Abstract:
A gas sensor including a housing containing a potassium permanganate element sandwiched between two polytetrafluoroethylene elements, a carbon element, a polytetrafluoroethylene element located adjacent to the carbon element, a sensing electrode, a reference electrode, and a counter electrode with attached current collectors, and an electrolyte.

Description:
FIELD 
       [0001]    The application pertains to gas sensors. More particularly, the application pertains to gas sensors that include a novel filter element to provide reduced cross-sensitivity to other gases and to provide longer life for the gas sensor. 
       BACKGROUND 
       [0002]    Gas sensors are used in many commercial and industrial applications, including workplace monitoring for the presence of toxic or otherwise hazardous or deleterious gases and in other applications where health and safety issues require detection of specific gases in the ambient environment. 
         [0003]    In these various applications, it is frequently necessary to monitor concentration of selected gas species down to levels of a few parts per million and less. In doing so, there is usually a need to remove, from the sampled air, other gases or volatile organic compounds that would likewise react at the sensor and generate an unwanted response. These gases normally have a myriad of different chemical properties, which therefore requires the use of a multitude of chemicals to remove each of them. For example, high surface area carbon is frequently used to adsorb most organic volatile species but the carbon is not effective at removing some of the common industrial inorganic gases. Thus, a second type of chemical must be used to remove those and so on. Typically, the carbon is coated with the second type of chemical or the two chemicals can be mixed then impregnated onto a solid support. Such an arrangement can quickly become self-destructive as the chemicals cross-react with each other, leading to decreased efficiency and longevity of the filter and gas sensor. 
         [0004]    Gas sensors used in the foregoing applications include electrochemical gas sensors, which may operate to electrochemically reduce the gas species to be monitored. Alternatively, the gas sensor may operate by electrochemically oxidizing the target gas species sought to be detected. As a still further alternative, the electrochemical gas sensor may operate by indirect oxidation or reduction reaction of a compound that is produced in the gas sensor device involving the target gas to be detected in the monitored gaseous environment. 
         [0005]    Electrochemical gas sensors utilize sensor cells that typically contain three electrodes—the working electrode, the reference electrode, and the counter electrode, although gas sensor cells are known having two-electrode and four-electrode structures. The electrodes are conventionally mounted within a housing that additionally contains an electrolyte, contacts, and electrical wires forming electronic circuitry of the sensor, and a gas permeable membrane that keeps the electrolyte within the cell and allows the gas to contact the measuring electrode. 
         [0006]    Electrochemical sensor cells require an electrolyte as a component of the electrochemical cell. The electrolyte performs the transport of electrical charge between the different electrodes and therefore enables an electrical current to flow. The transport of electrical charge by the electrolyte is ionic in character rather than involving charge transport by electrons. 
         [0007]    Conventional gas sensors contain filters that often use mixtures of chemicals to achieve multiple functionalities. Such gas sensors can have a limited lifespan due to the chemical components of the sensor reacting with each other or otherwise degrading due to environmental factors. The art therefore continues to seek improvements in electrochemical cell gas sensors. The current gas sensor comprises a novel filter that separates these materials into isolated chambers, which removes the risk of cross-reactions leading to improved overall filter efficiency and life without greatly increasing the complexity of the design. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a view of a detector in accordance herewith. 
           [0009]      FIG. 2  is a view of a graph detailing results of longevity of various gas detectors. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    While disclosed embodiments can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles thereof as well as the best mode of practicing same, and is not intended to limit the application or claims to the specific embodiment illustrated. 
         [0011]    A gas sensor having a filter that includes two or more chemicals that are incompatible or cross-reactive, which are physically separated by a gas permeable, inert barrier is described herein.  FIG. 1  illustrates a gas sensor  10  in accordance herewith. It will be understood that neither the exact shape, nor the exact configuration of the sensor  10 , except to the extent described below, are limitations hereof. The sensor includes a housing  12 , which is formed of a relatively inert, nonconductive, gas impervious and acid resistant material. The housing can be made of ceramic, acrylonitrile butadiene styrene, polyethylene, polypropylene, polyvinylchloride, respective derivatives thereof or mixtures thereof. 
         [0012]    Within the housing is an opening or capillary  14 , through which the gas enters the housing  12 . The housing can contain one or multiple openings or capillaries. Alternatively, the gas sensor can include a solid membrane sensor. 
         [0013]    A gas permeable, inert barrier  22   a,  shown as “PTFE seal” in  FIG. 1 , can be located adjacent to the capillary  14 . The gas permeable, inert barrier  22   a  is a solid material selected from the group consisting of fluorinated plastic, polyethylene, inorganic materials, ceramic materials, metallic foils, and mixtures thereof. One such fluorinated plastic is polytetrafluoroethylene (PTFE). Adjacent to the inert barrier  22   a  is a gas permeable chemical component  26 , shown as “potassium permanganate impregnated filter paper” in  FIG. 1 , that is used to remove or degrade a gas that is present in the sample that is not the analyte of interest. For example, potassium permanganate impregnated filter paper can be used in this regard. Adjacent to the gas permeable chemical component  26  is a gas permeable, inert barrier  18 , shown as “PTFE separator disk” in  FIG. 1 . The inert barrier  18  may be the same or different than inert barrier  22   a.  Next to the inert barrier  18  is a second gas permeable, chemical component  28 , shown as “carbon cloth” in  FIG. 1 . A gas permeable, inert barrier  22   b,  shown as “PTFE seal” in  FIG. 1 , can be located next to the second chemical component  28 . A number of materials could, in principle, be used as the inert barrier provided that they meet the criteria of being chemically inert and possess sufficient porosity so as not to significantly restrict gas flow through the filter compartment. Also, a physical porous separator can be used when combinations of the filter material include one component that is a strong oxidant that is capable of oxidizing the other filter component, or a strong reducing agent that is capable of reducing the other filter component, or an acidic filter component and a basic filter component. 
         [0014]    The interior volume of the housing includes an electrolyte compartment  16  containing an electrolyte  16   a,  and an electrode assembly including a counter electrode  20   b,  a reference electrode  20   a  and a sensing electrode  20   c . The electrolyte can be sulfuric acid. The sensing electrode  20   c  can be adjacent to the inert barrier  22   b  and a current collector  24   c.  The reference electrode  20   a  is adjacent to the current collector  24   c  and the counterelectrode  20   b.  The current collector  24   a  is adjacent to the counter electrode  20   b.    
         [0015]    A control circuit  30  is connected to the housing and controls the sensor. Alternatively, the control center can be separated from the housing. The control circuit refers to the external circuit, which might be a potentiostat or a simple load resistor plus downstream signal acquisition and display hardware. 
         [0016]    The gas sensor can operate in a diffusion mode or in an in-line mode, and includes a gas inlet and gas outlet. 
       EXAMPLE 1 
       [0017]    Several different gas sensors were exposed to 5 minutes of air, followed by 5 minutes of 200 ppm carbon monoxide (CO), followed by 5 minutes of air, then the detection capacity of the sensors for carbon monoxide was noted. The T90 is calculated as the time taken to for the sensor&#39;s output to reach 90% when stabilized in 200 ppm CO (response after 5 minutes). The graph of  FIG. 2  shows the T90 of three different CO gas sensors, namely 7EF, 7E/F and the claimed gas sensor (partitioned 7EFF) when those gas sensors were new as compared to those sensors after 8 months of use. (The 7E and 7E/F are electrochemical CO sensors manufactured by City Technology Ltd, UK.) The graph shows that there is very little if any change in the claimed gas sensor after 8 months of use whereas the other gas sensors show changes in response times after 8 months of use. Thus, the claimed gas sensor has a longer life than currently available gas sensors. 
         [0018]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope hereof. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. Further, logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be add to, or removed from the described embodiments.