Abstract:
A gas sensor includes a gas detector, a reference gas generator, and a circuit. The reference gas generator includes a heater and a gas releasing material. The gas releasing material is in proximity to the heater such that, when the heater is energized during calibration, the gas releasing material releases an overpressure of a reference gas to the gas detector and such that, when the heater is not energized, the gas releasing material releases no substantial overpressure of the reference gas to the gas detector. The circuit energizes the heater during calibration and is responsive to an output of the gas detector during the period when the gas detector is provided the reference gas so as to calibrate the gas sensor.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention relates to the internal generation of a reference gas that is used to calibrate a gas sensor.  
       BACKGROUND OF THE INVENTION  
       [0002]     Gas sensors have been in use for some time to sense various gases such as hydrogen, oxygen, carbon monoxide, etc. One form of a gas sensor is an electrochemical cell that uses a catalytic electrode so that the gas to be detected is either oxidized or reduced with the exchange of electrons. The flow of current due to the oxidation or reduction of the gas is then detected as a measure of the concentration of the gas to be detected.  
         [0003]     However, a known problem with gas sensors is that they lose sensitivity over time. For example, the working life of an electrochemical cell is determined by the activity of the cell&#39;s catalytic electrode that is used to detect gas within the sensor. This activity is gradually reduced over time by contaminant gases and poisons such that the sensitivity of the sensor drifts downward.  
         [0004]     Other types of gas sensors, such as pellistor sensors, biomimetic sensors, and tin oxide sensors that may be formed as thin film, thick film, sintered or MOSFET devices, may have similar problems.  
         [0005]     If the instrument into which the gas sensor is built is calibrated regularly, this downward sensitivity drift can be compensated for by adjusting the gain of the gas sensor, and any faulty gas sensors can be replaced immediately. However, if the instrument is in a difficult position to service, or if calibration of the gas sensor is not otherwise freely available, it is often impossible to confirm that the gas sensor is functioning correctly. Therefore, as the gas sensor reaches the end of its working life, the output of the sensing cell may be insufficient to generate an alarm condition. As a result, a situation could arise where toxic levels of gas are present, but the gas sensor is incapable of providing the requisite warning.  
         [0006]     A substantial effort has been invested in determining a method by which the function of a gas sensor, such as an electrochemical cell, can be checked without the need for an externally generated calibration gas. For example, it has been proposed to use additional electronic components in order to check conductive pathways through the gas sensor. While such methods can uncover broken connections, they do not provide any information on the condition of the electrodes in terms of their ability to react with the gas to be detected.  
         [0007]     When external gas source are used, gas detectors for industrial applications are normally calibrated to correct for drift. Toxic gas detectors are normally calibrated to measure around the Occupational Exposure Level, which for most toxic gases will be less than 50 ppm, an extremely low level. Because of the difficulty in preparing gas/air mixtures at this dilution, because some toxic gases such as hydrogen sulphide and sulphur dioxide are readily absorbed by the materials used to make the calibration gas cylinder housings, and because the stability of these mixtures can be a problem, calibration gas cylinder have a limited shelf life.  
         [0008]     For certain gases, calibration can be done using another gas to which a gas sensor is cross sensitive. Some examples are given in the following table:  
                                                       Sensor   Calibration Gas   Equivalent to                           0 10 ppm acid gas   10 ppm chlorine   10 ppm acid gas           0 10 ppm nitrogen   10 ppm chlorine   9 ppm nitrogen           dioxide       dioxide           0 25 ppm hydrogen   10 ppm sulphur   28 ppm hydrogen           cyanide   dioxide   cyanide           0 10 ppm chlorine   10 ppm chlorine   4 ppm chlorine           dioxide       dioxide           0 2.5 ppm phosphine   10 ppm sulphur   2 ppm phosphine               dioxide           0 1 ppm ozone   2 ppm chlorine   1 ppm ozone           0 10 ppm hydrogen   5 ppm hydrogen   10 ppm hydrogen           fluoride   chloride   fluoride                      
 
 These equivalent values may vary when electrode materials vary and filters vary. 
 
         [0009]     The present invention relates to the use of internally-generated H2 as calibration gas for reducing and oxidizing gas sensors.  
         [0010]     The present invention relates to an apparatus and method for self-calibration of gas sensors.  
       SUMMARY OF THE INVENTION  
       [0011]     According to one aspect of the present invention, a self-calibrating gas sensor comprises a gas detector and a reference gas generator. The reference gas generator includes a heater and a gas releasing material. The gas releasing material is in proximity to the heater such that, when the heater is energized during calibration, the gas releasing material releases an overpressure of a reference gas to the gas detector and such that, when the heater is not energized, the gas releasing material releases no substantial overpressure of the reference gas to the gas detector.  
         [0012]     According to another aspect of the present invention, a self-calibrating gas sensor comprises a gas detector, a reference gas generator, and a circuit. The reference gas generator includes a heater and a gas releasing material. The gas releasing material is in proximity to the heater such that, when the heater is energized during calibration, the gas releasing material releases an overpressure of a reference gas to the gas detector and such that, when the heater is not energized, the gas releasing material releases no substantial overpressure of the reference gas to the gas detector. The circuit energizes the heater during calibration and calibrates the gas sensor in response to an output of the gas detector during the period when the gas detector is provided the reference gas.  
         [0013]     According to still another aspect of the present invention, a self-calibrating gas sensor comprises a gas detector and a hydrogen generator. The gas detector comprises an electrochemical cell. The hydrogen generator includes a heater and a metal hydride. The metal hydride is in proximity to the heater such that, when the heater is energized during calibration, the metal hydride releases an overpressure of hydrogen to the gas detector and such that, when the heater is not energized, the metal hydride releases no substantial overpressure of hydrogen to the gas detector.  
         [0014]     According to still another aspect of the present invention, a self-calibrating gas sensor comprises a gas detector, a reference gas generator, and a continuous housing. The reference gas generator includes a heater and a gas releasing material. The gas releasing material is in proximity to the heater such that, when the heater is energized during calibration, the gas releasing material releases a reference gas to the gas detector and such that, when the heater is not energized, the gas releasing material releases no substantial overpressure of the reference gas to the gas detector. The reference gas generator and the gas detector are housed within the continuous housing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which:  
         [0016]      FIG. 1  illustrates a self-calibrating gas sensor according to one embodiment of the present invention;  
         [0017]      FIG. 2  shows the reference gas generator of the self-calibrating gas sensor shown in  FIG. 1  in additional detail; and,  
         [0018]      FIG. 3  shows a circuit that can be sued in the self-calibration process. 
     
    
     DETAILED DESCRIPTION  
       [0019]     As shown in  FIG. 1 , a self-calibration gas sensor  10  has a housing that, for example, may be in the form of a can  12  that is covered at its open end by a cover  13 . The can  12  and the cover  13 , for example, may be made of nickel plated steel. The can  12  holds a substance  14  such as water or water gel or hydrophilic oxides like silica gel in an antiseptic solution. The substance  14  provides a source of water vapor for the self-calibration gas sensor  10 . As shown in  FIG. 1 , the substance  14  has a level  16 , although the can  12  can contain more or less of the substance  14 . Other materials can be used for the substance  14  depending upon the particular application temperature range for the self-calibration gas sensor  10 .  
         [0020]     A support plate  18  is provided in the can  12  above the level  16  of the substance  14 . The support plate  18  has a hole  20  therethrough to permit the flow of vapor from the substance  14  through the support plate  18 . The support plate  18 , for example, may be a stainless steel washer.  
         [0021]     A gas detector  22  is supported by the support plate  18 . The gas detector  22 , for example, may be in the form of an electrochemical cell. As such, the gas detector  22  includes lower and upper cell plates  24  and  26 , a solid electrolyte membrane  28 , and lower and upper catalyst electrodes  30  and  32 . The lower and upper cell plates  24  and  26 , for example, may be hydrophobic Teflon™ disks.  
         [0022]     The lower cell plate  24  is sandwiched between the support plate  18  and lower catalyst electrode  30 , the lower catalyst electrode  30  is sandwiched between the solid electrolyte membrane  28  and the lower cell plate  24 , and the upper catalyst electrode  32  is sandwiched between the solid electrolyte membrane  28  and the upper cell plate  26 . The catalyst electrode  30  and  32 , for example, may comprise an element from the group Au, Pt, Pd, Ru, Rh, Ir, Os, Ag, etc., or an alloy or mixture of the elements from this group, or porous elements of the group mixed with carbon black, or porous elements of the group mixed with carbon black and Nafion particles. The solid electrolyte membrane  28  may be Nafion or Nafion composite like Nafion/7SiO 2 —2P 2 O 5 —ZrO 2 , and Nafion/ZrP particles or the Sandia Polymer Electrolyte Alternative (SPEA) for higher temperature applications. The gas detector  22  may be of the type shown in one or more of U.S. Pat. Nos. 4,025,412, 4,123,700, and 4,171,253.  
         [0023]     A reference gas generator  34  internally generates a reference gas that is provided to the gas detector  22  so that the gas detector can be self-calibrated. The reference gas generator  34  includes a reference gas generating chamber  36  and a gas diffusion control plate  38 . The gas sensor  10  also includes an active charcoal filter  40 . The gas diffusion control plate  38  separates the reference gas generating chamber  36  and the active charcoal filter  40  from the gas detector  22  and abuts the upper cell plate  26 .  
         [0024]     As shown in  FIG. 2 , the reference gas generating chamber  36  houses a heater  42  and a material  44  that is in proximity to the heater  42 . The material  44 , when heated, produces the reference gas. For example, the material  44  may be a metal hydride that, when the heater  42  is energized, is heated to a known temperature and consequently produces a known overpressure of hydrogen. The overpressure causes the reference gas, such as hydrogen, to flow through holes  48   b  in the gas diffusion control plate  38  directly into to the gas detector  22 .  
         [0025]     There are many possible metal hydride materials that could be used to generate hydrogen when heated. Preferable metal hydride materials include titanium hydride, magnesium hydride and magnesium nickel hydride.  
         [0026]     Accordingly, when the self-calibration gas sensor  10  is to be calibrated, the heater  42  is energized to heat the material  44  to a predetermined temperature and for a predetermined time that causes the material  44  to release an overpressure of the reference gas which is supplied to the gas detector  22 . The gas detector  22  senses the reference gas thus generating a reference signal from the lower and upper catalyst electrodes  30  and  32 . This signal is used to perform self-calibration. After such self-calibration, the heater  42  is de-energized so that the overpressure of the reference gas falls to a negligible level. Such self-calibration of the self-calibration gas sensor  10  can be intermittently repeated as desired.  
         [0027]     As shown in  FIG. 1 , the can  12  forms a continuous housing that houses the gas detector  22  and the reference gas generator  34 . Accordingly, in this construction of the present invention, the gas detector  22  and the reference gas generator  34  are not housed in separate and separated housings.  
         [0028]     As shown in  FIG. 3 , a controller  50  provides an output  52  based on the gas detected by the gas detector  22  and controls the reference gas generator  34  to calibrate the gas detector  22 . The output  52  may be coupled to various devices. For example, the output  52  may be coupled to an alarm indicator to produce a warning when the level of the detected gas exceeds a predetermined limit, or the output  52  may be coupled to an apparatus such as a ventilator to control the effects of the gas being detected. The self-calibration could be pre-programmed to operate twice a year or once a year. Calibration could also be initiated through pushing an external button. When self-calibration is in process, the controller  50  should provide an alarm/warning that self-calibration is being performed, and that the controller  50  may be out of function momentarily.  
         [0029]     The lower and upper catalyst electrodes  30  and  32  are coupled between the terminals of a source through a resistor  54 . The junction between the resistor  54  and the gas detector  22  is coupled to an amplifier  56  having a gain controlling element  58  in a feedback circuit around the amplifier  56 . The output of the amplifier  56  is coupled to a processor  60  that provides the output  52 , that controls the gain controlling element  58 , and that controls a switch  62  to selectively connect a source S to the heater  42  so as to energize the reference gas generator  34 .  
         [0030]     During normal operation, the processor  60  provides the output  52  based on the output of the amplifier  56  and controls the switch  62  so that the switch  62  is open. Thus, the reference gas generator  34  is de-energized and the output  52  indicates the level of ambient gas normally being detected by the gas detector  22 . This ambient gas normally being detected by the gas detector  22  enters the gas sensor  10  through one or more suitable holes (not shown) in the can  12 , flows through the active charcoal filter  40 , then flows through one or more holes  48   a  of the gas diffusion control plate  38  into the gas detector  22 .  
         [0031]     During self-calibration, the processor  60  controls the switch  62  so that the switch  62  is closed. Thus, the reference gas generator  34  is energized to produce the reference gas and to provide the reference gas to the gas detector  22  as described above. The processor  60  receives the output of the amplifier  56  and controls the gain controlling element  58  accordingly until the output of the amplifier  56  is at a desired calibration level. Accordingly, the self-calibration gas sensor  10  is calibrated.  
         [0032]     The controller  50  may intermittently repeat the above described calibration as many times as necessary or desired. The time periods between such repeated calibrations may be periodic or aperiodic and may be of any length as desired.  
         [0033]     The circuit  50  can be mounted as a chip or otherwise on a board or other support within the can  12 . The output  52  may then be run to the exterior of the can  12 .  
         [0034]     Certain modifications of the present invention have been discussed above. Other modifications will occur to those practicing in the art of the present invention. For example, the gas detector  22  as discussed above may be an electrochemical cell. Alternatively, the gas detector  22  may be a pellistor sensor, a biomimetic sensor, and a tin oxide sensor, or other gas detector.  
         [0035]     Moreover,  FIG. 1  illustrates an embodiment of the present invention in which the can  12  forms a continuous housing that houses the gas detector  22  and the reference gas generator  34 . However, the gas detector  22  and the reference gas generator  34  may instead be housed in separate and separated housings.  
         [0036]     Furthermore, metal hydrides typically produce an over pressure of hydrogen at all temperatures, but the over pressure usually increases strongly with increasing temperature. Thus, while metal hydrides may produce a small amount of hydrogen even though they are unheated, metal hydrides can be chosen (e.g., those listed above) in which the hydrogen released at normal (unheated) temperatures is so small as to be undetectable, but in which a detectable amount is released when heated.  
         [0037]     Also, the reference gas generating chamber  36  is preferably, although not necessarily, sealed. If sealed, it may be possible for the over pressure in the reference gas generating chamber  36  to become excessive. In this case, a suitable pressure relief valve or other compensator may be provided to maintain the over pressure below an acceptable limit.  
         [0038]     Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.