Abstract:
An electrochemical sensor system for monitoring emissions includes locating first and second electrodes in a position to sense the emissions. At least one of the first and second electrodes is made of a dense electrode material. An ion-conductor material that acts as an electrolyte is operatively connected to the first and second electrodes. The first electrode is excited at a frequency f 1 , A response is received from the first electrode at frequency f 1 . A second signal is received base on the emissions and a response is produced indicating the emissions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/138,735 filed Dec. 18, 2008 and titled “Frequency Technique for Operating Electrochemical Sensors” which is incorporated herein by this reference. 
         [0002]    This application is a Continuation-in-Part of U.S. patent application Ser. No. 11/893,751 filed Aug. 16, 2006 and titled “Multiple Frequency Method for Operating Electrochemical Sensors” which is incorporated herein by this reference. 
     
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0003]    The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory. 
     
    
     BACKGROUND 
       [0004]    1. Field of Endeavor 
         [0005]    The present invention relates to electrochemical sensors and more particularly to a multiple frequency method for operating electrochemical sensors. 
         [0006]    2. State of Technology 
         [0007]    The article, “Impedancemetric NOx Sensing Using YSZ Electrolyte and YSZ/Cr2O3 Composite Electrodes,” by L. Peter Martin, Leta Y. Woo, and Robert S. Glass, in  Journal of The Electrochemical Society,  154 (3) J97-J104 (2007), provides the following state of technology information, “Increasingly stringent emissions regulations require the development of advanced gas sensors for a variety of applications. For example, compact, inexpensive sensors are needed for detection of regulated pollutants, including hydrocarbons (HC), CO, and NOx, in automotive exhaust. Because many emerging applications, particularly monitoring of automotive exhaust, involve operation in harsh environments, which can include high temperature and corrosive or chemically reactive conditions, ceramic oxide-based electrochemical sensors have received considerable interest.” The article, “Impedancemetric NOx Sensing Using YSZ Electrolyte and YSZ/Cr2O3 Composite Electrodes,” is incorporated in this application in its entirety for all purposes. Additional information about the sensor systems is provided in the following four articles: (1) “Effect of electrode composition and microstructure on impedancemetric nitric oxide sensors based on YSZ electrolyte,” by L. Y. Woo, L. P. Martin, R. S. Glass, W. Wang, S. Jung, R. J. Gorte, E. P. Murray, R. F. Novak, and J. H. Visser, in J. Electrochem. Soc., 155(1):J32-40, (2008); (2) “Impedance characterization of a model Au/yttria-stabilized zirconia/Au electrochemical cell in varying oxygen and NOx concentrations,” by L. Y. Woo, L. P. Martin, R. S. Glass, and R. J. Gorte, in J. Electrochem. Soc., 154(4):J129-135 (2007); (3) “Development of NOx Sensing Devices Based on YSZ and Oxide Electrode Aiming for Monitoring Car Exhausts,” by N. Miura, M. Nakatou, S. Zhuiykov, Ceramics International, 30, pp. 1135-1139 (2004); and (4) “Impedancemetric gas sensor based on zirconia solid electrolyte and oxide sensing electrode for detecting total NOx at high temperature,” by N. Miura, M. Nakatou, and S. Zhuiykov, in Sensors and Actuators B, 93:221-228 (2003). The four articles are incorporated in this application in their entirety for all purposes. 
         [0008]    U.S. Pat. No. 6,551,149 issued Apr. 23, 2003 to Yunzhi Gao et al for measuring NOx concentration provides the following state of technology information: “Emissions of NOx from internal combustion engines used mainly in automotive vehicles and from the combustion equipment of thermal power stations and plants are a cause of photochemical smog and acid rain, are harmful to the human respiratory system and represent a major source of global environmental pollution. For these reasons the detection of noxious gases such as NOx is a major concern and a gas sensor that contributes to a reduction in the size and cost of measurement equipment and that is usable in a variety of environments has been sought. In recent years much attention has been focused on all solid-state NOx sensors inserted directly into the exhaust gas of an automotive vehicle to sense the gases continuously, and results of related research have been reported.” 
       SUMMARY 
       [0009]    Features and advantages of the present invention will become apparent from the following description. Applicants are providing this description, which includes drawings and examples of specific embodiments, to give a broad representation of the invention. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description and by practice of the invention. The scope of the invention is not intended to be limited to the particular forms disclosed and the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims. 
         [0010]    The present invention provides an electrochemical sensor system for monitoring emissions that includes locating first and second electrodes in a position to sense the emissions. At least one of the first and second electrodes is made of a dense electrode material. An ion-conductor material that acts as an electrolyte is operatively connected to the first and second electrodes. The first electrode is excited at a frequency f 1 , A response is received from the first electrode at frequency f 1 . A second signal is received base on the emissions and a response is produced indicating the emissions. 
         [0011]    The present invention provides a multiple frequency method for the operation of a sensor. The present invention is a multiple frequency method for the operation of a sensor to measure a parameter of interest using calibration information wherein interfering parameters may be present. The method includes the steps of exciting the sensor at a first frequency providing a first sensor response, exciting the sensor at a second frequency providing a second sensor response, using the second sensor response at the second frequency and the calibration information to produce a calculated concentration of the interfering parameters, using the first sensor response at the first frequency, the calculated concentration of the interfering parameters, and the calibration information to measure the parameter of interest. The method has advantages over more traditional potentiometric (open circuit) or amperometric (dc-biased) sensors. 
         [0012]    In one embodiment the present invention utilizes an alternating current (ac) signal across the electrodes of an electrochemical cell, and measurement of the impedance characteristics associated with the cell at the frequency of the ac signal, in particular the phase difference between the excitation signal and the sensor response at the excitation frequency. Multiple frequencies may be used, simultaneously or sequentially, to provide real-time compensation for aging, interfering species, and environmental variations (i.e., temperature). Another embodiment of the present invention is focused on sensing NOx gas in high temperature automotive exhaust gas using a solid state cell composed of a ceramic electrolyte and electrodes. 
         [0013]    The present invention is not specific to any particular electrolyte or electrode materials, or to any particular species being sensed. The sensing methodology should be broadly applicable to the use of electrochemical cells for detecting species of interest. It does appear that the physical mechanisms resulting in the sensor response to the analyte of interest and to any interfering species or effect must be sufficiently different as to cause them to have different frequency dependencies. 
         [0014]    The present invention has many uses. For example, the present invention can be used for the detection of pollutant gasses in a hot, flowing gas stream. Applications include the monitoring of industrial exhaust gasses and vehicle emissions. Broader applications include any application where electrochemical sensors are of interest. 
         [0015]    The invention is susceptible to modifications and alternative forms. Specific embodiments are shown by way of example. It is to be understood that the invention is not limited to the particular forms disclosed. The invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]    The accompanying drawings, which are incorporated into and constitute a part of the specification, illustrate specific embodiments of the invention and, together with the general description of the invention given above, and the detailed description of the specific embodiments, serve to explain the principles of the invention. 
           [0017]      FIG. 1  illustrates an embodiment of the multiple frequency method for the operation of a sensor of the present invention. 
           [0018]      FIG. 2  shows additional details of steps of the multiple frequency method for the operation of a sensor of the present invention. 
           [0019]      FIG. 3  shows additional details of steps of the multiple frequency method for the operation of a sensor of the present invention. 
           [0020]      FIG. 4  illustrates the final step of the multiple frequency method for the operation of a sensor of the present invention. 
           [0021]      FIG. 5  illustrates another embodiment of the multiple frequency method for the operation of a sensor of the present invention to detect NO in a varying O 2  background. 
           [0022]      FIG. 6  illustrates an alternate embodiment of the invention where frequencies f 1  and f 2  are excited simultaneously. 
           [0023]      FIG. 7  illustrates schematically the process of determining the NO concentration by using the multiple frequency technique to correct for an unknown O 2  concentration. 
           [0024]      FIG. 8  illustrates another embodiment of the multiple frequency method for the operation of a sensor of the present invention to detect CO in a varying O 2  background. 
           [0025]      FIG. 9  illustrates yet another embodiment of the multiple frequency method for the operation of a sensor of the present invention to detect NO in a varying temperature environment. 
           [0026]      FIG. 10  illustrates the operation of a H 2  sensor trying to measure the concentration of H 2  in a background with varying H 2 O concentration. 
           [0027]      FIG. 11  illustrates the operation of a NO 2  sensor trying to measure the concentration of NO 2  in a background with varying O 2  concentration. 
           [0028]      FIG. 12  illustrates a sensing method for measuring NO in a background with varying O 2  concentration. 
           [0029]      FIG. 13  illustrates a sensing method for measuring CO in a background with varying O 2  concentration. 
           [0030]      FIG. 14  illustrates a sensing method for measuring NO in a background with varying temperature. 
           [0031]      FIG. 15  illustrates a sensing method for measuring NO 2  in a background with varying O 2  concentration. 
           [0032]      FIG. 16  illustrates an embodiment of a sensor system for monitoring emissions. 
           [0033]      FIG. 17  illustrates operation of the sensor system for monitoring emissions. 
           [0034]      FIG. 18  illustrates another embodiment of a sensor system for monitoring emissions. 
           [0035]      FIG. 19  illustrates yet another embodiment of a sensor system for monitoring emissions. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    Referring to the drawings, to the following detailed description, and to incorporated materials, detailed information about the invention is provided including the description of specific embodiments. The detailed description serves to explain the principles of the invention. The invention is susceptible to modifications and alternative forms. The invention is not limited to the particular forms disclosed. The invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims. 
         [0037]    The present invention provides a mode of operation of sensors, particularly electrochemical sensors. The method includes the steps of exciting the sensor at a first frequency providing a first sensor response, exciting the sensor at a second frequency providing a second sensor response, using the second sensor response at the second frequency and the calibration information to produce a calculated concentration of the interfering parameters, using the first sensor response at the first frequency, the calculated concentration of the interfering parameters, and the calibration information to measure the parameter of interest. Additional information about the present invention is included in the article, “Impedancemetric NOx Sensing Using YSZ Electrolyte and YSZ/Cr2O3 Composite Electrodes,” by L. Peter Martin, Leta Y. Woo, and Robert S. Glass, in  Journal of The Electrochemical Society,  154 (3) J97-J104 (2007). The article, “Impedancemetric NOx Sensing Using YSZ Electrolyte and YSZ/Cr2O3 Composite Electrodes,” by L. Peter Martin, Leta Y. Woo, and Robert S. Glass, in  Journal of The Electrochemical Society,  154 (3) J97-J104 (2007) is incorporated in this application by this reference. 
         [0038]    One embodiment of the present invention provides solid (ceramic oxide based) sensors for detection of small concentrations (ppm levels) of pollutant gasses in automotive exhaust. However, it is to be understood that applications for the invention may be significantly broader, and this application is not limited to gas sensors, or solid oxide sensors. 
         [0039]    Another embodiment of the present invention provides an electrochemical cell consisting of an electrolyte and two (or possibly more) electrodes. The sensor is operated by applying an excitation signal which consists of a varying (typically sinusoidal) voltage difference between the two electrodes. The excitation signal consists of a fixed frequency, for example 10 Hz. A phase meter, phase lock loop, or other electronic measuring circuit is used to measure the changes in amplitude and phase of the excitation signal, after it interacts with the sensor, relative to a fixed, reference signal of the same frequency. The sensor response, which can be correlated with the impedance |Z| or the phase, is sensitive to the changes which the sensor is trying to detect (for example the NO x  concentration in an exhaust gas) as well as to changes in some interfering species or effect (O 2  or temperature). To correct for the uncertainty introduced by the unknown effect of the interference on the sensor response, the sensor is excited at a second frequency, for example 1000 Hz, where the response is only sensitive to the interfering species or effect. Comparison of the two responses, in conjunction with the appropriate calibration information, allows calculation of the concentration of the species of interest (i.e., NO) and of the interfering species. 
         [0040]    One aspect of the present invention is the operation at non-zero (ac) frequency. Electrochemical gas sensors are traditionally operated either passively (no excitation at all) or using a zero-frequency (dc) excitation. Operating at non-zero frequency provides several advantages over the traditional dc modes of operation. There is more ‘information’ in the ac response because it is, by definition, a dynamic (non-steady state) response and therefore contains not only amplitude information, but also some measure of the time-dependence of the response. Also, the frequency determines response and sampling times (with 1/frequency representing a general limitation for the sampling rate). Thus, it is desirable to operate at the highest frequency at which sufficient sensitivity can be obtained. Additionally, the sensor response to different species (say O 2  and NO) often can be distinguished by virtue of the differences in the frequency dependence of the responses to the different species. This is not possible using the traditional dc approach. This provides a third point of novelty of the proposed sensor . . . that the sensor can be simultaneously operated at two (or more) widely different frequencies to provide a compensation for these interfering effects. That is, for example, at 10 Hz the sensor senses both changes in the concentrations of NOx and O2, while at 1000 Hz it senses only the changes in O 2 . Thus, by comparing these signals the competing effects of variations of several percent in the O 2  background can be deconvolved from the effects of ppm changes in the NO x  concentration. 
         [0041]    One embodiment of the present invention includes the use of an ac excitation for the sensor at frequencies above ˜1 Hz. In particular, the use of the phase response of the sensor as the metric which is correlated with the gas composition. Another embodiment of the present invention includes the use of multiple frequencies to compensate for interfering gasses and or environmental variations. As an example, the target application of the ongoing project is to detect 2-25 ppm NO in a background of 5-20% O 2 . At low frequencies, &lt;40 Hz, the sensors we have fabricated are sensitive to both the NO and O 2  concentrations. However, at higher frequencies, &gt;500 Hz, the sensor is only sensitive to O 2 . By measuring at both frequencies, we can compensate the effect of large variations (several %) in the oxygen concentration in a way that allows us to clearly resolve changes in the NO concentrations on the ppm level. 
       Method for the Operation of a Sensor 
       [0042]    Referring now to the drawings and in particular to  FIG. 1 , an embodiment of the multiple frequency method for the operation of a sensor of the present invention is illustrated. This embodiment of the multiple frequency method for the operation of a sensor of the present invention is designated generally by the reference numeral  100 . The method  100  is a multiple frequency method for the operation of a sensor to measure a parameter of interest using calibration information, wherein interfering parameters may be present. The method  100  includes the steps of exciting the sensor at a first frequency providing a first sensor response, exciting the sensor at a second frequency providing a second sensor response, using the second sensor response at the second frequency and the calibration information to produce a calculated concentration of the interfering parameters, using the first sensor response at the first frequency, the calculated concentration of the interfering parameters, and the calibration information to measure the parameter of interest. 
         [0043]    The method  100  is an embodiment of the multiple frequency method for the operation of a general sensor trying to measure 1 species of interest with correction for 1 interfering species. The actual sensing element is designed so that the relative sensitivities to the two species are different at two different frequencies—f 1  and f 2 . Step  101  is to generate a calibration curve for sensor response to species of interest at frequency f 1 . Step  102  is to generate a calibration curve for sensor response to interfering species at frequency f 1 . Step  103  is to generate a calibration curve for sensor response to interfering species at frequency f 2 . Steps  101 ,  102 , and  103  are performed once, prior to sensor operation. 
         [0044]    Step  104  is to excite the sensor at frequency f 1 . Response at f 1  contains contributions from both species of interest and interfering species. Step  105  is to measure sensor response at frequency f 1 . Step  106  is to excite the sensor at frequency f 2 . Step  107  is to measure sensor response at frequency f 2 . Response at f 2  contains contributions only from interfering species. 
         [0045]    Step  108  is to use sensor response at frequency f 2  and calibration curves to calculate concentration of interfering species. In Step  109  concentration of ‘interfering species’ may also be considered an ‘output.’ 
         [0046]    Step  110  is to use sensor response at frequency f 1 , calculated concentration of interfering species and calibration curves to calculate concentration of species of interest. Step  111  provides output concentration of species of interest. 
         [0047]    The method  100  illustrated in  FIG. 1  is a computer implemented multiple frequency method for the operation of a sensor to measure a parameter of interest using calibration curves, wherein interfering parameters may be present. The method  100  illustrated in  FIG. 1  includes the step  104  of exciting the sensor at a first frequency providing a first sensor response recorded on a computer-readable medium, the step  106  exciting the sensor at a second frequency providing a second sensor response recorded on a computer-readable medium, the step  108  using said second sensor response at said second frequency and the calibration information to produce a calculated concentration of the interfering parameters recorded on a computer-readable medium, and the step  110  using said first sensor response at said first frequency, said calculated concentration of the interfering parameters, and the calibration information to measure the parameter of interest. 
         [0048]    Referring now to  FIG. 2 , additional details of Steps  104  and  105  are illustrated. The method  100  includes the step  104  of exciting the sensor at a first frequency providing a first sensor response recorded on a computer-readable medium and step  105  of measure sensor response at frequency f 1  and recorded it on a computer-readable medium. Step  104  is to excite the sensor at frequency f 1 . This is accomplished by the step  200  generate ac excitation at frequency=f 1 . Step  105  is to measure sensor response at frequency f 1 . Step  201  includes convert sensor response at frequency=f 1  to output signal. This is illustrated by the plot of “Sensor Response” vs. “Concentration.” This produces the lines  202  “Total response =signal of interest+interference,” the line  203  “Interference,” and line  204  “Signal of interest.” Response is strong, but sensitivity to interference is high. Interference could come from any source that affects sensor output (concentration of other gasses, temperature, etc.). 
         [0049]    Referring now to  FIG. 3 , additional details of Steps  106  and  107  are illustrated. Step  106  is to excite the sensor at frequency f 2 . This includes step  300  generate ac excitation at frequency=f 2 . Step  107  is to measure sensor response at frequency f 2 . Response is negligible, but sensitivity to interference is still high. Response at f 2  contains contributions only from interfering species. The method  100  includes the step  106  exciting the sensor at a second frequency providing a second sensor response recorded on a computer-readable medium. Step  107  is to measure sensor response at frequency f 2  and record it on a computer-readable medium. This includes step  301  convert sensor response at frequency=f 2  to output signal. This is illustrated by the plot of “Sensor Response” vs. “Concentration.” This produces the line  301  “Total response interference only.” 
         [0050]    Referring now to  FIG. 4 , the final step of using the first sensor response at the first frequency, the calculated concentration of the interfering parameters, and the calibration information to measure the parameter of interest is illustrated. This step is to calculate signal of interest from the responses at the two frequencies. The response at f 1  yields signal and interference data and the response at f 2  yields only interference data. A comparison yields the desired signal. 
       Method for the Operation of a NO Sensor in a Background of Varying O 2    
       [0051]    Referring now to the drawings and in particular to  FIG. 5 , an embodiment of the multiple frequency method for the operation of a NO sensor of the present invention trying to measure the concentration of NO in a background with varying O 2  concentration is illustrated. The actual sensing element must be designed so that the relative sensitivities to NO and O 2  are different at two different frequencies—f 1  and f 2 . This embodiment of the multiple frequency method for the operation of a NO sensor of the present invention is designated generally by the reference numeral  500 . The method  500  is a multiple frequency method for the operation of a NO sensor to measure a parameter of interest using calibration information, wherein interfering Varying O 2  may be present. 
         [0052]    The method  500  includes the steps of exciting the NO sensor at a first frequency providing a first NO sensor response to both NO and O 2 , exciting the NO sensor at a second frequency providing a second sensor response to only O 2 , using the second sensor response at the second frequency and the calibration information to produce a calculated concentration of O 2 , using the first NO sensor response at the first frequency, the calculated concentration of O 2 , and the calibration information to calculate the NO concentration. 
         [0053]    The method  500  is an embodiment of the multiple frequency method for the operation of a NO sensor trying to measure the concentration of NO in a background with varying O 2  concentration. The actual sensing element must be designed so that the relative sensitivities to NO and O 2  are different at two different frequencies—f 1  and f 2 . Step  501  is to generate a calibration curve for sensor response to NO at frequency f 1 . Step  502  is to generate a calibration curve for sensor response to O 2  at frequency f 1 . Step  503  is to generate a calibration curve for sensor response to O 2  at frequency f 2 . Steps  501 ,  502 , and  503  are performed once, prior to NO sensor operation a (i.e., prior to placing the sensor ‘in service’). 
         [0054]    Step  504  is to excite the NO sensor at frequency f 1 . Response at f 1  contains contributions from both species of interest and interfering species (NO and O 2 ). Step  505  is to measure NO sensor response at frequency f 1 . Step  506  is to excite the NO sensor at frequency f 2 . Step  507  is to measure NO sensor response at frequency f 2 . Response at f 2  contains contributions only from the interfering species O 2 . 
         [0055]    Step  508  is to use NO sensor response at frequency f 2  and calibration curves to calculate concentration of interfering species O 2 . In Step  509  concentration of ‘interfering species O 2 ’ may also be considered an ‘output.’ 
         [0056]    Step  510  is to use NO sensor response at frequency f 1 , calculated concentration of interfering species O 2  and calibration curves to calculate concentration of species of interest NO. Step  511  provides output concentration of species of interest NO. 
         [0057]    The method  500  illustrated in  FIG. 5  is a computer implemented multiple frequency method for the operation of a NO sensor to measure a parameter of interest using calibration curves, wherein interfering parameters may be present. The method  500  illustrated in  FIG. 5  includes the step  504  of exciting the NO sensor at a first frequency providing a first NO sensor response from both species of interest and interfering species (NO and O 2 ) recorded on a computer-readable medium, the step  506  exciting the NO sensor at a second frequency providing a second NO sensor response to O 2  recorded on a computer-readable medium, the step  508  using said second NO sensor response at said second frequency for the interfering species (O 2 ) and the calibration information to produce a calculated concentration of the interfering parameter O 2  recorded on a computer-readable medium, and the step  510  using said first NO sensor response at said first frequency, said calculated concentration of the interfering parameters O 2 , and the calibration information to calculate the parameter of interest NO. 
       Example—NO Sensor in a Background of Varying O 2    
       [0058]    An example of the multiple frequency method for the operation of a NO sensor of the present invention trying to measure the concentration of NO in a background with varying O 2  concentration is provided to further explain the principles of the invention. As illustrated in  FIG. 5 , a calibration curve for sensor response to NO at frequency f 1  is produced to provide calibration information. A calibration curve for sensor response to O 2  at frequency f 1  is produced to provide calibration information. A calibration curve for sensor response to O 2  at frequency f 2  is produced to provide calibration information. 
         [0059]    In Step  504  the NO sensor was excited at frequency f 1  of 10 Hz. The response at f 1  contains contributions from both NO and O 2 . 
         [0060]    In Step  505  the NO sensor phase response at frequency f 1  was measured as −40.5 degrees. 
         [0061]    In Step  506  the NO sensor was excited at frequency f 2  of 1000 Hz. 
         [0062]    In Step  507  the NO sensor response at frequency f 2  was measured as −32.3 degrees. Response at f 2  contains contributions only from O 2 . 
         [0063]    In Step  508  the NO sensor response at frequency f 2  and calibration curves were used to calculate concentration of interfering species O 2  as 7.0%. 
         [0064]    In Step  510  the NO sensor response at frequency f 1 , calculated concentration of interfering species O 2 , and calibration curves were used to calculate concentration of species of interest NO as 15 ppm. 
         [0065]    The concentration of species of interest NO 15 ppm is the output as shown in Step  511 . 
         [0066]    Note: in Step  509  the concentration of “interfering species O 2  7.0%” may also be considered an “output.” 
         [0067]    Referring now to  FIG. 6 , an alternate embodiment of the invention is illustrated wherein frequencies f 1  and f 2  are excited simultaneously. The alternate embodiment is designated generally the reference numeral  600 . The method  600  includes the following steps. In step  601  excitation signals at frequencies f 1  and f 2  are generated. In step  602 , the excitation signals are electronically mixed (added) together to produce a single excitation signal with two frequency components at f 1  and f 2 . In step  603  the sensor is excited by the combined signal. In step  604 , the sensor response is electronically separated into the frequency components at f 1  and f 2 . In step  605 , measurement electronics are used to measure the sensor response at f 1  and f 2 . These responses are then passed (step  606 ) to a computer for further analysis. 
         [0068]    Referring now to  FIG. 7 , the process of determining the NO concentration by using the multiple frequency technique to correct for an unknown O 2  concentration is illustrated. The process is designated generally by the reference numeral  700 . Note that in this example the sensor has been configured so that the response at f 1  is sensitive to both NO and O 2  concentrations, while the response at f 2  is sensitive only to the O 2  concentration. The process  700  begins with the measured sensor response at f 1  and f 2 . These responses may be measured via simultaneous excitation at the two frequencies, as described in  FIG. 6 , or via consecutive excitations at each frequency (i.e., f 1  then f 2 ). The process of determining the NO concentration in  FIG. 7  consists of the following steps. In step  701  the measured sensor response at f 2  is read from the computer. In step  702 , a predetermined calibration curve is used to determine the O 2  concentration from the measured sensor response at f 2 . In step  703  the O 2  concentration determined from the f 2  response is used to predict the portion of the sensor response at fl that corresponds only to the contribution of the O 2  concentration. This is accomplished using a 2 nd , predetermined calibration curve. In step  704  the portion of the sensor response at f 1  due to the NO concentration is determined from the total measured response and the O 2  response calculated in step  703 . In step  705  a 3 rd  predetermined calibration curve is used to determine the NO concentration from the NO response determined in step  704 . Finally, in step  706  the NO concentration is output from the measurement system. 
       Method for the Operation of a CO Sensor in a Background Varying O 2    
       [0069]    Referring now to the drawings and in particular to  FIG. 8 , an embodiment of the multiple frequency method for the operation of a CO sensor of the present invention trying to measure the concentration of CO in a background with varying O 2  concentration is illustrated. The actual sensing element must be designed so that the relative sensitivities to CO and O 2  are different at two different frequencies—f 1  and f 2 . This embodiment of the multiple frequency method for the operation of a CO sensor of the present invention is designated generally by the reference numeral  800 . The method  800  is a multiple frequency method for the operation of a CO sensor to measure a parameter of interest using calibration information, wherein interfering varying O 2  may be present. 
         [0070]    The method  800  includes the steps of exciting the CO sensor at a first frequency providing a first CO sensor response at f 1  that contains contributions from both CO and O 2 , exciting the CO sensor at a second frequency providing a second sensor response at f 2  that contains contributions only from O 2 , using the first sensor response of both CO and O 2  at f 1  and the calibration information to produce a calculated concentration of O 2 , using the first CO sensor response at the first frequency, the calculated concentration of O 2 , and the calibration information to measure CO. 
         [0071]    Step  801  is to generate a calibration curve for sensor response to CO at frequency f 1 . Step  802  is to generate a calibration curve for sensor response to O 2  at frequency f 1 . Step  803  is to generate a calibration curve for sensor response to O 2  at frequency f 2 . Steps  801 ,  802 , and  803  are performed once, prior to CO sensor operation. 
         [0072]    Step  804  is to excite the CO sensor at frequency f 1 . Response at f 1  contains contributions from both CO and O 2 . Step  805  is to measure CO sensor response at frequency f 1 . Step  806  is to excite the CO sensor at frequency f 2 . 
         [0073]    Step  807  is to measure CO sensor response at frequency f 2 . Response at f 2  contains contributions only from O 2 . Step  808  is to use CO sensor response at frequency f 2  and calibration curves to calculate concentration of O 2 . In Step  809  concentration of ‘O 2 ’ may be considered an ‘output.’ 
         [0074]    Step  810  is to use CO sensor response at frequency f 1 , calculated concentration of O 2  and calibration information to calculate concentration of CO. Step  811  provides output concentration of species of interest CO. 
         [0075]    The method  800  illustrated in  FIG. 8  is a computer implemented multiple frequency method for the operation of a CO sensor to measure a parameter of interest using calibration curves, wherein interfering parameters may be present. The method  800  illustrated in  FIG. 8  includes the step  804  of exciting the CO sensor at a first frequency providing a first CO sensor response from both species of interest and interfering species (CO and O 2 ) recorded on a computer-readable medium, the step  806  exciting the CO sensor at a second frequency providing a second CO sensor response recorded on a computer-readable medium, the step  808  using said second CO sensor response at said second frequency from both species of interest and interfering species (CO and O 2 ) and the calibration information to produce a calculated concentration of the interfering parameters O 2  recorded on a computer-readable medium, and the step  810  using said first CO sensor response at said first frequency, said calculated concentration of the interfering parameters O 2 , and the calibration information to measure the parameter of interest CO. 
       Method for the Operation of a NO Sensor with Uncertain Temperature 
       [0076]    Referring now to the drawings and in particular to  FIG. 9 , an embodiment of the multiple frequency method for the operation of a NO sensor of the present invention trying to measure the concentration of NO where the temperature is uncertain is illustrated. The actual sensing element must be designed so that the relative sensitivities to NO and temperature are different at two different frequencies—f 1  and f 2 . This embodiment of the multiple frequency method for the operation of a NO sensor of the present invention is designated generally by the reference numeral  900 . The method  900  is a multiple frequency method for the operation of a NO sensor to measure a parameter of interest using calibration information, wherein interfering varying temperature may occur. 
         [0077]    The method  900  includes the steps of exciting the NO sensor at a first frequency providing a first NO sensor response at f 1  that contains contributions from both NO and temperature, exciting the NO sensor at a second frequency providing a second sensor response at f 2  that contains contributions only from temperature, using the first sensor response of both NO and temperature at f 1  and the calibration information to produce a calculated temperature, using the first NO sensor response at the first frequency, the calculated temperature, and the calibration information to measure NO. 
         [0078]    The method  900  is an embodiment of the multiple frequency method for the operation of a NO sensor trying to measure the concentration of NO with uncertain temperature. The actual sensing element must be designed so that the relative sensitivities to NO and temperature are different at two different frequencies—f 1  and f 2 . Step  901  is to generate a calibration curve for sensor response to NO at frequency f 1 . Step  902  is to generate a calibration curve for sensor response to temperature at frequency f 1 . Step  903  is to generate a calibration curve for sensor response to temperature at frequency f 2 . Steps  901 ,  902 , and  903  are performed once, prior to NO sensor operation. 
         [0079]    Step  904  is to excite the NO sensor at frequency f 1 . Response at f 1  contains contributions from both NO and temperature. Step  905  is to measure NO sensor response at frequency f 1 . Step  906  is to excite the NO sensor at frequency f 2 . 
         [0080]    Step  907  is to measure NO sensor response at frequency f 2 . Response at f 2  contains contributions only from temperature 
         [0081]    Step  908  is to use NO sensor response at frequency f 2  and calibration curves to calculate temperature. In Step  909  “temperature” may be considered an “output.” 
         [0082]    Step  910  is to use NO sensor response at frequency f 1 , calculated temperature and calibration information to calculate concentration of NO. Step  911  provides output concentration of species of interest NO. 
         [0083]    The method  900  illustrated in  FIG. 9  is a computer implemented multiple frequency method for the operation of a NO sensor to measure a parameter of interest using calibration curves, wherein interfering parameters may be present. The method  900  illustrated in  FIG. 9  includes the step  904  of exciting the NO sensor at a first frequency providing a first NO sensor response from both species of interest and interfering species (NO and temperature) recorded on a computer-readable medium, the step  906  exciting the NO sensor at a second frequency providing a second NO sensor response recorded on a computer-readable medium, the step  908  using said second NO sensor response at said second frequency from both species of interest and interfering species (NO and temperature) and the calibration information to produce a calculated temperature recorded on a computer-readable medium, and the step  910  using said first NO sensor response at said first frequency, said calculated temperature, and the calibration information to measure the parameter of interest NO. 
       Method for the Operation of a H 2  Sensor in a Background Varying H 2 O 
       [0084]    Referring now to the drawings and in particular to  FIG. 10 , an embodiment of the multiple frequency method for the operation of a H 2  sensor of the present invention trying to measure the concentration of H 2  in a background with varying H 2 O concentration is illustrated. The actual sensing element must be designed so that the relative sensitivities to H 2  and H 2 O are different at two different frequencies—f1 and f 2 . This embodiment of the multiple frequency method for the operation of a H 2  sensor of the present invention is designated generally by the reference numeral  1000 . The method  1000  is a multiple frequency method for the operation of a H 2  sensor to measure the H 2  concentration using calibration information, wherein varying H 2 O concentration may interfere with the sensor response. 
         [0085]    The method  1000  includes the steps of exciting the H 2  sensor at a first frequency providing a first H 2  sensor response to both H 2  and H 2 O concentrations, exciting the H 2  sensor at a second frequency providing a second sensor response to H 2 O, using the second sensor response at the second frequency and the calibration information to produce a calculated concentration of H 2 O, using the first H 2  sensor response at the first frequency, the calculated concentration of H 2 O, and the calibration information to calculate the H 2  concentration. 
         [0086]    The method  1000  is an embodiment of the multiple frequency method for the operation of a H 2  sensor trying to measure the concentration of H 2  in a background with varying H 2 O concentration. The actual sensing element must be designed so that the relative sensitivities to H 2  and H 2 O are different at two different frequencies—f 1  and f 2 . Step  1001  is to generate a calibration curve for sensor response to H 2  at frequency f 1 . Step  1002  is to generate a calibration curve for sensor response to H 2 O at frequency f 1 . Step  1003  is to generate a calibration curve for sensor response to H 2 O at frequency f 2 . Steps  1001 ,  1002 , and  1003  are performed once, prior to H 2  sensor operation. 
         [0087]    Step  1004  is to excite the H 2  sensor at frequency f 1 . Response at f 1  contains contributions from both species of interest and interfering species (H 2  and H 2 O). Step  1005  is to measure H 2  sensor response at frequency f 1 . Step  1006  is to excite the H 2  sensor at frequency f 2 . Step  1007  is to measure H 2  sensor response at frequency f 2 . Response at f 2  contains contributions only from the interfering species H 2 O. 
         [0088]    Step  1008  is to use H 2  sensor response at frequency f 2 and calibration curves to calculate concentration of interfering species H 2 O. In Step  1009  concentration of ‘interfering species H 2 O’ may also be considered an ‘output.’ 
         [0089]    Step  1010  is to use H 2  sensor response at frequency f 1 , calculated concentration of interfering species H 2 O and calibration curves to calculate concentration of species of interest H 2 . Step  1011  provides output concentration of species of interest H 2 . 
         [0090]    The method  1000  illustrated in  FIG. 10  is a computer implemented multiple frequency method for the operation of a H 2  sensor to measure a parameter of interest using calibration curves, wherein interfering parameters may be present. The method  1000  illustrated in  FIG. 10  includes the step  1004  of exciting the H 2  sensor at a first frequency providing a first H 2  sensor response from both species of interest and interfering species (H 2  and H 2 O) recorded on a computer-readable medium, the step  1006  exciting the H 2  sensor at a second frequency providing a second H 2  sensor response recorded on a computer-readable medium, the step  1008  using said second H 2  sensor response at said second frequency from both species of interest and interfering species (H 2  and H 2 O) and the calibration information to produce a calculated concentration of the interfering parameters H 2 O recorded on a computer-readable medium, and the step  1010  using said first H 2  sensor response at said first frequency, said calculated concentration of the interfering parameters H 2 O, and the calibration information to measure the parameter of interest H 2 . 
       Method for the Operation of a NO 2  Sensor in a Background Varying O 2    
       [0091]    Referring now to the drawings and in particular to  FIG. 11 , an embodiment of the multiple frequency method for the operation of a NO 2  sensor of the present invention trying to measure the concentration of NO 2  in a background with varying O 2  concentration is illustrated. The actual sensing element must be designed so that the relative sensitivities to NO 2  and O 2  are different at two different frequencies—f 1  and f 2 . This embodiment of the multiple frequency method for the operation of a NO 2  sensor of the present invention is designated generally by the reference numeral  1100 . The method  1100  is a multiple frequency method for the operation of a NO 2  sensor to measure a parameter of interest using calibration information, wherein interfering varying O 2  concentration may be present. 
         [0092]    The method  1100  includes the steps of exciting the NO 2  sensor at a first frequency providing a first NO 2  sensor response sensitive to both NO 2  and O 2 , exciting the NO 2  sensor at a second frequency providing a second sensor response to only O 2 , using the second sensor response at the second frequency and the calibration information to produce a calculated concentration of O 2 , using the first NO 2  sensor response at the first frequency, the calculated concentration of O 2 , and the calibration information to calculate the NO 2  concentration. 
         [0093]    The method  1100  is an embodiment of the multiple frequency method for the operation of a NO 2  sensor trying to measure the concentration of NO 2  in a background with varying O 2  concentration. The actual sensing element must be designed so that the relative sensitivities to NO 2  and O 2  are different at two different frequencies—f 1  and f 2 . Step  1101  is to generate a calibration curve for sensor response to NO 2  at frequency f 1 . Step  1102  is to generate a calibration curve for sensor response to O 2  at frequency f 1 . Step  1103  is to generate a calibration curve for sensor response to O 2  at frequency f 2 . Steps  1101 ,  1102 , and  1103  are performed once, prior to NO 2  sensor operation. 
         [0094]    Step  1104  is to excite the NO 2  sensor at frequency f 1 . Response at f 1  contains contributions from both species of interest and interfering species (NO 2  and O 2 ). Step  1105  is to measure NO 2  sensor response at frequency f 1 . Step  1106  is to excite the NO 2  sensor at frequency f 2 . Step  1107  is to measure NO 2  sensor response at frequency f 2 . Response at f 2  contains contributions only from the interfering species O 2 . 
         [0095]    Step  1108  is to use NO 2  sensor response at frequency f 2 and calibration curves to calculate concentration of interfering species O 2 . In Step  1109  concentration of ‘interfering species O 2 ’ may also be considered an ‘output.’ 
         [0096]    Step  1110  is to use NO 2  sensor response at frequency f 1 , calculated concentration of interfering species O 2  and calibration curves to calculate concentration of species of interest NO. Step  1111  provides output concentration of species of interest NO 2 . 
         [0097]    The method  1100  illustrated in  FIG. 11  is a computer implemented multiple frequency method for the operation of a NO 2  sensor to measure a parameter of interest using calibration curves, wherein interfering parameters may be present. The method  1100  illustrated in  FIG. 11  includes the step  1104  of exciting the NO 2  sensor at a first frequency providing a first NO 2  sensor response from both species of interest and interfering species (NO 2  and O 2 ) recorded on a computer-readable medium, the step  1106  exciting the NO 2  sensor at a second frequency providing a second NO 2  sensor response recorded on a computer-readable medium, the step  1108  using said second NO sensor response at said second frequency from both species of interest and interfering species (NO 2  and O 2 ) and the calibration information to produce a calculated concentration of the interfering parameters O 2  recorded on a computer-readable medium, and the step  1110  using said first NO 2  sensor response at said first frequency, said calculated concentration of the interfering parameters O 2 , and the calibration information to measure the parameter of interest NO 2 . 
       Sensing Method for Measuring NO in a Background Varying O 2    
       [0098]    Referring now to  FIG. 12 , an embodiment of a sensing method for measuring NO in a background with varying O 2  concentration is illustrated. This embodiment of the present invention is designated generally by the reference numeral  1200 . The method  1200  includes the steps of exciting the sensor at a first frequency producing a first frequency sensor response, measuring the first frequency sensor response producing a first frequency sensor measurement, exciting the sensor at a second frequency producing a second frequency sensor response, measuring the second frequency sensor response producing a second frequency sensor measurement, using the second frequency sensor measurement and the calibration information to produce a calculated concentration of O 2  measurement, and using the first frequency sensor measurement, the calculated concentration of O 2  measurement, and the calibration information to measure NO. 
         [0099]    The method  1200  is an embodiment of the multiple frequency method for the operation of a NO sensor trying to measure the concentration of NO in a background with varying O 2  concentration. The actual sensing element is designed so that the relative sensitivities to NO and O 2  are different at two different frequencies—f 1  and f 2 . The method  1200  includes the following steps: Step  1201  is to generate calibration curves. Step  1201  is performed prior to NO sensor operation. Step  1202  is to excite the NO sensor at frequency f 1 . Response at f 1  contains contributions from both NO and O 2 . Step  1203  is to measure NO sensor response at frequency f 1 . Step  1204  is to excite the NO sensor at frequency f 2 . Step  1205  is to measure NO sensor response at frequency f 2 . Response at f 2  contains contributions only from O 2 . Step  1206  is to use NO sensor response at frequency f 2 and the calibration curves to calculate the concentration of interfering O 2 . Step  1207  is to use NO sensor response at frequency f 1 , calculated concentration of interfering O 2  and the calibration curves to calculate concentration of NO. Step  1208  provides output concentration of NO. 
         [0100]    In summary, the method  1200  is sensing method to measure NO using calibration information wherein varying amount of O 2  may be present. The method  1200  includes the steps of exciting the sensor at a first frequency producing a first frequency sensor response that includes both NO and O 2 , measuring the first frequency sensor response that includes both NO and O 2  producing a first frequency sensor measurement that includes both NO and O 2 , exciting the sensor at a second frequency producing a second frequency sensor response that contains only O 2 , measuring the second frequency sensor response producing a second frequency sensor measurement that contains only O 2 , using the second frequency sensor measurement that contains only O 2  and the calibration information to produce a calculated concentration of O 2  measurement, and using the first frequency sensor measurement includes both NO and O 2 , the calculated concentration of O 2  measurement, and the calibration information to measure NO. 
       Sensing Method for Measuring CO in a Background Varying O 2    
       [0101]    Referring now to  FIG. 13 , an embodiment of a sensing method for measuring CO in a background with varying O 2  concentration is illustrated. This embodiment of the present invention is designated generally by the reference numeral  1300 . The method  1300  includes the steps of exciting the sensor at a first frequency producing a first frequency sensor response, measuring the first frequency sensor response producing a first frequency sensor measurement, exciting the sensor at a second frequency producing a second frequency sensor response, measuring the second frequency sensor response producing a second frequency sensor measurement, using the second frequency sensor measurement and the calibration information to produce a calculated concentration of O 2  measurement, and using the first frequency sensor measurement, the calculated concentration of O 2  measurement, and the calibration information to measure CO. 
         [0102]    The method  1300  is an embodiment of the multiple frequency method for the operation of a CO sensor trying to measure the concentration of CO in a background with varying O 2  concentration. The actual sensing element is designed so that the relative sensitivities to CO and O 2  are different at two different frequencies—f 1  and f 2 . The method  1300  includes the following steps: Step  1301  is to generate calibration curves. Step  1301  is performed prior to CO sensor operation. Step  1302  is to excite the CO sensor at frequency f 1 . Response at f 1  contains contributions from both CO and O 2 . Step  1303  is to measure CO sensor response at frequency f 1 . Step  1304  is to excite the CO sensor at frequency f 2 . Step  1305  is to measure CO sensor response at frequency f 2 . Response at f 2  contains contributions only from O 2 . Step  1306  is to use CO sensor response at frequency f 2 and the calibration curves to calculate the concentration of interfering O 2 . Step  1307  is to use CO sensor response at frequency f 1 , calculated concentration of interfering O 2  and the calibration curves to calculate concentration of CO. Step  1308  provides output concentration of CO. 
         [0103]    In summary, the method  1300  is a sensing method to measure CO using calibration information wherein varying amount of O 2  may be present. The method  1300  includes the steps of exciting the sensor at a first frequency producing a first frequency sensor response that includes both CO and O 2 , measuring the first frequency sensor response that includes both CO and O 2  producing a first frequency sensor measurement that includes both CO and O 2 , exciting the sensor at a second frequency producing a second frequency sensor response that contains only O 2 , measuring the second frequency sensor response producing a second frequency sensor measurement that contains only O 2 , using the second frequency sensor measurement that contains only O 2  and the calibration information to produce a calculated concentration of O 2  measurement, and using the first frequency sensor measurement includes both CO and O 2 , the calculated concentration of O 2  measurement, and the calibration information to measure CO. 
       Sensing Method for Measuring NO in a Background of Varying Temperature 
       [0104]    Referring now to  FIG. 14 , an embodiment of a sensing method for measuring NO in a background with varying temperature is illustrated. This embodiment of the present invention is designated generally by the reference numeral  1400 . The method  1400  includes the steps of exciting the sensor at a first frequency producing a first frequency sensor response, measuring the first frequency sensor response producing a first frequency sensor measurement, exciting the sensor at a second frequency producing a second frequency sensor response, measuring the second frequency sensor response producing a second frequency sensor measurement, using the second frequency sensor measurement and the calibration information to produce a calculated temperature measurement, and using the first frequency sensor measurement, the calculated temperature measurement, and the calibration information to measure NO. 
         [0105]    The method  1400  is an embodiment of the multiple frequency method for the operation of a NO sensor trying to measure the concentration of NO in a background with varying temperature. The actual sensing element is designed so that the relative sensitivities to NO and temperature are different at two different frequencies—f 1  and f 2 . The method  1400  includes the following steps: Step  1401  is to generate calibration curves. Step  1401  is performed prior to NO sensor operation. Step  1402  is to excite the NO sensor at frequency f 1 . Response at f 1  contains contributions from both NO and temperature. Step  1403  is to measure NO sensor response at frequency f 1 . Step  1404  is to excite the NO sensor at frequency f 2 . Step  1405  is to measure NO sensor response at frequency f 2 . Response at f 2  contains contributions only from temperature. Step  1406  is to use NO sensor response at frequency f 2 and the calibration curves to calculate temperature. Step  1407  is to use NO sensor response at frequency f 1 , calculated temperature and the calibration curves to calculate concentration of NO. Step  1408  provides output concentration of NO. 
         [0106]    In summary, the method  1400  is sensing method to measure NO using calibration information wherein varying temperature may be present. The method  1400  includes the steps of exciting the sensor at a first frequency producing a first frequency sensor response that includes both NO and temperature, measuring the first frequency sensor response that includes both NO and temperature producing a first frequency sensor measurement that includes both NO and temperature, exciting the sensor at a second frequency producing a second frequency sensor response that contains only temperature, measuring the second frequency sensor response producing a second frequency sensor measurement that contains only temperature, using the second frequency sensor measurement that contains only temperature and the calibration information to produce a calculated temperature measurement, and using the first frequency sensor measurement includes both NO and temperature, the calculated temperature measurement, and the calibration information to measure NO. 
       Sensing Method for Measuring NO 2  in a Background Varying O 2    
       [0107]    Referring now to  FIG. 15 , an embodiment of a sensing method for measuring NO 2  in a background with varying O 2  concentration is illustrated. This embodiment of the present invention is designated generally by the reference numeral  1500 . The method  1500  includes the steps of exciting the sensor at a first frequency producing a first frequency sensor response, measuring the first frequency sensor response producing a first frequency sensor measurement, exciting the sensor at a second frequency producing a second frequency sensor response, measuring the second frequency sensor response producing a second frequency sensor measurement, using the second frequency sensor measurement and the calibration information to produce a calculated concentration of O 2  measurement, and using the first frequency sensor measurement, the calculated concentration of O 2  measurement, and the calibration information to measure NO 2 . 
         [0108]    The method  1500  is an embodiment of the multiple frequency method for the operation of a NO 2  sensor trying to measure the concentration of NO 2  in a background with varying O 2  concentration. The actual sensing element is designed so that the relative sensitivities to NO 2  and O 2  are different at two different frequencies—f 1  and f 2 . The method  1500  includes the following steps: Step  1501  is to generate calibration curves Steps is performed prior to NO 2  sensor operation. Step  1502  is to excite the NO 2  sensor at frequency f 1 . Response at f 1  contains contributions from both NO 2  and O 2 . Step  1503  is to measure NO 2  sensor response at frequency f 1 . Step  1504  is to excite the NO 2  sensor at frequency f 2 . Step  1505  is to measure NO 2  sensor response at frequency f 2 . Response at f 2  contains contributions only from O 2 . Step  1506  is to use NO 2  sensor response at frequency f 2  and the calibration curves to calculate the concentration of interfering O 2 . Step  1507  is to use NO 2  sensor response at frequency f 1 , calculated concentration of interfering O 2  and the calibration curves to calculate concentration of NO 2 . Step  1508  provides output concentration of NO 2 . 
         [0109]    In summary, the method  1500  is sensing method to measure NO 2  using calibration information wherein varying amount of O 2  may be present. The method  1500  includes the steps of exciting the sensor at a first frequency producing a first frequency sensor response that includes both NO 2  and O 2 , measuring the first frequency sensor response that includes both NO 2  and O 2  producing a first frequency sensor measurement that includes both NO 2  and O 2 , exciting the sensor at a second frequency producing a second frequency sensor response that contains only O 2 , measuring the second frequency sensor response producing a second frequency sensor measurement that contains only O 2 , using the second frequency sensor measurement that contains only O 2  and the calibration information to produce a calculated concentration of O 2  measurement, and using the first frequency sensor measurement includes both NO 2  and O 2 , the calculated concentration of O 2  measurement, and the calibration information to measure NO 2 . 
       Electrochemical Sensors for Monitoring Emissions 
       [0110]    Increasingly stringent emissions regulations require the development of advanced gas sensors for a variety of applications. For example, compact, inexpensive sensors are needed for detection of regulated pollutants, including hydrocarbons (HCs), CO, and NOx, in automotive exhaust. Of particular importance will be a sensor for NOx to ensure the proper operation of the catalyst system in the next generation of diesel (CIDI) automobiles. Compact, inexpensive sensors are particularly in demand for monitoring and control of regulated pollutants including hydrocarbons, carbon monoxide, and oxides of nitrogen (NOx). 
         [0111]    Referring now to  FIG. 16 , an embodiment of a sensor system for monitoring emissions is illustrated. The sensor system is designated generally by the reference numeral  1600 . The sensor system  1600  includes an alumina substrate  1602  and a first electrode  1604 . The first electrode  1604  is made of a dense electrode material. As used in this application, the term “dense electrode material” means any electronically conducting material (i.e., metal or ceramic oxide) that acts as an electrode in the electrochemical cell and has a density of more than 95% of theoretical density. 
         [0112]    In the embodiment illustrated in  FIG. 16  the first electrode is made of the dense electrode material gold  1604 . In other embodiments the first electrode  1604  is made of other dense electrode materials. A platinum layer  1606  on the alumina substrate  1602  forms a second electrode. A porous electrolyte material  1608  is located between the first electrode  1602  and the second electrode  1606 . The porous electrolyte material  1608  illustrated in  FIG. 16  is yttria-stabilized zirconia which is an oxygen-ion conducting ceramic. In other embodiments the porous electrolyte material  1608  is made of other oxygen-ion conductors and can be made of any ion-conductor acting as an electrolyte. An electronic processing unit  1610  is connected to the first electrode by lead(s)  1612  and is connected to the second electrode  1606  by lead  1614   
         [0113]    Referring now to  FIG. 17 , the operation of the sensor system will be illustrated.  FIG. 17  is a flow chart that show various steps involved in the operation of the sensor system. In step #1, designated by the reference numeral  1700 , one sensor is excited at a frequency f 1 . In step #2, designated by the reference numeral  1702 , the sensor response is measured at frequency f 1 . 
         [0114]      FIG. 17  shows alternatives for steps #3 and #4. In the first alternative, step #3, designated by the reference numeral  1704  is that another sensor is excited at a frequency f 2 . In step #4, designated by the reference numeral  1706 , the other sensor response is measured at frequency f 2 . In the second alternative, step #3 and step #4 are designated by the reference numeral  1708  which is to obtain a second signal for oxygen-only behavior. 
         [0115]    In step #5, designated by the reference numeral  1710 , is to use previous calibration from the electronic processing unit. In step #6, designated by the reference numeral  1712 , the output of NOx concentration is provided. 
         [0116]    Referring now to  FIG. 18 , another embodiment of a sensor system for monitoring emissions is illustrated. The sensor system shown in  FIG. 18  is designated generally by the reference numeral  1800 . The sensor system  1800  includes a first electrode  1804 . The first electrode  1804  is made of a dense electrode material. In this embodiment the first electrode  1804  is made of the dense electrode material LSM (La 0.85 Sr 0.15 MnO 3 ), an electronically conducting oxide. In this embodiment the second electrode  1806  is made of porous LSM. A porous electrolyte material  1808  is located between the first electrode  1804  and the second electrode  1804 . The porous electrolyte material  1808  illustrated in  FIG. 18  is yttria-stabilized zirconia which is an oxygen-ion conducting ceramic. In other embodiments the porous electrolyte material  1808  is made of other oxygen-ion conductors and can be made of any ion-conductor acting as an electrolyte. An electronic processing unit  1810  is connected to the first electrode  1804  by lead  1814  and is connected to the second electrode  1806  by lead  1812 . 
         [0117]    The structural elements of the sensor system  1800  having been described, the operation of the sensor system  1800  will now be described. The operation of the sensor system  1800  involves various steps. In step #1, one of the sensors is excited at a frequency f 1 . In step #2, the sensor response is measured at frequency f 1 . The next step actually involves alternatives steps #3 and #4. In the first alternative, step #3, the other sensor is excited at a frequency f 2 . In step #4, the other sensor response is measured at frequency f 2 . In the second alternative, step #3 and step #4 is to obtain a second signal for oxygen-only behavior. Step #5, is to use previous calibration from the electronic processing unit  1810 . In step #6, the output of NOx concentration is provided by the processing unit  1810 . 
         [0118]    Referring now to  FIG. 19 , another embodiment of a sensor system for monitoring emissions is illustrated. The sensor system shown in  FIG. 19  is designated generally by the reference numeral  1900 . The sensor system  1900  includes an alumina substrate  1902 . In the embodiment illustrated in  FIG. 19  the alumina substrate  1902  is a heated alumina substrate. A first electrode  1904  is deposited on the alumina substrate  1902 . The first electrode  1904  is made of a dense electrode material. In this embodiment the first electrode  1904  is made of the dense electrode material LSM (La 0.85 Sr 0.15 MnO 3 ), an electronically conducting oxide. A second electrode  1904  is deposited on the alumina substrate  1902 . In this embodiment the second electrode  1906  is made of the dense electrode material LSM (La 0.85 Sr 0.15 MnO 3 ), an electronically conducting oxide. 
         [0119]    A porous electrolyte material  1908  is located between the first electrode  1904  and the second electrode  1904 . The porous electrolyte material  1908  illustrated in  FIG. 19  is yttria-stabilized zirconia which is an oxygen-ion conducting ceramic. In other embodiments the porous electrolyte material  1908  is made of other oxygen-ion conductors and can be made of any ion-conductor acting as an electrolyte. An electronic processing unit  1910  is connected to the first electrode  1904  by lead  1914  and is connected to the second electrode  1906  by lead  1912 . 
         [0120]    The structural elements of the sensor system  1900  having been described, the operation of the sensor system  1900  will now be described. The operation of the sensor system  1900  involves various steps. In step #1, one of the sensors is excited at a frequency f 1 . In step #2, the sensor response is measured at frequency f 1 . The next step actually involves alternatives steps #3 and #4. In the first alternative, step #3, the other sensor is excited at a frequency f 2 . In step #4, the other sensor response is measured at frequency f 2 . In the second alternative, step #3 and step #4 is to obtain a second signal for oxygen-only behavior. Step #5, is to use previous calibration from the electronic processing unit  1910 . In step #6, the output of NOx concentration is provided by the processing unit  1910 . 
       Operation of Electrochemical Sensors for Monitoring Emissions 
       [0121]    The sensor system is operated by applying a varying (typically sinusoidal) voltage difference between the two electrodes. The excitation signal is chosen at a fixed frequency, for example 10 Hz. A phase meter, phase lock loop, or other electronic measuring circuit is used to measure the changes in amplitude and phase of the excitation signal, after it interacts with the sensor, relative to a fixed, reference signal of the same frequency. 
         [0122]    Applicants have demonstrated the effective use of a circuit board, which was configured to apply the alternating signal and then output the in-phase (conductive) and out-of-phase (capacitive) portions of the sensor response, for operation of the sensor. The amplitude of the response, which can be correlated with the impedance |Z|, is sensitive to the changes which the sensor is trying to detect (for example the NOx concentration in an exhaust gas), has been reported as a sensing metric at low frequency (1 Hz). However, Applicants&#39; observation, is that the phase angle signal is a better metric of the sensor response. The phase is more stable, responds more quickly, and maintains the response to higher frequency than |Z|. Also, Applicants&#39; work has determined compositional and microstructural criteria for the sensing electrode materials to optimize the sensor for sensitivity to ppm NOx in a large background of O2, which can be modified and extended for sensing other types of gases and for use in other types of electrochemical cells. 
         [0123]    The sensing electrode is defined by its ability to change polarization when the desired concentration of the sensing gas to be detected is introduced. In general, the sensing gas and other gases present that may cause undesired cross-sensitivity can have parallel kinetic contributions on the resulting polarization of the sensing electrode depending on its catalytic behavior. Due to the parallel kinetic contributions, relatively small amounts of the sensing gas can be detected in a background of competing gas of much larger concentrations for a sensing electrode with an appropriately low catalytic activity towards the competing gas of much larger concentration. For sensing ppm NOx in percent levels of oxygen, the above criteria have been shown to be met using porous yttria-stabilized zirconia as the electrolyte, and dense gold as symmetric electrodes. (See attached journal articles for more details.) The sensing behavior is not limited to gold electrodes, or to symmetric electrodes, and we have demonstrated sensing using materials and compositions with the appropriate catalytic activities, including dense electronically conducting perovskites. 
         [0124]    Another aspect of the proposed invention is the issue of the frequency of operation. Sensitivity is typically higher at lower frequencies, which is why the work of Miura reports sensing at 1 Hz; due to a combination of their sensor properties and their measurement of |Z| rather than phase angle, they cannot operate at higher frequencies while maintaining sufficient sensitivity. However, since the frequency determines response and sampling times (with 1/frequency representing a general limitation for the sampling rate), it is probably not practical to operate below ˜5 Hz, and desirable to operate at the highest frequency at which sufficient sensitivity can be obtained. At much higher frequencies, however, such as 1000 Hz or more, the sensor has no response to (in our case) NOx but responds only to changes in the O2 background, temperature, and other interfering effects. This provides another point of novelty in the proposed sensor, that the sensor can be simultaneously operated at two (or more) widely different frequencies to provide a compensation for these interfering effects. That is, for example, at 10 Hz the sensor senses both changes in the concentrations of NOx and O2, while at 1000 Hz it senses only the changes in O2. Thus, by comparing these signals the competing effects of variations of several percent in the O2 background can be deconvolved from the effects of ppm changes in the NOx concentration. 
         [0125]    The design and operation of the sensor system uses the response to alternating current excitation as the sensing signal, also known as impedancemetric operation, and consists of a porous electrolyte material and dense electrode material(s). In one embodiment the porous electrolyte is yttria-stabilized zirconia. The porous electrolyte is not limited to yttria-stabilized zirconia, or even to oxygen-ion conductors, but could be any ion-conductor acting as an electrolyte. 
         [0126]    In the design and operation of the sensor system the porous electrolyte is either fabricated using graphitic pore formers and fired to temperatures greater than 1500° C., or fabricated using a ceramic powder slurry containing binders and/or plasticizers and fired to temperature greater than 900° C., but less than 1500° C. The green porous electrolyte can be processed prior to firing using tape-cast methods or direct application methods such as painting or spraying. 
         [0127]    In one embodiment of the design and operation of the sensor system both electrodes in contact with the electrolyte are exposed to the same gas composition (i.e., the sensing gas) or only one electrode is exposed to the sensing gas, and the other electrode is in a reference gas composition (e.g., air). In another embodiment of the design and operation of the sensor system both electrodes are different but both act as sensing electrodes, or where both electrodes are different and only one electrode serves as the sensing electrode while the other electrode serves primarily as a counter electrode. The sensing electrode is defined by its ability to change polarization when the desired concentration of the sensing gas to be detected is introduced. The counter electrode is defined by the relative insensitivity of its polarization to changes in the concentration of the sensing gas. In general, the sensing gas and other gases present that may cause undesired cross-sensitivity can have parallel kinetic contributions on the resulting polarization of the sensing electrode depending on its catalytic behavior. Due to the parallel kinetic contributions, relatively small amounts of the sensing gas can be detected in a background of competing gas of much larger concentrations for a sensing electrode with an appropriately low catalytic activity towards the competing gas of much larger concentration. 
         [0128]    In one embodiment of the design and operation of the sensor system the sensing electrode has a dense microstructure and material composition that has low catalytic activity towards oxygen. In another embodiment of the design and operation of the sensor system the counter electrode has relatively high catalytic activity towards oxygen. 
         [0129]    In one embodiment of the design and operation of the sensor system the frequency of the applied low-amplitude alternating current excitation ranges from 1 to 1000 Hz. The frequency of operation is chosen based on maximizing sensitivity while reducing sampling time. The specific type of reactions (diffusion, adsorption, charge-transfer) occurring on both electrodes and the specific type of species (neutral, ionized) present will alter the optimum frequency of operation. Controlling the specific behavior can be achieved by altering the composition, microstructure, and geometry of the electrodes. For sensing ppm NO x  in a background of 2 to 20% O 2 , the sensing electrode should have a composition with low catalytic activity towards oxygen and a dense microstructure, which then allows optimum performance at frequencies near 10 Hz. 
         [0130]    In one embodiment of the design and operation of the sensor system the temperature of operation is chosen to reduce cross-sensitivity to interfering species (e.g., water vapor) and optimize the sensor sensitivity and accuracy. In another embodiment of the design and operation of the sensor system an oxidation catalyst located upstream of the operating sensor is used to control the resulting gas composition in order to reduce cross-sensitivity to interfering species (e.g., NO 2  and NH 3 ) and optimize the sensor sensitivity and accuracy. 
         [0131]    In one embodiment of the design and operation of the sensor system the sensing electrode is dense gold and the counter electrode is porous platinum. In one embodiment of the design and operation of the sensor system the frequency of operation is 10 Hz and the temperature of operation is 650° C. In one embodiment of the design and operation of the sensor system the sensing electrode is dense gold that is covered by a thin layer of porous YSZ electrolyte material, less than about 50 micron layer, to provide identical responses to either NO or NO 2 . The identical response could be due to the larger thickness layers of YSZ, greater than about 50 microns, providing additional reaction sites for the NO 2  species and therefore producing a larger, and unequal, signal when responding to NO 2  compared to NO. 
         [0132]    In one embodiment of the design and operation of the sensor system the sensing electrode is dense strontium-doped lanthanum manganite and the counter electrode is porous platinum. The frequency of operation is 10 Hz and the temperature of operation is 575° C. In another embodiment of the design and operation of the sensor system the sensing electrode is dense strontium-doped lanthanum chromite and the counter electrode is porous platinum. The frequency of operation is 10 Hz and the temperature of operation is 575° C. In another embodiment of the design and operation of the sensor system the sensing electrode is dense magnesium-doped lanthanum chromite and the counter electrode is porous platinum. The frequency of operation is 10 Hz and the temperature of operation is 575° C. In another embodiment of the design and operation of the sensor system the sensing electrode is dense strontium-doped lanthanum manganite and the counter electrode is porous strontium-doped lanthanum manganite. The frequency of operation is 10 Hz and the temperature of operation is 575° C. 
         [0133]    In one embodiment of the design and operation of the sensor system the sensing electrode is a dense electronically conducting oxide or an inert metal (e.g., gold) or an electronically conducting material coated with either a dense electronically conducting oxide or an inert metal and the counter electrode is also a sensing electrode of similar or different composition as the other sensing electrode, or a porous electronically oxide of the same or different composition than the sensing electrode, or a porous layer of metal. The frequency of operation is between 1 and 1000 Hz, and chosen based on maximizing sensitivity while reducing sampling time. The temperature of operation is between 400° C. and 800° C., and chosen to reduce cross-sensitivity to interfering species and optimize the sensor sensitivity and accuracy. 
         [0134]    In one embodiment of the design and operation of the sensor system both electrodes are co-located on the same surface of the sensor allowing the bottom non-active surface to be mounted onto a separate device such as a heated substrate. In one embodiment of the design and operation of the sensor system a circuit board is configured to apply the alternating signal and then output the in-phase (conductive) and out-of-phase (capacitive) portions of the sensor response. 
         [0135]    While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.