阅读说明：本技术 数字pH传感器及数字pH传感器的测量方法 (Digital pH sensor and measuring method thereof ) 是由 托马斯·威廉 迈克尔·汉克 于 2020-10-15 设计创作，主要内容包括：本发明涉及数字pH传感器及数字pH传感器的测量方法。数字pH传感器包括电子单元和具有第一和第二半电池的传感器壳体,如果数字pH传感器与测量介质接触,则第一和第二半电池在第一与第二电极之间形成电位差,电子单元将电位差转换成是电压值或pH值的数字测量值,选择第一电解质和/或第一电极的材料和/或第二电解质和/或第二电极的材料,以在测量介质的pH值为7且测量介质的温度为25℃时,两个电极的电位差不等于0mV,并且电子单元将电位差转换为数字测量值,以在测量介质的pH值为7且测量介质的温度为25℃且电位差不等于0mV时,如果将数字测量值输出为电压值时则数字测量值为0mV,或者如果将数字测量值输出为pH值则数字测量值为7。(The invention relates to a digital pH sensor and a measuring method of the digital pH sensor. The digital pH sensor includes an electronics unit and a sensor housing having first and second half-cells, the first and second half-cells form a potential difference between the first and second electrodes if the digital pH sensor is in contact with the measurement medium, the electronic unit converts the potential difference into a digital measurement that is a voltage value or pH value, the material of the first electrolyte and/or the first electrode and/or the material of the second electrolyte and/or the second electrode is selected, so that when the pH value of the measuring medium is 7 and the temperature of the measuring medium is 25 ℃, the potential difference of the two electrodes is not equal to 0mV, and the electronic unit converts the potential difference into a digital measurement value, so that when the pH value of the measurement medium is 7 and the temperature of the measurement medium is 25 ℃ and the potential difference is not equal to 0mV, the digital measurement is 0mV if the digital measurement is output as a voltage value or 7 if the digital measurement is output as a pH value.)

1. A digital pH sensor (1) for measuring pH in a measurement medium, comprising:

-an electronic unit (2),

a sensor housing (3), the sensor housing (3) having a first half-cell (4) and a second half-cell (5),

wherein the first half-cell (4) has a first electrolyte (13) and a first electrode (7) in contact with the first electrolyte (13), and the first electrode (7) is electrically connected to the electronic unit (2),

wherein the second half-cell (5) has a second electrolyte (14) and a second electrode (8) in contact with the second electrolyte (14), and the second electrode (8) is electrically connected to the electronic unit (2),

wherein the first half-cell (4) and the second half-cell (5) are adapted to form a potential difference between the first electrode (7) and the second electrode (8) if the digital pH sensor (1) is in contact with a measurement medium,

wherein the electronic unit (2) is adapted to convert the potential difference into a digital measurement value (DM), wherein the digital measurement value (DM) is a voltage value or a pH value,

wherein the first electrolyte (13) and/or the material of the first electrode (7) and/or the second electrolyte (14) and/or the material of the second electrode (8) are selected such that the potential difference of the two electrodes is different such that, at a pH value of the measurement medium of 7 and a temperature of the measurement medium of 25 ℃, the potential difference of the two electrodes is set such that the potential difference is not equal to 0mV, preferably greater than 30mV or less than-30 mV, preferably greater than 50mV or less than-50 mV, preferably greater than 100mV or less than-100 mV,

and the electronic unit (2) converts the potential difference into the digital measurement value (DM) in such a way that, when the pH value of the measurement medium is 7 and the temperature of the measurement medium is 25 ℃ and the potential difference is not equal to 0mV, preferably greater than 30mV or less than-30 mV, preferably greater than 50mV or less than-50 mV, preferably greater than 100mV or less than-100 mV, the digital measurement value (DM) is 0mV if the digital measurement value (DM) is output as a voltage value or the digital measurement value (DM) is 7 if the digital measurement value (DM) is output as a pH value.

2. The digital pH sensor (1) according to claim 1, wherein the pH of the second electrolyte (14) differs from 7 by more than 0.5, preferably by more than 0.85, preferably by more than 1.7 at 25 ℃.

3. The digital pH sensor (1) according to claim 2, wherein the pH of the second electrolyte (14) is 6 at 25 ℃, wherein the first electrolyte (13) comprises e.g. potassium chloride and the second electrolyte (14) comprises e.g. potassium chloride and a phosphate buffer.

4. The digital pH sensor (1) according to any of the preceding claims, wherein the first electrode (7) and the second electrode (8) are Ag/AgCl electrodes and the chloride ion activities of the first electrolyte (13) and the second electrolyte (14) are different such that the potential difference is larger than 30mV, preferably larger than 50mV, preferably larger than 100 mV.

5. The digital pH sensor (1) according to any of the preceding claims, wherein the material of the first electrode (7) and the second electrode (8) comprises silver or copper or platinum or conductive carbon, wherein the first electrolyte (13) and the second electrolyte (14) comprise halide or sulfate ions such that the potential difference is more than 30mV, preferably more than 50mV, preferably more than 100 mV.

6. A measuring method of a digital pH sensor (1), comprising at least the steps of:

-providing a digital pH sensor (1) according to any of the preceding claims,

-measuring a potential difference between the first electrode (7) and the second electrode (8) in a measurement medium using the electronic unit (2),

wherein the potential difference at a pH value of the measurement medium of 7 and a temperature of the measurement medium of 25 ℃ is not equal to 0mV, preferably more than 30mV or less than-30 mV, preferably more than 50mV or less than-50 mV, preferably more than 100mV or less than-100 mV,

-converting, using the electronic unit (2), the potential difference measured at a pH value of the measurement medium of 7 and a temperature of the measurement medium of 25 ℃ into a digital measurement value (DM) such that the digital measurement value (DM) is 0mV if the digital measurement value (DM) is output as a voltage value or 7 if the digital measurement value (DM) is output as a pH value.

7. Measuring method according to claim 6, wherein in the step of converting the potential difference into the digital measurement value (DM), the electronic unit (2) uses a correction function (KF).

8. Measuring method according to claim 7, wherein the correction function (KF) comprises an adjustable correction factor (K).

9. Measuring method according to any of claims 7 or 8, wherein the correction function KF comprises a temperature factor (T), wherein the temperature factor (T) is determined based on a temperature value of the measuring medium, wherein the temperature value of the electronics unit (2) is transmitted by a temperature sensor (18) integrated in the digital pH sensor (1) or by a measuring transmitter.

10. The measurement method according to any one of claims 7 to 9, wherein the correction function (KF) comprises an operating time factor (B), wherein the operating time factor (B) is determined based on the operating time of the digital pH sensor (1), wherein the electronic unit (2) is adapted to detect the operating time of the pH sensor (1).

11. A digital sensor system (100), comprising:

-a digital pH sensor (1) according to any one of claims 1 to 5,

-a measurement transmitter (110), the measurement transmitter (110) being adapted for receiving and evaluating a digital measurement value (DM), wherein the measurement transmitter (110) is adapted to be connected to the digital pH sensor (1).

12. A method of evaluating a measurement transmitter (110), comprising at least the steps of:

-providing a digital sensor system (100) according to claim 11,

-receiving a digital measurement value (DM) from the digital pH sensor,

-comparing the digital measurement value (DM) with an upper and a lower limit value,

-outputting an error message if the digital measurement value (DM) exceeds the upper limit value or is below the lower limit value.

Technical Field

The invention relates to a digital pH sensor and a measuring method of the digital pH sensor, and also relates to a digital sensor system and an evaluation method of a measuring transmitter.

Background

In environmental technology and almost all industrial processes involving water, pH is a key metric and influencing variable, since it influences the thermodynamics and kinetics of almost all chemical reactions involving water. This concerns, for example, the target reactions involving water, separation processes in aqueous systems, corrosion on and in reactors and conduit systems, solubility of environmentally active substances, and living conditions of organisms. Therefore, the real-time determination of pH-values, in particular their sensor measurements, is an important task of environmental and process measurement techniques. The pH glass electrode is established as the most important sensor for measuring the pH of a medium; it can measure pH values over a large measurement range with low cross-sensitivity, is chemically stable and robust, and delivers electrical measurement signals from the beginning.

Although pH glass electrodes or pH single-rod measuring elements contained therein differ in many technically relevant details, the basic structure is always the same (see fig. 6): fundamentally, a pH single-rod measuring element, also referred to as a pH sensor, or in many cases as a pH electrode, consists of two electrochemical half-cells which are in contact with the measuring medium in different ways and between whose electrodes the voltage across the measuring medium is measured. The first half-cell, the so-called "measurement half-cell", is in contact with the measurement medium via a thin glass layer, the so-called "pH glass film". The second half-cell, the so-called "reference half-cell", is in contact with the measurement medium via an electrochemical junction, which allows a limited ion exchange between the reference electrolyte and the measurement medium, thus achieving ionic conduction, but largely prevents substantial mass exchange, mainly by convection. Thus, the potential drop across the junction, the so-called "membrane", is 0mV, while the potential drop across the glass membrane depends in a known manner on the composition of the electrolyte of the measuring half-cell (the so-called "internal electrolyte").

The composition of the glass film determines, inter alia, the sensitivity, linearity, measuring range and chemical and temperature stability of the sensor and is therefore an essential quality characteristic. Such glass compositions containing a great deal of technical knowledge are often trade secrets of the sensor manufacturer.

particular embodiments of the pH sensor use an additional electrolyte chamber as bridging electrolyte, multiple junctions or even a solid-state reference electrode in direct contact with the measurement medium. Depending on the configuration, the pH voltage is not measured directly between the two electrodes but between the two electrodes and the additional discharge electrode. In many cases, the sensor also contains other measuring elements, such as redox electrodes or thermal sensors.

Certain features of these sensors have been established as standards: typically, a second type of electrode, especially an Ag/AgCl electrode, is used as the reference electrode and the internal electrode. In both electrolytes, the chloride ion activity adjusted with potassium chloride was 3mol/l, or the reference electrolyte was saturated with KCl. Optionally, known internal electrolytes may include up to 50% ethylene glycol or glycerol. The pH of the internal electrolyte was 7. In a standard pH sensor, the zero point pH is 7. The pH glass membrane has a slope of less than-59.16 mV/pH, typically between 57mV/pH to 59mV/pH, measuring the difference between the medium and the internal electrolyte. As a result, when the pH value of the measurement medium was 7 and the measurement temperature was 25 ℃, the measurement voltage output by the sensor was 0mV or the pH output was 7.

An important issue for pH measurements using glass electrodes is the low conductivity of the glass film that must be measured through it, so the high impedance of the whole sensor is in the range of 50MOhm to 1 GOhm. What results is a significantly complex and dense plug, pluggable impedance transmitter, short measuring cable, moisture-proof measuring fitting, and in particular a coaxial sensor design, in which a low-impedance reference half-cell is arranged around a high-impedance measuring half-cell and the high-impedance measuring half-cell is shielded against electromagnetic interference.

In sensors of high quality, systems have been established in recent years by which a measuring amplifier and electronics for AD conversion are integrated into the sensor plug and only digital signals are transmitted from the sensor to the superordinate unit. Particularly noteworthy here are the Memosens, which have an inductive interface, are closed and completely impermeable to liquids, and in addition, store the important operating parameters of the sensor in an internal memory, and already have simple measurement value calculation and diagnostic functions.

In principle, it is possible to separate the digital sensor plug head from the high-quality sensor and to connect it to the analog sensor in order to feed the digital pH sensor with the measured values determined by the analog pH sensor. This may take the form of either a "digital" sensor or a plug adapter. However, there are serious potential drawbacks to the user:

sensors retrofitted from inexpensive parts often have poor measurement performance, particularly in terms of performance achieved using high quality film glass.

It is difficult to integrate a suitable and correctly placed temperature sensor, which means that the sensor temperature cannot be measured at all or is measured incorrectly.

In this case, special safety features such as resistance to ambient pressure will be lost.

The programming of the sensor electronics no longer fits the specifications of the half-cell used, which may lead to errors in the calculation and diagnosis functions.

The estimates of the measurement uncertainty made in the sensor or in the superordinate of the sensor no longer match.

If the user or a third party modifies the digital pH sensor in the manner described above, the safety advantages obtained by the digital pH sensor will no longer be guaranteed. Thus, for example, the user may suffer unnecessary injuries due to incorrect measurement values.

Disclosure of Invention

It is therefore an object of the present invention to provide a digital pH sensor that allows for safe measurement operations.

According to the invention, this object is achieved by a digital pH sensor.

The digital pH sensor according to the present invention comprises: an electronic unit; a sensor housing having a first half cell and a second half cell. The first half-cell has a first electrolyte and a first electrode in contact with the first electrolyte, and the first electrode is electrically connected to the electronics unit. The second half-cell has a second electrolyte and a second electrode in contact with the second electrolyte, and the second electrode is electrically connected to the electronic unit. The first half-cell and the second half-cell are adapted to form a potential difference between the first electrode and the second electrode if the digital pH sensor is in contact with the measurement medium. The electronic unit is adapted to convert the potential difference into a digital measurement value. The digital measurement is a voltage value or a pH value. The material of the first electrolyte and/or of the first electrode and/or of the second electrolyte and/or of the second electrode is/are selected such that, when the pH of the measurement medium is 7 and the temperature of the measurement medium is 25 ℃, the potential difference between the two electrodes is set such that the potential difference is not equal to 0mV, preferably greater than 30mV or less than-30 mV, preferably greater than 50mV or less than-50 mV, preferably greater than 100mV or less than-100 mV, and the electronic unit converts the potential difference into a digital measurement such that, when the pH of the measurement medium is 7 and the temperature of the measurement medium is 25 ℃ and the potential difference is not equal to 0mV, preferably greater than 30mV or less than-30 mV, preferably greater than 50mV or less than-50 mV, preferably greater than 100mV or less than-100 mV, the digital measurement is 0mV if it is output as a voltage value, or if the digital measurement is output as a pH value the digital measurement equals 7.

The digital pH sensor according to the invention makes it possible to check the presence of safety conditions necessary for the operation of the digital pH sensor. Thus, the user can be warned and/or protected from safety risks.

In one embodiment of the invention, the pH of the second electrolyte differs from 7 by more than 0.5, preferably by more than 0.85, preferably by more than 1.7 at 25 ℃.

In one embodiment of the invention, the pH of the second electrolyte is 6 at 25 ℃. The first electrolyte includes, for example, potassium chloride, and the second electrolyte includes, for example, potassium chloride and a phosphate buffer.

In one embodiment of the invention, the first and second electrodes are Ag/AgCl electrodes and the chloride ion activities of the first and second electrolytes are different such that the potential difference is greater than 30mV, preferably greater than 50mV, preferably greater than 100 mV.

In one embodiment of the invention, the material of the first and second electrodes comprises silver or copper or platinum or conductive carbon. The first electrolyte and the second electrolyte comprise halide ions or sulfate ions such that the potential difference is greater than 30mV, preferably greater than 50mV, preferably greater than 100 mV.

The object according to the invention is also achieved by a method.

The method according to the invention comprises the following steps:

-providing a digital pH sensor according to the invention,

-measuring a potential difference between the first electrode and the second electrode in the measurement medium using the electronic unit,

wherein the potential difference at a pH value of the measurement medium of 7 and a temperature of the measurement medium of 25 ℃ is not equal to 0mV, preferably more than 30mV or less than-30 mV, preferably more than 50mV or less than-50 mV, preferably more than 100mV or less than-100 mV,

-converting the potential difference measured at a pH of 7 and a temperature of 25 ℃ of the measurement medium into a digital measurement value using electronic means in the following manner: the digital measurement is 0mV if the digital measurement is output as a voltage value, or 7 if the digital measurement is output as a pH value.

It should be noted that said values, for example the voltage value 0mV or the pH value 7, are ideal values which are rarely reached with precision in actual measurements. The conversion of a potential difference which is not equal to 0mV at a temperature of 25 ℃ and a pH value of 7 of the measuring medium is understood to be a compensation for the desired specific structural and chemical properties of sensors which differ from sensors having a standard design, in particular the electrolyte and electrode materials used. In the conversion process, additional compensation can additionally be taken into account, which is applied to the output values, for example due to aging or wear of the sensor.

In one embodiment of the invention the electronic unit uses a correction function in the step of converting the potential difference into a digital measurement value.

In one embodiment of the invention, the correction function comprises an adjustable correction factor.

In one embodiment of the invention, the correction function comprises a temperature factor. The temperature factor is determined based on a temperature value of the measurement medium. The temperature value of the electronic unit is transmitted by a temperature sensor integrated in the digital pH sensor or by a measurement transmitter.

In one embodiment of the invention, the correction function includes an operating time factor. An operating time factor is determined based on an operating time of the digital pH sensor. The electronic unit 2 is adapted to detect the operating time of the pH sensor.

The object according to the invention is further achieved by a digital sensor system.

The digital sensor system according to the present invention comprises:

-a digital pH sensor according to the invention,

a measurement transmitter for receiving and evaluating the digital measurement value, wherein the measurement transmitter is adapted to be connected to a digital pH sensor.

The object according to the invention is also achieved by an evaluation method of a measurement transmitter.

The evaluation method according to the invention comprises at least the following steps:

-providing a digital sensor system according to the invention,

-receiving a digital measurement value from the digital pH sensor,

-comparing the digital measurement value with an upper limit value and a lower limit value,

-outputting an error message if the digital measurement value exceeds the upper limit value or is below the lower limit value.

Drawings

The invention will be explained in more detail in the following description with reference to the drawings. Shown below:

FIG. 1: a schematic diagram of a digital sensor system with a digital pH sensor according to the invention,

-figure 2: figure 1 shows a schematic view of an alternative embodiment of the sensor,

-figure 3: a schematic representation of pH dependent measurements measured by a pH sensor according to the invention,

-figure 4: a schematic representation of the time dependence of the measured values measured by the pH sensor according to the invention,

-figure 5: a schematic representation of the time dependence of the measured values measured by the pH sensor according to the invention,

-figure 6: schematic representation of a prior art pH sensor designed according to established standards.

Detailed Description

Fig. 1 shows an exemplary embodiment of a digital sensor system 100 with a measurement transmitter 110 and a digital pH sensor 1 according to the present invention.

The measurement transmitter 110 is adapted to communicate with the digital pH sensor 1 via a transmission device 120. The transmission means 120 comprises for example a wireless communication interface. Alternatively or additionally, the transmission means 120 comprises e.g. a cable for transmitting energy and/or data between the measurement transmitter 110 and the digital pH sensor 1. The cable is connected to the digital pH sensor 1, for example via a galvanically isolated interface.

The digital pH sensor 1 is adapted to measure the pH in the measurement medium and to provide it as a digital measurement value in units of pH and/or in units of mV and/or in any other unit as an arbitrarily calculated/corrected value to the measurement transmitter 110 or to transmit it via the transmission means 120 to the measurement transmitter 110.

In the following, the term digital measured value is understood to mean a value, for example a voltage value, which is present in digital form, i.e. as a coded signal. This means that the particular bit sequence represents a number, i.e. the value.

In the following, the term potential difference is understood to mean a value, for example a voltage value, which is present in analog form, i.e. as a continuous signal.

The digital pH sensor 1 comprises an electronics unit 2 and a sensor housing 3. The electronics unit 2 has a calculation module 6 for generating and sending digital measurement values DM to the measurement transmitter 110. The calculation module 6 may comprise, for example, a microcontroller and a wireless communication module. The wireless communication module is adapted to transmit and receive data, for example, via WiFi, bluetooth, or other communication methods.

The sensor housing 3 comprises a first half-cell 4 and a second half-cell 5. The first half-cell 4 is for example a reference half-cell, while the second half-cell 5 is for example a measurement half-cell. The first half-cell 4 has an electrolytic junction 15. The second half-cell 5 has a membrane 17 or active sensing surface.

The first half-cell 4 has a first electrolyte 13. The first half-cell 4 also comprises a first electrode 7 in contact with the first electrolyte 13 and is electrically connected to the electronic unit 2. The second half-cell 5 has a second electrolyte 14. The second half-cell 5 also comprises a second electrode 8 in contact with a second electrolyte 14 and electrically connected to the electronic unit 2.

The first half-cell 4 and the second half-cell 5 are designed such that if the digital pH sensor 1 is surrounded by, i.e. in contact with, a measuring medium, a potential difference is formed between the first electrode 7 and the second electrode 8. The electronic unit 2 is adapted to convert the potential difference into a digital measurement value DM. The conversion is performed by the calculation unit 6.

The material of the first electrolyte 13 and/or of the first electrode 7 and/or of the second electrolyte 14 and/or of the second electrode 8 is chosen such that, at a pH value of the measuring medium of 7 and a temperature of the measuring medium of 25 ℃, the potential difference of the two electrodes is set such that the potential difference is not equal to 0 mV. In a preferred embodiment, the potential difference with the same measured medium properties is greater than 30mV or less than-30 mV. In an alternative embodiment, the potential difference with the same measured media characteristic is greater than 50mV or less than-50 mV. In an alternative embodiment, the potential difference with the same measured medium properties is preferably greater than 100mV or less than-100 mV. The electronic unit 2 is adapted to convert the potential difference into a digital measurement value DM in the following way: the digital measurement DM is 0mV when the temperature of the measurement medium is 25 ℃ and the pH of the measurement medium is 7 and the potential difference is not equal to 0mV or, depending on the exemplary embodiment, more than 30mV or less than-30 mV, or more than 50mV or less than-50 mV, or more than 100mV or less than-100 mV (see also table 1).

In other words, the electronic unit 2 or the calculation unit 6 is configured to convert the potential difference into a digital measurement value DM such that the value of the potential difference is different from the value of the digital measurement value DM. For example, the potential difference is 50mV, while the corresponding digital measurement DM after conversion is 0mV or a pH value of 0.

In other words, the material of the first electrolyte 13 and/or the first electrode 7 and/or the material of the second electrolyte 14 and/or the second electrode 8 are selected such that a different potential difference is set compared to conventional pH sensors. This means that in the pH sensor according to the invention with potassium chloride as electrolyte and silver-silver chloride electrodes, a potential difference not equal to 0V is set in the measuring medium at 25 ℃ and pH 7. At other pH values of the measuring medium, the potential difference in conventional pH sensors varies according to a linear function, hereinafter referred to as the standard function, which has a slope of-59 mV/pH and passes through the point 0V, as described in the introduction. Other suitable standard functions are also known if the temperature of the measuring medium changes.

In an embodiment compatible with the previous embodiments, the material of the first electrolyte 13 and/or of the first electrode 7 and/or of the second electrolyte 14 and/or of the second electrode 8 is chosen such that the potential difference differs from the known standard function by a predetermined factor for a preset pH value of the measurement medium and a preset temperature of the measurement medium. The electronic unit 2 or the calculation unit 6 is configured to convert the potential difference into a corresponding digital measurement value DM such that the digital measurement value DM corresponds to a digital measurement value of a known standard function. The known standard functions are stored in the electronic unit 2 or the calculation unit 6.

Since the potential difference in the pH sensor 1 according to the invention is preset to be different from the potential difference expected in a conventional pH sensor, if the pH sensor 1 is modified to feed it with the potential difference of a conventional pH sensor, the digital measurement DM produced by the electronic unit 2 will deviate considerably from the desired digital measurement, allowing the presence of a safety problem to be detected easily.

In one embodiment, the second electrolyte 14 is selected such that its pH is pH 7 at 25 ℃.

In such an embodiment, the first electrolyte 13 comprises, for example, potassium chloride having an activity of 3mol/l, while the second electrolyte 14 comprises, for example, potassium chloride having an activity of 1mol/l or 0.3 mol/l.

In one embodiment, the second electrolyte 14 is selected such that its pH is < 6.5 at 25 ℃; especially < 6.15; in particular <5.3 (see also table 1).

In an alternative embodiment, the second electrolyte 14 is selected such that its pH is > 7.5 at 25 ℃; especially > 7.85; especially >8.7 (see also table 1).

In one exemplary embodiment, the second electrolyte 14 has a pH of pH 6 at 25 ℃. In such an exemplary embodiment, the first electrolyte 13 includes, for example, potassium chloride, and the second electrolyte 14 includes, for example, potassium chloride and a phosphate buffer at pH 6.

If the potential difference deviates from the potential difference determined according to the standard function due to the properties of the electrolytes, the first electrolyte 13 and the second electrolyte 14 have to be selected such that an electrode potential difference exists between the reference electrode and the measuring electrode. For example: if the chloride ion activity in the reference electrolyte is 3mol/l, the potential difference, which depends on the Cl activity of the internal electrolyte, is 30mV at 0.93mol/l, 50mV at 0.43mol/l and 100mV at 0.06mol/l (see also Table 1).

The potential forming system may be, for example, silver/silver chloride, silver/silver sulfate, copper/copper sulfate or iodine/iodide. The reference electrode and the measurement electrode may form a system based on the same or different potentials. If the reference electrode system and the measuring electrode system differ and the temperature ranges differ, the temperature correction function of the electrode potential must be adapted appropriately.

Fig. 2 shows an alternative embodiment of a digital pH sensor 1'. In such an embodiment, the pH sensor 1' additionally has a third electrode 16 for measuring the oxidation-reduction potential of the measuring medium. For redox measurements, a potential difference is measured between the third electrode 16, also called redox electrode, and the first electrode 7, which is the reference electrode in fig. 2. The material of the redox electrode should be a chemically stable, electrically conductive material that is as inert as possible. Examples are platinum, gold or conductive carbon (glassy carbon, graphite, doped diamond).

In such an embodiment, the pH sensor 1' also has a temperature sensor 18 in order to detect the temperature in the measuring medium. In such an embodiment, the function of the digital pH sensor 1 'is otherwise identical to the function of the pH sensor 1' shown in fig. 1.

If the digital pH sensor used in the digital sensor system 100 does not have a temperature sensor 18, a temperature sensor separate from the pH sensor may also be connected to the measurement transmitter 110.

In one embodiment, the pH sensor according to the present invention comprises a second electrolyte 14, the pH value of the second electrolyte 14 being different from 7, such that a predetermined potential difference is formed between the first electrode 7 and the second electrode when the temperature of the measurement medium is 25 ℃ and the pH value of the measurement medium is 7 (see the first and second columns of table 1).

In one embodiment, the pH sensor 1 according to the invention comprises Ag/AgCl electrodes and comprises a second electrolyte 14, which second electrolyte 14 has a preset chloride activity different from 3mol/l, such that a preset potential difference is formed between the first electrode 7 and the second electrode 8 when the temperature of the measurement medium is 25 ℃ and the pH of the measurement medium is 7 (see the first and third and fourth columns of table 1).

In an embodiment compatible with the above described embodiments, the potential difference between the first electrode 7 and the second electrode 8 is achieved by a combination of a preset pH difference between the second electrolyte 14 and pH 7 combined with a preset chloride ion activity ratio of the second electrolyte 14. For example, a difference in pH of the internal electrolyte of 0.51 results in a potential difference of 30mV, and further a chlorine ion activity ratio of the internal electrolyte of 3.22mol/l results in an additional potential difference of 30mV, thus achieving a total potential difference of 60mV between the first electrode 7 and the second electrode 8 (see Table 1).

Table 1:

potential difference (mV)	Difference in pH	Chlorine activity ratio (mol/l)	C(cRef＝3)
				30	0.51	3.22	0.93
50	0.85	7.04	0.43
				60	1.02	10.40	0.29
100	1.69	49.54	0.06
				120	2.03	108.12	0.03

The measurement method of the digital pH sensor 1 will be described below.

In a first implicit step, a digital pH sensor 1, 1' is provided. This means that the pH sensor 1, 1' is ready and immersed in the measurement medium.

Then, the potential difference is measured by the electronic unit 2. That is, the electronic unit 2 evaluates the potential difference between the first electrode 7 and the second electrode 8.

Alternatively, the electronics can correct the potential difference even before the analog-to-digital conversion, for example by generating a correction voltage offset. If such analog electronic measurement correction is performed, the voltage difference from the standard sensor (0 mV at pH 7, 25 ℃) will be sufficiently compensated, as a result of which an analog-to-digital conversion of the potential difference into a digital measurement DM is carried out without correcting the digital measurement DM.

Next, the potential difference is converted by the electronic unit 2 into a digital measured value DM such that the digital measured value DM is preset to be different from the potential difference, i.e. for example with a preset voltage difference. For example, a potential difference of 30mV is measured and converted into a digital measurement DM of 0 mV. This results in a voltage difference of 30mV between the potential difference and the digital measurement DM. In another embodiment, the voltage difference between the potential difference and the digital measurement DM is 50 mV. In an alternative embodiment, the voltage difference between the potential difference and the digital measurement DM is 100 mV.

When converting the potential difference into a digital measured value DM, a known standard function stored in the electronic unit 2 can be used to achieve a preset deviation from the corresponding potential difference according to the standard function. In addition, temperature values, which are obtained, for example, by a temperature sensor 18 inside the sensor, can be transmitted to the pH sensor 1 by means of the measurement transmitter and can be taken into account when selecting the criterion function.

The converted digital measurement values DM preferably correspond to digital measurement values of a standard function. If the sensor is designed or configured to output a digital measurement DM in mV, a measurement adapted to the standard function is output.

In one embodiment, the correction function KF is used in the step of converting the potential difference into a digital measurement DM. The correction function KF may be, for example, an exponential function or a polynomial or any function stored in a table.

The correction function KF includes an adjustable correction factor K. The correction factor K may be preset by the manufacturer. The correction factor K makes it possible, for example, to set a desired voltage difference, for example 30mV, 50mV, 100mV, between the potential difference and the digital measured value DM.

In an alternative embodiment compatible with the above-described embodiment, the correction function KF contains a temperature factor T, taking into account the temperature of the measuring medium. The temperature factor T is determined based on the temperature value determined by the temperature sensor 18. The temperature sensor is for example integrated in a pH sensor and connected to the electronics unit 2. Instead, the temperature value of the electronics unit 2 is transmitted by the measurement transmitter 110.

The temperature factor T can be applied in one calculation step with AD conversion and measurement value adjustment, but also in a downstream calculation step. In the latter case, both the digital measurement DM and the temperature corrected measured pH value may be output. To achieve temperature compensation of the output measured pH value, an approximation function or interpolation stored in the sensor may also be used as an alternative application of the temperature factor T. Alternatively, a table stored in the sensor may also be used. The table allows the application of a specific temperature coefficient depending on the prevailing temperature.

In an alternative embodiment compatible with the above described embodiments, the correction function KF contains an operating time factor B, taking into account the operating time of the digital pH sensor 1. The operation time factor is determined based on the operation time of the pH sensor 1. The operating time of the pH sensor 1 is detected by the electronic unit 2. The operation time includes, for example, a measurement period in which the pH sensor is in contact with the measurement medium and a storage period in which the pH sensor is not in contact with any measurement medium.

The evaluation method of the measurement transmitter 110 will be described below.

In a first step, a digital measurement value DM is transmitted from the digital pH sensor 1 to the measurement transmitter 110 via the transmission means 120. The transmission device 120 is, for example, a wireless communication module or a data cable. For example, the transmission device 120 may be a cable with a potential isolation plug.

The digital measured value DM is then compared with an upper limit value and a lower limit value. The upper and lower limits are manufacturer-set values that define a tolerance range within which expected measurements can be found.

If the digital measured value DM exceeds the upper limit value or falls below the lower limit value, an error message is output. For example, the error message may be output by an audible or visual alarm signal.

In one embodiment of the evaluation method, the energy supply to the digital pH sensor 1 is interrupted if the digital measured value DM exceeds an upper limit value or falls below a lower limit value. In this variant, the measurement converter 110 supplies the digital pH sensor 1 with energy via the transmission means 120.

Fig. 3 shows a calibration line of the digital pH sensor 1.

The dashed line represents a measurement curve based on the actual potential difference between the electrodes as determined by the digital pH sensor in a calibration medium, e.g. a measurement medium with a pH value of 7 and a temperature of 25 ℃.

The solid line represents a measurement curve based on a digital measurement value DM determined by potential difference conversion.

The dotted line represents a measurement curve based on a digital measurement value DM obtained by conversion from a potential difference, which is fed as a potential difference from a conventional pH sensor to the digital pH sensor 1.

Fig. 4 shows the experimental zero drift of the digital pH sensor 1.

The solid line with unfilled dots represents the time offset of the zero point of the sensor output. This deviation is called zero drift.

The dashed line represents the change in electrochemical potential of the reference electrode over time due to depletion of the electrolyte in the reference half-cell.

The solid line with filled dots represents the corrected electrochemical potential of the measuring electrode.

The dotted line represents the uncorrected potential of the measurement electrode from the standard.

Fig. 5 shows the zero point drift of a digital pH sensor 1 fed with the potential difference of another sensor. Thus, with a suitable choice of the potential deviation, a more aged appearance of the electrode can be obtained, which electrode has drifted by 59mV at the start of the measurement (time 0). Thus, this action results in the output of an alarm/error message.

The solid line with unfilled dots represents the time offset of the zero point.

The dashed line represents the change in electrochemical potential of the reference electrode over time due to depletion of the electrolyte in the reference half-cell.

The solid line with filled dots represents the incorrectly corrected electrochemical potential of the measuring electrode.

The dotted line indicates the uncorrected potential of the measurement electrode.

If the pH sensor 1 is fed with the potential difference of another sensor, the pH sensor 1 can generate a systematic measurement error in the digital measurement value DM. Thus, the digital sensor system 100 may have a safety risk, as a result of which the sensor system 100 outputs an error message to the user. Alternatively, the error message may also be output by the digital pH sensor 1.