Method for determining the conductivity of an analysis sample, in particular of an electrolyte solution, measuring device and measuring system

Abstract

The invention relates to a method for determining the conductivity of an analysis sample (3), in particular of an electrolyte solution, comprising the steps of: applying a test voltage to the analysis sample (3), detecting a first measurement signal which correlates with a voltage between two measurement electrodes, introduced into the analysis sample, at a first analysis time, and determining the conductivity of the analysis sample (3) based on the first measurement signal, wherein the test voltage is an alternating voltage, and wherein the first analysis time is in the middle of one of the two half-waves of a test voltage period of the test voltage. The invention also relates to a measuring device (2) for determining the conductivity of the analysis sample (3), and to a measuring system (1) having such a measuring device (2).

Description

[0001] Method for determining the conductivity of an analysis sample, in particular an electrolyte solution, measuring device and measuring system

[0002] The content of the German patent application DE 10 2023 209 480.0 is incorporated herein by reference.

[0003] The invention relates to a method for determining the conductivity of an analysis sample, in particular an electrolyte solution. Furthermore, the invention relates to a measuring device for determining the conductivity of an analysis sample. The invention is also directed to a measuring system comprising such a measuring device.

[0004] Through prior public use, it is known that the conductivity of an electrolyte solution can be determined using measuring electrodes in galvanic contact with it, in particular by measuring the ohmic resistance of the electrolyte solution. A disadvantage is that the required test current, which is then passed through the electrolyte solution, leads to polarization of the electrolyte solution, which impairs the measurement result. Computational correction of the measurement results requires considerable computing power, is energy-intensive, and has limited precision.

[0005] It is an object of the invention to provide an improved method for determining the conductivity of an analysis sample, which is economically feasible, energy-efficient and particularly precise with regard to the resulting measurement results.

[0006] This object is achieved by a method having the features of claim 1. It has been recognized that a method for determining the conductivity of an analysis sample, in particular an electrolyte solution, based on a first measurement signal that correlates with a voltage between two measuring electrodes introduced into the analysis sample at a first analysis time, is particularly economical to implement, energy-efficient to carry out, and particularly precise with regard to the resulting measurement results if the analysis sample is subjected to a test voltage in the form of an alternating voltage and if the first analysis time lies in the middle of one of the two half-waves, in particular the first half-wave, of a test voltage period of the test voltage. Because the test voltage is an alternating voltage, the extent of polarization of the analysis sample that impairs the measurement result is reduced.In particular, it was found that the influence of polarization on the measurement result can be significantly further reduced by a suitable choice of the analysis time. If the analysis time is in the middle of one of the two half-waves of the test voltage period, the influence of the polarization caused by the test voltage on the measurement result is particularly small, in particular minimal, or neutralized. In other words, a zero crossing of the temporal progression of the effect of polarization on the measurement signal can occur at the first analysis time. At this first analysis time, the effects of the frequency response limitation of an optional amplifier device on the first measurement signal are also particularly small. Computing-intensive methods for correcting corrupted measurement results and the associated energy consumption can be dispensed with.The method is therefore particularly cost-effective and ensures the determination of the conductivity of an analysis sample with particularly high precision. As the ion concentration in the analysis sample increases, polarization effects increase. In principle, this can be counteracted by increasing the test voltage frequency, the inverse of the test voltage period. However, this has limitations, since a high test voltage frequency exceeds the performance of available and generally required amplifier circuits or drastically reduces their measurement accuracy. The proposed method offers a remedy in this regard by reducing the impairment of the measurement results due to polarization, regardless of the test voltage frequency. The test voltage frequency is also referred to below as the alternating voltage frequency.

[0007] When detecting the first measurement signal, the measuring electrodes are preferably in direct electrical contact with the analysis sample. This type of contact is also referred to as galvanic contact. A voltage is defined as an electrical potential.

[0008] For the test voltage period, a value is preferably selected which lies within an operating range of the amplifier circuit, in particular is greater than a predetermined minimum period at which the amplifier circuit still operates as intended or at which the amplification still follows a linear transfer function.

[0009] The test voltage period is preferably selected such that the polarization curve over time is substantially linear and / or that the polarization does not enter a saturation range. This may mean that the test voltage period is preferably selected to be shorter than a maximum period, in particular a predetermined one, which is dependent in particular on the analysis sample. With a linear polarization curve of the analysis sample, it is advantageously achieved that the effect of the polarization on the measurement signal has a zero crossing at the first analysis time, or that any deviation of the zero crossing from the first analysis time is small.

[0010] According to a simplified, model-based description, it can be assumed that polarization leads to a capacitive impedance. The time constant with which a capacitance is charged is determined by the product of resistance and capacitance. A capacitance can become largely saturated after a few, in particular around five, time constants have elapsed. In the time range before the first time constant, the charging or discharging process is largely linear. Based on this model-based approach, this can mean that the test voltage period is chosen to be so low that the polarization effect essentially has a linear temporal progression, in particular that the test voltage period is shorter than the time constant, in particular half the time constant, of a model-based capacitance describing the polarization.The predetermined maximum period can be determined based on the calculated time constant of a correspondingly modeled capacity.

[0011] Particularly preferably, the test voltage period is determined, in particular fixed or variably set, such that it lies above the predetermined minimum period and below the predetermined maximum period. This reliably ensures that the effects of the frequency response limitation of the amplifier device are minimal and that the polarization has a minimal effect on the measurement signal at the first analysis time, in particular has a zero crossing there. The test voltage frequency is preferably in a range from 100 Hz to 50 kHz, in particular from 200 Hz to 20 kHz, in particular from 500 Hz to 10 kHz. For example, a test voltage frequency of 0.3 kHz, 1 kHz, or 4 kHz can be specified for the resistance values ​​of the analysis sample, the reciprocal of the conductivity, of 4 kΩ, 1 kΩ, or 0.1 kΩ.

[0012] The cell constant has a certain influence on the behavior of the test setup, especially on polarization. The above specifications for the test voltage frequency are particularly suitable for a cell constant in the range of 0.01 cm'. 1 up to 0.5 cm' 1 , especially of 0.02 cm' 1 up to 0.3 cm' 1 , especially of 0.05 cm' 1 up to 0.1 cm' 1 .

[0013] The test voltage period can be determined based on measurement results, alternatively or in combination with the determination described above, in particular to find a particularly suitable test voltage period. For this purpose, at least three measurement signals can preferably be recorded at different times, in particular symmetrically around the first analysis time. One of the measurement signals can be the measurement signal at the first analysis time. The two further measurement signals preferably lie within the same half-wave of the test voltage period and / or at the beginning and / or end of the half-wave, but preferably at a distance from the beginning or end of the half-wave. Using the at least three measurement signals, it can be determined whether the measurement signal curve, in particular at the first analysis time, is linear or curved, or how strong a deviation from linearity is. A large deviation from linearity tends to mean a higher measurement inaccuracy.The test voltage period is preferably set in such a way that the measurement signal curve is as linear as possible, particularly in the area of ​​the first analysis time and / or over the entire measurement period.

[0014] Preferably, the linearity deviation is compared with a predetermined limit for a maximum linearity deviation. The quality of the measurement result is determined based on the comparison result. If the linearity deviation exceeds the specified limit, the measurement result is rejected as too inaccurate. The linearity of the measurement signal curve can be restored by adjusting the test voltage period. Alternatively or additionally, the linearity can be influenced by changing the measuring resistance and / or the gain. This will be discussed in more detail below.

[0015] The test voltage period is therefore preferably set so that it lies within the operating range of the amplifier device and that the polarization is low and / or has a substantially linear profile. Additionally, the test voltage period can be set so that energy consumption, particularly due to the current conducted through the analysis sample, is as low as possible.

[0016] The test voltage is particularly preferably a square-wave voltage, in particular a symmetrical square-wave voltage. This makes the measurement signal particularly stable, especially at the first analysis time. Alternatively, the test voltage can have other signal waveforms. The first measurement signal is preferably detected by a detection device, in particular a voltage measuring device and / or a current measuring device.

[0017] The analysis sample is preferably a fluid containing water, in particular a liquid, in particular an aqueous solution, for example an electrolyte solution with a water content of at least 50%.

[0018] The test voltage can have any desired waveform, at least in sections periodic. Preferably, the test voltage has no DC component or a negligibly small DC component.

[0019] The test voltage is preferably symmetrical. This means that the test voltage has a symmetrical profile, in particular a point-symmetrical profile with respect to half the test voltage period, particularly both in terms of time and voltage amplitude. The test voltage can have a sinusoidal, triangular, or preferably rectangular waveform. The test voltage period is also referred to as the period duration of the test voltage.

[0020] The respective half-wave of the test voltage period can be limited by two consecutive zero crossings and / or inflection points of the test voltage. In other words, the first half-wave extends over the first half of the test voltage period T, in particular from 0 to T / 2, and the second half-wave extends over the second half of the test voltage period T, in particular from T / 2 to T. The middle of the half-wave is understood to be the time range of + / - 20%, in particular + / - 10%, in particular + / - 5%, in particular + / - 2%, around the time at which the respective half-wave has been halfway through, i.e. around the time T / 4 or T*3 / 4 with respect to the test voltage period T.

[0021] To determine the conductivity, a first and / or a second additional measurement signal is preferably recorded, which correlates with the test current passed through the analysis sample, in particular at a first and / or a second analysis time. The first and / or the second additional measurement signal are preferably current measurement signals. The first and / or the second additional measurement signal and the first and / or the second measurement signal are preferably recorded synchronously, in particular synchronized in time, in particular simultaneously. The respective additional measurement signal can be recorded by means of an additional recording device, in particular a current measuring device and / or a voltage measuring device. Preferably, the first and / or the second additional measurement signal are recorded as a voltage across a measuring resistor. From this, the test current can be deduced. The conductivity of the analysis sample can be determined based on the first measurement signal and the test current.Particularly preferably, the conductivity can be determined based on, in particular, a difference and / or a mean value from the first additional measurement signal and the second additional measurement signal.

[0022] According to one aspect of the invention, the acquisition of the first measurement signal can be preceded by a transient phase. During the transient phase, the analysis sample can be subjected to the test voltage. The transient phase preferably comprises several test voltage periods. During the transient phase, the first measurement signal is not acquired. During the transient phase, a stable or regular profile of the measurement signal, in particular the polarization, can be achieved. The conductivity determined based on the first measurement signal acquired after the transient phase is particularly precise.

[0023] Preferably, a second measurement signal is acquired that correlates with the voltage between the two measuring electrodes introduced into the analysis sample at a second analysis time. The second analysis time can be in the middle of the other half-wave, in particular the second half-wave, of the test voltage period, particularly with respect to the first analysis time. This allows the precision of the measurement method to be further increased. The second measurement signal can be acquired using the same acquisition device as the first measurement signal.

[0024] Particularly preferably, the conductivity can be determined based on, in particular, a difference and / or an average value between the first measurement signal and the second measurement signal. This further reduces measurement deviations. The first measurement signal and the second measurement signal may differ, in particular due to a voltage offset. The difference and / or averaging can ensure a particularly precise determination of the conductivity, in particular without the need for corrective measures, in particular a correction calculation, to counteract corresponding offset effects.

[0025] According to one aspect, the first measurement signal is acquired at the first analysis time. This eliminates the need for a computational determination of the first measurement signal at the first analysis time, for example, through temporal interpolation and / or extrapolation and / or integration. In particular, the method requires relatively few measurements. This makes the method particularly robust and energy-efficient.

[0026] The first measurement signal can be recorded at a time interval from the first analysis time.

[0027] Preferably, at least two, in particular at least three, in particular at least five, in particular at least seven, measurement signals are determined within the same half-wave of the test voltage period, preferably symmetrically around the first analysis time and / or at the first analysis time and / or at the beginning and / or at the end of the half-wave and / or at a distance from the beginning and the end of the half-wave, in particular at a maximum distance of 178 from the first analysis time. On the basis of the plurality of measurement signals, determined in particular symmetrically around the first analysis time, it can be determined, for example, whether the measurement signal curve is, as desired, essentially linear and / or measurement deviations, in particular random ones, can be compensated for by averaging, in particular at the first analysis time.

[0028] Multiple measurement signals can be recorded at different time intervals, particularly symmetrically in time, at the first analysis time. The voltage between the two measuring electrodes can be determined at the first analysis time using statistical methods, for example by averaging and / or interpolation and / or extrapolation and / or median calculation and / or estimation methods and / or by artificial intelligence methods. Averaging increases the reliability of the measurement results. Interpolation and / or extrapolation can reduce the requirements for recording measured values ​​at a precisely specified time. For example, measurement errors can also be identified by determining a scatter value. However, corresponding statistical methods have the disadvantage, compared to recording measured values ​​at the respective analysis time, that they are computationally intensive and energy-consuming.

[0029] Preferably, the first measurement signal, in particular all measurement signals, are acquired using a sample-and-hold element. This allows the time of measurement acquisition to be set particularly precisely, in particular to the first analysis time. The acquisition device for acquiring the first measurement signal is preferably an analog-to-digital converter.

[0030] According to a further aspect, the first analysis time, in particular all analysis times, and the test voltage period or the test voltage frequency are controlled by the same control device. This allows the first analysis time to be synchronized particularly precisely with the test voltage period, in particular to be precisely set to time 174 or T*3 / 4. For this purpose, the control device is preferably designed to control the detection device, in particular the sample-and-hold element, and / or to control a test voltage supply, in particular a first switch, for providing the alternating voltage.

[0031] A test voltage period preferably lasts a maximum of 0.02 s, in particular a maximum of 0.01 s, in particular a maximum of 5 ms, in particular a maximum of 1 ms, in particular a maximum of 0.1 ms, and / or at least 0.1 s, in particular at least 1 ps. This allows polarization effects to be further reduced.

[0032] Preferably, a first diagnostic measurement signal can be acquired that correlates with the voltage between the two measuring electrodes at a first diagnostic time. The first diagnostic time can be at the end of the first and / or second half-wave of the test voltage period. At the first diagnostic time, the influence of polarization on the measurement signal can be at its maximum. The polarization effect can be monitored using the first diagnostic measurement signal. For this purpose, the difference between the first diagnostic measurement signal and the first measurement signal can be determined. Depending on the magnitude of the difference, the test voltage period or the AC frequency of the test voltage can be changed. A second diagnostic measurement signal can be determined in accordance with the first diagnostic measurement signal, but at a second diagnostic time, at the end of the other half-wave of the test voltage period.The second diagnostic measurement signal can be compared with the second measurement signal. This allows the polarization effect to be determined even more precisely.

[0033] To diagnose the measurement accuracy, the at least three measurement signals, in particular the at least three measurement signals acquired symmetrically around the first analysis time, can be evaluated, as described above. In particular, it can be checked whether a linearity deviation exists and, if applicable, whether this is below a limit value for the maximum linearity deviation. If a corresponding, in particular predetermined, limit value is exceeded, the measurement signal can be determined to be too inaccurate, in particular rejected. The diagnosis is preferably carried out by evaluating at least three, in particular at least five, in particular at least seven, measurement signals, in particular acquired symmetrically to the first analysis time.

[0034] A diagnosis of the measurement accuracy can alternatively or additionally be carried out by determining the at least one measurement signal for at least two, in particular at least three, in particular at least five, in particular at least ten, test voltage periods. The test voltage periods can each have a deviation in a range of 0.1% to 20%, in particular from 0.5% to 10%, in relation to a base test voltage period. In principle, the measurement signal should deviate at most slightly for different test voltage periods. It has been recognized that a defect in the sensor and / or unforeseen behavior of the measurement setup that is detrimental to the measurement signal can result in different measurement signals for different test voltage periods. Preferably, a deviation between the measurement signals for the different test voltage periods is determined.If the deviation is below a predetermined deviation limit, the measurement signal is determined to be valid. Otherwise, the measurement signal may be determined to be invalid or inaccurate, and in particular, rejected. Diagnosis based on the different test voltage periods may require that the analysis sample is stable over time, meaning that its properties that influence the measurement signal are stable across multiple measurements.

[0035] If the diagnosis reveals an invalid measurement signal, an error message can be output, particularly to a connected data processing device. Brief limit violations, particularly of individual diagnostic steps, can preferably be ignored during the respective diagnosis. Preferably, measurement signals are only discarded and / or an error message is only output if a limit violation has occurred at least three times, in particular at least five times, especially consecutively. The error message can, in particular, be a failure message.

[0036] The error message may trigger a cleaning process, particularly manual or automated, and / or replacement of the sensor, particularly the measuring electrodes introduced into the analysis sample.

[0037] Based on the respective diagnosis, it can be determined in particular whether a sensor, in particular the measuring electrodes introduced into the analysis sample, are defective and / or heavily contaminated.

[0038] According to a further aspect, statistical methods can be carried out for diagnosis and / or for determining the linearity of the measurement signal and / or for determining the test voltage period, for example for determining a scatter, in particular the variance and / or the standard deviation of several measurement signals, in particular the measurement signals recorded within the same half-wave of the test voltage period and / or in time-spaced test voltage periods.

[0039] According to a further aspect, the time of the at least one measurement, in particular the analysis time, and / or the time interval between two consecutive measurements and / or the test voltage period can be adjusted and / or changed in such a way that the effects of interference are reduced or that interference, in particular external interference, in particular aliasing effects, are reduced, in particular avoided. In principle, electromagnetic influences from external devices and electrical lines on a sensor, in particular the detection device, cannot be completely avoided, especially in complex industrial plants that may have a multitude of potential interference sources, such as large electrical consumers or radio communication devices.Measured value interference can be counteracted by appropriately setting the measurement time, in particular the first analysis time, and / or the time interval between two consecutive analysis times for recording the measurement signals. The respective measurement can comprise the recording of multiple measured values. A single measurement is also referred to as a pulse packet. A measurement or a pulse packet can have a duration of 0.1 ms to 100 ms, in particular from 0.2 ms to 50 ms, in particular from 0.4 ms to 25 ms. Two consecutive measurements can be carried out at a time interval according to the above time specifications. Preferably, the time interval between two consecutive measurements or pulse packets is changed. This makes it possible to reduce, and in particular to detect and avoid, aliasing effects.1 to 1000 measurements, in particular 2 to 100 measurements, in particular 3 to 10 measurements, can be performed per second, particularly based on pulse packets. Measurement is understood to mean the acquisition of at least one measurement signal, in particular multiple measurement signals.

[0040] According to one aspect, the measurement signals are processed using a filter, in particular a mean value filter and / or a low-pass filter, in particular reducing measured value deviations. The time for measuring value acquisition, in particular the start time for a pulse packet or the first analysis time, can be determined using a random generator and / or according to a fixed, predetermined system and / or based on, in particular triggered by, at least one event, in particular the start and / or end of a communication phase, in particular of the detection device, for example with a field device, and / or triggered by a signal from a sensor for, in particular electromagnetic, interference and / or a signal of an operating state of an external device, for example a motor and / or a pump and / or a heating unit and / or a radio communication means.The time for at least one measurement can thus be specifically set in a time range when electromagnetic interference is particularly low.

[0041] According to a further aspect, the test voltage period can be changed in such a way that interference, in particular aliasing effects, are reduced, in particular detected and avoided. For example, the test voltage period can be changed starting from a basic test voltage period in a range of 0.1% to 25%, in particular from 0.5% to 10%, in particular from 1% to 5%. With the changed test voltage period or the changed test voltage frequency, further measurements can be carried out. Preferably, at least two, in particular at least three, in particular at least five, in particular at least ten, and / or a maximum of 100, in particular a maximum of 40, in particular a maximum of 20, measurements are carried out, each with different test voltage periods. Using statistical methods, in particular by averaging and / or standard deviation, interference, in particular aliasing effects, can be reduced, in particular compensated ordampened and / or detected and avoided. Mean value filters and / or low-pass filters can also be used here.

[0042] A method or a measuring device which comprises the measures described above for reducing interference, in particular aliasing effects, ensures particularly precise measured value acquisition with high reliability, in particular in a measuring environment subject to external interference.

[0043] The test voltage can be applied to the analysis sample at two supply electrodes, which are separate from the two measuring electrodes. This type of method for determining the conductivity of an analysis sample is also known as the four-pole measuring method. The supply electrodes are also referred to as current electrodes. Alternatively, the additional supply electrodes can be omitted. The test voltage can be introduced directly into the analysis sample via the two measuring electrodes. This type of method is also known as the two-pole measuring method. With the four-pole measuring method, the polarization effect is reduced. The measuring device for the two-pole measuring method is particularly economical to manufacture.

[0044] Preferably, the test voltage is switched off during a waiting period, in particular, disconnected from the analysis sample. During the waiting period, the supply electrodes and / or the measuring electrodes can be electrically connected to a reference potential, in particular a ground potential, or switched to a high-ohmic resistance, preferably of at least 100 kΩ. During the waiting period, no current flows through the analysis sample. This allows the method to be implemented in a particularly energy-efficient manner. A corresponding waiting period can follow each test voltage period.

[0045] A measurement cycle can comprise several test voltage periods, in particular at least two, in particular at least three, in particular at least five, in particular at least seven, and / or a maximum of twenty, test voltage periods, in particular also the transient phase. The measurement cycle can be followed by the waiting period. The ratio between the waiting period and the duration of the measurement cycle is preferably at least 0.5:1, in particular at least 1:1, in particular at least 10:1. The waiting period is preferably set variably, in particular depending on the energy consumption. This allows the method to be implemented in a particularly energy-efficient manner.

[0046] The measuring range is preferably adjusted by changing a measuring resistance value and / or a gain, in particular of the first measuring signal, and / or the test voltage period. The aforementioned parameters can be changed simultaneously or not simultaneously, in particular always only sequentially, in particular in the above order, beginning with the switching of a multi-range measuring resistor. Changing the gain of the first measuring signal is preferably carried out by switching an amplifier device for changing an amplification factor. The amplifier device can be designed as a multi-range amplifier and / or as a differential amplifier. The amplifier device is preferably arranged between the measuring electrodes and the detection device. Changing the test voltage period is preferably carried out by changing the switching frequency of a switch used to generate the alternating voltage, in particular from a direct voltage.Preferably, the switching of the aforementioned parameters is carried out by a control device, in particular a single, common control device.

[0047] The measuring resistor can be adjusted so that a differential voltage across the measuring resistor corresponds as closely as possible to the differential voltage across the measuring electrodes. The multi-range measuring resistor can have resistances in the range of, for example, 100 Ω to 1 MΩ. For example, the multi-range measuring resistor can provide at least 3, in particular at least 5, and / or a finite number, in particular a maximum of 20, in particular a maximum of 10, different measuring resistors. The measuring resistor, in particular the measuring resistance value, can also be adjusted so that the energy consumed, in particular the energy delivered to the analysis sample, is as low as possible. Furthermore, the measuring resistor can be adjusted so that the detection range for determining the measurement signal is not exceeded.

[0048] The amplification of the measurement signal can be adjusted so that the voltage at the detection device, especially the measurement signal, is as strong as possible without exceeding the measurement range. Amplification is not necessary for low conductivities of the analysis sample. Furthermore, the amplification can be adjusted so that the resulting frequency response limitation has as little impact on the respective measurement signal as possible.

[0049] The test voltage period can be set such that the polarization and / or energy consumption are as low as possible. With a long test voltage period, in particular of at least 1 ms, several measurement signals are preferably acquired within the same test voltage period, as described above, in particular for determining the first measurement signal using statistical methods. With a short test voltage period, in particular of less than 100 ps, ​​preferably only a single measurement signal is determined per half-wave, in particular at the first analysis time. This can reduce the required signal processing performance.

[0050] Preferably, the test voltage period, particularly for setting the measuring range, can be varied by at least 0.1 ps, particularly at least 1 ps, particularly at least 100 ps, ​​particularly at least 1 ms, and / or a maximum of 100 ms, particularly a maximum of 10 ms, particularly a maximum of 10 ms. This enables the conductivity of different analysis samples to be determined precisely and energy-efficiently. By determining the measurement signal at the analysis time described above, the measurement results are particularly robust against corresponding changes in the test voltage period.

[0051] Another measure for adjusting the measuring range can be to adjust the waiting time based on the conductivity and / or the test current. If the conductivity and / or test current are high, the waiting time can be increased to reduce energy consumption.

[0052] Conductivity is preferably checked for plausibility. For this purpose, the scatter of measured values ​​can be evaluated using statistical methods. For example, the degree of polarization can also be monitored by comparing the respective diagnostic measurement signal with the measurement signal and / or the additional measurement signal. Measurement errors can also be identified, for example, by monitoring the temporal progression of the measurement signals and / or the additional measurement signals and / or the conductivity. A measurement error can be identified if the corresponding measurement signals and / or additional measurement signals and / or conductivity values ​​change rapidly and / or fluctuate significantly in magnitude and / or fall below or exceed a specified threshold.

[0053] Plausibility can also be determined based on the test current. If the test current is too low, especially below a specified threshold, especially zero, a fault can be detected. A correspondingly low test current may indicate insufficient contact between the electrodes, particularly the measuring electrodes and / or the supply electrodes, with the analysis sample.

[0054] The measurement signals can be stabilized with a filter, in particular a low-pass filter, in particular a variable low-pass filter.

[0055] Preferably, the method is carried out, particularly by the measuring device, with an electrical power of at most 500 mW, particularly at most 100 mW, particularly at most 50 mW, particularly at most 20 mW, particularly at most 10 mW, particularly at most 5 mW, particularly at most 1 mW, and / or at least 0.1 mW. This minimizes energy consumption. The method can be carried out safely, particularly in potentially explosive environments.

[0056] A further object of the invention is to provide an improved measuring device for determining the conductivity of an analysis sample, which is economically feasible, energy-efficient in operation and particularly precise with regard to the resulting measurement results.This object is achieved by a measuring device for determining the conductivity of an analysis sample, in particular an electrolyte solution, comprising a test voltage supply for providing a test voltage, two measuring electrodes for introduction into the analysis sample, a detection device for detecting a first measurement signal that correlates with a voltage between the two measuring electrodes introduced into the analysis sample at a first analysis time, and a control device for determining the conductivity of the analysis sample based on the first measurement signal. The test voltage supply is designed to provide the test voltage in the form of an alternating voltage, and the first analysis time lies in the middle of one of the two half-waves of a test voltage period of the test voltage. The detection device and / or the control device are preferably designed to set the first analysis time accordingly.In particular, the control device can be configured to temporally synchronize the test voltage period and the acquisition of the first measurement signal, so that the analysis time reliably lies in the middle of one of the two half-waves of the test voltage period. The control device can be configured, in particular, to carry out the method described above. The advantages of the measuring device preferably correspond to the advantages of the method described above. The measuring device can be further developed with at least one of the features described above in connection with the method.

[0057] The measuring device can have a connecting part, in particular a plug-in connection part, for the inductive transmission of energy and / or signals. Preferably, an energy of a maximum of 500 mW, in particular a maximum of 100 mW, in particular a maximum of 50 mW, in particular a maximum of 20 mW, in particular a maximum of 10 mW, in particular a maximum of 5 mW, and / or at least 0.5 mW, in particular at least 1 mW, can be transmitted via the connecting part. Such a connecting part can be operated reliably and safely, in particular in an explosive environment. The connecting part can be designed for reversible connection to a mating connecting part, in particular for creating a clamping or locking connection, in particular a bayonet connection, with the mating connecting part.

[0058] Preferably, the measuring device comprises an amplifier device, in particular with an adjustable amplification factor, for amplifying the respective measuring signal.

[0059] The measuring electrodes can be arranged in a sensor head. The test voltage supply, the detection device, and / or the control device can be components of an electronic assembly. The components of the electronic assembly are preferably arranged on a common circuit board.

[0060] According to one aspect of the invention, the measuring electrodes, the electronic assembly, and / or the connecting part are rigidly connected to one another, in particular arranged on a common support structure, in particular in a common housing. Alternatively, a cable connection, in particular a flexible one, can be provided between the measuring electrodes, in particular the sensor head, and / or the electronic assembly and / or the connecting part. A further object of the invention is to provide an improved measuring system that is economically feasible, energy-efficient in operation, and particularly precise with regard to the resulting measurement results.

[0061] This object is achieved by a measuring system comprising a measuring device as described above and a transmitter for processing a signal correlating with the conductivity. The advantages of the measuring system can correspond to the advantages of the method or measuring device described above. Preferably, the measuring system is further developed with at least one of the advantages described above in connection with the method and / or the measuring device.

[0062] The transmitter is also referred to as a signal processing device. The transmitter can be designed to exchange signals with the measuring device, in particular to send them to and / or receive them from the device. Preferably, the transmitter is designed to supply energy to the measuring device, in particular to provide the electrical power required to operate the measuring device.

[0063] The connecting part can be designed to transmit signals and / or energy. The transmitter is preferably connected to a mating connecting part for signal and / or energy transmission. The connecting part and the mating connecting part preferably form a reversibly detachable coupling device between the measuring device and the transmitter. Further features, details, and advantages of the invention will become apparent from the following description of an exemplary embodiment with reference to the figures. They show:

[0064] Fig. 1 is a schematic representation of a measuring system with a measuring device for determining the conductivity of an analysis sample and a transmitter for processing a signal correlating with the conductivity,

[0065] Fig. 2 is a schematic detailed view of the measuring device in Fig. 1, comprising a sensor head with two measuring electrodes for insertion into the analysis sample, a test voltage supply, a detection device for detecting measurement signals and a control device for determining the conductivity of the analysis sample based on the measurement signals,

[0066] Fig. 3 Courses of the measurement signals over time, taking into account different effects acting on them,

[0067] Fig. 4 idealized curves of control signals, test voltage and measurement signals over time, respectively.

[0068] Fig. 5 realistic curves of the measurement signals over time.

[0069] A first embodiment of a method or a measuring system 1 with a measuring device 2 for determining the conductivity of an analysis sample 3 is described with reference to Figs. 1 to 5. The measuring system 1 has a transmitter 4, which is designed to exchange signals and / or energy with the measuring device 2.

[0070] In particular, the transmitter 4 can receive signals, in particular a signal correlating with the conductivity o, from the measuring device 2 and / or transmit signals, in particular control signals, in particular for setting a measuring range, to the measuring device 2 and / or provide the energy required to operate the measuring device 2. For this purpose, a signal and / or energy transmitting connection 5 is formed between the measuring device 2 and the transmitter 4. The transmitter 4 can have a radio module 6 for the wireless exchange of signals and / or a connection for the wired exchange of signals, for example with a central processing unit, and / or a user interface 7, in particular with a display means 8, in particular a display, and / or with an input means, for example a touch-sensitive screen.

[0071] The measuring device 2 can be designed to adjust its measuring range independently, in particular based on measurement signals detected by it and / or based on the control signals of the transmitter 4.

[0072] The measuring system 1, in particular the signal and / or energy-transmitting connection 5, has a coupling device 9. The connection 5 can be reversibly released by means of the coupling device 9. The coupling device 9 is designed for the transmission, in particular inductive transmission, of energy and / or signals, in particular in the manner of a plug connection, with a plug connection part 10 and a counter plug connection part 11. The analysis sample 3 is a liquid, in particular an electrolyte solution. The analysis sample 3 can be conducted through a fluid channel 12 or held in a fluid container.

[0073] The measuring device 2 has two measuring electrodes 13.1, 13.2, which are only shown schematically in Fig. 2. In addition, the measuring device 2 has two supply electrodes 14.1, 14.2 for applying a test voltage Up to the analysis sample 3. Alternatively, the supply electrodes 14.1, 14.2 can be omitted, and the test voltage Up can be applied to the measuring electrodes 13.1, 13.2.

[0074] The measuring electrodes 13.1, 13.2 and the supply electrodes 14.1, 14.2 are components of a sensor head 15. The sensor head 15 is attached to a wall of the fluid channel 12 such that the electrodes 13.1, 13.2, 14.1, 14.2 protrude into the fluid channel 12 and are in galvanic contact with the analysis sample 3, wherein an electrical connection 16 of the sensor head 15 is arranged outside the fluid channel 12.

[0075] The measuring device 2 has a test voltage supply 17 for providing the test voltage Up, in particular an alternating voltage. The test voltage supply 17 is electrically connected to the supply electrodes 14.1, 14.2. To generate the alternating voltage from a direct voltage, the test voltage supply 17 has a first switch 19, in particular a transistor. For this purpose, the switch 19 is connected to two different supply potentials, in particular a positive and a negative supply potential. The upward and downward arrows shown to the right of the switch 19 in Fig. 2 indicate high and low, in particular positive and negative, supply potentials. The test voltage supply 17 can have a capacitor 20 connected to the first switch 19.

[0076] The test voltage supply 17 has a measuring resistor 18. The measuring resistor 18 is also referred to as a shunt. The measuring resistor 18 is preferably a switchable measuring resistor 18, in particular a multi-range shunt, with an adjustable measuring resistance value R.

[0077] Preferably, the test voltage supply 17 comprises a second switch 21 for reversibly switching off the test voltage Up, in particular for separating the test voltage Up from the analysis sample 3, during a waiting period Ti.

[0078] The measuring device 2 has a detection device 22. The detection device 22 is an analog-to-digital converter. The detection device 22 is preferably designed as a sample-and-hold element. The detection device 22 is electrically connected to the measuring electrodes 13.1, 13.2.

[0079] The measuring device 2 preferably comprises an amplifier device 23 for amplifying the voltage difference existing between the measuring electrodes 13.1, 13.2 or for providing an amplified signal to the detection device 22. The amplifier device 23 is preferably designed as a differential amplifier. In particular, the amplifier device 23 can be designed as a switchable amplifier, in particular as a multi-range amplifier, for providing different amplification factors A. The measuring device 2 has an additional detection device 24 for detecting an additional measurement signal that correlates with a test current Ip flowing between the supply electrodes 14.1, 14.2. The additional measurement signal is preferably detected as a voltage present across the measuring resistor 18.The additional measurement signal could alternatively be detected as a current, in particular as a test current, which flows through the measuring resistor 18, in particular by means of a measuring current converter. The additional detection device 24 is designed as a sample-and-hold element, in particular as an analog-to-digital converter.

[0080] Furthermore, the measuring device 2 comprises a communication device 25 for transmitting and / or receiving digital information.

[0081] The measuring device 2 has a control device 26. The control device 26 comprises a programmable computing unit, in particular a processor.

[0082] A stimulus controller 27a of the measuring device 2 is configured to control the first switch 19, in particular to generate the test voltage Up. The stimulus controller 27a can also be configured to adjust the multi-range measuring resistor 18, i.e., the measuring resistance value R, and / or the second switch 21, i.e., the waiting time Ti.

[0083] An amplifier control 27b of the measuring device 2 is designed to adjust the amplifier device 23, i.e. the amplification factor A.

[0084] Preferably, the detection device 22, the additional detection device 24, the communication device 25, the control device 26, the stimulus control 27a and the amplifier control 27b are components of a signal processing unit 28, which can be designed as an integrated circuit, in particular as a microcontroller.

[0085] Furthermore, the measuring device 2 comprises a transmission element 29 for transmitting digital signals to the transmitter 4 and for receiving digital signals and / or energy from the transmitter 4, in particular via the coupling device 9. The transmission element 29 is designed to provide the digital signals transmitted jointly via the coupling device 9 and the energy for operating the measuring device 2 separately from one another.

[0086] The test voltage supply 17, the amplifier device 23, the signal processing unit 28 and the transmission element 29 are components of an electronic assembly 30. The components of the electronic assembly 30 are preferably arranged on a, in particular common, printed circuit board.

[0087] The electronic assembly 30 can be connected to the sensor head 15 and / or the plug-in connector 10 rigidly or flexibly, in particular by means of a cable. Preferably, the electronic assembly 30 and / or the plug-in connector 10 are integrated into a housing of the sensor head 15. The connection between the transmitter 4 and the mating plug-in connector 11 can be implemented as a cable connection.

[0088] The measuring device 2 may comprise a filter device, in particular a digital and / or adjustable filter device, for example a low-pass filter. The method, the measuring system 1, and the measuring device 2 function as follows:

[0089] The sensor head 15 is attached to the wall of the fluid channel 12 such that the measuring electrodes 13.1, 13.2 and the supply electrodes 14.1, 14.2 are in contact with the analysis sample 3. The plug-in connector 10 is reversibly and detachably connected to the mating plug-in connector 11. The transmitter 4 is in energy- and signal-transmitting connection with the measuring device 2.

[0090] The transmitter 4 provides the energy required to operate the measuring device 2 via the inductive coupling device 9 on the electronic assembly 30. The average, in particular the maximum, electrical power supplied to the measuring device 2 is preferably a maximum of 100 mW, in particular a maximum of 20 mW, in particular a maximum of 10 mW.

[0091] By means of the control device 26, a first measuring resistance value R is set via the stimulus control 27a, preferably a high measuring resistance value R, in particular the highest available measuring resistance value Rmax.

[0092] The test voltage Up is generated as an alternating voltage with a test voltage period T by means of the control device 26 via the stimulus controller 27a and the first switch 19. The test voltage Up has a symmetrical profile, in particular a point-symmetrical profile with respect to half the test voltage period T, in particular both in terms of time and voltage amplitude. After each test voltage period T of the test voltage Up or after a measurement cycle with a plurality of test voltage periods T, the test voltage Up can be disconnected from the sensor head 15 by means of the second switch 21. A corresponding waiting period Ti is controlled by the control device 26 via the stimulus controller 27a and reduces the energy consumption of the measuring device 2.

[0093] The control device 26 sets the amplification factor A via the amplifier control 27b and the amplifier device 23, preferably to a low value.

[0094] The measuring resistance value R can be 100 kΩ, the amplification factor A can be 1 and the test voltage period T can be 10 ms.

[0095] The test voltage Up is applied to the analysis sample 3 at the supply electrodes 14.1, 14.2. A voltage U is established between the measuring electrodes 13.1, 13.2. The measuring voltage UM, amplified by the amplification factor A, is applied to the detection device 22. Its time profile UM(1) is determined as

[0096] U M (t) = A * (U13.1(t) - U13.2(t)).

[0097] Figure 3 shows three curves of the measurement voltage UM applied to the detection device 22 plotted over time. The actual curve 31 of the measurement voltage UM is shown as a solid line. This actual curve takes into account effects due to the polarization of the analysis sample 3 and due to the frequency response limitation by the amplifier device 23. The first simplified curve 32 of the measurement voltage UM, shown as a dash-dot line, only takes into account effects due to the polarization. The second simplified curve 33 of the measurement voltage UM, shown as a dashed line, takes into account neither the frequency response limitation nor the polarization.

[0098] The second simplified curve 33 qualitatively corresponds to the time profile of the test voltage Up. The curve of the test voltage Up can therefore be seen from the simplified curve 33. In particular, the first half-wave 34.1 and the second half-wave 34.2 of the test voltage period T can be seen from the simplified curve 33 of the measurement voltage UM.

[0099] A first measurement signal UMI correlates with the voltage U between the measuring electrodes 13.1, 13.2 at a first analysis time ti. The first measurement signal UMI corresponds to the measurement voltage UM at the first analysis time ti, which is in the middle of the first half-wave 34.1 of the test voltage period T. In other words, UMI = UM(4 = ti = T / 4).

[0100] It was advantageously found that at time t1 the real curve 31, the idealized curve 32 and the simplified curve 33 have approximately the same value, in particular that these curves 31, 32, 33 intersect at time t1. Based on this, it was found that the influences of the polarization of the analysis sample 3 or the frequency response limitation by the amplifier device 23 are small at the first analysis time t1, in particular at t1 = T / 4. At the first analysis time t1, the conductivity of the analysis sample 3 can therefore be determined particularly precisely. High measurement accuracy can be achieved even if the test voltage period T is short and / or the amplification factor A is high and / or due to the high conductivity of the analysis sample 3, a high test current Ip between the supply electrodes 14.1, 14.2 leads to a polarization with a non-negligible influence on the curve of the measurement voltage UM.

[0101] Because the detection device 22 is designed as a sample-and-hold element, the first measurement signal UMI can be determined particularly precisely at the first analysis time ti.

[0102] Preferably, a second measurement signal UM2 is determined at a second analysis time t2, which is in the middle of the second half-wave 34.2. In particular, UM2 = UM(1 = t2 = T3 / 4). The conductivity o is preferably determined based on the first measurement signal UM1 and the second measurement signal UM2.

[0103] The conductivity o can also be determined based on the test current Ip conducted through the analysis sample 3. For this purpose, the voltage Us across the measuring resistor 18 is measured by the additional detection device 24. The known measuring resistance value R is used to determine the test current Ip, in particular the test current Ipi at the first analysis time ti. Taking into account the cell constant C, the conductivity of the analysis sample 3 can be determined as follows:

[0104] O = C * IPI / UMI .

[0105] The respective time t1, t2 for recording the measurement signal UMI, UM2 and the voltage Usi, Us2 at the measuring resistor 18 is preferably controlled by the same control device 26 in order to ensure the most synchronized, simultaneous measurement value recording possible. The present sensor head 15 is designed as a 4-pole sensor. Alternatively, a sensor head in the form of a 2-pole sensor can be used. The 2-pole sensor does not have separate supply electrodes 14.1, 14.2. The measuring electrodes 13.1, 13.2 also serve as supply electrodes 14.1, 14.2. In other words, with a 2-pole sensor, the test voltage U p applied to the measuring electrodes 13.1, 13.2.

[0106] Figs. 4 and 5 show the course of different voltages within the measuring device 2 over time. The indices of the voltage values ​​entered therein indicate the measuring position, unless otherwise specified. The voltages U13.1, U13.2, U14.1, U14.2 are applied to the electrodes 13.1, 13.2, 14.1, 14.2. The voltages U19, U21 are applied to the switches 19, 21. The voltages U24.1, U24.2 are applied to the measuring terminals 24.1, 24.2 of the additional detection device 24. The voltages U22.3, U24.3 are applied to the respective control terminals 22.3, 24.3 of the detection device 22 and the additional detection device 24, respectively. The respective voltages are measured relative to the ground potential UGND. UMI to UM4 are differential voltages and are not related to the ground potential UGND. Ipi to Ip4 are differential currents, for which the following applies in particular:

[0107] Ip(t) = (U24.1(t) - U24.2(t)) / R.

[0108] The voltage curves shown in Fig. 4 correspond to simplified curves, without taking polarization or frequency response limitation into account. The voltage curves shown in Fig. 5 correspond to the actual curves, taking into account the effects of polarization and frequency response limitation. The test voltage period T is initially set as short as possible in order to reduce the influence of polarization. Depending on the conductivity o of the analysis sample, the test voltage Up can be disconnected from the sensor head 15 after one or more test voltage periods T, in particular via the waiting period Ti, in order to save energy.

[0109] To adjust the measuring range, the resistance value R and / or the gain factor A and / or the periods T can be changed, preferably one after the other, in particular in this order. Preferably, only one of these parameters is changed per measuring cycle. A measuring cycle can comprise one or more test voltage periods T.

[0110] The measuring resistance value R is preferably set so that the differential voltage across the measuring resistor 18 and the differential voltage across the measuring electrodes 13.1, 13.2 are as equal as possible. This setting is deviated from if the available energy is insufficient. A correspondingly higher measuring resistance value R is then selected.

[0111] The amplifier device 23 is adjusted so that the maximum detection range of the detection device 22 is not exceeded, in particular, it is not exceeded or undershot, and so that the differential voltage at the measuring terminals 22.1, 22.2 of the detection device 22 is as large as possible in order to reduce measurement inaccuracies of the detection device. Amplification of the voltage U applied to the measuring electrodes 13.1, 13.2 is not always necessary. With a short test voltage period T or a high test voltage frequency f, the amplification factor A should generally be reduced to reduce the effect of frequency response limitation.

[0112] The test voltage period T is set such that the effect of polarization on the voltage U is as small as possible and that the cutoff frequency of the amplifier device 23 is not exceeded, in particular for the set amplification factor A. A short test voltage period T is generally advantageous with regard to the energy consumption of the measuring device 2, in particular if, after one or more test voltage periods T, the test voltage Up is disconnected from the sensor head 15 for the waiting period Ti.

[0113] For a particularly reliable determination of the conductivity o of the analysis sample 3, the measurement signals UMI, UM2 can be determined multiple times, in particular for several consecutive test voltage periods T, in particular of the same measurement cycle. Alternatively or additionally, measurement signals can be recorded before and / or after the respective analysis time t1, t2, in particular temporally symmetrical to the respective analysis time t1, t2. The conductivity o of the analysis sample 3 can be determined using statistical methods, in particular as the mean value of the multiple measurement signals. Preferably, in particular with a short test voltage period T, measurements are only taken at the analysis times t1, t2. With a longer test voltage period T, a corresponding statistical evaluation can be carried out to increase the measurement accuracy.

[0114] Particularly preferably, the analysis sample 3 is subjected to several consecutive test voltage periods T, wherein the acquisition of the first measurement signal UMI occurs only after the expiration of at least one, in particular several, test voltage periods T. Thus, the first test voltage periods T can serve as idle periods to ensure a stable course of the measurement signal UMI ZU, in particular a steady state with regard to polarization. This further increases the measurement accuracy.

[0115] The waiting time Ti following a single test voltage period T and / or a plurality of test voltage periods T, in particular a measurement cycle, is preferably selected to be longer the higher the conductivity o of the analysis sample 3. This reduces the energy consumption of the measuring device 2.

[0116] The method for determining the conductivity o can comprise fault detection. For this purpose, at least one further measurement signal UM3, UM4 can be acquired at a further analysis time t3, t1. Preferably, the measurement signals UM3, UM4 are acquired at the end of the first and / or second half-wave 34.1, 34.2, in particular at t3 = T / 2 or t4 = T. The at least one further measurement signal UM3, UM4 is preferably compared with the first and / or second measurement signal UM1, UM2. The comparison result allows conclusions to be drawn about the degree of polarization and / or the influence of the frequency response limitation of the amplifier device 23. If the voltage difference is high, there is also a high degree of polarization. In order to reduce the influence of the polarization, the test voltage period T can be reduced.

[0117] A fault can also be detected if the test current Ip is too low, in particular 0. A fault can also be detected if the determined conductivity o is subject to excessive temporal fluctuation. Preferably, the test voltage period T and / or the measuring resistance value R and / or the gain factor A are set using control data, in particular tabular control data, in particular a frequency table. This makes the method particularly reliable in operation and easy to control.

[0118] By determining the first measurement signal UMI at the first analysis time ti according to the method described above, which lies in the middle of one of the half-waves 34.1, 34.2 of the test voltage period T, measurement deviations can be significantly reduced. In particular, a correction of measurement signals, which is complex in terms of computing capacity and energy consumption, can be avoided. The measuring device 2 described above, in particular the measuring system 1 formed therewith, and the method for determining the conductivity o of the analysis sample 3, are particularly energy-efficient in operation and ensure the determination of the conductivity o in a reliable manner and with particularly high precision.

Claims

Patent claims 1. A method for determining the conductivity (a) of an analysis sample (3), in particular an electrolyte solution, comprising the steps: 1.1 Applying a test voltage (Up) to the analysis sample (3), 1.2 Acquisition of a first measurement signal (UMI) which is associated with a voltage (U) between two measuring electrodes (13.1, 13.2) introduced into the analysis sample (3) at a first analysis time (ti), and 1.3 Determining the conductivity (o) of the analysis sample (3) using the first measurement signal (UMI), 1.4 where the test voltage (Up) is an alternating voltage, and 1.5 wherein the first analysis time (ti) is in the middle of one of the two half-waves (34.1, 34.2) of a test voltage period (T) of the test voltage (Up).

2. Method according to claim 1, characterized by detecting a second measurement signal (UM2) which correlates with the voltage (U) between the two measuring electrodes (13.1, 13.2) introduced into the analysis sample (3) at a second analysis time (t2), wherein the second analysis time (t2) lies in the middle of the other half-wave (34.2) of the test voltage period (T).

3. Method according to claim 2, characterized in that the conductivity (o) is determined based on the difference between the first measurement signal (UMI) and the second measurement signal (UM2).

4. Method according to one of the preceding claims, characterized in that the detection of the first measurement signal (UMI) takes place at the first analysis time (ti).

5. Method according to one of the preceding claims, characterized in that the detection of the first measurement signal (UMI) takes place at a time interval from the first analysis time (ti), wherein the voltage (U) between the two measuring electrodes (13.1, 13.2) at the first analysis time (ti) is determined by means of statistical methods.

6. Method according to one of the preceding claims, characterized in that the first measurement signal (UMI) is detected by means of a sample-and-hold element (22).

7. Method according to one of the preceding claims, characterized by controlling the first analysis time (ti) and the test voltage period (T) by means of the same control device (26).

8. Method according to one of the preceding claims, characterized by a test voltage period (T) of a maximum of 0.01 s.

9. Method according to one of the preceding claims, characterized by detecting a first diagnostic measuring signal (UM3, UM4) which correlates with the voltage between the two measuring electrodes (13.1, 13.2) at a first diagnostic time (t3, U), wherein the first diagnostic time (t3, U) is at the end of the first and / or the second half-wave (34.1, 34.2) of the test voltage period (T).

10. Method according to one of the preceding claims, characterized in that the test voltage (Up) is applied to the analysis sample (3) at two supply electrodes (14.1, 14.2) which are formed separately from the two measuring electrodes (13.1, 13.2).

11. Method according to one of the preceding claims, characterized by switching off the test voltage (Up) during a waiting period (Ti).

12. Method according to one of the preceding claims, characterized by setting the measuring range by changing the test voltage period (T) and / or by changing a measuring resistance value (R) and / or by changing an amplification (A) of the first measuring signal (UMI).

13. Method according to one of the preceding claims, characterized by checking the determined conductivity (o) for plausibility.

14. Method according to one of the preceding claims, characterized in that it is carried out with an average, in particular a maximum, electrical power of at most 20 mW.

15. Measuring device (2) for determining the conductivity (o) of an analysis sample (3), in particular an electrolyte solution, comprising 15.1 a test voltage supply (17) for providing a test voltage (Up), 15.2 two measuring electrodes (13., 13.2) for insertion into the analysis sample (3), 15.3 a detection device (22) for detecting a first measurement signal (UMI) which correlates with a voltage (U) between the two measuring electrodes (13.1, 13.2) introduced into the analysis sample (3) at a first analysis time (ti), and 15.4 a control device (26) for determining the conductivity (o) of the analysis sample (3) based on the first measurement signal (UMI), 15.5 where the test voltage (Up) is an alternating voltage, and 15.6 wherein the first analysis time (ti) is in the middle of one of the two half-waves (34.1, 34.2) of a test voltage period (T) of the test voltage (Up).

16. Measuring device (2) according to claim 15, characterized by a connecting part (10), in particular a plug-in connection part for the inductive transmission of signals.

17. Measuring system (1), with 17.1 a measuring device (2) according to one of claims 15 or 16 and 17.2 a transmitter (4) for processing a signal correlating with the conductivity (o).

Images