Abstract:
A differential pH probe design uses a container having an outer surface and an inner volume, where the inner volume is divided into a first chamber and a second chamber. A first pH-sensitive area is located on the outer surface of the first chamber where the first pH-sensitive area is configured to be exposed to a sample. A second pH-sensitive area is located on the outer surface of the second chamber where the second pH-sensitive area is shielded from the sample and is exposed to a buffer solution. A first electrode is configured to detect a first voltage in the first chamber and a second electrode is configured to detect a second voltage in the second chamber. Circuitry is coupled to the first and second electrodes and configured to process the first voltage and the second voltage to determine a pH of the sample.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application No. 60/785,339 filed on Mar. 23, 2006 entitled “Differential pH probe,” which is hereby incorporated by reference into this application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention is related to the field of pH measurements, and in particular, to a differential pH probe. A pH probe typically operates using an active chamber that measures a voltage across a pH sensitive material immersed in a sample. Differential pH sensors also use a reference chamber that measures a voltage across a pH sensitive material immersed in a buffer solution having a known pH, typically with a pH of 7. The differential probe uses the active voltage and the reference voltage to determine the pH of the sample. Current pH probes are typically complex designs with many fluid seals and may be large and costly to manufacture. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  illustrates glass piece  100  used in differential pH probe  150 , in an example embodiment of the invention. 
           [0004]      FIG. 2  illustrates glass piece  100  with seals, in an example embodiment of the invention. 
           [0005]      FIG. 3  illustrates glass piece  100  with seals and circuitry, in an example embodiment of the invention. 
           [0006]      FIG. 4  illustrates differential pH probe  150 , in an example embodiment of the invention. 
           [0007]      FIG. 5  illustrates differential pH probe  150  with temperature sensors, in an example embodiment of the invention. 
           [0008]      FIG. 6  illustrates glass piece  130  used in a differential pH probe in an example embodiment of the invention. 
           [0009]      FIG. 7  illustrates a variation for conductive enclosure  120  in another example embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0010]      FIGS. 1-7  and the following description and exhibits depict specific examples to teach those skilled in the art how to make and use the best mode of the invention. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these examples that fall within the scope of the invention. Those skilled in the art will appreciate that the features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to the specific examples described below, but only by the claims and their equivalents. 
         [0011]      FIG. 1  illustrates glass piece  100  used in differential pH probe  150  (shown in  FIG. 4 ), in an example embodiment of the invention. Glass piece  100  is depicted as a tube, although other suitable shapes could be used. A generalized cylinder is a cylinder where the cross section can be any shape. Glass piece  100  includes active areas  101  and  103 , in addition to, non-active areas  102  and  104 . Active areas  101  and  103  are formed by pH sensitive glass. An example of pH-sensitive glass is lithium-ion conductive glass. Non-active areas  102  and  104  are formed by non-pH sensitive glass. Note that alternative materials other than glass could be used for piece  100 , such as pH-sensitive polymers and plastics. 
         [0012]    Note that both the active and non-active areas are integrated together to form a single piece of glass—glass piece  100 . This integration could be accomplished by treating a single glass tube to form the active and non-active areas. Alternatively, the active and non-active areas could be formed separately from one another and then fused together to form glass piece  100 . 
         [0013]    Note that active areas  101  and  103  share the same axis making them co-axial with one another. The co-axial configuration allows for a large active area  101  while reducing the overall size of probe  150 . The single piece configuration provides structural strength and requires fewer seals than a multiple piece configuration. 
         [0014]      FIG. 2  illustrates glass piece  100  from  FIG. 1 , in an example embodiment of the invention. Glass piece  100  now has seals  105 ,  106 , and  107 . Seals  105 - 107  could be rubber, silicon, or some other suitable insulating material. Active area  101  and seal  105  form a first chamber referred to as the active chamber. Active area  103  and seals  106 - 107  form second chamber referred to as the reference chamber. Both the active and reference chambers are filled with an electrolyte solution. In one example embodiment of the invention, glass piece  100  may be called a container that is divided into a number of different chambers. 
         [0015]      FIG. 3  illustrates glass piece  100  from  FIG. 2  and also shows circuitry  110 . Glass piece  100  includes active electrode  111  that is exposed within the active chamber and then runs to circuitry  110 . Note that insulating tube  113  is used so that active electrode  111  runs through the center of the reference chamber, but is not exposed within the reference chamber. Glass piece  100  also includes reference electrode  112  that is exposed within the reference chamber and then runs to circuitry  110 . 
         [0016]      FIG. 4  illustrates differential pH probe  150  in an example of the invention. Probe  150  includes glass piece  100  and circuitry  110  as described in  FIGS. 1-3 . Probe  150  also includes conductive enclosure  120 . Conductive enclosure  120  could be tube-shaped like glass piece  100 , although other shapes could be used. Glass piece  100  and circuitry  110  are placed within conductive enclosure  120 . 
         [0017]    Conductive enclosure  120  includes seals  121 ,  122 , and  123 . In this example with glass piece  100  and enclosure  120  being tube-shaped, seals  121 - 123  could be doughnut-shaped discs, although other shapes could be used in other examples. These disks could have much larger contact areas than conventional o-rings to provide better seals. Seals  121 - 123  could be rubber, silicon, or some other insulating material. Seals  121 - 122  provide a junction that allows electrical conductivity, but not fluid transfer, between the buffer chamber and the sample being tested. To provide this junction, seals  121 - 122  could be silicon disks with ceramic frits (tubes), where seals  121 - 122  are separated by a salt gel to form a salt bridge. 
         [0018]    Seal  121  seals the end of enclosure  120  so that active area  101  of the active chamber may remain exposed to an external sample, but so that the external sample will not enter enclosure  120 . Enclosure  120 , seals  122 - 123 , and active area  103  form a buffer chamber around active area  103  of glass piece  100 . This buffer chamber is filled with a buffer solution that maintains a constant pH—typically seven. 
         [0019]    Circuitry  110  is grounded to conductive enclosure  120  by electrical line  113 . Circuitry  110  is coupled to plug  115  by electrical lines  114 . Thus, circuitry  110  communicates with external systems through lines  114  and plug  115 . In other embodiments, circuitry  110  may communicate with an external system using a wireless or non-contact technology, for example an optical link or an RF link. 
         [0020]    In operation, active area  101  of probe  150  is dipped into the sample whose pH will be determined. Note that seal  121  prevents the sample from entering enclosure  120 . The sample (with unknown pH) interacts with active area  101  to produce a first voltage across active area  101 . This first voltage is referred to as the active voltage and corresponds to the unknown pH of the sample. Active electrode  111  detects the active voltage and indicates the active voltage to circuitry  110 . 
         [0021]    In a similar manner, the buffer solution (with known pH) interacts with active area  103  to produce a second voltage across active area  103 . This second voltage is referred to as the reference voltage and corresponds to the known pH of the buffer solution. Reference electrode  112  detects the reference voltage and indicates the reference voltage to circuitry  110 . 
         [0022]    Circuitry  110  processes the active and reference voltages in the conventional manner to determine the pH of the sample. Circuitry  110  indicates the pH of the sample to external systems (not shown) that are plugged into plug  115 . 
         [0023]    Conductive enclosure  120  is typically held by hand during testing. Note that conductive enclosure  120  electrically shields the internal components of probe  150  (electrodes  111 - 112 , circuitry  110 ) from hand capacitance. Conductive enclosure  120  also provides a ground. Note that conductive enclosure  120  could be stainless steel, aluminum, or some other conductive material. In one example embodiment of the invention, conductive enclosure  120  may have a conducting part and a non-conducting part. The conductive part would begin just below seal  123  and would cover and shield the lower portion of the probe, including the circuitry  110 . The upper portion starting just below seal  123  would be made from a non-conductive material or have a non-conductive coating. When using the two part enclosure a separate ground rod may be located in the outer salt bridge seal  121 . 
         [0024]      FIG. 5  illustrates differential pH probe  150  in an example of the invention. Thermistor T 1  has been added to the active chamber to detect the temperature near active electrode  111 . Thermistor T 2  has been added to the reference chamber to detect the temperature near reference electrode  112 . Thermistors T 1  and T 2  could be integrated within seals  105 - 107 . Thermistor T 1  transfers its temperature information to circuitry  110  over electrical line  116 . Thermistor T 2  transfers its temperature information to circuitry  110  over electrical line  117 . Circuitry processes the temperature information from thermistors T 1  and T 2  to provide temperature compensation during the pH determination. In another embodiment of the invention, thermistor T 1  may be located on the outside of the active chamber (not shown) and be exposed to the sample and used to detect the temperature of the sample. In another embodiment of the invention, thermistor T 2  may be located in the buffer chamber. 
         [0025]      FIG. 6  illustrates an alternative to glass piece  100 . Note that some details from the previous figures are omitted for clarity. Glass piece  130  is now used for probe  150  instead of glass piece  100 . Glass piece  130  is similar to glass piece  100  with active areas  101  and  103  and non-active areas  102  and  104  separated by seals  105 - 107  to form the active and reference chambers. The variation from glass piece  100  is in the shape of the active chamber. Active area  101  is no longer a dome at the top of the glass piece, but is now formed by the walls of glass piece  130  in the same way that active area  103  forms the reference chamber. Thus, the active chamber has the same geometry as the reference chamber. Non-active glass  108  is used at the top of the active chamber, although a seal could be used instead of non-active glass  108  if desired. The top of the active chamber is protected by cap  122 . Cap  122  could be rubber, metal, or some other protective material that is adhered to glass piece  130 . 
         [0026]      FIG. 7  illustrates a variation for conductive enclosure  120 . Note that some details from the previous figures are omitted for clarity. Glass piece  130  is used, but glass piece  100  could be used as well. Enclosure  120  now extends above the active chamber of glass piece  130  to provide protection. The extension of enclosure  120  must still allow the sample to contact active area  101 , so openings in enclosure  120  should be provided for this purpose. The sample should still not be allowed to pass seal  121 .