Interrogation and output correction in capillary-limited oxygen sensors.

Abstract

1. A method of operating a gas detection device, the method comprising: applying an electrical signal to an electrochemical sensor of the gas detection device, thereby generating a current flow between a working electrode and a counter electrode of the electrochemical sensor through an electrolyte of the electrochemical sensor; measuring a parameter of the electrochemical sensor's response to the electrical signal; and comparing the measured parameter to a predetermined characterization of the parameter, the predetermined characterization providing a relationship between the parameter and the response of the electrochemical sensor over varying conditions of the electrochemical sensor; and determining an output value of the gas detection device from a comparison of an output signal of the electrochemical sensor detecting oxygen in an ambient environment and the predetermined characterization of the measured parameter.

Description

[Background technology]

[0001] The following information is provided to aid the reader in understanding the technology disclosed below and the environments in which such technology is typically used. Terms used herein are not intended to be limited to any particular narrow interpretation unless expressly stated otherwise herein. References listed herein may facilitate understanding of the technology or its background. The disclosures of all references cited herein are incorporated by reference.

[0002] Electrochemical sensors are effective for detecting a variety of gases. Their low cost, response speed, and selectivity are just a few of the characteristics that make them attractive as safe products. However, one of the requirements for using these sensors is frequent calibration with a test gas containing a known concentration of the target gas. Furthermore, the functionality of gas detectors, including electrochemical gas sensors, must be tested periodically. For example, it is common practice to perform a "bump check" or functionality check on portable gas detectors daily. The purpose of this test is to ensure the functionality of the entire gas detection system (including the sensor and transport path), commonly referred to as the instrument. Periodic bump checks or functionality checks may also be performed on permanent gas detectors, for example, to extend the time between full calibrations. A gas detection system includes at least one gas sensor, electronic circuitry for driving the sensor, interpreting its response, and displaying the response to the user, and a power source. The system further includes a housing to enclose and protect these components. A bump check typically involves a) applying a gas of interest (usually a gas containing a known concentration of the analyte gas or its simulant), b) collecting and interpreting the sensor response, and c) indicating to the end user the functional status of the system (i.e., whether the equipment is functioning properly or not).

[0003] For example, many systems and methods have been proposed to reduce the number of periodic tests using test gases on diffusion-limited electrochemical gas sensors while frequently measuring the sensor's life and health. Such systems may include, for example, electronic interrogation of the sensor in the absence of a test gas. Sensitivity fluctuations resulting from moisture loss or gain in a diffusion-limited electrochemical gas sensor occur gradually but predictably as the average relative humidity in such a sensor slowly changes. Similarly, the sensor's response to electronic interrogation (in the absence or application of a test gas containing a known concentration of the analyte gas or its surrogate) similarly changes. Electronic interrogation may be used, for example, to measure changes in sensitivity and correct the sensor output for such changes in sensitivity.

[0004] Electronic interrogation techniques and resulting calibration for diffusion-limited electrochemical gas sensors are disclosed, for example, in U.S. Patent Nos. 7,413,645, 7,959,777, 9,784,755, and 9,528,957, as well as U.S. Patent Application Publication Nos. 2013 / 0186777 and 2017 / 0219515, the disclosures of which are incorporated herein by reference. In such electronic interrogation approaches, an electrical signal, such as a potential pulse, is typically applied to the sensor, and the resulting response is measured and recorded. Electronic interrogation of capillary-limited gas sensors is described in U.S. Patent No. 11,112,378, the disclosure of which is incorporated herein by reference.

[0005] Electronic interrogation of capillary-limited electrochemical oxygen gas sensors is sensitive to failure modes, but such interrogation has not provided consistent data to correct for changes in sensor behavior / sensitivity in the field. Therefore, there is a need for the development of an off-the-shelf electronic interrogation technique for use in conjunction with capillary-limited electrochemical oxygen gas sensors. Summary of the Invention

[0006] In one aspect, a method of operating a gas detection device (including a capillary-limited electrochemical sensor analytically responsive to oxygen, the electrochemical sensor including a housing containing a capillary through which gas diffuses from the environment into the housing, a working electrode within the housing, a counter electrode within the housing, and an electrolyte within the housing, the electrolyte in ionic contact with the working and counter electrodes) includes applying an electrical signal to the electrochemical sensor, thereby generating a flow of current between the working and counter electrodes through the electrolyte; measuring a parameter of the electrochemical sensor's response to the electrical signal; comparing the measured parameter to a predetermined characterization of the parameter, the predetermined characterization providing a relationship between the parameter and the response of the electrochemical sensor over varying conditions of the electrochemical sensor; and determining an output value of the gas detection device from a comparison of an output signal of the electrochemical sensor detecting oxygen in an ambient environment and the predetermined characterization of the measured parameter. In some embodiments, the method further includes operating the electrochemical sensor in a detection mode, wherein an output signal is generated, the output signal being representative of the oxygen concentration in the environment; and operating the electrochemical sensor in an interrogation mode, during which the electrochemical sensor is electronically interrogated by applying an electrical signal to the electrochemical sensor, and in which a measured parameter is measured.

[0007] The gas detection device may further include a control system including a processor system and a memory system, wherein the working electrode is operably connected to the control system and the counter electrode is operably connected to the control system. The memory system includes one or more algorithms stored therein and executable by the processor system to perform one or more actions of the method. The predetermined characterization may be stored in the memory at the time of manufacture.

[0008] In some embodiments, the parameters are chronoamperometric parameters, which may be, or functions of, the maximum peak value, area under the curve, minimum peak value, peak-to-peak value, and inverse area under the curve for the response of the electrochemical sensor to an electrical signal applied in interrogation mode.

[0009] The predetermined characteristic may be determined over varying states of the electrochemical sensor, which may be induced by environmental conditions.

[0010] In some embodiments, the predetermined characterization may be determined as a function of the varying baseline response of the sensor in the absence of oxygen, which may be determined under a nitrogen atmosphere.

[0011] In some embodiments, the ambient current output of the sensor is less than 300 μA, optionally less than 150 μA, or optionally less than 30 μA.

[0012] In another aspect, a gas detection apparatus includes a control system including a processor system and a memory system; and an electrochemical sensor analytically responsive to oxygen, the electrochemical sensor including a housing including a capillary through which gas diffuses from the environment into the housing, a working electrode within the housing operably connected to the control system, a counter electrode within the housing operably connected to the control system, and an electrolyte within the housing, the electrolyte in ionic contact with the working and counter electrodes. The control system is configured to apply an electrical signal to the electrochemical sensor, thereby causing a current to flow between the working and counter electrodes through the electrolyte, by executing software stored in the memory system by the processor system; measure a parameter of the electrochemical sensor's response to the electrical signal; and compare the measured parameter to a predetermined characterization of the parameter, the predetermined characterization providing a relationship between the parameter and the response of the electrochemical sensor over varying conditions of the electrochemical sensor. An output value of the gas detection apparatus is determined from a comparison of the analytical response of the electrochemical sensor to detect oxygen in the ambient environment and the measured parameter to the predetermined characterization.

[0013] The control system may be further configured to operate the electrochemical sensor in a detection mode, where an output signal is generated, the output signal being representative of the oxygen concentration in the environment, and to operate the electrochemical sensor in an interrogation mode, where during the interrogation mode the electrochemical sensor is electronically interrogated by applying an electrical signal to the electrochemical sensor, where in the interrogation mode the measured parameter is measured. The default characterization may be stored in the memory system at the time of manufacture.

[0014] In some embodiments, the parameters are chronoamperometric parameters, such as the maximum peak value, area under the curve, minimum peak value, peak-to-peak value, and inverse area under the curve for the response of the electrochemical sensor to an electrical signal applied in interrogation mode, or functions thereof.

[0015] In some embodiments, the ambient current output of the electrochemical sensor is less than 300 μA, optionally less than 150 μA, or optionally less than 30 μA.

[0016] In another aspect, a method for characterizing changes in the response of an electrochemical sensor (capillary restricted and analytically responsive to oxygen, the electrochemical sensor including a housing containing a capillary through which gas diffuses from the environment into the housing, a working electrode within the housing, a counter electrode within the housing, and an electrolyte within the housing, the electrolyte in ionic contact with the working and counter electrodes) over varying conditions includes applying an electrical signal to the electrochemical sensor to generate a current flow between the working and counter electrodes through the electrolyte. The method includes determining a predetermined characterization that provides a relationship between a parameter measured during the test and the response of the electrochemical sensor over varying conditions of the electrochemical sensor.

[0017] The apparatus, system and method of the present invention, together with its attributes and attendant advantages, will best be understood and appreciated by considering the following detailed description taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0018] [Figure 1A] FIG. 1A is a schematic cross-sectional view of a capillary-limited electrochemical gas sensor according to the present invention.

[0019] [Figure 1B]FIG. 1B is a schematic cutaway perspective view of a capillary-type electrochemical gas sensor of the present invention.

[0020] [Figure 1C] FIG. 1C is a schematic diagram showing an enlarged view of the capillary inlet of the capillary-limited electrochemical gas sensor of FIG. 1A.

[0021] [Figure 2] Figure 2 shows a representative example of a sensor's response (for two different working electrode sizes) to a pulse test herein, where the energy to the working electrode is varied, resulting in a change in the current passing through the working electrode.

[0022] [Figure 3] Figure 3 shows a study of a representative capillary-limited electrochemical gas sensor herein, showing the changes in the AUC, DeltaPeak, and MaxPeak parameters as a result of pulse testing, demonstrating that these parameters contain the same or very similar analytical information.

[0023] [Figure 4A] FIG. 4A shows the dependence of the nitrogen baseline (capacitive) current on changes in the parameter AUC.

[0024] [Figure 4B] FIG. 4B shows a measurement of the nitrogen baseline above the baseline sensor current in the absence of oxygen.

[0025] [Figure 5] Figure 5 shows the ambient sensor output linearly correlated with the change in nitrogen baseline, demonstrating that the effect of oxygen is constant across the environmental changes evaluated.

[0026] [Figure 6] FIG. 6 shows that the ambient output of the sensor correlates with changes in the chronoamperometric pulse test parameters.

[0027] [Figure 7] FIG. 7 shows that the chronoamperometric pulse test parameters result in a reasonably predictable sensor output. DETAILED DESCRIPTION OF THE INVENTION

[0028] It will be readily understood that the components of the embodiments as generally described and illustrated in the Figures herein could be arranged and designed in a wide variety of different configurations in addition to the exemplary embodiments described. Thus, the following more detailed description of the exemplary embodiments as illustrated in the Figures is not intended to limit the scope of the embodiments as claimed, but merely to illustrate exemplary embodiments.

[0029] References throughout this specification to "one embodiment" or "an embodiment" (or the like) mean that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment.

[0030] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to provide a thorough understanding of the embodiments. However, one skilled in the art will recognize that various embodiments can be practiced without one or more of the specific details, or without other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring them.

[0031] As noted above, response measurements obtained by assessing correlation between sensor life and health analysis data and sensor sensitivity via electronic interrogation have not previously correlated sufficiently to allow for sufficiently accurate calibration of output or measurement values ​​in capillary-limited electrochemical oxygen sensors. As used herein, "sensor sensitivity" generally refers to the ratio between the change in output signal (e.g., current or voltage) and the measured property.

[0032] Without being limited to the underlying mechanism, studies of the devices, systems, and sensors herein demonstrate that electrochemical parameters measured during electronic interrogation, in which energy to the sensor's working electrode changes, do not correlate with changes in sensor sensitivity. Studies of electrochemical, capillary-limited oxygen sensors demonstrate that the height or amplitude of the sensor signal is determined solely by the capillary and diffusion characteristics at the sensor's gas inlet. Primarily, such characteristics include the effective diameter and length of the capillary and the carrier gas background. Again, without being limited to any mechanism, the present studies demonstrate that the sensor's nitrogen baseline (or oxygen-free baseline) depends on changes in sensor state resulting from environmental conditions, such as increased humidity, and sensor history. The overall sensor signal or response is the sum of both the oxygen-free sensor baseline (sometimes referred to herein as the nitrogen baseline) and the response to oxygen. The present studies further demonstrate that characterization of the sensor state and corresponding sensor response can be determined as a function of one or more parameters determined during electronic interrogation, in which an electrical signal is applied to an electrochemical sensor to pass a current through the electrolyte between the working and counter electrodes. Such a characterization or predefined characterization may provide, for example, a relationship between one or more parameters and the response of the electrochemical sensor over varying conditions of the electrochemical sensor. The response of the electrochemical sensor measured in a predefined characterization of the electrochemical sensor may include only the baseline output / response of the electrochemical sensor in the absence of oxygen, or may be the ambient output / response (including both the baseline response of the electrochemical sensor in the absence of oxygen and its response to oxygen).

[0033] During a given characterization determination, variations in the state of an electrochemical sensor (e.g., variations in electrolyte concentration and local water content at the working and reference electrodes of an electrochemical sensor system) or one or more representative sensors, such as an electrochemical sensor, may be induced, for example, by exposure over time to ambient conditions that differ from the "standard" ambient conditions at calibration. The parameter or sensor response being characterized may be measured at different times (e.g., periodically) over the time period during which it is exposed to the non-standard ambient conditions. It is also possible to induce a change in state without prolonged exposure to ambient conditions that differ from those at calibration. For example, the electrolyte concentration / water content of the electrochemical sensor being studied may be varied manually or automatically, and one or more parameters may be measured after each such variation, for example, while maintaining ambient conditions at or near the same as those at calibration.

[0034] The output value or sensor reading (e.g., provided as % O2 by volume) of an electrochemical, capillary-restricted oxygen sensor can be determined from two sensor parameters. In this regard, first, the sensor current (Iamb, or current output at ambient conditions) is determined while operating in ambient air at 20.8% O2 by volume. The output current is also determined while operating in a known calibration gas concentration. The calibration gas may be, for example, pure nitrogen with 0.0% O2 by volume, as discussed above. Also, as discussed above, the current output while operating in a nitrogen atmosphere is referred to herein as the sensor's nitrogen baseline (Initrogen). The sensor sensitivity can be calculated from these two values ​​as follows: Sensitivity [A / volume %] = (Iamb - Initrogen) [A] / 20.8 [volume %]

[0035] Other sets or sets of O2 concentrations may be used to calculate sensitivity. The choice of concentration may be determined by the desired measurement range and the context of the technical application. From the above equation for sensitivity, the sensor output reading for an electrochemical capillary-limited oxygen sensor may be calculated from the following equation: Sensor reading [vol%] = (actual / measured sensor current - Initrogen) [A] / sensitivity [A / vol%].

[0036] Currently, the nitrogen baseline of a sensor is treated as a constant value. However, this study suggests that the sensor output reading or value can be affected by changes in both sensitivity and nitrogen baseline. Regardless of the underlying mechanism, the devices, systems, and methods herein provide a predefined characterization that provides a relationship between parameters determined in an electronic interrogation of the sensor and the response of the electrochemical sensor over varying sensor states. The response of the electrochemical sensor at the predefined characterization may be determined as the change in response of the electrochemical sensor compared to the response at values ​​determined at an initial state or a calibration state under defined conditions. Without being limited to any mechanism, the varying state / response of the electrochemical sensor may be related to changes in electrolyte concentration and local water content, for example, at the working and reference electrodes of the electrochemical sensor system. The predefined characterization may be used in determining the sensor output reading or value, which may be considered the corrected sensor output reading or value. In this regard, the output value of the gas detection device may be determined from (i) a measurement output signal (e.g., a current signal) in response to exposure to a gas in the ambient environment, and (ii) a comparison of a measured parameter in an electronic interrogation (contemporaneous with the measurement output signal) with a predetermined characterization of that parameter. Implementation of a correction method to improve the accuracy of a sensor output reading or value in the devices, systems, and methods herein may be achieved in a variety of ways. In this regard, a change in a sensor output reading (relative to a measured parameter in an electronic interrogation that references a predetermined characterization of the parameter) may be implemented, for example, in an algorithm that changes the overall sensitivity of the sensor, an algorithm that changes the nitrogen baseline, an algorithm that changes a general correction factor, etc. The implementation of the sensor output determination may be independent of the actual naming of the variables described above.

[0037] Currently available methodologies for determining the life and health of capillary-limited electrochemical gas sensors for oxygen attempt to determine either the signal height or the overall signal. In some embodiments, the devices, systems, and methods herein provide a determination or characterization of the sensor state (and associated response) as a function of parameters determined in an electronic interrogation to predict the overall signal through adding a certain signal height provided by the presence of atmospheric oxygen. As described above, the characterization of the sensor state / associated response as a function of parameters determined in an electronic interrogation may be determined, for example, by parameter dependency by varying the sensor state of an electrochemical sensor over time. In many studies, the variation in sensor state has been induced by exposure to predetermined environmental conditions over time. Such environmental conditions may differ significantly (e.g., in temperature and / or relative humidity) from the environmental conditions during calibration.

[0038] Prior to this work, studies of capillary-limited oxygen sensors (typically operated at relatively high power) suggested that the height or amplitude of the ambient output signal was independent of environmental conditions such as humidity. Given the relatively small effect of capacitive or non-faradaic current changes associated with changes in the nitrogen baseline due to changing conditions in electrochemical sensors (e.g., resulting from environmental conditions such as those described herein), the signal-to-noise ratio in the electronics of such sensors was typically insufficient to observe and characterize the phenomenon.

[0039] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. Thus, for example, a reference to "a parameter" includes a plurality of such parameters and their equivalents known to those of ordinary skill in the art, and a reference to "the parameter" is a reference to one or more such parameters and their equivalents known to those of ordinary skill in the art. The recitation of ranges of values ​​herein is merely intended to serve as a shorthand method for individually referencing each separate value falling within the range. Unless otherwise indicated herein, each separate value and intermediate range is incorporated herein as if it were individually set forth herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or clearly contraindicated in the context.

[0040] As used herein, the terms "electronic circuitry," "circuitry," or "circuit" include, but are not limited to, hardware, firmware, software, or a combination thereof for performing a function or operation. For example, a circuit may include a software-controlled microprocessor, discrete logic such as an application-specific integrated circuit (ASIC), or other programmed logic device, depending on the desired function or need. A circuit may also be embodied entirely in software. As used herein, "circuitry" is considered synonymous with "logic." As used herein, "logic" includes, but is not limited to, hardware, firmware, software, or a combination thereof that performs a function or operation or causes a function or operation from other components. For example, a logic may include a software-controlled microprocessor, discrete logic such as an application-specific integrated circuit (ASIC), or other programmed logic device, depending on the desired application or need. A circuit may also be embodied entirely in software.

[0041] As used herein, the term "processor" includes one or more of virtually any number of processor systems or stand-alone processors, such as, but not limited to, microprocessors, microcontrollers, central processing units (CPUs), and digital signal processors (DSPs), in any combination. A processor may be associated with various other circuits that support the operation of the processor, such as random access memory (RAM), read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), clocks, decoders, memory controllers, or interrupt controllers. These support circuits may be internal or external to the processor or its associated electronic package. The support circuits are in operative communication with the processor. The support circuits are not necessarily shown separately from the processor in block diagrams or other figures.

[0042] As used herein, the term "controller" includes, but is not limited to, any circuit or device that coordinates and controls the operation of one or more input and / or output devices. A controller may include, for example, a device having one or more processors, microprocessors, or central processing units that can be programmed to perform functions.

[0043] As used herein, the term "software" includes, but is not limited to, one or more computer-readable or executable instructions that cause a computer or other electronic device to perform a function, act, or function in a desired manner. The instructions may be embodied in various forms, such as a routine, algorithm, module, or program, including code from a separate application or a dynamic link library. Software may also be implemented in various forms, such as a stand-alone program, a function call, a servlet, an applet, instructions stored in memory, part of an operating system, or other type of executable instructions. Those skilled in the art will understand that the form of software will depend, for example, on the requirements of the desired application, the environment in which it will run, and the desires of the designer / programmer.

[0044] When an electrical signal is applied to the working electrode of an electrochemical gas sensor during electronic interrogation, the response may be measured, for example, in the following manner (or as a function of): (i) maximum peak (MaxPeak), which is the maximum current observed upon application of the potential pulse; (ii) area under the curve (AUC), which is the integrated current response of the working electrode after application of the potential pulse (corresponding to the charging response of the sensor); (iii) minimum peak (minPeak), which is the minimum current obtained upon removal or reversal of the potential pulse; typically, it is the difference between the current observed immediately before and immediately after removal or reversal of the potential pulse, but can also be summarized and used as the difference between the minimum current and the baseline; (iv) peak-to-peak (PP), which is the algebraic difference between the maximum and minimum current; and (v) reversal area under the curve (rAUC), which is more precisely the area under the curve after removal, which is the charging current obtained by integrating the current response after removal or reversal of the potential pulse. In some embodiments, values ​​of one or more of these parameters determined during the period in which the sensor is used to monitor the concentration of an analyte may be compared to the value of the parameter, for example, in a pre-defined characterization in which the sensor's response (e.g., as represented by a variation in the nitrogen baseline) relates to that parameter over varying sensor conditions. The pre-defined characterization may be stored, for example, in the memory of a device containing the sensor at the time of manufacture. In an interrogation mode test or pulse cycle, energy to the working electrode is increased or decreased (e.g., via a variation in current or voltage) over a predetermined period of time (typically a small period of time), and the resulting response is measured.

[0045] For example, electronic interrogation may be performed for a fairly short period of time to minimize the time the sensor is offline to perform sensor test diagnostics (i.e., between sensor electronic interrogation cycles). In some embodiments, electronic interrogation may result in the electrochemical sensor of the present invention returning to normal (gas sensing) mode operation in less than 10 seconds, less than 5 seconds, less than 1 second, or even less than 0.5 seconds. Devices, systems, and methods for electronic interrogation of sensors may allow instruments containing one or more sensors to remain “online.” Furthermore, such devices, systems, and methods may provide active, automatic monitoring of sensor status as a background operation without user initiation. The frequency of electronic interrogation may vary. For example, providing sensor interrogation several times per hour may provide near-constant sensor life and health monitoring.

[0046] For gas sensors, detection is preferably performed within the gas phase or at the phase boundary. In general, this observation indicates that the speed of a sensor is limited only by the rate of gas-phase diffusion of the target gas molecules into the sensor. To limit the sensor output, gas sensors, such as electrochemical gas sensors, may be, for example, permeation-controlled / diffusion-controlled or permeation-limited / diffusion-limited, in which a permeable membrane is used to limit the diffusion of the target gas into the sensor, or capillary-controlled or capillary-limited, in which a capillary inlet is used to limit the diffusion of the target gas into the sensor.

[0047] In this regard, in electrochemical gas sensors, the gas to be measured (sometimes called the target gas or analyte gas) typically enters the sensor housing from the surrounding atmosphere or environment, for example, through a gas-porous or gas-permeable membrane or through a capillary inlet to a first or working electrode (sometimes called the sensing electrode) where a chemical reaction occurs. A complementary chemical reaction occurs at a second electrode, known as the counter electrode (or auxiliary electrode). Electrochemical sensors generate an analytical signal through the generation of a current resulting directly from the oxidation or reduction of the analyte gas (i.e., the gas to be detected) at the working electrode. A comprehensive discussion of electrochemical gas sensors is also found in Cao, Z. and Stetter, JR, "The Properties and Applications of Amperometric Gas Sensors," Electroanalysis, 4(3), 253 (1992), the disclosure of which is incorporated herein by reference.

[0048] The combination of the working and counter electrodes produces an electrical signal that is (1) related to the concentration of the analyte gas and (2) strong enough to provide a signal-to-noise ratio suitable for distinguishing between concentration levels of the analyte gas over the entire range of the analyte. In other words, the current flow between the working and counter electrodes should be measurably proportional to the concentration of the analyte gas over the concentration range of interest.

[0049] In addition to the working and counter electrodes, electrochemical sensors often include a third electrode, commonly referred to as a reference electrode. The reference electrode is used to maintain the working electrode at a known voltage or potential. It is desirable for the reference electrode to be physically and chemically stable in the electrolyte.

[0050] The electrical connection between the working electrode and the counter electrode is maintained through an electrolyte. The functions of the electrolyte are as follows: (1) efficiently carry ionic current, (2) solubilize the analyte gas, (3) support reactions at both the counter electrode and the working electrode, and (4) establish a stable reference potential with the reference electrode. Criteria for an electrolyte include, for example, the following: (1) electrochemical inertness, (2) ion conductivity, (3) chemical inertness, (4) temperature stability, (5) low cost, (6) low toxicity, (7) low flammability, and (8) appropriate viscosity.

[0051] Generally, the electrodes of an electrochemical cell provide surfaces where oxidation or reduction (redox) reactions occur, providing a mechanism by which ionic conduction in the electrolyte solution and electronic conduction in the electrode combine to provide a complete circuit for electrical current. The measurable current resulting from a cell reaction in an electrochemical cell is directly proportional to the extent of the reaction occurring at the electrode. Therefore, maintaining high reaction rates in an electrochemical cell is desirable. To that end, the counter electrode and / or working electrode of an electrochemical cell typically include a suitable electrocatalyst on their surface to support the reaction rate.

[0052] As a result of electrostatic forces, the volume of solution very close to the working electrode surface is a very highly ordered structure. This structure is important for understanding electrode processes. The volume of solution very close to the electrode surface is variously referred to as the diffuse layer, scattering layer, or Helmholtz layer or plane.

[0053] The magnitude of resistance and capacitance present in an electrochemical cell is a result of the properties and identities of the materials used in its construction. The resistance of an electrolyte is due to the number and type of ions dissolved in the solvent. The capacitance of an electrode is primarily a function of the active surface area of ​​the electrocatalyst. In an ideal environment, these quantities remain constant. However, amperometric gas sensors using aqueous electrolytes can experience changes in solution resistance as a result of exposure to various changes in the ambient relative humidity. As water evaporates from the sensor, the chemical concentration of the ionic electrolyte increases. This change in concentration can cause an increase or decrease in the electrolyte's resistivity, depending on the actual electrolyte used.

[0054] Furthermore, even substances that are typically considered insoluble in a particular solvent can only exist at a low, upper limit of their concentration in the solvent. For example, the electrolyte of an electrochemical sensor contains dissolved electrode metal, but at a very low, upper limit. This small concentration of dissolved metal is constantly in flux; that is, metal atoms are constantly being dissolved from the electrode and redeposited elsewhere. The net effect of this process is to reduce the effective surface area of ​​the electrode, which has the effect of reducing the sensor's capacitance over time. Both of these effects have the net effect of modifying the sensor over its lifetime.

[0055] 1A and 1B show schematic diagrams of a representative embodiment of a capillary-limited electrochemical sensor 10 in the devices, systems, and methods of the present invention. Sensor 10 includes a housing 20 having a gas inlet 30 in the form of a capillary tube for admitting one or more target or analyte gases to sensor 10. As the name suggests, capillary-limited sensors such as sensor 10 use a very small inlet hole 30 (i.e., a capillary tube) with a common or typical aspect ratio (length:diameter or l:d) of about 100:1 (see, e.g., FIG. 1C , which shows axial and radial cross-sections of inlet 30 and the surrounding cylindrical portion of housing 20).

[0056] In FIG. 1C, p2 is the partial pressure of the target gas outside the inlet 30, p1 is the partial pressure of the target gas at the inner opening of the inlet 30, c2 is the concentration of the target gas outside the inlet 30, and c1 is the concentration of the target gas at the inner opening of the inlet 30 (or at the surface working electrode 50, which is essentially zero). What is often called "ordinary capillary diffusion" is actually a special case of Graham's law of effusion. See, for example, Barrow, GM: Physical Chemistry, 4th edition. New York, NY: McGraw Hill (1979). In general, "diffusion" refers to the bulk flow of gas from a region of high pressure (or partial pressure) or high concentration through a porous wall or tube with a very small diameter to a region of low pressure or low concentration, respectively. Furthermore, "effusion" refers to a process of transport due to molecular flow, rather than bulk flow through an orifice or membrane.

[0057] Capillary-restricted oxygen sensors (O2 sensors) have become the dominant O2 sensor on the market. This dominance is largely due to the fact that many performance standards are expressed in volume percent (vol%) O2 concentration. Capillary-type O2 sensors measure vol% O2 without being affected by O2 partial pressure (which changes with total atmospheric pressure even when the vol% O2 concentration is constant). In other words, capillary-type sensors simply respond to the vol% target gas in the sample, regardless of pressure. The output of a capillary-type sensor is given by the following equation:

number

[0058] In some embodiments, electrolyte-saturated wicks 40a, 40b, and 40c may separate the reference and counter electrodes 70 and 80 from the working electrode 50 within the sensor 10 and / or provide ionic conduction therebetween via electrolyte 44 absorbed within the wicks 40a, 40b, and 40c within the housing 20. Electronic circuitry 100, as known in the art, provides, for example, maintaining a desired potential difference between the working electrode 50 and the reference electrode 70, varying or pulsing the potential difference as described herein, and processing the output signal from the sensor 10. The sensor electrodes are disposed in connection with the electrical circuitry 100 via a connector 90, which provides conductive electrical conductivity / connectivity through the housing 20.

[0059] In the illustrated embodiment, the working electrode 50 may be formed, for example, by depositing a first electrocatalytic layer 54 onto a gas diffusion membrane 52 (e.g., using catalyst deposition techniques known in the sensor art). The sensor 10 may include a gas diffusion membrane 52 at a backside portion of the capillary inlet 30; however, unlike permeation- or diffusion-limited sensors, diffusion through the gas diffusion membrane 52 is not rate-limited. The membrane 52 serves to retain the electrolyte 44 within the housing 20 and to support the electrocatalytic layer / surface 54 within the sensor 10. Gases readily move or transport through the diffusion membrane 52 (e.g., via diffusion), but the electrolyte 44 does not readily move or transport through the diffusion membrane 52. The diffusion membrane 54 of the working electrode 50 may be attached (e.g., via heat sealing) to the inner surface of the top, cap, or lid 22 of the housing 20. An exemplary working electrode 50 may include, for example, platinum or platinum dispersed on carbon as the electrocatalytic layer 54. Alternatively, an acidic electrolyte such as H2SO4 may be used.

[0060] The electronic circuit 100 includes a processor or controller system 102 including, for example, one or more processors or microprocessors for controlling various aspects of the operation of the sensor 10. A memory system 104 may be operatively or communicatively coupled to the processor system 102 and may store software for control, measurement, and / or analysis in the sensor 10. A user interface system 106 (e.g., including a display, a speaker, etc.) may also be operatively or communicatively coupled to the processor system 102. A communication system 108, such as a transceiver, may be operatively or communicatively coupled to the processor system 102 for wired and / or wireless communication. A power source 110 (e.g., a battery system and / or line power) may provide power to the electronic circuit 100.

[0061] 1A and 1B, the sensor housing 20 is formed with a vent 26 that is in spatial communication with the counter electrode 80. The vent 26 allows O2 generated at the counter electrode 80 to escape from the housing 20. The amount of O2 generated is very small (on the order of a few nanoliters per second). However, over the lifetime of the sensor 10, the amount of O2 generated can become significant. If the sensor 10 is not effectively vented, pressure can build up within the sensor housing 20, disrupting the sensor signal or causing electrolyte leakage.

[0062] Several studies herein used sensors based on the XCELL® oxygen sensor, available from MSA Safety, Inc., Cranberry Township, Pennsylvania, to characterize capillary-limited oxygen sensors. Oxygen sensors with a 12 μm capillary diameter were tested over a six-month period under various storage conditions. After calibration at ambient conditions, one research group placed the sensors in a chamber at 25°C and 10% relative humidity (rh). Ambient conditions used during calibration ranged from approximately 22°C and 40-50% relative humidity. Another research group placed the sensors in a chamber at 25°C and 85% rh after calibration at ambient conditions. All sensors were periodically tested at a nitrogen baseline (i.e., a baseline in the absence of oxygen or at 0% oxygen output by volume), 20.8% oxygen output by volume, and 10.4% oxygen output by volume. After approximately two months of storage in these chambers, the conditions were reversed, and sensors previously maintained under dry conditions were placed in humid conditions, and vice versa. Each sensor test was accompanied by a chronoamperometric pulse or interrogation test. Chronoamperometry is an electrochemical technique in which the potential of the working electrode is changed (e.g., stepwise) and the resulting current is monitored as a function of time. Electronic interrogation herein is performed without applying a test gas or its simulant with a known analyte gas concentration to the sensor from a container. In some studies, the pulse or interrogation test involved a 10 mV bias change per second. This voltage change resulted in a current curve from which representative parameters were determined: baseline, maximum current (MaxPeak), area under the curve (AUC), and signal height between baseline and MaxPeak (DeltaPeak). All values ​​were corrected for the baseline value.

[0063] To study the effect of working electrode size, the sensors tested included two different sizes of working electrodes with the same specific surface area. One set of sensors had a standard-sized working electrode (0.19 inches, 4.83 mm diameter), while the other set had a larger working electrode (0.312 inches, 7.92 mm diameter). This study of working electrode size resulted in two distinct groups of pulse test responses, as shown, for example, in Figure 2. In this regard, it was found that working electrode size alters the absolute value of the current response during the pulse test, but does not alter the gas response. The gas response current is determined solely by the capillary characteristics. While working electrode size affects the pulse pattern, conclusions drawn from chronoamperometry data are independent of working electrode size. This study demonstrates the generality of the system response assessments described herein, enabling the devices, systems, and methods described herein to predict system behavior across a wide variety of electrochemical O2 sensing embodiments.

[0064] The predictive evaluation of pulse test data or electronic interrogation data involves two steps and highlights newly discovered aspects of the system behavior of capillary-restricted oxygen sensors. Generally, all pulse parameters are strongly correlated with each other, and this prediction is independent of parameter selection, as shown, for example, in the matrix scatter plot of FIG. 3. As noted above, evaluation of all environmentally dependent data has previously shown no detectable direct correlation between pulse data and sensor ambient output, typically for sensors operated at relatively high output. However, in some embodiments of the devices, systems, and methods herein, the sensor's nitrogen baseline is correlated with one or more parameters determined, for example, via chronoamperometric pulse data and / or other electrochemical characterization, in dependence of the sensor's state on environmental conditions. The nitrogen baseline current in the absence of oxygen represents the background current of the sensor's electrochemical system. This parameter depends on environmental conditions / history and varies in a predictable manner with the chronoamperometric response, as shown, for example, in FIG. 4A.

[0065] FIG. 4B shows the difference between the nitrogen baseline (i.e., the baseline in the absence of oxygen) and the ambient baseline (i.e., the baseline at 20.8% by volume of oxygen or O2, the standard oxygen concentration in ambient air). In the absence of oxygen (e.g., in a pure nitrogen or N2 atmosphere), the current through the sensor is almost entirely capacitive and driven by double-layer processes within the sensor. In the presence of oxygen, the sensor signal includes a combination of the capacitive "nitrogen baseline" signal and a signal resulting from the Faradaic reduction current. As shown in the studies herein, the capacitive nitrogen baseline current detectably changes with changes in environmental factors (e.g., humidity).

[0066] As noted above, the actual sensor reading or output is a combination of its nitrogen baseline and an additional signal resulting from the influence of incoming oxygen. The influence of oxygen entering the sensor is capillary driven only and is constant with humidity fluctuations that affect the nitrogen baseline (see, e.g., Figure 5). Without being limited to any mechanism, it is believed that the electrolyte concentration and local water content at the working and reference electrodes of an electrochemical sensor system affect the nitrogen baseline across humidity and environmental conditions. This correlation is also seen in the ambient output of the sensors studied herein, which is a combination of the baseline and the influence of oxygen (see, e.g., Figure 6). Because the actual sensor reading is determined solely by the actual current output, the correlation herein does not allow for a chronoamperometric function or other electrochemical evaluation function (see, e.g., Figure 7). Figure 7 shows the parameter AUC (μA 2 This allows us to predict the change in sensor reading as a function of the change in sensor output (volume %). The output change represents the difference between the measured value of the parameter and the value determined when calibrating the sensor at the ambient conditions described above. Therefore, a change in reading (no signal change) of 0.0 volume % occurs at a parameter output change of 0.0.

[0067] Characterization of the chronoamperometric data (and / or other parameters measured after inducing current flow) over a range of sensor states (e.g., changes in sensor state induced by environmental conditions such as relative humidity, temperature, etc.) provides a predetermined relationship between such parameters and changes in sensor state, for example, as described in certain embodiments herein. Such changes in state may be indicated, for example, by changes in the nitrogen baseline response (and thereby the sensor output) of the sensors of the present invention in response to changes in sensor state induced by environmental conditions / history. The determined relationship between one or more measured electrochemical parameters and the sensor output / response over varying sensor states may be stored (e.g., as an equation or algorithm or as a look-up table) within the sensor's memory system. Suitable measurement parameters are not limited to chronoamperometric parameters, but may be parameters measured in any type of electrochemical measurement (e.g., measured in EIS electrochemical impedance spectroscopy, cyclic voltammetry, CV, etc.) in which an electrical signal is applied to an electrochemical sensor, thereby generating a current flow between the working and counter electrodes of the electrochemical sensor through an electrolyte. One or more algorithms may also be stored in the memory system for execution by the processor system to correct the analyte gas (oxygen) concentration output of the sensor based on one or more electrochemical parameters measured in close proximity to or contemporaneous time (e.g., within a day, an hour, minutes, or seconds) of the sensor measurement of the analyte gas concentration that produces the sensor output. In this regard, one or more parameters, such as chronoamperometric parameters, are measured over a period of time (post-manufacture) while the sensor is in operation to measure the analyte (oxygen) of interest. As noted above, in some embodiments, the selection of parameters is not critical, as various parameters provide similar or identical analytical data.For example, with reference to FIG. 7 , data on the change in output reading as a function of a chronoamperometric parameter or other parameter may be stored in memory (e.g., at the time of manufacture—e.g., as identified over a range of sensor conditions) and applied to the sensor output based on simultaneous measurements of the chronoamperometric parameter and / or another electrochemical parameter. In this regard, the sensor device displays a predetermined output value, which may be calculated using the stored sensitivity and nitrogen baseline, as described above. The predetermined characterization data determined in the methodology herein may be used in the calculations and / or may be used to adjust the output value determined in the calculations to provide a more accurate output value.

[0068] A predetermined characterization or relationship between variations in sensor state / output readings and chronoamperometric or other electrochemical parameters may be determined, for example, for an individual sensor or for one or more sensors representing a class of similar or like sensors. As used herein, the terms "similar sensor" or "similar sensor" refer to sensors having the same or similar design parameters (e.g., the same or similar electrodes, electrolytes, etc.). As described herein, a predetermined characterization may be determined based on changes in nitrogen baseline response or ambient response over induced changes in sensor state. Alternatively, one or more mathematical models may be used to determine the predetermined characterization, for example, based on characterization of the design parameters of a sensor or class of sensors and relationships developed from theory and / or experimental data.

[0069] Thus, the devices, systems, and methods herein provide prediction / correction of capillary-restricted oxygen sensor readings over a range of relevant and practically available environmental conditions. In addition to improving the maintenance of sensor output accuracy, this functionality can be used for various improvements in oxygen detection technology, such as, for example, predictive maintenance, improved operator safety, reduced sensor downtime, and providing sensor operation over a wider range of environmental conditions.

[0070] A recent trend in gas sensor technology is toward sensors that require less power. Sensors operating at relatively high power have a relatively large Faradaic current compared to environmental changes in capacitive current. As discussed above, sensors operating at relatively high power have a greater difficulty detecting changes in capacitive current, which are a small fraction of the ambient output current. As the sensor current decreases, typically associated with smaller capillaries in capillary-limited gas sensors, the relative influence of the capacitive nitrogen baseline increases. The capacitive baseline change can be large enough that changes in the sensor output as a result of environmental changes become noticeable. For a typical, relatively high-output, capillary-limited oxygen sensor, the ambient output can range, for example, from about 300 to about 250 μA. In some embodiments of the sensors herein, the ambient current output is 300 μA or less, 250 μA or less, 150 μA or less, 100 μA or less, 80 μA or less, or 30 μA or less. Generally, the lower the ambient current output of a sensor, the greater the proportion of capacitive current attributable to environmental changes in its output. The effect of changes in capacitive current due to environmental conditions is observable in the ambient output of the sensor, as shown in Figure 6, but given that a larger proportion of the total signal is related to changes in capacitive current in the case of measuring the nitrogen baseline signal, a default characterization of that change measured over a range of sensor conditions by studying the change in the nitrogen baseline may provide better characterization results.

[0071] The devices, systems, and methods herein may be used in conjunction with relatively high-power capillary-restricted oxygen sensors, however, the electronics of such sensors (e.g., high-power, low-noise potentiostats) must provide an adequate signal-to-noise ratio to accurately and reproducibly determine the effect of capacitive current changes described herein.

[0072] In summary, in the devices, systems, and methods herein, a characterization of the change in the sensor's response (e.g., as evidenced by a change in the nitrogen baseline value) over a change in the sensor's state (e.g., which may result from varying environmental conditions or history) is determined. In this regard, the parameter characterization determined during electronic interrogation, or a pre-defined characterization, provides a relationship between the parameter and the electrochemical sensor's response over the electrochemical sensor's changing state. The electrochemical parameter is measured, for example, during a pulse test / electronic interrogation in which energy to the electrochemical sensor's working electrode is varied. The pre-defined characterization is used to modify or correct the sensor output reading or value based on a relationship or comparison of the pre-defined characterization to simultaneous measurements of the electrochemical parameter. In this manner, the sensor output reading or value may be adjusted or corrected for changes in the sensor's state, for example, resulting from the effects of changing environmental conditions or history.

[0073] The foregoing description and accompanying drawings illustrate a number of representative embodiments presently in accordance with the present invention. Of course, various modifications, additions, and alternative designs will be apparent to those skilled in the art in light of the foregoing teachings without departing from the scope of the invention, which is indicated by the following claims rather than the foregoing description. All changes and variations that come within the meaning and range of equivalency of the claims are intended to be embraced within their scope.

Claims

1. A method for operating a gas detection device comprising a capillary-restricted electrochemical sensor (10) that analytically responds to oxygen, the electrochemical sensor comprising a housing (20) including a capillary tube (30) through which gas diffuses from the environment into the housing (20); a working electrode (50) within the housing (20); a counter electrode (80) within the housing (20); and an electrolyte (44) within the housing (20) that is in ionic contact with the working electrode (50) and the counter electrode (80), The method is, The steps include applying an electrical signal to the electrochemical sensor (10), thereby generating a current flow between the working electrode (50) and the counter electrode (80) via the electrolyte (44), A step of measuring the parameters of the response of the electrochemical sensor (10) to the electrical signal, A step of comparing the measured parameter with a default characterization of the parameter, wherein the default characterization of the parameter provides a relationship between the parameter and the response of the electrochemical sensor (10) over a fluctuating state of the electrochemical sensor (10), The steps include determining the output value of the gas detection device by comparing the output signal of the electrochemical sensor (10) that detects oxygen in the surrounding environment with the predetermined characteristic evaluation of the measured parameters, and Methods that include...

2. A step of operating the electrochemical sensor (10) in detection mode, wherein the output signal is generated, and / or A step of operating the electrochemical sensor (10) in query mode, wherein in query mode, the measured parameter is measured, and during query mode, the electrochemical sensor (10) is electronically queryed by applying the electrical signal to the electrochemical sensor (10). The method according to claim 1, further comprising:

3. The method according to claim 2, wherein the gas detection device further comprises a control system (100) including a processor system (102) and a memory system (104), the working electrode (50) being operably connected to the control system (100) and / or the counter electrode (80) being operably connected to the control system (100), and / or the memory system (104) including one or more algorithms that can be executed by the processor system (102) to perform one or more actions of the method, stored in the memory system (104).

4. The method according to claim 1, wherein the predetermined characteristic evaluation is stored in memory during manufacturing.

5. The method according to claim 1, wherein the parameter is a chronoamperometry parameter.

6. The method according to claim 3, wherein the parameters are the maximum peak value, area under the curve, minimum peak value, peak-to-peak value, and / or inverse area under the curve for the response of the electrochemical sensor to the electrical signal applied in the query mode, or are functions thereof.

7. The method according to claim 1, wherein the predetermined characteristic evaluation is determined over the fluctuating state of the electrochemical sensor (10).

8. The method according to claim 7, wherein the fluctuating state of the electrochemical sensor (10) is induced by environmental conditions.

9. The method according to claim 1, wherein the predetermined characterization is determined as a function of the fluctuating baseline response of the electrochemical sensor (10) in the absence of oxygen.

10. The method according to claim 9, wherein the baseline response of the electrochemical sensor (10) in the absence of oxygen is determined under a nitrogen atmosphere.

11. The method according to claim 1, wherein the ambient current output of the electrochemical sensor (10) is less than 300 μA.

12. A gas detection device A control system (100) including a processor system (102) and a memory system (104), An electrochemical sensor (10) that is analytically responsive to oxygen, comprising: a housing (20) including a capillary tube (30) through which gas diffuses from the environment into the housing (20); a working electrode (50) within the housing (20) operably connected to the control system (100); a counter electrode (80) within the housing (20) operably connected to the control system (100); and an electrolyte (44) within the housing (20) that is in ionic contact with the working electrode (50) and the counter electrode (80); Includes, The control system (100) applies an electrical signal to the electrochemical sensor (10) by having software stored in the memory system (104) executed by the processor system (102), thereby generating a current flow between the working electrode (50) and the counter electrode (80) via the electrolyte (44), measuring the parameters of the response of the electrochemical sensor (10) to the electrical signal, comparing the measured parameters with a predetermined characterization of the parameters, where the predetermined characterization of the parameters provides a relationship between the parameters and the response of the electrochemical sensor (10) over fluctuating states of the electrochemical sensor (10), and the gas detection device is configured to determine the output value of the gas detection device from the analytical response of the electrochemical sensor (10) for detecting oxygen in the ambient environment and a comparison of the measured parameters with the predetermined characterization.

13. The apparatus according to claim 12, wherein the control system (100) is further configured to operate the electrochemical sensor (10) in detection mode so that the output signal is generated and / or to operate the electrochemical sensor (10) in query mode so that the measured parameter is measured in the query mode, and the electrochemical sensor (10) is electronically queryed by applying the electrical signal to the electrochemical sensor (10) during the query mode.

14. The apparatus according to claim 12, wherein the predetermined characteristic evaluations are stored in the memory system during manufacturing.

15. The apparatus according to claim 12, wherein the parameter is a chronoamperometry parameter.

16. The apparatus according to claim 13, wherein the parameters are the maximum peak value, area under the curve, minimum peak value, peak-to-peak value, and / or inverse area under the curve for the response of the electrochemical sensor (10) to the electrical signal applied in the query mode, or are functions thereof.

17. The apparatus according to claim 12, wherein the ambient current output of the electrochemical sensor (10) is less than 300 μA.

18. A method for characterizing changes in the response of an electrochemical sensor (10) over a fluctuating state, wherein the electrochemical sensor (10) is capillary-limited and analytically responsive to oxygen, and the electrochemical sensor (10) comprises a housing (20) including a capillary tube (30) through which a gas diffuses from the environment into the housing (20); a working electrode (50) within the housing (20); a counter electrode (80) within the housing (20); and an electrolyte (44) within the housing (20) which is in ionic contact with the working electrode (50) and the counter electrode (80). The method is, A step to determine a predetermined characterization that provides a relationship between parameters measured when an electrical signal is applied to the electrochemical sensor (10) to generate a current flow between the working electrode (50) and the counter electrode (80) via the electrolyte (44), and the response of the electrochemical sensor (10) over a fluctuating state of the electrochemical sensor (10). Methods that include...

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