DE9205313U1 - Device for monitoring the operating status of electrochemical sensors - Google Patents

Description

2517/ot/mu
30.3.1992

Conducta GmbH & Co., Dieselstr. 24, 7016 Gerungen

Device for monitoring the operating status of electrochemical sensors

State of the art

The invention is based on a device according to the preamble of claim 1 and is specifically directed to monitoring the internal resistance of potentiometric measuring half-cells and their reference half-cells. In a known monitoring circuit of this type (EP 86200640.0), which is also shown in Fig. 2 as prior art, to which special reference is made here, a test voltage U ' which differs from the measuring potential is periodically applied to the measuring input E ' or to the reference input via a series resistor R ₁ ' and a switch S-'.

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and the step response of the RC element consisting of series resistance, cell internal resistance R'pH and cable capacitance Ck is determined by sampling the voltage curve, e.g. using a sample and hold element, at different times after the test voltage is applied. If the internal resistance of the monitored cell changes, this is expressed in a characteristic change in the charge transfer rate, from which a possible error can be concluded. If necessary, an additional capacitance is connected to the measuring input via another switch in order to achieve longer and therefore easier to evaluate charge transfer times. In order not to load the measuring cell with a direct current over time, which would lead to disruptive polarization or electrolytic decomposition processes in the cell, positive and negative test voltages can be switched on alternately.

Since a potential equalization pin PA' is usually immersed in the measuring medium next to such a pH-sensitive half-cell, which is used here as a non-limiting example for better understanding of the invention, and the reference half-cell, the described scheme makes it possible to monitor both half-cells separately for their fault status. Due to the very different internal resistances, the two sub-circuits must be dimensioned differently.

In this known test procedure, which is shown in Fig. 1, as well as in all other

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With other monitoring circuits, which will be discussed below and which are based on the fact that the measuring input is electrically directly or at least indirectly intervened, considerable problems can arise because the one or more switches required for this, such as Sl' in Fig. 1, load the signal node between the pH measuring cell and the downstream input amplifier. According to the state of the art, pH electrodes usually have internal resistances of up to several gigaohms

9
(several 10 ohms). In contrast, even the best electronic switches do not have an arbitrarily high resistance when open, so that they may distort the signal from the pH sensor. The required measurement accuracy is therefore not achievable in many applications. In the aforementioned EP 86200640 (Yokogawa), but also, for example, in the monitoring circuit known from EP 0419769, which is explained further below, there is an additional requirement that the query time must be adhered to exactly for evaluation; this makes it difficult to implement the evaluation software that is usually implemented on a microcomputer.

In general, the measurement of the pH potential with glass electrodes is one of the most common measuring tasks in process engineering. The pH-sensitive half-cell can be described in a simple approximation, as shown in Fig. 1, by a primary voltage U " and an internal resistance

P"

resistance R ". Typical values for pH

U "are between -400 mV and +400 mV, extreme values P"

for R„ between 50 KQ and 10 &THgr;&OHgr;, depending on the used

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sensor glass and the operating temperature. Other potentiometric sensors, for example ion-sensitive measuring electrodes (ISE), have comparable properties; if, for the sake of simplicity, only pH measuring cells are referred to below, these and all other comparable sensors and measuring cells are therefore always included.

The pH-sensitive glass membrane of the measuring electrode separates the interior of the half-cell from the surrounding measuring medium. If this membrane is damaged (hairline cracks) or destroyed, the measuring potential can be falsified or short-circuited against the reference potential. This can simulate incorrect measured values; this can be particularly dangerous if a measuring voltage of 0 volts is expected, but in reality the corresponding correct pH value has long since been exceeded.

On the other hand, measurement difficulties can also arise if chemical processes change the structure of the glass membrane in such a way that a serious increase in resistance occurs and then the measuring system becomes very slow due to its unavoidable input capacitance.

Therefore, monitoring the glass membrane, for example, is an important safety-related area in electrochemical analysis technology.

A corresponding monitoring task also exists on the side of the reference half-cell, which complements the pH half-cell to form a complete measuring chain.

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The purpose of this reference half-cell is to establish a conductive contact with a well-defined contact potential to the measuring medium. For this purpose, an electrolyte bridge is formed, for example, using capillaries, porous materials or other means, which connects the outside with the inside of the half-cell. There is a risk that this conductive bridge will be blocked by dirt or deposits during operation, thus creating an incorrect pH value. Typical values for the volume resistance of reference half-cells are less than or around 1 kti; in the event of a fault, this resistance changes to several kf2 or more. The detection of an increase in resistance can be used to initiate a cleaning process for the measuring chain (if necessary automatically); if cleaning is unsuccessful, the sensor must be replaced.

In connection with these monitoring areas, a large number of other solutions have already become known.

For example, in US 3661748, an alternating current is fed into the measuring medium via an auxiliary electrode. If the glass membrane is damaged, an alternating voltage signal appears at the contact of the measuring electrode, which serves as an error indicator.

In US 49611163 (Bryan) it is proposed to periodically allow a direct current to flow between two auxiliary electrodes. The resulting water electrolysis briefly raises the pH value,

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so that the correct function of the measuring chain can be checked.

Both of the methods mentioned can be problematic at least in that they make sensor installation unnecessarily difficult due to the auxiliary electrodes and also require a considerable amount of electronic circuitry.

In US 4189367, a current of varying amplitude is also periodically impressed between the measuring cells and a potential equalization pin using switching means. The resulting voltage drop is a measure of the level of the internal resistance of the cells. A similar method is used in the aforementioned EP 0419769A2, whereby the temporal progression of the test signal is also used to classify different errors. A disadvantage of this technology is that different cable capacities result in different recharge times when installing the measuring point; the evaluation must take this into account in an appropriate manner. The above-mentioned restriction applies to all methods that utilize the time process of recharge.

Finally, in DE 4001274A1, namely in its subclaim 18, a fundamental solution for the problems mentioned is indicated by using several electronic switches and an additional amplifier stage to ensure that during the measuring phase a voltage U ¹ is applied to the actual test switch Sl which is identical to the

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Voltage at the measuring input. Therefore, no current flows through the switch S^ and the measuring voltage is not distorted. However, the electronic effort specified in DE 4001274 for achieving this goal is particularly high.

The present invention is therefore based on the object of creating a monitoring circuit for sensors in analytical chemistry, in particular an electronic monitoring circuit, which does not impair the highly precise measuring process through its presence and at the same time achieves the set goal inexpensively and with very simple circuit means, therefore without major expenditure.

Advantages of the invention

The invention solves this problem with the features of claim 1 or the first device claim and has the advantage that for measuring the internal resistance of electrochemical sensors, especially potentiometric sensors and their reference half-cells, a test voltage can be applied to the measuring input, for example via a resistor, without the switching means required for this representing a load on the sensor voltage in the measuring phases and without a precise time schedule having to be observed for the evaluation.

It is particularly advantageous that the electrode monitoring according to the invention can be implemented with

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The simplest circuit means can be used, whereby there is no need to resort to precision switching means or precision resistors when implementing the invention. The electrode monitoring provided by the invention is suitable both for the actual measuring sensor, for example a pH-sensitive half-cell, and for its reference half-cells in the same way, provided that suitable dimensions are selected.

Even if the internal resistance of the pH measuring cell assumes very low values, for example around 50 kfl, it is possible to ensure correct measurement values by appropriately dimensioning the simple resistors used.

Thus, the circuit on which the invention is based can be dimensioned in such a way that a quantitative determination of the internal resistance RpH of a pH measuring cell in the range from 50 kΩ to 1 μΩ is possible. In this way, in addition to complete destruction of the glass membrane, the occurrence of hairline cracks can also be reliably detected.

On the other hand, the circuit according to the invention also allows for dimensions that can reliably detect an increase in resistance due to membrane changes.

The control and evaluation process can be set up so that an entire test cycle can be completed very quickly, for example in less than 300 ms.

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can be worked on. For measurements with pH electrodes or ion-sensitive electrodes, the duration of such an interruption is in any case irrelevant.

A dimensioning can also be specified for a reference electrode, which allows reliable detection, for example, of blockage of the electrolyte path.

The measures listed in the subclaims enable advantageous further developments and improvements of the invention. Particularly advantageous is the possibility of introducing a second switch in the positive feedback branch included in the electrode monitoring circuit, which is also open in the measuring phases and closed during the test phase. This second switch ensures that the operational amplifier in the form of a voltage follower provided here as an amplifier functions correctly even with a very low-impedance feedback path.

Finally, a further advantage is that the resistance Rl, via which the test voltage is switched on via a switch during the test cycles, can be selected to be so low-resistance that the charge reversal of the RC element comprising this resistance Rl is completed after a very short time, for example after a few 10 ms.

One is therefore not forced to measure the time process of the reloading, as is the case with other proposed

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This is not the case with any other method, but the final voltage can be used to assess the condition of the membrane. This increases the accuracy of the evaluation and the requirements for the control circuit remain low at the same time, because the query time does not have to be precisely determined. There is also no need to wait very long until the voltage is stable, as is the case with other proposals. The test time can therefore be kept short compared to the measurement time.

drawing

Embodiments of the invention are shown in the drawing and are explained in more detail in the following description. They show:

Fig. 1 shows a first simple embodiment of a monitoring circuit according to the present invention for electrodes in electrochemical analysis measurement technology, wherein in comparison

Fig. 2 shows the electrical circuit structure of a prior art electrode monitoring circuit, in which the test voltage is applied via a switch to the input of an operational amplifier connected to the measuring electrode,

Fig. 3 an electrode monitoring circuit extended by an additional switch to maintain the correct function of the operational amplifier used as a voltage follower,

Fig. 4 a completed evaluation circuit under

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Inclusion of the electrode monitoring circuit of Figures 2 and 3 and

Fig. 5 shows the voltage curves of a complete test cycle in diagram form.

Description of the embodiments

The basic idea of the present invention is to prevent a disturbing fault current resulting from the electrode monitoring circuit during the measuring operation, which can flow from the switch to the measuring input of a downstream amplifier via which the test circuit is connected.

One way to prevent this error current is to ensure that the amplifier input and the switch connection have the same potential during the measurement cycle. Under these circumstances, it is irrelevant how the switch itself is designed, whether it has a very high or low resistance when open, because the potential identity clearly prevents the flow of any current that could have a distorting effect on the actual measurement signal.

In the illustration in Fig. 1, the sensor is represented by the equivalent circuit of its internal resistance R„

ptl

and the original voltage or measuring voltage U " emitted by it. The sensor is an operating

pti.

amplifier OPV, whose non-inverting, i.e. positive input E _M is the output voltage

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voltage of the sensor, whereby a capacitance C^ is connected in parallel to the sensor, which represents all capacitances occurring in the area of the sensor, but also, for example, as cable capacitance.

The operational amplifier OPV represents a perfect voltage follower, since the output terminal A is directly connected to the inverting input terminal E _v .

The test voltage U is supplied via a switch S, which determines the operating or test cycle by its opening or closing, whereby the test voltage reaches a circuit point P ₁ , formed by the connection point of two resistors R ₁ and R-, which also feed back the output A of the operational amplifier to its input terminal E..

As is known, the operational amplifier OPV represents a very high-impedance voltage follower with gain V=I due to the circuitry shown in Fig.1, therefore the voltage at output A is equal to the voltage at measuring input E _M.

If the resistance R-, which in series with the resistance R^ returns the output voltage to the positive input (measurement input E _M ) of the operational amplifier, is kept small compared to the residual resistance of the switch S in the open state, which can also be achieved very easily using commercially available components,

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chen, then, regardless of the size of the test voltage U applied to the switch, the same voltage is obtained at the circuit point P. as at the circuit point P-, which forms the signal node and is connected to the input of the amplifier. In other words, the potential of the two circuit points or nodes P. and P- is identical. This phase therefore represents the measuring operation of the circuit shown in Fig.1, in which, due to the identical potentials of P. and P- to the measuring input E _M , no fault current can flow through the resistor R,, so that the measuring signal of the pH cell shown in this case from the original voltage U "and internal resistance R_" also

ptl-ptl-ptl

cannot be falsified.

For the correct function, the circuit shown in Fig.1 only requires that during measuring operation the (positive) feedback formed by the resistors R-, and R- and fed back to the positive input terminal E _M of the operational amplifier must not become so low-impedance that the operational amplifier can no longer work as a voltage follower with V=I.

The further function of the circuit in Fig. 1 is then such that during a test cycle replacing the measuring operation or measuring cycle, for example checking the measuring cell membrane, the switch S is closed. The forward resistance of the switch is now small compared to the value of R _2. Then the node P. assumes the test potential U in very good approximation. The measuring input E" is therefore recharged with a time

. 1 &id; *

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constant resulting from the total capacitance C and the parallel connection of the resistors R " and R ₁ .

ptl &igr;

C is essentially formed by the cable capacitance of the sensor connection cable; typical values can be between 100 pF and several nF. The target voltage of the charge transfer is calculated in a known manner from the voltages U _o , U and the resistances R " and R ₁ .

pn S pn X

There is a great deal of freedom in the dimensioning of R ₁ and U, so that the most suitable combination for a particular evaluation method can be selected. The time-varying voltage can be taken from output A of the amplifier and evaluated.

As mentioned, the arrangement described is equally suitable for a reference half-cell if a suitable dimensioning is chosen.

If the internal resistance of the pH measuring cell assumes very low values of around 50 kTi , which can occur, for example, with special electrodes at very high measuring temperatures, the resistance _R1 must also have a small value so that the division factor between the internal resistance and R is large enough for the evaluation. In this case, the feedback path R2+R-1 can become so low-resistance that correct function of the operational amplifier as a voltage follower becomes difficult or is no longer guaranteed.

The solution is then, as shown in Fig. 4, to introduce a second switch S- in the positive feedback branch, which is also open during the measurement phases and closed during the test phase.

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A new control signal is therefore not required. The additional effort for S ₂ is negligible, since several electronic switches are usually combined in one housing.

According to Fig. 4, two switches S ₁ and S ₂ are now provided in the positive feedback branch, formed by the resistors R. and R ₂ , whereby the second switch S ₂ opens or closes the connection between the two resistors R. and R ₂ - synchronously with the switch movement of S ₁ , while the switch S ₁ switches the test voltage U present at the output of a low-pass filter with operational amplifier PPO.

switches to the positive feedback branch. Due to the presence of this second switch S ₂ , the resistance R- can now also be selected to be (arbitrarily) low-resistance, since the switch S ₂ being open during measurement operation means that this positive feedback branch is high enough to ensure the correct function of the operational amplifiers as voltage sequences.

Figures 4 and 5 show a preferred simple evaluation method for monitoring electrodes, whereby Figure 4 comprises the circuit components of Figure 3, expanded by a microcontroller 11 and its circuitry. The output port P of the computer supplies the control signal for the two synchronous switches S ₁ and S ₂ - The test voltage U can be generated in the usual way by a pulse width modulated pulse train PWM with a downstream low-pass filter with operational amplifier PPO or by another conventional analog/digital converter. Finally, the

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The output signal U- of the impedance converter (operational amplifier OPV) is measured via an analog-digital converter ADW which can be part of the computer or a separate component.

A complete test cycle is shown in Fig. 5.

- At time t < T ₁ (measurement operation) U _g has arbitrary values, but preferably U is adjusted to approximately match the measuring voltage U 1 at the output A of the amplifier.

- At time T ₁ the switches are closed and U is set to a first test voltage U .. _£1 . The voltage at the pH measuring cell now charges at different speeds depending on the cable capacity and internal resistance - in order to reach a final value

^U testl' ^R pH ^} / ^{( R} l ^{+ R} pH

2 follows this voltage. From the final value it is calculated whether R "is in the safe range or whether pn

there is damage or degeneration of the membrane.

- At time T_, with the switches U still closed, a second test voltage U _γ ^�PS7 is set. The time period and value are selected so that on average the current through the measuring cell = 0, so that no damage to the cell can occur.

If the initial measuring voltage is negative, first a positive test voltage is applied, then a negative

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tive test voltage is applied, otherwise vice versa. The test signal that can be evaluated is then always sufficiently large.

- At time T-, U is reset to the original measuring voltage, or possibly to a measuring voltage that results from a prediction from the time course before the test cycle. After the measuring input has adjusted to this voltage, the measuring operation can be resumed at time T ₄ .

The times given in Fig. 5 below are approximate values; they are essentially determined by the operating speed of the connected computer.

Finally, it should be noted that the claims and in particular the main claim are attempts to formulate the invention without comprehensive knowledge of the state of the art and therefore without restrictive prejudice. Therefore, we reserve the right to regard all features presented in the description, the claims and the drawing, both individually and in any combination with one another, as essential to the invention and to set them down in the claims and to reduce the feature content of the main claim.

Claims (6)

DIPL-ING. PETER OTTE PATENT ATTORNEY: f;* - f"?-^ ^Leonber 3 Representative at the European Patent Office/ European Patent Attorney - '''&Iacgr;_&idiagr;'^; - ^: "'T^oJeYStraße 2517/ot/mu
30.3.1992 Conducta GmbH & Co., Dieselstr. 24, 7016 Gerungen Protection claims 1. Device for monitoring the operating state of electrochemical sensors, in particular the behavior of the internal resistance of potentiometric measuring half-cells and their reference half-cells, wherein the measuring input (E _M ) connected to the electrochemical sensor to be monitored is supplied with a test voltage (U ) via a test resistor (R.) from a switch (S.) which is closed during the test cycles, characterized in that the resistance-side connection point (P.) of the switch (S.) is connected to the output (A) of the amplifier (OPV) via a further resistor (R ₂ ) ^{with sufficiently} low resistance that in measuring mode with the switch (S ₁ ) open, the amplifier input (E") of the operational amplifier and the connection point (P ₁ ) of the switch (S.) have the same potential in order to avoid a fault current from the switch to the measuring input. 2. Device according to claim 1, characterized in that the second resistor (R^) which is low-resistance with respect to the contact resistance of the switch (S ₁ ) in the open state, together with the test resistor (R ₁ ) forms a positive feedback branch 2517/ot/mu '^; 30.3.1992 - 2 - for the operational amplifier (OPV) designed as a voltage follower (gain V = 1), whose output (A) is connected via the series connection of the resistors (R ₁ and R-) to the non-inverting measuring input (E-.) of the operational amplifier. 3. Device according to claim 1 or 2, characterized in that a further switch (S ₂ ) is arranged in series with the test resistor (R..) and therefore in the feedback branch (R ₂ / Ri) & ^the operational amplifier (OPV). 4. Device according to claim 3, characterized in that the further switch (S ₂ ) is actuated synchronously with the first switch (S..) and, due to its open state in measuring operation, makes the positive feedback path from the amplifier output (A) to the amplifier input (E..) high-impedance. 5. Device according to one of claims 1 to 4, characterized in that a microcomputer or microcontroller (11) is provided for evaluating the voltage values determined by supplying the test voltage, which microcomputer or microcontroller generates the control signal for the switches (S ₁ , S ₂ ) at an output terminal (P) and supplies the adjustable test voltage (U ) via a digital/analog converter. 6. Device according to claim 5, characterized in that to generate the test voltage (U ) a pulse width modulated pulse train generated by the microcomputer (11) is generated and fed to the test circuit via a downstream low-pass filter with operational amplifier (PPO).