WO2000014523A2 - Apparatus for monitoring the operability of an electrochemical sensor - Google Patents

Abstract

An apparatus for monitoring the status of an electrochemical sensor includes a measuring device for monitoring various electrical characteristics of the sensor and a processor for comparing such electrical characteristics to pre-determined values. The apparatus signals the operator when such characteristics deviate a pre-determined amount from a set point thereby signaling a failure of the sensor. Sensors for which the present invention is applicable include concentration sensors such as those used for quantifying the concentration of elements in molten metal samples. The electrical characteristics that are measured include AC impedance or conductance, DC resistance or conductance, and the EMF output of the sensor. Such measured characteristics can also be compared to calculated values by imposing temperature perturbations into the sample being tested.

Description

Apparatus For Monitoring Electrochemical Sensors And Method Therefor

FIELD OF THE INVENTION This invention relates to a means for monitoring the status of electrochemical concentration sensors. More specifically, such sensors are employed in the determination of concentrations of minor components in molten metals or alloys.

BACKGROUND OF THE INVENTION In recent years numerous electrochemical sensors have been developed for determining the concentration of elements in molten metals or alloys. These include but are not limited to the continuous monitoring of the aluminum concentration in galvanizing bath applications, the magnesium content in aluminum melts, and the oxygen content of molten metals. A number of electrochemical concentration sensors are known and selected products are commercially available. These sensing techniques provide the capability to continuously monitor concentrations of selected elements and are a satisfactory or preferred alternative to the various analytical techniques currently employed, which rely on taking bath samples for chemical assaying.

An example of one such electrochemical sensor is taught by Yamaguchi in US patent number 5,393,400 (1995) which describes a molten salt electrolyte sensor to measure the active aluminum level in zinc melts according to Nernst's equation. Bocage, as described in WO 97/33170, and others have developed similar devices for detecting active aluminum in zinc.

Solid electrolyte sensors for determining the aluminum concentration in hot dip galvanizing applications have been described in US patent number 5,256,272 (1993) and in J. Japan Inst. Metals, Vol. 58, No. 8 (1994), pp. 929-935.

The most significant limitation with the known electrochemical concentration sensors is the relatively rapid degradation of the sensor ultimately leading to erroneous readings and sensor failure. Depending on the sensor and the application, typical lifetimes of such sensors range from a few hours to several days, and on occasion, several weeks. As the sensor's reading is relied on for the concentration value of the selected component, a deteriorated sensor may provide erroneous readings; hence, the need exists to determine when the sensor's accuracy has deteriorated beyond an acceptable limit, and therefore the sensor must be replaced. To date, no means has been identified that is capable of detecting when an electrochemical concentration sensor provides an erroneous electromotive force (EMF) output and in addition, no means have been identified to predict or diagnose sensor characteristics to alert the user when the sensor deteriorates or requires replacement. To overcome this lack of diagnostic criteria to detect the end-of- life, it is a current practice to operate two sensors side by side. The two sensors give identical readings until one of the sensors fails. Once the readings from the two sensors differ by a pre-set amount, it is up to the operator to determine which sensor has likely failed and to replace it. Such a system leads to increased costs as two sensors are used for the same purpose.

It has been found that various factors influence the life span of the sensors including the characteristics of the sensor itself (life spans can vary between sensors from the same batch) and of the bath (i.e. a turbulent bath reduces the sensor life from several days to a few hours).

Various methods have been proposed to monitor the status of such sensors. Examples of such systems are provided in US patent numbers 3,661,748, 4,189,367, and 4,468,608.

However, there exists a need for a reliable and cost efficient method for monitoring the status of an electrochemical sensor so as to ensure that the desired concentration values being obtained are accurate.

SUMMARY OF THE INVENTION

In its broad aspect, the present invention relates to a method of monitoring the condition of electrochemical sensors comprising one or more, or a combination thereof, electrical techniques that include AC impedance or conductance, DC resistance or conductance, electrochemical noise, pulsed current, and current interruption measurements. A further aspect is to analyze the transient response of the electrochemical sensors to these electrical characteristics or the sensor EMF to other non- electrical perturbations such as bath temperature fluctuations or the like (signal conditioning). In one of its typical forms, the product of the invention is a device comprised of an electrochemical concentration sensor, a temperature sensor and at least one monitoring means. The EMF of the sensor and the temperature are used to continuously calculate and display the concentration of a component of the metal melt. At certain time intervals, or continuously, characteristics of the sensors are determined and compared to reference values. When the difference between the actual and expected characteristic exceeds a specified value, means are activated to alert an operator who can exchange the failed sensor. The characteristics of the electrochemical concentration sensor which can be monitored include its resistance by employing AC or DC methods, conductance, electrochemical noise, and response of sensor characteristics (e.g., EMF, AC or DC resistance) to changes in the bath temperature or composition.

Thus, in one embodiment, the present invention provides, as an active system, an apparatus for monitoring the status of an electrochemical sensor comprising: a) a current generator for applying a current to the sensor; b) a measuring device for monitoring an electrical characteristic of the sensor, the electrical characteristic being selected from the group including AC and/or DC impedance, AC and/or DC resistance, AC and/or DC conductance, electrochemical noise and sensor EMF output; and, c) a processor for comparing the sensor electrical characteristic to a set, reference point and for signaling when the characteristic deviates a pre-determined amount from the selected set point.

The current generator can be an active power supply or a passive load, e.g. a load resistor, causing the sensor and zinc melt to form a galvanic cell producing a discharge current. In another embodiment, as a passive system, the invention provides an apparatus for monitoring the status of an electrochemical sensor, the sensor being used to measure the concentration of an element in a liquid test sample by measuring the EMF between the sample and a reference sample, the apparatus comprising: a) a measuring device for monitoring at least one electrical characteristic of the sensor, the electrical characteristic being selected from the group including AC and/or DC impedance, AC and/or DC resistance, AC and/or DC conductance, electrochemical noise and sensor EMF output; b) a means for varying and measuring the temperature of a sample in which the sensor is used and wherein the electrical characteristics are measured; and, c) a processor for calculating a theoretical value for the electrical characteristic and for comparing the measured and the calculated values of the characteristic and for signaling when the measured and calculated characteristics deviate a pre-determined amount from a set point.

In yet another embodiment, as an active/passive system, the present invention provides an apparatus for monitoring the status of an electrochemical sensor, the sensor being used to measure the concentration of an element in a liquid test sample by measuring the EMF between the sample and a reference sample, the apparatus comprising: a) a measuring device for monitoring the electrochemical noise from the EMF measurement of the sensor; b) a processor for monitoring the noise from the sensor and for signaling when the amplitude of the noise exceeds a pre-determined set point.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein: Figure 1 is a perspective view of a preferred embodiment of the invention showing a typical electrochemical sensor arrangement including the end-of-life detection system. Figure 2 is a graph illustrating the change with time of: a) the total impedance of the sensor of Figure 1, at a frequency of 1 kHz; and, b) of the measured Al concentration. Figure 3 is a graph illustrating the change with time of: a) the total impedance of the sensor of Figure 1, at a frequency of 1 kHz; b) the electrochemical potential noise; and, c) the Al concentration in a molten zinc bath.

Figure 4 is a graph illustrating the changes in bath temperature and signal conditioning output of the sensor of Figure 1 monitoring the Al concentration in a molten zinc bath in a continuous sheet galvanizing plant. DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to Figures 1 through 4, various preferred embodiments of the present invention are shown. Although the description will proceed with reference to an electrochemical sensor for monitoring the aluminum content in zinc melts using a molten salt electrolyte, it will be understood that the invention will equally apply to other sensors such as those using aqueous and non-aqueous electrolyte, molten-salt electrolyte, or solid- electrolyte electrochemical sensors. Such sensors can also be employed in a variety of liquid metal applications over a vast temperature range.

Figure 1 illustrates, schematically, a typical electrochemical sensor system, shown generally at 10, as known in the art for monitoring the concentration of aluminum in a molten zinc bath. An example of such a system is taught by Yamaguchi in US patent number 5,393,400, which is incorporated herein by reference. As shown in Figure 1, the sensor system comprises an Al standard electrode 12 (containing a sample of pure Al for use as a reference electrode), a working electrode 26 (for comparing the standard electrode with the sample being tested), and a thermocouple 14 which are immersed in a bath 16 containing a molten zinc-aluminum (Zn-Al) mixture 18. The electrode 26 is used to create an electrical contact with the molten zinc alloy 18. The standard and working electrodes, 12 and 26, of the sensor system and thermocouple 14 are connected to a data processing unit 20 by means of electrical leads 22, 28 and 24, respectively. The processing unit serves various functions. Firstly, the unit measures the electromotive force (EMF) between the standard electrode 12 and the working electrode 26 in the Zn-Al bath. Further, the processing unit 20 also measures the temperature of the bath by means of the thermocouple 14. With these values, the concentration of Al in the molten zinc bath is calculated by the processing unit 20 using the Nernst equation. Further detail concerning this measurement and calculation method is provided in US patent 5,393,400, which is incorporated herein by reference. The data processing unit 20 performs the necessary calculations and displays the relevant information, such as bath temperature and Al concentration, on a display screen. In other embodiments, the processing unit can provide the required information in any number of ways as known in the field. As mentioned previously, one of the problems associated with an electrode sensor system as above lies in the fact that such sensors are prone to physical deterioration as a result of the harsh environments to which they are exposed. Under these conditions, sensors have been found to develop defects that lead to inaccurate EMF measurements and, thereby, inaccurate calculations of Al concentration. The present invention provides a means of monitoring the status of the electrode, and specifically the standard electrode, or sensor, using electrical methods wherein certain parameters are monitored. These methods include the following: a) monitoring of the sensor impedance and/or conductance upon subjecting the sensor to an AC current; b) monitoring the resistance and/or conductance of the sensor upon application of a positive and/or negative DC current; and c) monitoring the electrochemical noise inherent in the sensor. Furthermore, in another approach, monitoring at least one of the listed parameters and/or monitoring the variation in the EMF output as a function of temperature, can be employed to determine the difference between the "expected" and "actual" values. In embodiments of the invention where a current (either AC or DC) is applied to the sensor, such current may be applied via the electrical leads 22 and 28 that connect the sensor and the metal melt to the data processing unit 20. The electronics for the sensor monitoring, or diagnostic, system are typically mounted near the data processor 20 and the appropriate signal (AC or DC resistance/impedance, electrochemical noise, etc.) is fed into the processor. In addition to various hardware components, the processor also contains the software required to determine the Al concentration from the EMF and T measurements, the software used in signal processing and the logic to determine and display the sensor status. The data processor determines the value of the diagnostic measurement, compares it to a preset threshold value (e.g. minimum and maximum resistance/impedance), and signals the operator of a defective sensor when the preset threshold is exceeded. Any conventional electronics may be used for handling the signal comparison of the invention as will be apparent to those skilled in the art. The following examples serve to illustrate the various embodiments of the invention and are not intended to be limiting the scope thereof.

EXAMPLE 1 : AC Impedance

Experiments have been carried out to monitor the aluminum level in a zinc alloy bath as used in continuous galvanizing applications. The zinc melt was contained in a crucible furnace kept at approximately 450°C. The active Al bath concentration was adjusted to 0.16% and was monitored using a sensor as in US patent number 5,393,400, manufactured by Yamari Sangyo Kabushiki Kaisha, Osaka, Japan, and the AlSensor software commercially available through Cominco Ltd., Product Technology Centre, Mississauga, Ontario. To monitor the condition of the sensor, an AC impedance unit supplied by FC Software Inc., Campbellville, Ontario was used; however, it will be apparent to those skilled in the art that any suitable AC impedence applying systems may be used. The advantage of the AC impedance technique is the ability to utilize a very small signal that does not disturb the electrode processes being investigated. AC impedance also permits studying corrosion reactions in low-conductivity media, where traditional DC methods may be unsuitable. An AC bias signal was applied to the sensor and used to determine the impedance characteristics of the sensor. The real, complex or total impedance was used to trigger a preset alarm to signal the necessity to change a damaged sensor and, thus, assure that the Al concentration readings are not erroneous due to degradation of the electrochemical sensor. The apparatus used was similar to that illustrated in Figure 1. Figure 2 shows the total impedance of the sensor at 1kHz and the Al concentration over the duration of the experiment. The active Al concentration over the duration of the experiment was maintained constant at 0.16%.

Figure 2 illustrates that for the first 21 hours of operation, the Al concentration and AC impedance readings remained fairly stable. Thereafter, the concentration reading abruptly and drastically decreased indicating complete sensor failure. At the same time, the AC impedance reading also substantially decreased confirming the sensor failure. Therefore, this example illustrates that monitoring the impedance characteristics of the sensor provides an accurate means of signaling the deterioration or failure of an electrochemical sensor.

Other electrical characteristics were also successfully employed to determine sensor failure. These methods included: a) monitoring the conductance of the sensor, which is defined as the reciprocal value of the real impedance; b) monitoring the internal resistance using current-interruption techniques; c) DC-polarization techniques; and, d) current or load-resistor pulse techniques. These results surprisingly indicate that AC and DC impedance/resistance techniques can be employed to reliably predict the end-of-life condition of an Al sensor. EXAMPLE 2: Electrochemical Noise

The experiment performed utilized an identical set up as described in Example 1. In this experiment, however, both AC impedance and electrochemical noise of the sensor were monitored. The electrochemical noise was monitored using the Capsis March Ltd., Manchester, U.K., Electrochemical Noise Monitoring Unit, which recorded both the electrochemical current noise (ECN) and the electrochemical potential noise (EPN). The resistance noise (ERN) can be derived from EPN and ECN according to Ohm's law. Electrochemical noise is a generic term used to describe the spontaneous fluctuations of potential and current that occur at the interface of electrochemical electrodes. The stochastic processes generating the noise signals are generally related to the electrode's kinetics. The concentration of the active aluminum in the furnace for the duration of the experiment was 0.16%. Figure 3 illustrates that sensor failure, as evidenced by the Al concentration readings, occurred after approximately 105 hours of operation while both electrochemical potential noise and AC impedance successfully signaled the imminent failure after approximately 95 hours of operation.

Figure 3 indicates that the impedance of the sensor gradually increased over the first 90 hours, likely indicative of a loss of the A1C1 component of the NaCl-AlCl₃ electrolyte. After approximately 90 hours, the slope of the curve changes indicating the development of a crack in the sensor electrolyte matrix. The sensor eventually failed after about 105 hours. The electrochemical noise measurement, represented by the recorded potential noise, is also illustrated. Upon approaching sensor failure, the electrochemical noise drastically and transiently increased at approximately 95 hours, also signaling imminent sensor failure. Changes in the electrochemical current noise also were found to relate to sensor failure. Resistance noise, calculated from the EPN and ECN values was found to closely match the AC impedance data.

This experiment illustrates that noise measurements can also serve to monitor the status of the sensor. Moreover, this experiment also illustrates that it is possible to anticipate the failure of a sensor thereby providing the opportunity to change the sensor before actual failure occurs. It is believed that such anticipatory readings are possible in situations where a crack in the sensor begins to develop. Complete failure of the sensor would occur when the crack progresses until the sensor is broken. EXAMPLE 3: Transient Sensor Response Methods

This approach treats the sensor as a dynamic system, and assesses the variation of the EMF of a sensor with varying temperature. According to the Nernst equation, the EMF changes in proportion to changes in the absolute temperature. A software program was used to continuously compare the theoretically expected EMF changes with the actual ones. The EMF of a properly functioning sensor follows temperature fluctuations as expected; however, it was found that for a malfunctioning sensor, the thermodynamic relationship is no longer obeyed, as indicated in Figure 4. The failure index plotted in Figure 4 relates to the difference between the measured and expected Al concentration. As observed, the failure index for the first 45 hours of operation is quite small, then, suddenly, the index substantially increases. The sudden change in the failure index occurred simultaneously with a substantial drop in the apparent Al concentration at approximately 47 hours due to sensor failure, which was verified by analytical techniques. In addition to the EMF, variations of other electrical parameters (e.g. AC or DC resistance/impedance, electrochemical noise) as a function of temperature can also be monitored and assessed.

In practical applications, the process temperature in a zinc melt varies continuously, depending on the size of the furnace, the throughput of, for example, steel sheet and the frequency and size of bath additions and the sophistication of the furnace controllers (on-off, proportional, proportional with integral and derivative control).

Alternatively, the temperature variation can be introduced into the system by any variety of methods such as a heating/cooling apparatus etc.

As will be apparent to those skilled in the art, one or several detection systems can be employed in any given application. Generally the characteristics monitored are graphically displayed in real time on a suitable monitor, enabling the operator to follow the condition of the sensor at all times and providing an indication of the rate of sensor deterioration encountered. In a similar approach, the sensor characteristic is displayed digitally on a monitor. The colour of the field changes from green (signaling a properly functioning sensor) to yellow (deteriorating, but still accurate sensor) to red (failed sensor, requiring replacement). The need for replacement is generally also flashed on the screen and can incorporate an audio warning as well, e.g., a horn or buzzer. Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for monitoring the status of an electrochemical sensor comprising: a) a current generator for applying a current to the sensor; b) a measuring device for monitoring an electrical characteristic of the sensor, said electrical characteristic being selected from the group including AC and/or DC impedance, AC and/or DC resistance, AC and/or DC conductance, electrochemical noise and sensor EMF output; and, c) a processor for comparing the sensor electrical characteristic to a set point and for signaling when the characteristic deviates a pre-determined amount from the set point.

2. The apparatus of claim 1 wherein the current is AC.

3. The apparatus of claim 2 wherein the electrical characteristic is the AC impedance or AC conductance of the sensor.

4. The apparatus of claim 1 wherein the current is DC.

5. The apparatus of claim 4 wherein the electrical characteristic is the DC resistance or DC conductance of the sensor.

6. The apparatus of claim 1 wherein the sensor is monitored continuously.

7. An apparatus for monitoring the status of an electrochemical sensor, said sensor being used to measure the concentration of an element in a liquid test sample by measuring the EMF between the sample and a reference sample, the apparatus comprising: a) a measuring device for monitoring at least one electrical characteristic of the sensor, said electrical characteristic being selected from the group including AC and/or DC impedance, AC and/or DC resistance, AC and/or DC conductance, electrochemical noise and sensor EMF output; b) a means for varying and measuring the temperature of a sample in which the sensor is used and wherein the electrical characteristics are measured; and, c) a processor for calculating a theoretical value for said electrical characteristic and for comparing the measured and the calculated values of said characteristic and for signaling when the measured and calculated characteristic deviate a pre-determined amount from a set point.

8. An apparatus for monitoring the status of an electrochemical sensor, the sensor being used to measure the concentration of an element in a liquid test sample by measuring the EMF between the sample and a reference sample, the apparatus comprising: a) a measuring device for monitoring the electrochemical noise from the EMF measurement of the sensor; b) a processor for monitoring the noise from the sensor and for signaling when the amplitude of the noise exceeds a pre-determined set point.

9. The apparatus of claim 7 or 8 wherein the sensor is continuously monitored.

10. The apparatus of claim 8 wherein the electrochemical noise parameter is selected from the group consisting of potential, current and resistance noise.