Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/403—Cells and electrode assemblies
  • G01N27/406—Cells and probes with solid electrolytes
  • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
  • G01N27/4073—Composition or fabrication of the solid electrolyte
  • G01N27/4074—Composition or fabrication of the solid electrolyte for detection of gases other than oxygen
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/403—Cells and electrode assemblies
  • G01N27/404—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/403—Cells and electrode assemblies
  • G01N27/406—Cells and probes with solid electrolytes
  • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
  • G01N27/4073—Composition or fabrication of the solid electrolyte
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/0004—Gaseous mixtures, e.g. polluted air
  • G01N33/0006—Calibrating gas analysers

Definitions

• Prudence dictates that gas detection instrumentation be tested regularly for functionality. It is a common practice to, for example, perform a "bump check,” or functionality check on portable gas detection instrumentation on a daily basis. The purpose of this test is to ensure the functionality of the entire gas detection system, commonly referred to as an instrument. A periodic bump check or functionality check may also be performed on a permanent gas detection instrument to, for example, extend the period between full calibrations.
• Gas detection systems include at least one gas sensor, electronic circuitry and a power supply to drive the sensor, interpret its response and display its response to the user. The systems further include a housing to enclose and protect such components.
• a bump check typically includes: a) applying a gas of interest (usually the target gas or analyte gas the instrument is intended to detect); b) collecting and interpreting the sensor response; and c) indicating to the end user the functional state of the system (that is, whether or not the instrument is properly functioning).
• a gas of interest usually the target gas or analyte gas the instrument is intended to detect
• b) collecting and interpreting the sensor response and c) indicating to the end user the functional state of the system (that is, whether or not the instrument is properly functioning).
• bump tests are performed regularly and, typically, daily.
• Bump checks provide a relatively high degree of assurance to the user that the gas detection device is working properly.
• the bump check exercises all the necessary functionalities of all parts of the gas detection device in the same manner necessary to detect an alarm level of a hazardous gas.
• the bump check ensures that there is efficient gas delivery from the outside of the instrument, through any transport paths (including, for example, any protection and/or diffusion membranes) to contact the active sensor components.
• the bump check also ensures that the detection aspect of the sensor itself is working properly and that the sensor provides the proper response function or signal.
• the bump check further ensures that the sensor is properly connected to its associated power supply and electronic circuitry and that the sensor signal is being interpreted properly.
• the bump check ensures that the indicator(s) or user interface(s) (for example, a display and/or an annunciation functionality) of the gas detection instrument is/are functioning as intended.
• a periodic/daily bump check requirement has a number of significant drawbacks.
• bump checks are time consuming, especially in facilities such as industrial facilities that include many gas detection systems or instruments.
• the bump check also requires the use of expensive and potentially hazardous calibration gases.
• the bump check also requires a specialized gas delivery system, usually including a pressurized gas bottle, a pressure reducing regulator, and tubing and adapters to correctly supply the calibration gas to the instrument.
• the requirement of a specialized gas delivery system often means that the opportunity to bump check a personal gas detection device is limited in place and time by the availability of the gas delivery equipment.
• Such a system may, for example, include electronic interrogation of a sensor.
• a sensor is offline or unable to sense an anlayte or target gas or gases during such electronic interrogation.
• a number of sensors include functionality to electronically interrogate of one or more electrodes thereof, require a user to initiate an interrogation process which takes between 20-30 seconds.
• a potential change may be applied to an electrode for 5-10 seconds and the corresponding current decay curve is studied over a 20-30 second period.
• the sensor is offline and can't be used to sense the analyte(s). It is desirable to minimize the amount of time a sensor is offline, particularly in cases wherein a sensor is used to detect one or more hazardous analytes or target gases.
• a method of operating a sensor to detect an analyte in an environment includes performing a sensor interrogation cycle including applying electrical energy to the working electrode to generate a non-faradaic current, measuring a response to the generation of the non-faradaic current to determine a state of the sensor, and actively controlling the circuitry to dissipate the non-faradaic current.
• the sensor interrogation cycle lasts less than one second.
• the method may, for example, include periodically initiating the sensor interrogation cycle.
• Applying electrical energy to the working electrode may, for example, include applying a first potential difference to the working electrode.
• Actively controlling the circuitry may, for example, include applying at least a second potential difference to the working electrode of opposite polarity to the first potential difference.
• actively controlling the circuitry includes decreasing a load resistance in electrical connection with the working electrode.
• Applying electrical energy to the working electrode may, for example, include changing the potential of the working electrode for a period of time.
• the period of time may, for example, be no greater than 1/2 seconds, no greater than 1/16 seconds, or no greater than 1/64 seconds.
• decreasing the load resistance occurs at the same time or after measuring the response.
• the senor comprises a load resistor and a bypass switch to bypass the load resistor.
• the bypass switch may, for example, include a field effect transistor switch, wherein activating a field effect transistor switch decreases the load resistance and deactivating the field effect transistor switch increases the load resistance.
• the method further includes adjusting the output of the sensor at least in part on the basis of the sensor interrogation cycle.
• An electrochemical sensor is operable to detect an analyte in an environment during an operational mode of the sensor and includes a working electrode and circuitry in operative connection with the working electrode, which is adapted to carry out an electronic interrogation cycle.
• the circuitry includes a power source via which electrical energy is applied to the working electrode during the electronic interrogation cycle to generate a non- faradaic current.
• the electrochemical sensor further includes a system to measure a response of the sensor and a control system to actively control the circuitry to dissipate the non- faradaic current.
• the circuitry is adapted to complete the sensor interrogation cycle in less than one second.
• the circuitry may, for example, be adapted to periodically initiate the sensor interrogation cycle.
• the circuitry may, for example, be adapted to apply a first potential difference to the working electrode.
• the control system may, for example, actively control the circuitry to apply at least a second potential difference to the working electrode of opposite polarity to the first potential difference to dissipate the non-faradaic current.
• the control system may, for example, decrease a load resistance in electrical connection with the working electrode to dissipate the non-faradaic current. Decreasing the load resistance may, for example, occur at the same time or after measuring the response to the generation of the non-faradaic current.
• Applying electrical energy to the working electrode may, for example, include changing the potential of the working electrode for a period of time.
• the period of time is no greater than 1/2 seconds, no greater than 1/16 seconds, of no greater than 1/64 seconds.
• the circuitry include a load resistor and a bypass switch to bypass the load resistor.
• the bypass switch may, for example, include a field effect transistor switch. Activating the field effect transistor switch may, for example, decrease the load resistance and deactivating the field effect transistor switch may, for example, increase the load resistance.
• control system is further adapted to adjust output of the sensor at least in part on the basis the sensor interrogation cycle.
• a method of operating a sensor operable to detect an analyte in an environment includes performing a sensor interrogation cycle including applying electrical energy to the working electrode to generate a non-faradaic current and measuring a response to the generation of the non-faradaic current to determine a state of the sensor.
• the amount of energy applied is low enough in amplitude and short enough in duration such that the non-faradaic current dissipates quickly enough so that a baseline current is reached less than one second from application of the electrical energy at which an analytic response of the sensor can be measured to detect the analyte.
• FIG. 1A illustrates schematically an embodiment of an electrochemical sensor hereof.
• Figure IB illustrates a schematic circuit diagram of an embodiment of a sensor hereof.
• Figure 2A illustrates recovery of a sensor signal of a sensor for hydrogen sulfide (FhS) after imposition of a l/16 th second +10mV pulse for the case in which a load resister of a predetermined resistance is in series with the working electrode and for the case that the resister is bypassed or short circuited via an FET switch at the end of the pulse.
• FhS hydrogen sulfide
• Figure 2B illustrates a portion of the results of Figure 2A over expanded output and time scales.
• Figure 3A illustrates recovery of a sensor signal of a sensor for hydrogen sulfide (FhS) after imposition of a l/64 th second +10mV pulse for the case in which a load resister of a predetermined resistance is in series with the working electrode and for the case that the resister is bypassed or short circuited via an FET switch at the end of the pulse.
• FhS hydrogen sulfide
• Figure 3B illustrates a portion of the results of Figure 3A over expanded output and time scales.
• Figure 4A illustrates recovery of a sensor signal of a sensor for carbon monoxide (CO) after imposition of a l/16 th second +10mV pulse for the case in which a load resister of a predetermined resistance is in series with the working electrode and for the case that the resister is bypassed or short circuited via an FET switch at the end of the pulse.
• CO carbon monoxide
• Figure 4B illustrates a portion of the results of Figure 4A over expanded output and time scales.
• Figure 5A illustrates recovery of a sensor signal of a sensor for carbon dioxide after imposition of a l/64 th second +10mV pulse for the case in which a load resister of a predetermined resistance is in series with the working electrode and for the case that the resister is bypassed or short circuited via an FET switch at the end of the pulse.
• Figure 5B illustrates a portion of the results of Figure 5A over expanded time and output scales.
• Figure 6 illustrates the output of a sensor hereof in response to a +10 mV test potential difference wherein an FET switch is activated after the measured peak value of output (MPV), but prior to collection of number of additional data points.
• Figure 7 illustrates the output of a sensor hereof wherein an applied potential is toggled between lower and higher values over two interrogations cycles (a "low” interrogation cycle and a "high” interrogation cycle).
• Figure 8 illustrates the output of a sensor hereof wherein a series of potential step changes is applied to discharge the current, and wherein each consecutive potential step change is of a smaller magnitude and opposite polarity than the previous one.
• Figure 9 illustrates the output of a sensor hereof wherein longer pulses of potential change than applied in Figure 8 are applied so that data from abbreviated decay curves may be collected.
• Figure 10 illustrates the output of a sensor hereof wherein a potential step change or a pulse of sufficiently small magnitude and short duration is applied such that active control of the electronic circuitry of the sensor is not required to dissipate the current spike in less than, for example, 1 second.
• Figure 11 illustrates the output of a sensor hereof wherein a series of potential step changes or pulses of a magnitude and duration as described in connection with Figure 10 are applied, but instead of applying a potential perturbation as a separate interrogation event, a potential waveform is applied across the sensor, and data points are sampled at predetermined intervals within a cycle.
• circuit includes, but is not limited to, hardware, firmware, software or combinations of each to perform a function(s) or an action(s).
• a circuit may include a software controlled microprocessor, discrete logic such as an application specific integrated circuit (ASIC), or other programmed logic device.
• a circuit may also be fully embodied as software.
• controller includes, but is not limited to, any circuit or device that coordinates and controls the operation of one or more input or output devices.
• a controller can include a device having one or more processors, microprocessors, or central processing units (CPUs) capable of being programmed to perform input or output functions.
• processor includes, but is not limited to, one or more of virtually any number of processor systems or stand-alone processors, such as 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 operation of the processor, such as a memory system (for example, 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, etc.
• RAM random access memory
• ROM read-only memory
• PROM programmable read-only memory
• EPROM erasable programmable read only memory
• These support circuits may be internal or external to the processor or its associated electronic packaging.
• the support circuits are in operative communication with the processor.
• the support circuits are not necessarily shown separate from the processor in block diagrams or other drawings.
• devices, systems and/or methods described herein generally allow for a return to a normal mode operation for the electrochemical sensors hereof that is under 10 seconds, under 5 seconds or even under 1 second.
• the devices, systems and methods hereof not only allow an instrument including one or more sensor to remain "online", but also provide for active, automatic sensor status monitoring as a background operation, without the requirement of user initiation.
• the frequency of the interrogations hereof may vary. Providing for sensor interrogation at a frequency of, for example, several times an hour can provide for nearly constant sensor life and health status monitoring.
• the gas to be measured typically passes from the surrounding atmosphere or environment into a sensor housing through a gas porous or gas permeable membrane to a first electrode or working electrode (sometimes called a sensing electrode) where a chemical reaction occurs.
• a complementary chemical reaction occurs at a second electrode known as a counter electrode (or an auxiliary electrode).
• the electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte gas (that is, the gas to be detected) at the working electrode.
• a comprehensive discussion of electrochemical gas sensors is also provided in Cao, Z. and Stetter, J.R., "The Properties and Applications of Amperometric Gas Sensors," Electroanalvsis. 4(3), 253 (1992), the disclosure of which is incorporated herein by reference.
• the working and counter electrode combination produces an electrical signal that is (1) related to the concentration of the analyte gas and (2) sufficiently strong to provide a signal-to-noise ratio suitable to distinguish between concentration levels of the analyte gas over the entire range of interest.
• the current flow between the working electrode and the counter electrode must be measurably proportional to the concentration of the analyte gas over the concentration range of interest.
• an electrochemical sensor In addition to a working electrode and a counter electrode, an electrochemical sensor often includes a third electrode, commonly referred to as a reference electrode.
• a reference electrode is used to maintain the working electrode at a known voltage or potential.
• the reference electrode should be physically and chemically stable in the electrolyte.
• Electrode Electrical connection between the working electrode and the counter electrode is maintained through the electrolyte.
• Functions of the electrolyte include: (l) to efficiently carry the ionic current; (2) to solubilize the analyte gas; (3) to support both the counter and the working electrode reactions; and (4) to form a stable reference potential with the reference electrode.
• Criteria for an electrolyte may, for example, include the following: (1) electrochemical inertness; (2) ionic conductivity; (3) chemical inertness; (4) temperature stability; (5) low cost; (6) low toxicity; (7) low flammability; and (8) appropriate viscosity.
• the electrodes of an electrochemical cell provide a surface at which an oxidation or a reduction (a redox) reaction occurs to provide a mechanism whereby the ionic conduction of the electrolyte solution is coupled with the electron conduction of the electrode to provide a complete circuit for a current.
• the measurable current arising from the cell reactions of the electrochemical cell is directly proportional to the extent of reaction occurring at the electrode.
• the counter electrode and/or the working electrode of the electrochemical cell generally include an appropriate electrocatalyst on the surface thereof to support the reaction rate.
• the volume of solution very close to the working electrode surface is a very highly ordered structure. This structure is important to understanding electrode processes.
• the volume of solution very close to the electrode surface is variously referred to as the diffusion layer, diffuse layer, and or the Helmholtz layer or plane.
• the magnitudes of the resistance and capacitance present in an electrochemical cell are a result of the nature and identities of the materials used in its fabrication.
• the resistance of the electrolyte is a result of the number and types of ions dissolved in the solvent.
• the capacitance of the electrode is primarily a function of the effective surface area of the electrocatalyst. In an ideal world, these quantities are invariant.
• the solution resistance present in an amperometric gas sensor that utilizes an aqueous (water-based) electrolyte may change, for example, as a result of exposure to different ambient relative humidity levels. As water transpires from the sensor, the chemical concentration of the ionic electrolyte increases. This concentration change can lead to increases or decreases in the resistivity of the electrolyte, depending on the actual electrolyte used.
• FIG. 1A illustrates a schematic diagram of a representative embodiment of an electrochemical sensor 10 used in the studies hereof.
• Sensor 10 includes a housing 20 having a gas inlet 30 for entry of one or more target gases or analyte gases into sensor 10.
• electrolyte saturated wick materials 40a, 40b and 40c separate a working electrode 50 from a reference electrode 70 and a counter electrode 80 within sensor 10 and/or provide ionic conduction therebetween via the electrolyte absorbed therein.
• Electronic circuitry 90 as known in the art is provided, for example, to maintain a desired potential difference between working electrode 50 and reference electrode 70, to vary or pulse the potential difference as described herein, and to process an output signal from sensor 10.
• Electronic circuitry 90 may include or be in operative connection with a controller 90a such as a microprocessor to control various aspects of the operation of sensor 10.
• working electrode 50 may be formed by, for example, depositing a first layer of catalyst 54 on a first diffusion membrane 52 (using, for example, catalyst deposition techniques known in the sensor arts).
• Working electrode 50 may be attached (for example, via heat sealing) to an inner surface of a top, cap or lid 22 of housing 20.
• Figure IB illustrates schematically an embodiment of a portion or part of electronic or control circuitry 90 used in a number of studies of the sensors hereof.
• electronic circuitry is sometimes referred to as a potentiostatic circuit.
• a predetermined potential difference or voltage is maintained between reference electrode 70 and sensing or working electrode 50 to control the electrochemical reaction and to deliver an output signal proportional to the current produced by the sensor.
• working electrode 50 responds to the analyte or target gas by either oxidizing or reducing the gas.
• the redox reaction creates a current flow that is proportional to the gas concentration. Current is supplied to sensor 10 through counter electrode 80.
• the measuring circuit for electrical circuitry 90 includes a single stage operational amplifier or op amp IC l .
• the sensor current is reflected across a gain resistor 91 (having a resistance of 5kQ in the illustrated embodiment), generating an output voltage.
• a load resistor 92 (having a resistance of 56 ⁇ in the illustrated embodiment) may be chosen, for example, via a balance between the fastest response time and best signal-to-noise ratio.
• a control operational amplifier IC2 provides the potentiostatic control and provides the current to counter electrode 80 to balance the current required by working electrode 50.
• the inverting input into IC2 is connected to the reference electrode, but does not draw any significant current from the reference electrode.
• a non-faradaic current is induced (for example, via application of energy to working electrode 50).
• a step change in potential may be created which generates a non-faradaic current.
• the generated non-faradaic current can be used to monitor the sensor functionality or health as a result of the charging of the electrodes.
• the sensor should be returned to its normal bias potential or potential range for normal operation in sensing a target or analyte gas.
• the process of returning the sensor to its operating bias or operating potential difference (which may be zero) produces a current peak (a charge build-up) in the opposite direction.
• the current peak arising on return to the operating potential difference can take many of seconds to dissipate.
• the present inventors have discovered that information regarding sensor health or the state of the sensor may be obtained upon application of energy /electrode potential changes that are quite small and/or short in duration, and measuring/analyzing single data points or multiple data points over short time spans in a resultant response/current curve. Moreover, the present inventors have discovered that a rapid discharge of even relatively large current peaks arising when inducing a non-faradaic current in sensor 10 (or another sensor hereof) and/or in returning sensor 10 (or another sensor hereof) to its operating potential difference may be achieved via active control of sensor electronics 90 (for example, by decreasing a load resistance in electronic circuitry 90 between working electrode 50 and the point at which the output/response is measured after the test potential difference has been applied).
• the load resistance between working electrode 50 and the output of operational amplifier ICl is decreased to a low value. Subsequently, the load resistance between working electrode 50 and the output of operational amplifier ICl is restored to its normal or operational load resistance (or to within an operation range of load resistance) after the charge is substantially dissipated or fully dissipated.
• load resistor 92 (see Figure IB) is bypassed to decrease the load resistance between working electrode 50 and the inverting terminal of operational amplifier IC1.
• a bypass circuit 94 may, for example, be provided to bypass load resistor 92.
• a field effect transistor (FET) 94a was used as a switch in a bypass circuit 94 to controllably effect a bypass or short circuit around load resistor 92.
• FET field effect transistor
• MOSFET metal-oxide-semiconductor
• FIGS 2A and 2B illustrate the output of sensor 10 including a working electrode 50 designed to detect hydrogen sulfide or H2S.
• working electrode 50 was formed by depositing an iridium catalyst on a diffusion membrane
• reference electrode 70 was formed by depositing an iridium catalyst on a diffusion membrane
• counter electrode 80 was formed by depositing an iridium catalyst on a diffusion membrane .
• the bias potential or operating potential difference of the sensor was 0 mV.
• FIG. 2A at a time represented by point A, an electronic interrogation procedure is initiated. After 0.5 seconds (represented by point B), a test potential difference is applied. In the illustrated studies, a test potential of +10 mV was applied.
• FIG. 2A illustrates the sensor output when load resistor 92 is bypassed by activation of FET 94a and the sensor output when load resistor 92 is not bypassed. In the case where load resistor 92 is bypassed, FET 94a was activated at generally the same time or contemporaneously with return of the potential to the operating potential difference.
• the significantly lower load resistance causes a significantly greater negative current spike (which would be viewed as a very high negative gas ppm reading in the normal mode of operation) than the case in which load resistor 92 is not bypassed. It was, therefore, surprising that the rapid discharge which occurs upon bypassing load resistor 92 returns the sensor output to the baseline in a very short period of time (that is, in less than 1 second). The contrast with the case in which load resistor 92 is not bypassed is best illustrated in Figure 2B in which the output scale is expanded.
• the output current is below a value that would be discerned by the end user. This value is typically in the range of approximately 0 to ⁇ 2 ppm of the target gas
• Figure 3A illustrates both the sensor output when load resistor 92 is bypassed by activation of FET 94a, and the sensor output when load resistor 92 is not bypassed.
• FIGs 4A and 4B illustrate the output of sensor 10 including a working electrode 50 designed to detect carbon monoxide or CO in response to a +10 mV test potential difference lasting for 1/16 th of a second.
• working electrode 50 was formed by depositing an platinum catalyst on a diffusion membrane
• reference electrode 70 was formed by depositing an platinum catalyst on a diffusion membrane
• counter electrode 80 was formed by depositing an platinum catalyst on a diffusion membrane .
• FIG. 4A illustrates sensor output when load resistor 92 is bypassed by activation of FET 94a and sensor output when load resistor 92 is not bypassed.
• FET 94a was activated at generally the same time or contemporaneously with return of the potential to the operating potential difference.
• the significantly lower load resistance causes a significantly greater negative current spike than is the case when load resistor 92 is not bypassed.
• Figure 6 illustrates another embodiment of a sensor interrogation methodology hereof.
• Figure 6 illustrates the output of a sensor hereof in response to a +10 mV test potential difference lasting for approximately l/16 th of a second.
• FET 94a is applied 3/64* of a second after the measured peak value of output (MPV), but prior to collection of a number of additional data points.
• MPV measured peak value of output
• the potential is increased from OmV to +10MV.
• the measured maximum peak value and a small portion of the positive decay curve are recorded.
• the FET is activated and the potential is returned to OmV. The current is then allowed to decay to near zero.
• This methodology allows more sensor data to be retrieved before FET 94a is activated while reducing the accumulated charge compared to techniques in which an increased potential is applied for, for example, 5-10 seconds.
• the smaller accumulated charge hereof translates into a shorter recovery time, but provides significant information regarding the state/health of the sensor.
• the measured peak value, the slope of the decay curve, the positive area under curve (+AUC) of the abbreviated decay curve, and the negative area under curve (-AUC) of the current discharge during FET activation are among the parameters that may collected. More data may be collected and potentially used for sensor interrogation while shortening the recovery time.
• Figure 7 illustrates another embodiment of a methodology hereof in which an applied potential is toggled between lower and higher values over two interrogations cycles (a "low” interrogation cycle and a "high” interrogation cycle).
• a positive potential step is applied at point B (for example, from 0 mV to +10 mV) during the first of two interrogation cycles.
• Data collected may include the positive measured peak value as well as the nature of the abbreviated decay curve.
• FET 94a is activated at point C. However, the potential is not returned to the original potential at point C, but is maintained at the increased potential initiated at point B. Activating FET 94a quickly discharges the curve from the positive step initiated at point B.
• FET 94a is de-activated at point D, before the sensor/instrument resumes operation at the increased potential established at point B (for example, +10 mV).
• the sensor's operating potential is now at a higher positive from its original potential (0 mV for instance), even after the sensor has returned to normal gas detection operation.
• the sensor performs nominally as long as the original potential is chosen to be within a plateau region wherein small changes in potential do not significantly change sensor performance.
• the potential is toggled back to the original value (for example, 0 mV).
• the resultant negative response is characterized to draw data regarding sensor state/health.
• data collected may include the negative measured peak value as well as the nature of the abbreviated decay curve.
• FET 94a is activated at point F and data is collected in the same manner as described above. However, the current direction is opposite in polarity.
• additional information may, for example, be obtained by comparing the positive and negative responses (MPV+ vs. MPV-, AUC+ vs. AUC-, slope of decay+ vs. slope of decay-).
• a similar toggling approach may be applied to the representative examples described in connection with Figures 2A through 5B.
• the FET may be activated immediately after the MPV is taken and the data available would include the MPV+ and the MPV-.
• Actions other than decreasing resistance may be taken to rapidly discharge current arising from, for example, a change in potential.
• Figure 8 illustrates a representative example in which a series of potential step changes is applied to discharge the current.
• each consecutive potential step change is of a smaller magnitude and opposite polarity than the previous one. This process occurs until the potential steps are so small that the resulting current has returned to approximately zero.
• the current is driven in the opposite direction from the previous step, and the potential may, for example, be changed to the next potential when the current is measured to be approximately zero.
• the potential is altered with very fast pulses and only the MPV's are collected. Both the positive and negative MPV's may be collected. Sensor data may be collected and relationships analyzed. For example, one may analyze all of the MPV+ values and determine changes over time to predict sensor health.
• the system instead of having a series of potential steps with predetermined magnitudes and durations, calculates, in real time, an improved or optimized sequence with the goal of more quickly dissipating the current.
• the information from only the first potential step would typically be used as a predictor of sensor state/health as the first potential step would be the only one guaranteed to be the same between interrogation events.
• the subsequent pulses may be optimized in real time with the sole purpose of rapidly discharging the current. As such, the subsequent potential step pulses will be variable over time, potentially making it difficult to predict sensor performance from responses thereto.
• a potential waveform may be constantly applied across the sensor, and data points may be sampled at predetermined intervals within a cycle (see Figure 11).
• This waveform may, for example, be a step function.
• other waveforms may be used (for example, a sine wave, a triangle wave, etc.).
• This methodology is a variation of Figure 10.
• the magnitudes and duration of the potential steps are of small enough magnitude and short enough duration to allow the current to discharge quickly.
• Data/Information may be collected at predetermined intervals during the potential waveform. For example, MPV's, AUC's and normal (analytical) gas readings may be taken at the same time during each cycle.
• a short circuit was created via a FET to quickly dissipate charge.
• one may, at the same time as activation of a FET or separately from activation of a FET/switch, apply a pulse in the opposite direction of a determined magnitude to pull off the original charge that was applied minus any loss of charge.
• the amount of charge can be readily determined by a person skilled in the electrical arts given the proper context. This result may also be accomplished with a current pulse rather than a voltage pulse.
• Sc is the corrected sensitivity of the sensor
• Ro and So were the initial values of response function and sensitivity, respectively
• Ri and Si were the response function and sensitivity, respectively, at any point in time during the experiment, and a was an adjustable parameter.
• the form of this equation is not unique; other correction functions may be used as well.
• the application of this correction factor to the experimental data brought the indicated response of the instrument back into the specified range over the entire course of the experiment, thereby eliminating the need to recalibrate the sensor against a known standard calibration gas.

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