FR2762395A1 - Device for individually testing the electrodes of a potentiometer device used for measuring pH values - Google Patents

Abstract

The device includes a stabilised continuous voltage generator (1), a galvanic insulation system, and an auxiliary electrode (2). There is a first high quality capacitor (5a) associated with an operational amplifier (10a) which is designed to measure , in normal operation, the voltage delivered by the measuring electrode (3), and in the test operation , the voltage at the terminals of the capacitor associated with the measuring electrode (3). There is a second high quality capacitor (5b) associated with an operational amplifier (10b) which is designed to measure, in normal operation, the voltage delivered by a reference electrode (4), and in test operation, the voltage at the terminals of this second capacitor associated with the reference electrode (4). There is also an analogue/digital converter which takes into account signals from temperature probes as well as those from the electrodes operating in normal mode or those signals from the capacitor terminals in test mode. There is also a computer which processes the magnitude of the measuring signals to be evaluated and the values of the electrode impedance's during in the test mode. In the test mode the stabilised voltage generator applies a voltage to the auxiliary electrode. The capacitors charge them selves through the measuring electrode and reference electrode resistance's. The voltage measurement at the capacitors terminals is done by an electronic system during a predetermined time period. The signals from the electronic system are corrected by a temperature computer so as to provide a value compensated to 25 degrees C.

Description

DEVICE AND METHOD FOR INSPECTING THE INTEGRITY OF
ELECTRODES OF A POTENTIOMETRIC MEASUREMENT SYSTEM.

 The invention relates to a device and a method for monitoring the state of the electrodes of a potentiometric measurement system. More particularly, it makes it possible to regularly monitor the evolution of the state of each of the electrodes, that is to say of the measurement electrode (of glass) and that of the reference one.

PRIOR ART
A technique generally used to detect an electrode defect is to calibrate a measurement system in buffer solutions. If the characteristics of the slope or zero (asymmetry) are outside the preset limits, one of the electrodes is considered to be damaged. Another method is to manually measure the impedance of the two electrodes.

If this method is easy to use for the reference electrode, it is quite complicated for the glass electrode which has a very high impedance.

On the other hand, these two techniques have the disadvantage of requiring the removal of the electrodes from the device.

 To facilitate and attempt to automate this detection of an electrode fault, various devices and methods have been envisaged.

 Patent EP 0 497 994 describes an electrical process making it possible to control the sum of the impedances of the glass and reference electrodes. It does not make it possible to obtain an individual value of the impedances and therefore to know the state of each electrode. It is based on current phase shift measurements.

 Patent WO 93/08477 proposes an electrical device for monitoring the development of the state of the measurement electrode. The principle used consists in injecting currents of determined shape (square signal) through a low value capacitance (1 nf) and then carrying out surface area measurements of the alternating signal from which the value of the resistance of the electrode. The process is essentially based on a measurement of the surface of a curve. It makes it possible to establish the sum of the impedances of the electrodes.

 Patent FR 2 439 400 describes a method consisting in sending two verification currents: a positive current followed by the same current of reverse polarity during the same time. The detection of the fault is based on the measurement of quantities of current and does not distinguish the faulty electrode.

 Patent EP 0 241 601 describes circuits making it possible to inject a voltage onto a ground electrode to fix the potential of the fluid, to switch a resistance on the input of the controlled electrode and to measure the voltage drop across the terminals of resistance. The disadvantage of this invention resides in the fact of using several switching devices (mechanical or electronic) for on the one hand putting the test circuit into service and on the other hand successively controlling the measurement and reference electrodes. This assembly uses electronic relays intended to avoid the leakage current. It is complex, and therefore subject to failure and costly in design and maintenance. The process is based on a voltage drop measurement.

The devices of the prior art therefore generally have one of the following two major drawbacks:
- they provide an overall result which does not allow the faulty electrode to be defined,
- They impose the constraining use of switching circuits successively switching on / off the test circuit, which must have an extremely low leakage current; the systems thus designed are therefore complex, difficult to implement and unreliable.

OBJECT OF THE INVENTION
The object of the present invention is to provide a device and a method for individually testing the electrodes of a potentiometric measuring apparatus, in particular for measuring pH. This device has the particularity of being fully integrated into the measurement system and thus allowing online control of the electrodes. It does not use any switching device and allows regular monitoring of the evolution of the state of each of the electrodes. It also makes it possible to keep the measurement system in service within the framework of the exploitation of the industrial process.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 represents the basic electrical diagram of an embodiment implementing the present invention.

 FIG. 2 represents the diagram of the part of a pH measurement system ensuring the control of the electrodes according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION
The measurement and control of certain specific solution parameters are carried out by carrying out potentiometric measurements.

 For example, pH is defined as the negative decimal logarithm of the activity in hydrogen ions. It is normally measured using a glass electrode and a reference electrode.

Nernst's formula: E = E0- (2,3RT! F) x log +
allows to establish it, - log a H + being by definition the pH of the solution.

 From this formula, we see that determining the pH involves a measurement of electromotive force as well as that of a temperature.

 The invention relates to a device and a method for checking the integrity of the electrodes of a measurement system.

A measurement error can have different origins:
- a cracking of the glass membrane resulting in a short circuit between the sample and the electrode,
- the contamination of an electrode by a deposit, resulting in a lower measurement sensitivity and a high impedance of this electrode,
- a lack of electrolyte in the reference electrode resulting in a very high impedance,
- poisoning of the reference electrode by chlorides or sulphides precipitating the silver ions and liable to obstruct the electrolytic junction; this poisoning also results in a significant increase in impedance.

 An electrode fault therefore generally results in an abnormal variation in its impedance and leads to the production of an erroneous measurement.

 The method according to the invention consists in charging capacitors through resistors which are the electrodes to be checked and in measuring the charge of these capacitors (the importance of which is directly a function of the resistance) after a determined duration.

 This process consists in measuring the importance of the impedance of each electrode by applying a stabilized continuous voltage step to a resistance / capacity circuit, the resistance being that of the controlled electrode, the capacity being of very high quality with a current negligible leakage. This operation is carried out by applying the voltage step to an auxiliary electrode made of stainless material, which fixes the potential of the liquid at zero measurement. Its triggering can be either manual or automatic at regular unwinding frequency. A second resistor, called additional resistor and of known value, can be added to the capacitance resistance circuit in order to protect the operational amplifiers described below. From time T = 0 the capacity is charged through the controlled electrode and the voltage across this capacity varies according to a well known law. The course of the operation for a predetermined period causes a voltage to appear across the capacitance, the value of which depends only on the resistance of the electrode.

 More explicitly, let E be a very stable voltage step applied.

After a time T1 a voltage V appears at the terminals of the capacitor according to the known formula:
V'E (1-. RC)
We deduce the value of the impedances of the electrodes by: R = C Ltt (E - 2
R therefore only depends on V.

 The time T1 is chosen according to the order of magnitude of the resistance to be measured. It is determined by a clock that is an integral part of the device.

 The device according to the invention is integrated into a more general measurement system. It provides online control of the electrodes. It mainly consists of a voltage generator circuit, an auxiliary electrode, high-quality capacitors associated with the electrodes to be checked and a voltage measurement system at a given time.

FIG. 1 represents a basic electric diagram of the device according to the invention in which:
- (1) represents a stabilized DC voltage generator,
(2) represents an electrode made of stainless material fixing the potential of the liquid at zero measurement,
- (3) is the measuring electrode (of glass) of the measuring device,
- (4) is the reference electrode,
- (5a) and (5b) are the high quality capacities (very high leakage resistance) associated respectively with the electrodes (3) and (4),
- (6) is a continuous signal proportional to the voltage appearing at the terminals of the capacitor (5a),
- (7) is a continuous signal proportional to the voltage appearing across the capacitor (5b),
- (8) is a switch enabling the stabilized voltage to be connected,
- (9) is an electrical signal proportional to the difference of the voltage values across the capacitors (5a) and (5b); in normal operation, it is representative of the quantity measured, for example the pH of the solution,
- (10a) and (10b) are impedance adapters (operational amplifiers) used to measure, in normal operation, the voltage delivered by
The electrode associated with them and, during a control period, the voltage across the terminals of the capacitance associated with this same electrode,
- (11) is a differential amplifier.

 The invention further includes an adjustable clock (not shown) predetermining the duration of the capacity loading operation before its measurement.

 Two calculation circuits (not shown) make it possible to calculate the impedances of the electrodes (3) and (4).

 Known conversion systems transform analog signals into digital signals.

 A known system for measuring the temperature of the measuring electrode (for example a thermocouple, a resistance probe or a thermistor, etc.) associated with a known calculation device makes it possible to establish the temperature of the electrode and to bring the impedance measurement at a value at 25 "C.

 The device according to the invention also comprises galvanic isolation. It can be located in different locations. We particularly prefer to place it between the analog part and the digital part of the device.

 The stabilized DC voltage generator can be operated manually or automatically at regular intervals. In the case of an automatic application of the voltage step to the auxiliary electrode, the device further comprises an electronic programming means adjustable by the user.

 The control device according to the invention can be designed so as to be used in intrinsic safety.

 According to a variant of the invention, the time taken by a given capacitance, associated with the electrode under test, is measured for the voltage across its terminals to reach a predetermined level, for example 63% of the voltage supplied by the generator. This time is a function of the resistance of the electrode tested and therefore of the extent of its deterioration. For this variant, the device according to the invention is fitted with two clocks and two comparison circuits making it possible to detect this instant.

EXAMPLE OF IMPLEMENTATION
This example concerns the production of a pH measuring device.

 FIG. 2 represents the main functional entities of a particular embodiment of the apparatus which is the subject of the present invention.

- (1) is a stabilized voltage generator supplied from the + or - 8 Volts source and delivering a stabilized voltage of either 0 volts or 1 volt;
- (12) is the terminal of the auxiliary electrode;
- (13) is the terminal of the measurement electrode;
- (14) is the terminal of the reference electrode;
- (15) are the terminals of the temperature probe;
- (16) is the analog temperature measurement computer;
- (10a) and (10b) are the operational amplifiers with high input impedance, for example 1012 ohms and without drift;
- (5a) and (5b) are high quality capacities (very high leakage resistance);
- (11) is the differential amplifier whose output is a function of the potential difference between the reference and measurement electrodes;
- (17) is an analog-to-digital converter;
- (18) is a calculator;
- (19) is a voltage generator (+ or - 8 Volts);
- (20) is a reference voltage generator (- 5 Volts);
- (21) are resistors of the RC circuits. They take values in relation to the orders of magnitude of the values of the resistances of the electrodes which one wants to measure. They serve as protective resistors for amplifiers.

 - (22) is a galvanic isolation device.

 The device as it is designed makes it possible to carry out on-line electrode checks without special handling at regular variable intervals, the frequency of which, programmable by the user, can range from a check every 6 minutes to a check every 24 hours.

 In normal operation, the generator (1) sends a potential 0 to the auxiliary electrode (12). The voltages collected across the electrodes (13) and (14) are amplified by the amplifiers (10a) and (10b). The output of the amplifier (9) is proportional to the potential difference between the electrodes. The computer (16) calculates the temperature value (Nernst law). The signals are sent to the analog-to-digital converter (17) which generates a digital signal of the pH value.

 When we want to make electrode control measurements, we send a signal actuating the switch (8). The emission of this signal can result from an automatic electronic programming. The voltage delivered by the generator (1) goes to a stable value of +1 Volt in this particular embodiment. This voltage step is applied to the auxiliary electrode. The capacitors (5a) and (5b) are charged through the resistors of the electrodes and the resistors (21). After a defined time T1 equal in this example to 0.4 seconds (0.8 seconds for high impedances), the application of the voltage on the electrode (12) is interrupted. This voltage returns to zero. The operational amplifiers (1 0a) and (1 0b) measure the voltages appearing at the terminals of the capacitors (5a) and (5b). The computer (18) develops the ohmic value of the resistances.

 The pH measurement is interrupted during the electrode impedance measurement operations. The duration of these operations does not exceed 5 to 10 seconds.

 This computer can be associated with a comparison circuit which emits an alarm signal warning the user if at least one of the two impedances takes a value outside the predetermined tolerances. The order of magnitude of these tolerances depends on the resistance, ie the electrode, to which they are linked. They correspond to the limit values (low or high) of impedance tolerated for the given electrode.

Claims (8)

 1. Device for individualized online checks of the integrity of the measurement electrode and of the reference electrode of a potentiometric measurement system equipped with one or more temperature measurement probes, characterized in that it includes:  - a stabilized DC voltage generator;  - a galvanic isolation device;  - an auxiliary electrode;  a first high-quality capacitor associated with an impedance adapter which is intended to measure, during normal operation, the voltage delivered by the measuring electrode and, during the test period, the voltage at the terminals of this first capacitor associated with the measuring electrode;  a second high-quality capacitor associated with an impedance adapter which is intended to measure, during normal operation, the voltage delivered by the reference electrode and, during the test period, the voltage at the terminals of this second capacitor associated with the reference electrode;  - an analog / digital converter taking into account the temperature signals as well as those from the electrodes in normal operation or those at the terminals of the capacitors during the test period;  - a computer developing the measurement signal of the quantity to be evaluated, in normal operation, and the values of the impedances of the electrodes during the test periods.  2. Online control device according to claim 1, characterized in that the galvanic isolation device is placed between the analog part and the digital part.  3. Online control device according to claim 1 or claim 2, characterized in that the auxiliary electrode is made of stainless material.  4. Online control device according to any one of the preceding claims, further characterized in that the computer is associated with a comparison circuit emitting an alarm signal when at least one of the two impedance measurements takes a value outside the predetermined tolerances.  5. Method for individualized online checks of the reference electrode and the measurement electrode of a potentiometric measurement system by using a device according to claim 1, characterized in that during the test period the generator of voltage applies a stabilized voltage to the auxiliary electrode, the high-precision capacitors are charged through the resistances of the measurement and reference electrodes, the measurement of the voltages across these capacitors is carried out by the electronic devices at the end a period T1 of a fixed duration.  6. Online control method according to claim 5, characterized in that the signals from the electronic devices are corrected by the temperature calculator to provide a value compensated at 25 "C.  7. Method of online checks according to claim 5 or claim 6, characterized in that the duration T1 is defined as a function of the order of magnitude of the impedance to be measured.  8. Online control method according to claim 5, characterized in that the quantity measured to evaluate the impedance of the electrodes is time  necessary for each capacity so that the voltage across its terminals reaches a determined value.