Abstract:
An electrochemical oxygen sensor includes a micro-porous plastic membrane supported on a sealing disk and located between a gas inflow port and the sensor&#39;s electrolyte. The membrane and disk minimize thermal shock effects due to using the sensor at a first location, at a first temperature, and then moving it to a second location at a different temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This continuation application claims the benefit of the filing date of parent U.S. patent application Ser. No. 11/877,331 filed Oct. 23, 2007, entitled “Oxygen Sensors”, incorporated herein by reference, which claimed the benefit of the filing date of U.S. Provisional Application Ser. No. 60/863,859 filed Nov. 1, 2006 and entitled “Oxygen Sensors”. 
     
    
     FIELD 
       [0002]    The invention pertains to electrochemical gas sensors. More particularly, the invention pertains to electrochemical oxygen sensors which are resistant to thermal shock. 
       BACKGROUND 
       [0003]    Electrochemical sensors are known and can be used to detect various types of gases including oxygen as well as other types of gases. 
         [0004]    Representative sensors have been disclosed in U.S. Pat. No. 5,668,302 to Finbow et al. entitled Electrochemical Gas Sensor Assembly, issued Sep. 16, 1997, and U.S. Pat. No. 5746,899 to Finbow et al. entitled Electrochemical Gas Sensor, issued May 5, 1998. The &#39;302 and &#39;899 patents have been assigned to the assignee hereof and are incorporated by reference. Useful as they have become, such sensors are not without some limitations. 
         [0005]    A fairly common problem experienced by users of portable oxygen gas detection equipment is that the instrument can be susceptible to thermal shock and generate false alarms when the user moves between locations at different temperature. Typical conditions that might generate the false alarm condition would be when the user exits a heated office or calibration station into a cold working environment. 
         [0006]    This situation is most noticeable in winter when the temperature difference often exceeds 30° C. Whilst the effect is often associated with a negative temperature change i.e. movement from a warm to cooler environment, the same effect can also manifest itself in the opposite sense when there is a positive temperature change. 
         [0007]    Thermal shock in an oxygen sensor or cell is usually characterized by a rapid change in output response other than that caused by normal diffusion, when a change in temperature is experienced by the cell. Thermal shock does not always happen immediately and thermal shocks have been noticed after time periods of over one hour after the initial temperature change occurred. This has implications in a finished product of false alarms where a false oxygen level is registered by the cell. 
         [0008]    The cause of the problem is related to the design and construction of the oxygen sensor which relies on controlled diffusion of oxygen into the sensor from the external environment via a capillary hole. Once oxygen has entered the cell it reacts and generates a current that is proportional to the oxygen concentration in the external environment. 
         [0009]    Large temperature excursions can cause an additional contribution to the signal when the internal cell pressure (caused by the temperature change) equilibrates with the environment. The causes of these pressure differences include air, or, gas pockets within the body of the cell which expand or contract with temperature. 
         [0010]    For the typical condition described above, the gas inside the cell contracts when the instrument is transferred to the cold environment. The pressure difference caused through contraction draws air into the cell through the capillary leading to an enhanced cell output and false alarm. 
         [0011]    Thus, there continues to be a need for improved oxygen sensors which minimize false alarms. Preferably such improved functionality could be achieved without substantially increasing the manufacturing complexity and cost of such units. 
         [0012]    Also, it would be preferable if such improved detectors could be implemented as portable or human wearable to facilitate use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a plan view of a sealing disk according to one embodiment of the present invention; 
           [0014]      FIG. 2  illustrates a side elevational view of a sealing disk as in  FIG. 1 ; 
           [0015]      FIG. 2A  is an enlarged partial view of an edge of the disk of  FIG. 2 ; 
           [0016]      FIG. 2B  is an enlarged partial view of a central region of the disk of  FIG. 2 ; and 
           [0017]      FIG. 3  is an exploded view that illustrates the components of an oxygen cell according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    While embodiments of this invention 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 of the invention, as well as the best mode of practicing same, and is not intended to limit the invention to the specific embodiment illustrated. 
         [0019]    Embodiments of the invention solve the thermal shock problem by creating a partition in the cell that prevents bulk flow or exchange of air between the cell and the environment. In a preferred embodiment of the invention, the partition comprises a micro-porous plastic membrane supported on a compressible foam adhesive gasket which is itself supported on a rigid plastic perforated disk. 
         [0020]    The disk serves a number of functions. The primary role is to support the foam gasket and membrane. Additionally, an air tight seal can be formed between the plastic disk and the cell body or housing. This is achieved, in a disclosed embodiment, by ultrasonically welding the plastic disk to the main outer plastic body of the cell. The weld also serves to form an air tight seal around the metallic current collector strip that passes between both compartments within the cell. 
         [0021]    It is preferable that the membrane be adequately supported and not be able to deform or flex under pressure, since this behavior can also generate pressure transient effects of sufficient magnitude to cause false alarms. The disk can be annular with a hole in the middle to allow electrolyte ions to pass through the partition between the upper and lower compartments of the cell thereby facilitating normal operation of the cell. Alternatively the single central hole in the disk could be replaced with a plurality of smaller holes, each also capable of allowing electrolyte transport across the disk to the electrode from the main body of the cell. 
         [0022]    The function of the micro-porous membrane is to prevent the bulk transport of gas (usually in the form of bubbles) through the cell. It relies on the fact that liquid is held in the pores of the membrane by surface tension and capillary action, and that pressure must be applied to overcome these forces before bubbles/air are able to cross the membrane. The minimum pressure (bubble point pressure) required to force the liquid out of the membrane pores is related to the membrane capillary geometry according to the following equation; P=(4k cosθσ)/d where P is the bubble point pressure, d is the pore diameter, k is a geometry correction factor, θ is the liquid solid contact angle and σ is the surface tension. Therefore if a membrane material is chosen with pores of sufficiently small diameter, the pressure required for gas to cross the membrane will exceed that created by environmental temperature changes, thereby eliminating the temperature transient effect. 
         [0023]    An estimate of the pressure difference that the membrane needs to withstand to eliminate the problem can be calculated from the gas law (PV=nRT). Where P, V, T, and n are pressure, volume, temperature, and amount of gas with R as the gas constant. For a 40° C. temperature reduction the internal cell pressure decreases by 13.3% which is equivalent to 0.133 bar. Therefore a suitable membrane material can be expected to have a minimum bubble point pressure of at least 0.13 bar. In practice materials are chosen with values that exceed this. 
         [0024]    The compressible foam gasket serves two purposes. The adhesive surfaces ensure an air tight seal between the supporting shelf and the membrane material; in addition the compressible nature of the material ensures that any “dead volume” in the upper partition is minimized. The amount of free “dead volume” in the upper partition is associated with the size of the initial thermal transient that all lead based oxygen sensors show on rapid changes of temperature. 
         [0025]    Embodiments of the invention include a micro-porous plastic membrane material. The membrane material is not swelled or deformed by electrolyte, nor chemically degraded by electrolyte, or reaction products of oxygen reduction. 
         [0026]    The Micro-pores in the membrane also allow transport of ions through the film unlike some solid membranes which only allow water migration. Therefore the membrane material does not promote osmosis which may under certain circumstances prove to be an issue. 
         [0027]    Use of a plastic support disk, or plate, as in  FIG. 1 , which can be ultrasonically welded to a cell body to provide air tight seal with the body and current collector, improves cell reliability and life-time. The disk also stops the membrane from flexing under pressure. Use of a compressible gasket minimizes “dead volume” in a cap or cover for the cell thereby reducing initial thermal transient effects. 
         [0028]      FIGS. 1 ,  2 - 2 B illustrate various aspects of a disk or partition  12  in accordance with the invention. As illustrated therein, disk  12  is a rigid, generally annular shaped member with first and second spaced apart planar surfaces  12 - 1 ,  12 - 2  bounded by a peripheral support  12 - 3 . The disk  12  has a central opening  12 - 4 . The disk  12  carries a compressible foam adhesive gasket  14 . The gasket in turn carries a selected micro-porous plastic membrane  16 . 
         [0029]    As noted above, the disk  12  can be ultrasonically welded to an external housing of a respective oxygen sensor. As will be understood by those of skill in the art, disk  12  could alternately be perforated by one or more openings therethrough. The openings need not be centrally located, but could be distributed across the disk. Alternately, disk  12  could be formed of a material permeable to a selected electrolyte. 
         [0030]      FIG. 3  is an exploded view of a representative oxygen sensor  30  which embodies the present invention. The cell  30  includes a hollow, cylindrical body  32  which defines an interior region  34  for an electrolyte for the cell. 
         [0031]    A top end  36  of the body  32  defines an annular region indicated generally at  38  which can receive and support a multiple element separator filter  46 . 
         [0032]    The sealing disk  12  as previously discussed, is supported by the annular surface  38  and can be ultrasonically welded to the body  32 . The compressible foam adhesive gasket  14 , also annular in shape, having a central opening  14   a,  overlays and is supported by the disk  12 . Filter  18  fills the opening  14   a  of the gasket  14 . 
         [0033]    Membrane  16  overlays the gasket  14 . The cell  30  is closed with a cap  40  which could be affixed to the body  32  by welding or adhesive. The cap  40  can carry a working electrode  42 . The body  32  can carry an internal current collector element  44 . 
         [0034]    In assembling one embodiment of the present invention, as in  FIG. 3 , the plurality of separator filters  46  is located centrally on the top of the cell body molding  32 . The plastic sealing disk,  12 , as in  FIG. 1  for example, is then placed centrally within the recess  38  of the body molding  32  on top of the separator filters  46  ensuring that the internal current collector  44  is located between the plastic sealing disc  12  and body molding  32 . The sealing disc  12  is then ultrasonically welded to the body molding  32 . 
         [0035]    Partition  10  is a multi-component separator structure. An adhesive foam annulus  14 , such as a closed cell EPDM, is located centrally on the plastic sealing disk  12 . A plurality of small separator filters  18  is then located in the opening  14   a  formed by the annular foam gasket  14  and plastic sealing disc  12 . The micro-porous membrane  16  is located centrally on top of the adhesive foam disc or gasket  14 . 
         [0036]    The internal current collector  44  is then folded over the nylon ionic membrane  16 . The pre-assembled cap molding  40  and working electrode  42  is then placed on to the cell body assembly  32  and ultrasonically welded into place to complete cell  30 . 
         [0037]    Embodiments of the invention remove thermal shock within an oxygen cell by sealing the lead chamber of the cell from the working electrode preventing gas transfer between the two. Thermal transients (The initial peak in cell output when exposed to a sudden reduction in temperature) can be reduced by removing free volume and therefore trapped gas in the top half of the cell within the area between ionic membrane and working electrode. 
         [0038]    By using a plastic disc such as disk  12  as a support for a nylon membrane, such as membrane  16  and welding the disc to the body  32  of the oxygen cell such as cell  30  it is possible to create a gas tight seal between upper and lower parts of the cell and remove thermal shock effects that are caused by transfer of gas between the two parts. Further, by using an EPDM Closed foam gasket, such as gasket  14 , between the plastic disc  12  and membrane  16  it is possible to lower the amount of free volume in the top part of the cell  30  and therefore lower the initial thermal transient. 
         [0039]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. 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.