Abstract:
A system and method are provided for monitoring the levels of combustible gas in a gas stream. The system includes means for controlling the relative humidity of the the gas stream to maintain a humidity level in the performance range of combustible gas sensors. A number techniques are illustrated for achieving the humidity control, including, secondary phase separations, mixing the gas with dry air and adjusting of the gas stream temperature.

Description:
FIELD OF INVENTION 
     This disclosure relates generally to the detection of combustible gases, and especially relates to the detection of hydrogen in a vent gas stream. 
     BRIEF DESCRIPTION OF THE RELATED ART 
     Hydrogen gas is used and produced in many applications. Since the amount of hydrogen in a gas stream produced by a given process may be an indicator of system efficiency, the systems typically utilize combustible gas sensors to determine the level of hydrogen. An example of a prior art system having an arrangement for monitoring combustible gas is shown in  FIG. 1A . The electrochemical system  12  receives water from an external source  14  and passes it through a deionizing bed  16 . Once the water has been properly conditioned, it is supplied to an electrochemical cell  18  which disassociates the hydrogen and oxygen. 
     One example of an electrochemical cell  18  is a proton exchange membrane electrolysis cell which can function as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gas, and can function as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity. Referring to  FIG. 1B , which is a partial section of a typical anode feed electrolysis cell  100 , conditioned water  102  is fed into cell  100  on the side of an oxygen electrode (anode)  116  to form oxygen gas  104 , electrons, and hydrogen ions (protons)  106 . The reaction is facilitated by the positive terminal of a power source  120  electrically connected to anode  116  and the negative terminal of power source  120  connected to a hydrogen electrode (cathode)  114 . The oxygen gas  104  and a portion of the process water  108  exit cell  100 , while protons  106  and water  110  migrate across a proton exchange membrane  118  to cathode  114 . At cathode  114 , hydrogen gas  112  is formed and removed. Water  110  is also removed from cathode  114 . 
     A typical fuel cell uses the same general configuration as is shown in  FIG. 1B . Hydrogen gas is introduced to the hydrogen electrode (the anode in fuel cells), while oxygen, or an oxygen-containing gas, such as air, is introduced to the oxygen electrode (the cathode in fuel cells). Water can also be introduced with the feed gas. The hydrogen gas for fuel cell operation can originate from a pure hydrogen source, hydrocarbon, methanol, or any other hydrogen source that supplies hydrogen at a purity suitable for fuel cell operation (i.e., a purity that does not poison the catalyst or interfere with cell operation). Hydrogen gas electrochemically reacts at the anode to produce protons and electrons, wherein the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water, which additionally includes any feed water that is dragged through the membrane to the cathode. The electrical potential across the anode and the cathode can be exploited to power an external load. 
     In other embodiments, one or more electrochemical cells can be used within a system to both electrolyze water to produce hydrogen and oxygen, and to produce electricity by converting hydrogen and oxygen back into water as needed. Such systems are commonly referred to as regenerative fuel cell systems. 
     After the electrochemical cell  18  ( FIG. 1A ) disassociates the water, oxygen and hydrogen gas exit the cell  18  through conduits  20  and  22 , respectively. As mentioned herein above, in addition to the gas products, water entrained in the gases exits with the oxygen and hydrogen. The hydrogen conduit  22  typically connects with a hydrogen phase separator  24  which extracts most of the water from the gas, with the water exiting the phase separator  24  through a valving arrangement which recycles the water back into the electrochemical cell water feed conduit. Depending on the needs of the application, additional water may be removed from the hydrogen gas by passing through an optional dessicant gas dryer  26  before exiting the process for use in the application. 
     The oxygen gas stream  20  also enters into a phase separator  28  with a majority of the water separating from the gas stream and dropping to the bottom of the separator  28 . As with the hydrogen separator  24 , this water is removed via a valving arrangement  30  and recycled into the electrochemical cell water feed conduit. The separated hydrogen gas exits the phase separator  28  via a conduit  32  to exit the process. Since it is desirable to monitor for the presence of hydrogen gas in the oxygen gas stream, the oxygen phase separator  28  includes a second outlet  34  which provides a sample gas stream through an orifice  40  to a combustible gas sensor  36 . A gas dryer  38 , such as a NAFION tube dryer, is usually placed inline between the phase separator  28  and the sensor  36  to remove water still entrained in the gas. Unfortunately, since the gas stream can still have a relative humidity greater than 95%, this high relative humidity results in lower monitoring performance than is desired. 
     Accordingly, what is needed in the art is a system for monitoring combustible gas levels in a gas stream that reduces or eliminates the effects of relative humidity on combustible gas sensing. 
     SUMMARY OF INVENTION 
     Disclosed herein are systems and methods for monitoring combustible gas levels in a gas stream. In an exemplary embodiment of a system for monitoring combustible gas that comprises: a first phase separator having first outlet; a second phase separator having an inlet and at least one outlet having a opening therefrom, said second separator inlet being fluidly connected to said first separator outlet; and a first combustible gas sensor adjacent said second separator outlet, said first sensor being spaced a predetermined distance from said second separator outlet opening. 
     Another embodiment includes a electrochemical system that comprises: an electrochemical cell stack having an oxygen outlet; a first phase separator having an inlet and at least one outlet, said inlet being connected to said cell stack oxygen outlet; a second phase separator having an inlet and at least one outlet having a opening therefrom, said second separator inlet being fluidly connected to said first separator outlet; and a first combustible gas sensor adjacent said second separator outlet, said first sensor being spaced a predetermined distance from said second separator outlet opening. 
     Another embodiment includes a system for monitoring combustible gas comprising: a first phase separator having at least one outlet; a housing having an inlet and at least one outlet, said housing inlet being connected to said first separator outlet; and a first combustible gas sensor mounted to said first housing, said first sensor having a sensing face being positioned generally perpendicular to said first housing inlet. 
     Another embodiment for an electrochemical system comprises: an electrochemical cell stack having an oxygen outlet; a first phase separator having an inlet and at least one outlet, said inlet being connected to said cell stack oxygen outlet; a housing having an inlet and at least one outlet, said housing inlet being connected to said first separator outlet; and a first combustible gas sensor mounted to said first housing, said first sensor having a sensing face being positioned generally perpendicular to said first housing inlet. 
     Another embodiment includes a system for monitoring combustible gas comprising: a gas temperature controller having an inlet and an outlet, said controller reducing the relative humidity of the gas to less than 95% relative humidity; and a combustible gas sensor coupled to said controller outlet. 
     Another embodiment includes a method for monitoring the level of combustible gas comprising: injecting a gas stream into a housing; impacting said gas stream into a wall; mixing said gas stream with air; and sensing levels of combustible gas in said mixed gas stream. 
     Another embodiment includes a method for monitoring the level of combustible gas comprising: separating water from a saturated gas stream in a first phase separator; flowing said gas stream through an orifice to restrict flow and decrease pressure of said gas stream; and monitoring the level of combustible gas in said gas stream. 
     Another embodiment includes a method for monitoring the level of combustible gas comprising: separating water from a saturated gas stream; controlling the temperature of said gas stream to reduce the relative humidity of said gas stream; and monitoring the level of combustible gas in said gas stream. 
     The above discussed and other features will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Referring now to the drawings, which are meant to be exemplary and not limiting, and wherein like elements are numbered alike: 
         FIG. 1A  is a schematic drawing of a electrochemical system having a combustible gas detection system used in the prior art; 
         FIG. 1B  is a schematic diagram of a partial prior art electrochemical cell showing an electrochemical reaction 
         FIG. 2A  is an illustration of the combustible gas sensor shown in  FIG. 1 ; 
         FIG. 2B  is an illustration of an exemplary embodiment of a combustible gas sensor; 
         FIG. 2C  is an illustration of an alternate embodiment of a combustible gas sensor; 
         FIG. 2D  is an illustration of another alternate embodiment of a combustible gas sensor; 
         FIG. 3  is a schematic drawing illustrating an exemplary embodiment of a system capable of detecting combustible gas in a vent stream; and 
         FIGS. 4–11  are a schematic drawings illustrating an alternate embodiments of a system capable of detecting combustible gas in a vent stream. 
     
    
    
     DETAILED DESCRIPTION 
     Hydrogen gas is a versatile material having many uses in industrial and energy applications ranging from the production of ammonia, to powering vehicles being propelled into space. Since the hydrogen molecule is one of the smallest known particles, containing and controlling leaks of hydrogen gas is very difficult. Monitoring of these leaks is important since it typically is an indicator of performance degradation and or component wear. Typically, prior art systems have used combustible gas sensors to monitor levels of combustible gas in the system. When unacceptable levels of hydrogen are detected in the system, the system is either shut down, or the operator is alerted that preventative maintenance is required. 
     Commercial combustible gas sensors typically use a technology referred to as a “catalytic bead” type sensor, such as the Detcon, Inc. Model FP-524C. These sensors monitor the percentage of “LEL” or lower explosive limit of combustible gas in a product gas stream. This LEL measurement represents the percentage of a combustible gas (hydrogen, propane, natural gas) in a given volume of air. One limitation of catalytic bead sensors is their sensitivity to moisture in the gas they are monitoring. Once the gas reaches 95% relative humidity, the ability of the sensor to detect combustible gas deteriorates resulting in less than desirable life and reliability performance. Many hydrogen applications, including but not limited to electrochemical cell, electrolyzers, fuel cells, and methane steam reformers, also utilize water in their process which tends to effect the relative humidity of the gas stream. It should be appreciated that while the examples described herein typically refer to electrochemical systems such as electrolyzers or fuel cells, the present invention can be equally applied in any application where a combustible gas needs to be monitored. 
     Since high relative humidity has undesirable effects, the present invention addresses these issues by three mechanisms: 1) through mixing of the gas stream with dry air, or 2) by controlling the temperature of the gas stream, or 3) by controlling the pressure of the gas stream. Referring to  FIGS. 2A–2D , four different combustible gas sensor arrangements are shown. As will be described in more detail herein, these sensor arrangements, either alone, or in combination with other components reduce the relative humidity of the sampled gas to increase the performance of combustible gas measurements. 
     The combustible gas sensor arrangement utilized by the prior art is shown in  FIG. 2A . In this arrangement, the CG sensor device  36  includes a CG sensor  42  and a housing  44 . The housing  44  is typically tubular in shape and attaches to the sensor  42  by any convenient means, such as a thread (not shown). The CG sensor  42  also includes a sensing face  43  which detects the levels of combustible gas, this face  43  is located opposite a housing open end  46 . A gas sample tube  48  is inserted into the open end  46 . During operation, the saturated gas stream  49  exits the sample tube  48  and mixes with the air in the housing allowing some drying of the saturated gas. 
     An exemplary embodiment of the CG sensor of the present invention is shown in  FIG. 2B . In this embodiment, the gas sample tube  48  is positioned a predetermined distance d from the sensor face  43 . An air stream  50  is moved through the gap defined by the distance d. During operation, the saturated gas stream  49  exits the sample tube  48  and mixes with the dry air stream  50  drying the gas stream  49  and reducing the relative humidity. When used inside an enclosure, the CG sensor arrangement  51  may additional benefits over the prior art when the air stream  50  is also the main ventilation path as well. This arrangement would allow the sensor arrangement to not only sense hydrogen originating from the phase separator  28 , but from elsewhere in the enclosure as well. 
     An alternate embodiment CG sensor arrangement of the present invention is shown in  FIG. 2C . This embodiment  53  is similar to that of  FIG. 2A , except that the CG sensor  42  is positioned above the housing  52  and is spaced from the sample tube  48  vertically by a predetermined distance y. The sample tube  48  enters the housing  52  through an opening  54  preferably at an angle generally perpendicular to the sensor face  43 . By sizing the distance y appropriately for a given gas flow rate, the sensor can be protected from inadvertent splashing or contamination by water from the gas stream  49 . The housing  52  includes an open end  56  opposite the CG sensor  42  to allow drainage of water. During operation, the saturated gas stream  49  enters the housing  52  from the sample tube  48 . In the housing, the gas stream mixes with dry air to reduce the relative humidity of the gas being monitored by the sensor  42 . Since the sensor  42  is vertically above the sample tube  48 , water and oxygen being heavier than air will drain away from the sensor  42  through the opening  56  while the lighter gases, such as hydrogen, will mix with the air and raise to the sensor face  43 . By adjusting the distance d′ between the end of the sample tube and the housing wall  58 , the mixing of the gas stream  49  with the dry air can be enhanced. Openings  55  may be optionally provided in the housing adjacent the sensor  42 , or between the housing  52  and the sensor  42  to prevent the buildup of gas resulting in erroneous measurements by the sensor  42 . 
     Another alternate embodiment of the CG sensor arrangement is shown in  FIG. 2D . This embodiment  59  is preferable in environments where excessive amounts of water or high levels of relative humidity may be expected. In this embodiment, the sample tube enters a housing  60  and mixes with dry air to reduce the relative humidity. At least one, and preferably several, openings  62  are located vertically above the sample tube  48  and generally opposite a drain opening  66 . The openings  62  can either be in the side wall (as shown in  FIG. 2D ), or in the top of the housing  60 . The openings allow the dried gas to disperse into a second housing  64  installed around the first housing  60 . The CG sensor  42  is coupled to the second housing  64  and is located generally above the first housing  60 . As the dried gas disperses, it enables the sensor  42  to monitor the levels of combustible gas. Optional holes  65  located in the second housing  64  prevent build up of gas in the second housing  64 . By arranging the sensor  42  in the second housing, the sensor  42  can be protected from liquid splashing onto the sensor, while minimizing the size of the assembly. 
     Referring now to  FIGS. 3–11 , the four CG sensor arrangements  36 ,  51 ,  53 ,  59  are arranged individually and in combination with each other and additional components to provide reduced relative humidity gas streams to the sensor  42 . 
     Referring to  FIG. 3 , an electrochemical system  12  of the present invention is shown. Electrochemical cells  18  typically include one or more individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode (hereinafter “membrane electrode assembly”, or “MEA”  119 ) as shown in  FIG. 1B . Each cell typically further comprises a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA  119  may be supported on either or both sides by screen packs or bipolar plates disposed within the flow fields, and which may be configured to facilitate membrane hydration and/or fluid movement to and from the MEA  119 . 
     Membrane  118  comprises electrolytes that are preferably solids or gels under the operating conditions of the electrochemical cell. Useful materials include, for example, proton conducting ionomers and ion exchange resins. Useful proton conducting ionomers include complexes comprising an alkali metal salt, alkali earth metal salt, a protonic acid, a protonic acid salt or mixtures comprising one or more of the foregoing complexes. Counter-ions useful in the above salts include halogen ion, perchloric ion, thiocyanate ion, trifluoromethane sulfonic ion, borofluoric ion, and the like. Representative examples of such salts include, but are not limited to, lithium fluoride, sodium iodide, lithium iodide, lithium perchlorate, sodium thiocyanate, lithium trifluoromethane sulfonate, lithium borofluoride, lithium hexafluorophosphate, phosphoric acid, sulfuric acid, trifluoromethane sulfonic acid, and the like. The alkali metal salt, alkali earth metal salt, protonic acid, or protonic acid salt can be complexed with one or more polar polymers such as a polyether, polyester, or polyimide, or with a network or cross-linked polymer containing the above polar polymer as a segment. Useful polyethers include polyoxyalkylenes, such as polyethylene glycol, polyethylene glycol monoether, and polyethylene glycol diether; copolymers of at least one of these polyethers, such as poly(oxyethylene-co-oxypropylene) glycol, poly(oxyethylene-co-oxypropylene) glycol monoether, and poly (oxyethylene-co-oxypropylene) glycol diether; condensation products of ethylenediamine with the above polyoxyalkylenes; and esters, such as phosphoric acid esters, aliphatic carboxylic acid esters or aromatic carboxylic acid esters of the above polyoxyalkylenes. Copolymers of, e.g., polyethylene glycol with dialkylsiloxanes, maleic anhydride, or polyethylene glycol monoethyl ether with methacrylic acid exhibit sufficient ionic conductivity to be useful. 
     Ion-exchange resins useful as proton conducting materials include hydrocarbon-and fluorocarbon-type resins. Hydrocarbon-type ion-exchange resins include phenolic resins, condensation resins such as phenol-formaldehyde, polystyrene, styrene-divinyl benzene copolymers, styrene-butadiene copolymers, styrene-divinylbenzene-vinylchloride terpolymers, and the like, that can be imbued with cation-exchange ability by sulfonation, or can be imbued with anion-exchange ability by chloromethylation followed by conversion to the corresponding quaternary amine. 
     Fluorocarbon-type ion-exchange resins can include, for example, hydrates of tetrafluoroethylene-perfluorosulfonyl ethoxyvinyl ether or tetrafluoroethylene-hydroxylated (perfluoro vinyl ether) copolymers and the like. When oxidation and/or acid resistance is desirable, for instance, at the cathode of a fuel cell, fluorocarbon-type resins having sulfonic, carboxylic and/or phosphoric acid functionality are preferred. Fluorocarbon-type resins typically exhibit excellent resistance to oxidation by halogen, strong acids, and bases. One family of fluorocarbon-type resins having sulfonic acid group functionality is NAFION™ resins (commercially available from E. I. du Pont de Nemours and Company, Wilmington, Del). 
     Electrodes  114  and  116  comprise catalysts suitable for performing the needed electrochemical reaction (i.e., electrolyzing water to produce hydrogen and oxygen). Suitable electrodes comprise, but are not limited to, platinum, palladium, rhodium, carbon, gold, tantalum, tungsten, ruthenium, iridium, osmium, and the like, as well as alloys and combinations comprising one or more of the foregoing materials. Electrodes  114  and  116  can be formed on membrane  118 , or may be layered adjacent to, but in contact with or in ionic communication with, membrane  118 . 
     Flow field members (not shown) and support membrane  118 , allow the passage system fluids, and preferably are electrically conductive, and may be, for example, screen packs or bipolar plates. The screen packs include one or more layers of perforated sheets or a woven mesh formed from metal or strands. These screens typically comprise metals, for example, niobium, zirconium, tantalum, titanium, carbon steel, stainless steel, nickel, cobalt, and the like, as well as alloys and combinations comprising one or more of the foregoing metals. Bipolar plates are commonly porous structures comprising fibrous carbon or fibrous carbon impregnated with polytetrafluoroethylene or PTFE (commercially available under the trade name TEFLON® from E. I. du Pont de Nemours and Company). 
     After hydrogen and oxygen have been disassociated from the water, the hydrogen exits the electrochemical cell  18  as described herein above via the separator  24  and an optional dryer  26 . The oxygen gas and excess water exit the electrochemical cell through a conduit  20  which carries the oxygen and water into a phase separation and gas monitoring subsystem  70 . 
     As the oxygen/water stream enters the phase separator  28  the stream experiences a slight pressure drop causing some of the water entranced in the stream to condense and drop to the bottom of the phase separator. The separated oxygen gas stream exits the phase separator  28  via a conduit  32  and exits the system  12 . It should be noted that while the phase separator  28  removes water from the gas stream, the oxygen gas typically exits the separator  28  in a saturated condition with a relative humidity in excess of 95%. 
     In the exemplary embodiment shown in  FIG. 3 , a sample conduit  72  connects the phase separator  28  with a second phase separator  74 . As the gas stream enters the second separator  74 , additional water is removed from the gas. An optional solenoid valve  76  is connected to the phase separator  74  to allow periodic draining of water for disposal, or recycling back into the electrochemical cell  18  feed loop. The separated gas in second separator  74  exits through a conduit  78 , passing through an orifice  80  which reduces the pressure of the gas and restricts the flow of gas into the combustible gas sensor  36 . The size of the orifice  80  will depend on the application, and the amount of flow restriction desired. In general, the smallest orifice that provides a minimal risk of becoming plugged is desired. In the exemplary embodiment, the preferred orifice  80  has an opening size of less and 0.025 inches, and more preferably has an opening size of less than 0.016 inches. It should be appreciated that while the phase separator  74  is illustrated as a standard phase separation device (long tubular vessel, mounted vertically), this device may also be a coalescing filter which is periodically replaced. The drop in pressure due to the orifice  80  lowers the relative humidity from near 100% when the gas enters the second separator  74  to less than 80%. 
     An alternate embodiment of the phase separation and gas monitoring system  70  is shown in  FIG. 4 . In this embodiment, the gas leaving separator  28 , passes through an orifice  81 , which drops the pressure and restricts the flow of gas into a second separator  74 . Preferably, the orifice  81  has an opening size of less than 0.025 inches, and more preferably has an opening size of less than 0.016 inches. In this configuration, since the gas in the second phase separator  74  is at a lower pressure, the second separator  74  can be drained using an orifice  82  which provides sufficient flow to prevent the second separator  74  from over-filling with water. The gas stream from the second separator  74  moves to the combustible gas sensor  36  via conduit  78 . The drop in pressure due to the orifice  81  and second separator  74  lowers the relative humidity from near 100% when the gas leaves the separator  28  to less than 95% when it reaches the combustible gas sensor. 
     Another alternate embodiment of the phase separation and gas monitoring system  70  is shown in  FIG. 5 . In this embodiment, the gas leaving second separator  74  through conduit  78 , passes through an orifice  80 , which drops the pressure and restricts the flow of gas to the CG sensor arrangement  51 . As the gas stream leaves the conduit  78 , it passes through a stream of dry air  86  which further reduces the relative humidity of the gas stream as it reaches the CG sensor  42 . A fan  84  either coupled to the combustible gas sensor arrangement  51  or elsewhere in the system  12 , provides the mechanism for creating dry air stream  86 . 
     Another alternate embodiment of the phase separation and gas monitoring system  70  is shown in  FIG. 6 . This embodiment uses the CG sensor arrangement  53  in combination with an orifice  80  connected to a conduit  72  which carries the gas stream from the separator  28 . As described herein above, as the gas stream enters the sensor arrangement  53 , the gas stream impacts on the housing wall enhancing the mixing of the gas stream with dry air allowing for improved detection of combustible gasses. 
       FIG. 7  and  FIG. 8  provide yet other embodiments utilizing redundant sensors to detect the presence of combustible gas in the gas stream. In addition to improving reliability in monitoring capability, each of these embodiments utilize a different CG sensor arrangement which increases the reliability further by lowering the risk that an environmental factor (air pressure, temperature, humidity and the like) will effect both sensors simultaneously. It should be appreciated that the specific CG sensor arrangements used in the embodiments shown in  FIGS. 7–8  are examples, and that any combination of CG sensor arrangements described herein could be utilized to achieve the same effect. 
     In the alternate embodiment shown in  FIG. 7 , the first sensing arrangement  53  monitors the main gas stream that exits separator  28  through conduit  32 . The gas stream enters the CG sensor arrangement  53  where it mixes with dry air to provide monitoring capability as described herein above. The gas stream exits the CG sensor arrangement  53  through opening  56  ( FIG. 2C ) and vents to the atmosphere. It should be appreciated the CG sensor arrangement  53  could be positioned internally, or externally to the system  12 . A second CG sensor arrangement  51  is provided through sampling conduit  72  which provides a gas stream from the phase separator  28  to a second phase separator  74  which provides further reduction in the gas stream&#39;s water content. The gas stream exits the phase separator  74  through conduit  78 , passes through an orifice  80 , and is monitored by sensor arrangement  51 . As described herein above, the gas stream mixes with a dry air stream  86  which provides further reduction in the relative humidity and improvement in monitoring performance. 
     The alternate embodiment in  FIG. 8  is arranged to also provide redundant monitoring of a sample stream. In some applications, this may provide additional benefits over the embodiment illustrated in  FIG. 7  in that the primary vent stream is not interrupted. In this embodiment, a sample conduit  72  allows the gas stream to move from the separator  28  to a second separator  74  through an orifice  81  which lowers the pressure and restricts the flow of the gas stream. In the second separator  74 , additional water is removed and the gas stream exits through both conduit  78 , that delivers the gas stream to CG sensor arrangement  51 , and through conduit  88  to the CG sensor arrangement  53 . It should be appreciated that similar to the embodiment in  FIG. 5 , the orifice  81  may be positioned after the second separator  74 . Additionally, instead of having two conduits exit the second separator  74 , a single conduit may be used with the conduits  78 ,  88  branching off from the single conduit to the respective CG sensor arrangements  51 ,  53 . 
       FIGS. 9–11  illustrate alternate embodiments utilizing temperature control as a means for reducing the relative humidity of the gas stream. A gas has a given relative humidity level depending on the temperature and pressure of the gas. Since the pressure of the system  12  will be generally known for a given application, by adjusting the temperature of the gas stream, the relative humidity of the monitored gas can be lowered to the operating range of the sensor. 
     The exemplary embodiment utilizing temperature control is shown in  FIG. 9 . Saturated gas from the system  12  enters a phase separator  28  where most of the water entrained in the gas stream is separated and recycled back into the system through valve arrangement  30 . A saturated gas stream exits the phase separator  28  and travels via conduit  72  through an orifice  80  which lowers the pressure and restricts the flow of the gas stream. A heat exchanger  90  raises the temperature of the gas to a sufficient level to lower the relative humidity to less than 95%. The gas exits the conduit  72  into the CG sensor arrangement  36 . The type of heat exchanger used can be of any suitable type, including but not limited to cross-flow, counter-flow or parallel-flow exchangers, or resistive heat elements such as heat tape. Additionally, any of the CG sensor arrangements described herein may be used in this arrangement and additional phase separation devices may be utilized as needed for a particular application. 
     An alternate embodiment shown in  FIG. 10  uses the cooling of the gas stream to condense additional water from the gas stream and thus lower the relative humidity. In this embodiment, a gas stream enters the phase separator  92  from conduit  20 . The phase separator  92  is cooled by a suitable device, including but not limited to thermoelectric cooling devices, to cause water vapor in the gas stream to condense and be captured within the phase separator  92 . The condensed water is removed from the separator  92  via valve arrangement  30  and either recycled, or similarly disposed of. The gas stream exits the phase separator  92  via conduit  72  through an orifice  80  which lowers the pressure and restricts the flow of the gas stream. The gas exits the conduit  72  into the CG sensor arrangement  36 . While the embodiment illustrated in  FIG. 10  shows the cooling device coupled with a single or primary phase separator, other arrangements would be equally effective including the addition and cooling of a second subsequent phase separator. Depending on the application, the cooling of a second phase separation may be preferable since it may reduce the amount of cooling necessary to achieve the desired final relative humidity. Additionally, while the CG sensor arrangement  36  is illustrated, any of the CG sensor arrangements  51 ,  53 ,  59  described herein may be used in this arrangement. 
     Another alternate embodiment utilizing both heating and cooling to lower the relative humidity is shown in  FIG. 11 . In this embodiment, a gas stream enters the phase separator  28  where entrapped water in the gas stream is separated. A sample gas stream  72  is cooled by a suitable cooling device  94  to condense additional water from the saturated gas stream. A second phase separator  74  separates the condensed water and drains it away for disposal or recycling. The gas stream then leaves the second separator  74  passing through an orifice  80  that restricts the gas flow and further drops the pressure of the gas stream. A heater  90  then heats the gas stream further reducing the relative humidity of the gas stream prior to monitoring by the CG sensor arrangement  36 . It should be appreciated that the heater device  90  and cooling device  94  can be any suitable device, including but not limited to the heating and cooling devices described herein above. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. For example, while the embodiments shown referred specifically to an electrochemical system generating hydrogen, this invention would apply equally to any system where there is a potential for mixing hydrogen with air or oxygen including, but not limited to photolysis, fuel cells, steam methane reformers or hydrocarbon reformers. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.