension for thin, relatively long rods of solid material is

where u (x, t) represents the spatially and temporally dependent heat distribution, α the thermal conductivity, x the location and t the time. This is advantageous: from the measured data of the at least two temperature sensors and the known geometry of the sensor, the at least one higher-order unit calculates the temperature along the reference half-cell. The temperature sensor assigned to the reference half cell can thus be positioned relatively freely, and the at least one higher-level unit uses the known position of the sensor and the measured data to calculate the temperature over the course of the reference half cell.

In an advantageous embodiment, the sensorically active component is an ion-selective membrane, in particular a pH glass membrane, a semiconductor-insulator layer stack, in particular an ISFET or a metal or metal / metal oxide or a non-metallic redox electrode , in particular a carbon electrode.

Preferably, the at least one higher-level unit determines the process variable via a potentiometric or amperometric method.

The object is further achieved by a combination electrode comprising the electrochemical sensor according to the invention,
wherein a housing is provided in which at least one chamber is formed, wherein the reference half cell is arranged in the chamber,
wherein at least one reference electrolyte is arranged in the chamber,
wherein the sensory component is in contact with the medium,
wherein a reference system for deriving a potential of the reference half cell is provided at the reference half cell,
wherein the reference electrolyte at least partially wets the lead-off system,
wherein at least one passage is provided by means of which the reference electrolyte with a medium surrounding the housing,
especially a measuring medium, in contact,
wherein at the sensorically active component a second discharge system for deriving a potential,
wherein the second delivery system is associated with the sensory component.

In an advantageous embodiment, the first lead-off system and the second lead-off system have different lengths. So it is possible to save material and thus costs.

The invention will be explained in more detail with reference to the following figures. It shows

1 a schematic structure of a sensor according to the invention in a first embodiment,

2 a schematic structure of a sensor according to the invention in a second embodiment, and

3 a schematic structure of a sensor according to the invention in a third embodiment.

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

1 shows schematically the structure of an inventively designed electrochemical sensor. In its entirety, the sensor has the reference number 1 , The sensor 1 gets into a medium 13 introduced, whose pH is to be determined. Shown is an example of an electrochemical sensor for determining the pH, which is realized as Einstabmesskette.

The sensor 1 includes a housing 2 , which preferably consists of glass and two chambers 3 and 4 includes. The first chamber 3 is coaxial around the second chamber 4 arranged. The first chamber 3 is at least partially with a reference electrolyte nine filled. The second chamber 4 is at least partially with an electrolyte 10 filled.

For sealing the upper housing section with respect to the electrolyte 10 and the reference electrolyte nine an unillustrated closure element is introduced into the housing. So are the two chambers 3 . 4 completely closed. The seal can be at least partially in the sensor plug head 17 are located.

The combination electrode consists of two half-cells 18 and 19 : the reference half cell 18 consisting of a reference electrode 5 and a reference electrolyte nine , where reference electrode 5 and reference electrolyte nine in the first chamber 3 are located; the measuring half cell 19 consisting of a measuring electrode 6 and an electrolyte 10 , where the measuring electrode 6 and electrolyte 10 in the second chamber 4 are located.

The reference electrode 5 consists of a potential-forming element at the media end and its derivative (not shown separately). The potential-forming element consists of a metallic part and a supply of a salt of the metal, for. For example, it is silver and silver chloride. This is for example from the EP 1 172 648 A1 known. The derivative consists of an electrical conductor, usually a metal wire. However, embodiments also leave one Wire to. Typically, a noble metal is used, for example, silver or platinum. In the following, the potential-forming element and the derivative will be referred to as the lead-off system 11 designated. The reference electrolyte nine wets at least partially the discharge system 11 ,

In the wall of the first chamber 3 there is a passage 14 , z. B. a diaphragm made of a ceramic, which the power key to the medium 13 forms.

The measuring electrode 6 consists of a potential-forming element at the media end and its derivative (not shown separately). The potential-forming element consists of a metallic part and a supply of a salt of the metal. For example, it is silver and silver chloride. This is for example from the EP 1 172 648 A1 known. The derivative consists of an electrical conductor, usually a metal wire. However, embodiments also allow for a stranded wire. Typically, a noble metal is used, for example, silver or platinum. In the following, the potential-forming element and the derivative will be referred to as the lead-off system 12 designated. The electrolyte 10 wets at least partially the discharge system 12 ,

In the wall of the second chamber 4 there is a sensory component 15 which the contact to the medium 13 forms. The sensory component 15 is designed as an ion-selective membrane, in particular as a thin glass membrane.

Apart from the possibility as a glass membrane, the sensory component is conceivable 15 as a semiconductor insulator layer stack with chemically sensitive semiconductor component, in particular as an ISFET or as a metal or metal / metal oxide or as a non-metallic redox electrode, in particular as a carbon electrode to design.

The measuring electrode 6 and the reference electrode 5 can differ in their length, in particular, the reference electrode 5 shorter than the measuring electrode. Consequently, the discharge system is also 11 correspondingly shorter than the lead-off system 12 ,

At the sensor plug head 17 an interface not shown is attached. About the sensor plug head 17 become the signals of the derivation systems 11 . 12 via the interface to a higher-level system 21 forwarded. The interface can be a sensor cable, which sends the signals to the higher-level system 21 passes. In this case, a suitable plug serves as a link. Both electrically coupled and galvanically isolated interfaces (eg optical, inductive, capacitive) are conceivable. A superordinate system 21 is for example a transmitter, a control center or in the simplest case only a display. In the parent system 21 the signals are processed, calculated, displayed and / or forwarded.

The method of pH measurement described here is a potentiometric method, ie ideally there is no current flow through the derivation systems 11 . 12 instead of. However, the invention can also be applied to electrochemical processes in which a current flow takes place, such. B. in amperometry or coulometry.

Electrochemical processes are highly temperature dependent according to the Nernst equation. That's why the measuring electrode is 6 a temperature sensor 8th assigned. The temperature sensor 8th is approximately at the level of the potential-forming element of the measuring electrode. Via a line, the measuring signals of the temperature sensor 8th to the sensor plug head 17 and possibly to the higher-level system 21 forwarded for processing, billing and / or presentation. The pipe of the temperature sensor 8th can also be in the case 2 be integrated.

The reference electrode 5 assigned is a temperature sensor 7 ,

1 shows a first embodiment. The temperature sensor 7 is in the first chamber 3 on a separate line. The line can also be in the case 2 be integrated. The line may be an electrical conductor, such as a metal wire. It should be opposite to the reference electrolyte nine be sufficiently isolated.

2 shows a second embodiment. In 2 is the first chamber 3 only about as long as in 1 , Thus, also the reference electrode 5 only about half as long. In this embodiment, significantly less reference electrolyte nine needed as in the 1 illustrated embodiment.

As already mentioned, the measurement signals are obtained in the embodiments in the figures 1 and 2 both from the reference electrode 5 , Measuring electrode 6 as well as from the temperature sensors 7 . 8th via a derivative or line to the sensor plug head 17 which, among other things, includes an interface. The interface can be designed both galvanically coupled and galvanically isolated and be realized for example as a cable.

Recently, digital interfaces with galvanic decoupling have become established. An embodiment of an electrochemical sensor 1 with digital interface is in 3 shown. The measuring signals are already in Sensor plug head 17 by means of an electronic circuit 16 billed, processed and / or digitized. Typically, in the electronic circuit 16 the measured potentials are amplified, converted into a pH value, digitized and forwarded via the interface. Even the measuring signals of the temperature sensors can already 7 . 8th will be charged.

On the electronic circuit 16 There may also be a memory element which stores sensor-typical data. Examples for this are:
Serial number, date of manufacture, device data, calibration data etc.

In the in 3 variant shown is a temperature sensor 7 in the electronic circuit 16 , The electronic circuit 16 is usually implemented as printed circuit board (PCB) in through hole technology (THT) or in surface mounting (surface-mounting technology (SMT)). So can the temperature sensor 7 as a THT or SMT component, as a so-called surface-mounted device (surface-mounted device (SMD)), be designed and mounted on the circuit board.

The measured temperature signals of the temperature sensor 7 then be in the electronic circuit 16 analyzed and possibly further processed. This is done by a higher-level unit 20 For example, by a microprocessor or microcontroller. The parent unit 20 can also be part of the electronic circuit 16 be. The parent unit 20 and / or the electronic circuit 16 are powered by the interface.

wobei u(x, t) die räumlich und zeitlich abhängige Wärmeverteilung in der Elektrode, α die Temperaturleitfähigkeit, x den Ort und t die Zeit darstellt, kann folglich auch die Temperatur an der Referenzelektrode 5 bestimmt werden. Insbesondere ist die Temperatur am Ort der Potentialbildung wichtig, d. h. am Potential bildenden Element.With at least one measurement signal of the temperature sensors 7 . 8th can be the course of temperature over the length of the sensor 1 be calculated. With the aid of the heat equation for a room dimension

where u (x, t) represents the spatially and temporally dependent heat distribution in the electrode, α the thermal diffusivity, x the location and t the time, consequently also the temperature at the reference electrode 5 be determined. In particular, the temperature at the point of potential formation is important, ie at the potential-forming element.

It is conceivable that the complete calculation in the sensor 1 , more precisely in the parent unit 20 , he follows. That is, the measurement signals are amplified, processed, charged and / or forwarded. In the case of a digital interface, the signals are additionally digitized. It is also conceivable that the measurement signals in raw form over the interface to the parent system 21 be transmitted in analog or digital form and possibly only be strengthened for better communication. The actual calculation of the process variable then happens in the higher-level system. Any measurements and key figures can be returned to the sensor 1 in the electronic circuit 16 be written in the memory. Partial solutions are also conceivable: for example, the process variable may already be in the sensor 1 be determined from the measured data, but the temperature compensation takes place in the higher-level system 21 , That is, parts of the tasks of the parent system 21 be from the parent unit 20 taken over and vice versa.

Of course, all combinations of the aforementioned embodiments are possible. So z. B. basically always a small chamber 3 possible as in 2 shown, a large chamber (off 1 ) with a short reference electrode 5 (out 2 ) or a digital interface as in 3 shown. Also, a temperature sensor 7 in the electronic circuit 16 be integrated and the measuring signals of the electrodes 5 . 6 as well as possibly temperature sensor 8th via an analog or digital interface to a connected higher-level system 21 where they are processed, analyzed and / or presented.

LIST OF REFERENCE NUMBERS

1

sensor

2

casing

3

First chamber

4

Second Chamber

5

reference electrode

6

measuring electrode

7

temperature sensor

8th

temperature sensor

9

reference electrolyte

10

electrolyte

11

Metal lead

12

Metal lead

13

medium

14

execution

15

Sensory effective component

16

Electronic switch

17

Sensor plug head

18

Reference half-cell

19

Measuring half cell

20

Parent unit

21

Parent system

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102008055082 A1 [0011]
• EP 1172648 A1 [0036, 0038]

Claims (11)

Method for determining at least one process variable of a medium ( 13 ) by means of an electrochemical sensor ( 1 ) comprising at least one reference half-cell ( 18 ), at least one sensory component ( 15 ), at least a first temperature sensor ( 8th ), characterized in that the reference half-cell ( 18 ) at least a second temperature sensor ( 7 ), the process variable with the electrochemical sensor ( 1 ), and wherein at least two of different temperature sensors ( 7 . 8th ) are taken into account in the determination of the process variable. Electrochemical sensor ( 1 ) for determining at least one process variable of a medium ( 13 ) comprising at least one reference half-cell ( 18 ), at least one sensory component ( 15 ), at least a first temperature sensor ( 8th ), characterized in that the reference half-cell ( 18 ) at least a second temperature sensor ( 7 ), wherein at least one higher-level unit ( 20 ), which is designed such that it can be used to determine the process variable at least two of different temperature sensors ( 7 . 8th ) taken into account temperatures. Electrochemical sensor ( 1 ) according to claim 2, wherein the second temperature sensor ( 7 ) in or at the reference half cell ( 18 ) is arranged. Electrochemical sensor ( 1 ) according to claim 2, wherein a sensor plug head ( 17 ) at the media-distal end of the electrochemical sensor ( 1 ), and the second temperature sensor ( 7 ) in the sensor plug head ( 17 ) is arranged. Electrochemical sensor ( 1 ) according to claim 2, wherein a sensor plug head ( 17 ) at the media-distal end of the electrochemical sensor ( 1 ) is provided, wherein an electronic circuit ( 16 ) provided in the sensor plug head ( 17 ), and the second temperature sensor ( 7 ) in the electronic circuit ( 16 ) is arranged. Electrochemical sensor ( 1 ) according to claim 5, wherein the at least one higher-level unit ( 20 ) Is part of the electronic circuit ( 16 ). Electrochemical sensor ( 1 ) according to at least one of claims 2, 4, 5 or 6, wherein the at least one higher-level unit ( 20 ) the temperature along the reference half cell ( 18 ), in particular at the media-side end of the reference half-cell ( 18 ), with the help of at least two temperature sensors ( 7 . 8th ) and the heat conduction equation. Electrochemical sensor ( 1 ) according to at least one of claims 2 to 7, wherein it is in the sensory active component ( 15 ) is an ion-selective membrane, in particular a pH glass membrane, a semiconductor-insulator layer stack, in particular an ISFET or a metal or metal / metal oxide or a non-metallic redox electrode, in particular a carbon electrode. Electrochemical sensor ( 1 ) according to at least one of claims 2 to 8, wherein the at least one higher-level unit ( 20 ) determines the process variable via a potentiometric or amperometric method. Combination electrode comprising the electrochemical sensor ( 1 ) according to at least one of claims 2 to 9, wherein a housing ( 2 ) is provided, in which at least one chamber ( 3 ), wherein the reference half-cell ( 18 ) in the chamber ( 3 ), wherein at least one reference electrolyte ( nine ) in the chamber ( 3 ), wherein the sensory component ( 15 ) in contact with the medium ( 13 ), wherein at the reference half cell ( 18 ) a first delivery system ( 11 ) for deriving a potential of the reference half-cell ( 18 ), the reference electrolyte ( nine ) the first derivation system ( 11 ) at least partially wetted, wherein at least one implementation ( 14 ) is provided by means of which the reference electrolyte ( nine ) with a housing ( 2 ) surrounding medium ( 13 ), in particular a measuring medium, is in contact, wherein at the sensory component ( 15 ) a second delivery system ( 12 ) for deriving a potential, wherein the second derivation system ( 12 ) of the sensory component ( 15 ) assigned. A combination electrode according to claim 10, wherein the first lead-off system ( 11 ) and the second delivery system ( 12 ) are different in length.

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