Abstract:
A breath analyzer detects a breath component in a breath sample. The analyzer includes a sensor for sensing the breath component and a temperature control system integrally formed with the sensor. The temperature control comprises a resistive temperature device configured to heat the sensor to a predetermined temperature. The resistive temperature device also senses the temperature of the sensor. A controller is operatively connected to the sensor to receive breath component information sensed by the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/834,642, filed Jun. 13, 2013, the entire disclosure of which is hereby incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    Exhaled human breath typically consists of approximately 78% nitrogen, 15-18% oxygen, 4-6% carbon dioxide, and 5% water. The remaining small fraction of exhaled breath generally consists of trace levels of more than 1000 volatile organic compounds (VOCs) with concentrations ranging from parts per trillion (pptv) to parts per million (ppmv). 
         [0003]    Acetone is a VOC in exhaled human breath that can indicate various health conditions such as diabetes, heart disease, epilepsy, and others. For example, a person with diabetes who is in a state of ketosis will have an increased breath concentration of acetone resulting from the body&#39;s production of ketone bodies. Acetone is also produced by ketosis resulting from a restricted calorie weight loss and/or exercise program. This acetone production is the result of metabolism of fat. Hence, a breath acetone content measurement can be used as an indication of a medical condition or of fat burning during a diet and/or program to show the effectiveness of the program. These examples should be considered non-limiting in that the present disclosure can be directed to any situation in which breath acetone levels are to be detected and/or monitored. 
         [0004]    The present disclosure is directed to an acetone sensor useful for detecting various health conditions and/or for monitoring the efficacy of diet and exercise programs. The acetone level for diet and exercise is lower than that caused by diabetes. Accordingly, a more sensitive sensor is required for to monitor increased acetone levels caused by diet and exercise. Thus, there is a need for an acetone sensor capable of detecting acetone levels corresponding to diet and exercise induced ketosis. 
       SUMMARY 
       [0005]    A first embodiment of a disclosed breath analyzer detects a particular breath component in a breath sample. The analyzer includes a sensor for sensing the breath component. A temperature control system is integrally formed with the sensor. The temperature control system has a resistive temperature device configured to heat the sensor to a predetermined temperature. The resistive temperature device also detects the temperature of the sensor. A controller is operatively connected to the sensor to receive breath component information sensed by the sensor. 
         [0006]    A second disclosed embodiment of a breath analyzer detects a breath component in a breath sample. The breath analyzer includes a metal oxide sensor for sensing the breath component and a temperature control system integrally formed with the sensor. 
         [0007]    The temperature control has a metal resistor with a positive temperature coefficient. The metal resistor is configured to heat the sensor to a predetermined temperature and to sense the temperature of the sensor. The breath analyzer further includes a controller operatively connected to the sensor to receive breath component information sensed by the sensor. 
         [0008]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0010]      FIG. 1  shows a cross-sectional side view of an acetone sensing device in accordance with the present disclosure; 
           [0011]      FIG. 2  shows a cross-sectional view of a first exemplary embodiment of a sensor assembly of the acetone sensing device of  FIG. 1 ; 
           [0012]      FIG. 3  shows a cross-sectional view of a second exemplary embodiment of a sensor assembly of the acetone sensing device of  FIG. 1 ; 
           [0013]      FIG. 4  shows a schematic diagram of a first exemplary embodiment of a temperature control system of the sensor assembly of  FIG. 2 ; and 
           [0014]      FIG. 5  shows a schematic diagram of a second exemplary embodiment of a temperature control system of the sensor assembly of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    This present disclosure relates to a device for detecting the concentration of a particular breath component, such as acetone, using a metal oxide sensor in combination with a temperature control system. The temperature control system uses a closed-loop control both to sense the sensor operating temperature and to heat the sensor as necessary to achieve the desired sensor operating temperature. Metal oxide based gas sensors typically require operation at relatively high temperatures (e.g. 300 C). The sensitivity of the sensor to a given gas is often highly dependent on the sensor temperature. Therefore the ability to directly monitor and control sensor temperature is advantageous. Accurate thermal control becomes especially critical when attempting to detect gases that are present at very low concentrations such as with the acetone vapor found in breath as a result of diet and exercise. The present disclosure relates to the temperature control system used to achieve the desired sensor operating temperature, and the advantageous use of the heating element itself as the temperature measurement device. 
         [0016]    While the present disclosure and exemplary embodiments are generally described with respect to devices used to detect acetone content in a breath sample, such embodiments are exemplary only and should not be considered limiting. In this regard, the described sensors can be sensors for detecting levels of gaseous breath components other than acetone, including VOCs and other gaseous compounds. Further, it will be appreciated that the sensors are not limited to sensors used for detecting components from breath samples, but can include sensors used to detect components of any other suitable gas sample. 
         [0017]      FIG. 1  shows an exemplary embodiment of an acetone sensing device  100  according to the present disclosure. The device  100  includes a breath sample collector  102  comprising an elongate body  104  with an inlet aperture  106  located at one end and an outlet aperture  108  located at the opposite end. The inlet aperture  106  has an optional mouthpiece  110  formed thereon. The mouthpiece  110  can be permanently fixed to the body  104  or may optionally be detachably coupled to the body  104  to allow for embodiments in which a disposable mouthpiece is utilized. The mouthpiece  110  optionally includes a check valve (not shown) that allows fluid to flow through the inlet aperture  106  in only one direction, i.e., into the elongate body  104 . In other contemplated embodiments, the optional check valve is disposed within the elongate body  104  rather than the mouthpiece  110 . 
         [0018]    A cavity  112  is formed in a central portion of the collector  102  in fluid communication with the inlet aperture  106  and the outlet aperture  108 . A sensor assembly housing  114  is positioned between the first and second ends of the elongate body and defines a sensor assembly cavity  116  in fluid communication with the cavity  112  of the elongate body  104 . A sensor assembly  200  is disposed within the sensor assembly housing  114 . 
         [0019]    The sensor assembly  200  is operatively connected to a processor  118 . As described in further detail below, the processor  118  receives data from the sensor assembly  200  related to sensed breath components, breath flow, sensor temperature, and other operating characteristics. In one contemplated embodiment, the processor  118  processes the data and selectively displays the information received from the sensor assembly  200  on a display (not shown) for the user. In yet another contemplated embodiment, the processor  118  stores the data locally, or makes the data available for transfer to a remote storage location or processor, such as a home computer, tablet, smart phone, etc. These and other processor functions suitable for receiving and processing diagnostic data are contemplated and should be considered within the scope of the present disclosure. 
         [0020]    The disclosed configuration is suitable for collecting a breath sample from a user and exposing the breath sample to the sensor assembly  200  for analysis. For acetone detection, it is preferable that the analyzed breath sample be alveolar air, i.e. air from deep within the lungs. While some alveolar air is generally exhaled during the entire exhalation, in a preferred embodiment, the sample is taken from the last third of the exhalation to maximize the amount of deep-lung air in the sample. The illustrated device  100  collects and isolates alveolar air for analysis. 
         [0021]    To utilize the device  100 , a user places his mouth to the mouthpiece and blows a long, continuous breath sample into the inlet aperture  106 . The breath sample flows through the cavity  112  in the direction indicated by the arrow and then exits through the outlet aperture  108 . The outlet aperture  108  has a reduced geometry that limits the flow of breath out of the cavity  112 . In this manner, the breath sample is contained within the device  100  until after the sensor assembly  200  has analyzed the breath sample.  FIG. 2  shows a first exemplary embodiment of a sensor assembly  200  suitable for use in the acetone sensing device  100  of  FIG. 1 . The sensor assembly  200  includes a substrate  202  with an acetone sensor  210  formed on a first side and a temperature control system  220  formed on a second side. Sensor leads  214  and  216  are in electronic communication with the sensor  210  and the temperature control system  220 , respectively, to provide information between the sensor assembly  200  and the processor  118 . It will be appreciated that the illustrated sensor leads are exemplary only, and that any suitable configuration for operatively connecting the sensor assembly  200  to the processor  118  can be utilized. Further, the positions of the acetone sensor  210  and the temperature control system  220  on the substrate are exemplary only. In this regard, the acetone sensor  210  and the temperature control system  220  can be formed on the same side of the substrate  202  or in any other suitable locations relative to each other on the substrate. 
         [0022]    In the illustrated embodiment, the substrate  202  is an alumina substrate with tungsten oxide (WO 3 ) coating  212  deposited thereon. It will be appreciated that metal oxide gas sensors are known in the art and the described WO 3  coating  212  disposed on an alumina substrate  202  is exemplary only. In this regard, other metal oxides or combination of metal oxides suitable for sensing acetone and alternate substrate materials are possible and should be considered within the scope of the present disclosure. Further, the disclosed sensor  210  is not limited to the use of metal oxide gas sensors made using any particular manufacturing method. It will also be appreciated that the substrate  202  is not limited to an aluminum oxide material, but can alternatively be formed of glass, other suitable high-temperature substrates, or a combination thereof. Exemplary metal oxide gas sensors and/or methods of forming the same are disclosed in U.S. Patent Publication Nos. 2011/0071446, and 2003/0217586, the disclosures of which are expressly incorporated herein by reference. 
         [0023]    In the illustrated embodiment, the surface area of the sensor  210  is approximately 1 mm 2 ; however, other embodiments are contemplated wherein the surface area of the sensor is larger or smaller than that of the illustrated embodiment. Because the surface area of the sensor is relatively small, the sensor heats-up and cools-down quickly. Metal oxide gas based sensors, such as the disclosed acetone sensor  210 , typically require relatively high operational temperatures, e.g., about 300° C. The sensitivity of the sensor to a given gas is often highly dependent on the sensor temperature. Accordingly, the ability to directly monitor and control sensor temperature is advantageous. 
         [0024]    Accurate thermal control becomes especially critical when attempting to detect gases that are present at very low concentrations such as with the acetone vapor found in breath. The presently disclosed acetone sensing device  100  incorporates a heating element to achieve the desired sensor operating temperature, and uses of the heating element itself as the temperature measurement device. Utilizing the temperature control system  220  enables the operating temperature of the acetone sensing device  100  to be maintained within a range of approximately 300° C. to 450° C. It will be appreciated that this range is exemplary only and that the actual range of the sensor operating temperature can be modified to be suitable for a particular type of sensor. Further, the operating temperature of the sensor can be maintained within a narrower range to provide increase accuracy. 
         [0025]    Still referring to the embodiment of  FIG. 2 , the temperature control system  220  is a circuit formed by depositing a platinum trace on the substrate  202 . Although other materials known in the art are contemplated for the trace, platinum based resistive temperature devices (RTDs) are commonly used as temperature sensing elements due to platinum&#39;s stable resistance temperature coefficient. As used herein, RTD refers to a metal resistor with a positive temperature coefficient. In contrast to thermistors, which generally use ceramic or polymeric materials, RTDs provide more accurate readings in the temperature ranges utilized for acetone detection. 
         [0026]      FIG. 3  shows an alternate embodiment, wherein the sensor assembly  300  includes a discrete acetone sensor  310  bonded to a discrete temperature control system  320 . The acetone sensor  310  comprises a combination of a substrate  304  with a metal oxide sensor  312  disposed thereon. The temperature control system  320  is a circuit formed by depositing a platinum trace  322  on a second substrate  324 . The acetone sensor  310  and the temperature control system  320  are bonded together and connected to the processor  118  by leads  314  and  316 , respectively. It will be appreciated that the illustrated sensor leads are exemplary only, and that any suitable configuration for operatively connecting the sensor assembly  300  to the processor  118  can be utilized. 
         [0027]    In order to minimize self-heating, typical RTD resistance sensing is conducted in a manner that minimizes the power applied to the RTD. Moreover, platinum is generally not used as a base material for resistive heaters due to its high cost. However, the disclosed temperature control system  220  requires a relatively small heater so that it is feasible to use the RTD itself as the resistive heating element. Integrating a resistive heating element with a temperature sensor in a single temperature control system  220  allows for significant advantages, including reduced cost, reduced sensor complexity, fewer interconnecting leads, and intimate thermal contact between heater and temperature sensor. 
         [0028]      FIG. 4  shows a schematic illustration of an exemplary embodiment of a heater/temperature sensor circuit  400  suitable for use with the temperature control system  220 . Generally speaking, the circuit  400  is a bridge circuit with an operational amplifier to provide closed loop control of the circuit. A first leg of the bridge includes a first resistor R 1  in series with the RTD. The second leg of the bridge includes a second resistor R 2  in series with a variable resistor R VAR . The junction between R 1  and the RTD is connected to the inverting input of an operational amplifier  402 , and the junction between R 2  and R VAR  is connected to the non-inverting input of the operational amplifier  402 . The operational amplifier  402  supplies voltage to the circuit according to the difference between the voltages received from the circuit legs so that the legs of the circuit balance, as shown in equation (1). 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       R 
                       RTD 
                     
                     
                       R 
                       1 
                     
                   
                   = 
                   
                     
                       R 
                       VAR 
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0029]    The operational amplifier  402  controls the voltage so that the temperature of the RTD is such that the resistance of the RTD balances the circuit. With the R 1 , R 2 , and R VAR  having known values, the circuit is balanced when the resistance or the RTD, is at a specific value, as defined in equation (2) below. 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     RTD 
                   
                   = 
                   
                     
                       
                         R 
                         1 
                       
                        
                       
                         R 
                         VAR 
                       
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0030]    The value of R RTD  corresponds closely to a specific RTD temperature, so that the equation is balanced when the RTD is at a predetermined temperature. In this manner, the operational amplifier  402  controls the voltage to maintain a predetermined RTD temperature. 
         [0031]    The circuit works by the operational amplifier  402  continuously balancing its inputs, V IN+  and V IN− , as shown below in Equation 3. 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     
                       IN 
                       + 
                     
                   
                   = 
                   
                     
                       V 
                       
                         IN 
                         - 
                       
                     
                     = 
                     
                       
                         
                           
                             R 
                             RTD 
                           
                           
                             
                               R 
                               RTD 
                             
                             + 
                             
                               R 
                               1 
                             
                           
                         
                          
                         
                           V 
                           OUT 
                         
                       
                       = 
                       
                         
                           
                             R 
                             VAR 
                           
                           
                             
                               R 
                               VAR 
                             
                             + 
                             
                               R 
                               2 
                             
                           
                         
                          
                         
                           V 
                           OUT 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0032]    When the RTD is below the temperature setpoint, its resistance is lower. Accordingly, the operational amplifier  402  input V IN−  is lower than V IN+ , which causes V OUT  to increase. When V OUT  increases, more power is delivered to the RTD, raising its temperature. Conversely, when the RTD is above the temperature setpoint, its resistance is higher. In this case, the operational amplifier  402  input V IN−  is higher than V IN+ , which causes V OUT  to decrease, delivering less power to the RTD and cooling it. Accordingly, the known value of R 1  can be used along with the measured values of V IN−  and V OUT  to calculate the resistance and, therefore, the temperature, of the RTD. 
         [0033]    In addition to providing the ability to maintain a particular RTD temperature and also to sense the temperature of the RTD, the disclosed circuit  400  is also suitable for use as an anemometer. When used with the acetone sensing device  100 , the heater/temperature sensor circuit  400  is subjected to a breath sample being blown past the sensor. As the breath flows past the sensor circuit  400 , the effects of forced convective heat transfer requires more power to the RTD to maintain a constant temperature. Because the characteristics of the exhaled breath are known, e.g., 37° C. (body temperature for a human) with a humidity of 100%, the added power used to maintain a constant RTD temperature, which is related cooling rate due to forced convective heat transfer, can be used to calculate the rate of flow of the breath sample as the user breathes into the acetone sensing device. Other embodiments of constant temperature anemometers are disclosed in U.S. Pat. No. 5,069,066, the disclosure of which is expressly incorporated herein. 
         [0034]    Using the anemometer features of the disclosed temperature control system  220 , it is possible to provide an acetone sensing device  100  that senses whether or not a breath sample is suitable for analysis. As previously discussed, it is preferable that the breath sample to be analyzed is from approximately the last third of a full, breath expiration. In one contemplated embodiment, the acetone sensing device  100  senses the flow rate of breath through the device and requires that the user maintain a minimum breath flow rate for a threshold amount of time before beginning acetone detection. 
         [0035]      FIG. 5  shows a schematic illustration of a second exemplary embodiment of a heater/temperature sensor circuit  500  suitable for use as the temperature control system  220 . Similar to the circuit  400  shown in  FIG. 4 , the circuit  500  of  FIG. 5  provides closed-loop control of the temperature of an RTD, while also functioning as a temperature sensor and anemometer. 
         [0036]    The circuit  500  includes an RTD connected in series with a shunt resistor, R shunt . A microprocessor  502  provides an excitation voltage (V excitation ) to the RTD. The junction between the microprocessor  502  and the RTD is connected to an analog input of the microprocessor  502  feeding V excitation  back to the microprocessor. In addition, the junction between the RTD and R shunt  is connected to a second analog input of the microprocessor  502  feeding V shunt  to the microprocessor. The circuit acts as a resistive divider, wherein the relationship of V shunt  to V excitation  is shown in equation (4). 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     shunt 
                   
                   = 
                   
                     
                       
                         R 
                         shunt 
                       
                       
                         
                           R 
                           RTD 
                         
                         + 
                         
                           R 
                           shunt 
                         
                       
                     
                      
                     
                       V 
                       excitation 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0037]    As previously noted, for a given RTD, a particular value R RTD  corresponds closely to a particular temperature of the RTD. To achieve a known R RTD-SETPOINT  and the corresponding target RTD temperature, the microprocessor  502  controls V excitation  according to equation (5). 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     excitation 
                   
                   = 
                   
                     
                       
                         
                           R 
                           
                             RTD 
                              
                             
                               - 
                             
                              
                             SETPOINT 
                           
                         
                          
                         
                           V 
                           shunt 
                         
                       
                       
                         R 
                         shunt 
                       
                     
                     + 
                     
                       V 
                       shunt 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0038]    The closed-loop feedback provided through the microprocessor  502  combined with the close correlation between the resistance and the temperature of the RTD allows for the circuit  500  to also be used as a temperature sensor and as an anemometer in the manner previously described with respect to the circuit  400  of  FIG. 4 . 
         [0039]    While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.