Patent Publication Number: US-2023152220-A1

Title: Rapid, sensitive hydrogen detector with flow path difference compensation

Description:
RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 17/178,696, filed on Feb. 18, 2021, by David D. Nelson, Jr. et al for a “Rapid, Sensitive Hydrogen Detector”, the contents of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates generally to gas detection, and more particularly to rapid, sensitive detection of molecular hydrogen. 
     Background Information 
     There is a growing need for rapid, sensitive detection of molecular hydrogen. As the world transitions away from fossil fuels as our primary energy source, it is likely that a hydrogen-based energy infrastructure will emerge. Both for economic and safety reasons it will be essential to have effective ways of measuring hydrogen concentration. For example, to detect hydrogen gas leaks it will be essential to have effective ways of measuring hydrogen concentration in sample gas (e.g., atmospheric air). Just as methane detectors play an important role in our existing natural gas-based energy infrastructure, hydrogen detectors will likely play an important role in hydrogen-based energy infrastructure. 
     Many currently deployed methane detectors utilize optical detection to measure methane concentration and detect methane gas leaks. Optical detection can be fast, sensitive, portable, and specific, and it would seemingly be an appealing option for use in detecting hydrogen. However, unlike methane, hydrogen has no strong absorption features in the near ultraviolet (UV), visible, infrared (IR) or microwave regions of the electromagnetic spectrum. Accordingly, it is very difficult to optically detect hydrogen molecules with conventional techniques and direct optical detectors for hydrogen have not proved viable. 
     Accordingly, there is a need for improved techniques for detecting molecular hydrogen that may enable rapid, sensitive hydrogen detection. 
     SUMMARY 
     In various embodiments, rapid, sensitive detection of molecular hydrogen is achieved by chemically converting hydrogen to water vapor (i.e., oxidizing the hydrogen) and then optically detecting the water vapor (e.g., using an optical detection technique such as laser spectroscopy, non-dispersive infrared (NDIR) absorption spectroscopy, etc.). The water vapor serves as a surrogate for hydrogen, such that hydrogen is indirectly detected. Indirect detection avoids the difficulties of optically detecting hydrogen molecules themselves and may provide other advantages. However, indirect detection may also introduce other challenges. In various embodiments described herein, these other challenges may be addressed. 
     One challenge is that the sample gas (e.g., atmospheric air) often includes significant ambient water vapor (e.g., 1% to 4%). The additional water vapor produced by chemically converting hydrogen will typically be very small compared to the ambient water vapor. Even when detecting nearby a hydrogen leak it may be hundreds of times smaller, and if remote from a hydrogen leak it may be tens of thousands of times smaller. In addition, the amount of ambient water vapor may change with time, and be correlated with air movements, causing further problems in specific detection. 
     In various embodiments described herein, this challenge may be addressed by separating a water vapor signal describing detected water vapor concentration into two components in the time domain, referred to as the “ambient water vapor signal” and the “hydrogen-derived water vapor signal.” Separation may be facilitated by dampening variation in the ambient water vapor signal to differentiate it from the more rapidly varying hydrogen-derived water vapor signal. Dampening may be achieved in various manners. In one embodiment, a gas dryer (e.g., a Nafion® sulfonated tetrafluoroethylene based fluoropolymer-copolymer membrane gas dryer) may be employed. 
     Various additional techniques may be employed to enhance such embodiments. In one embodiment, the detection may be enhanced by dampening variation in ambient water vapor and rapidly actively modulating the hydrogen-derived water vapor signal. After dampening variation in the ambient water vapor signal (e.g., by passing the sample gas through a gas dryer), the sample gas may be divided into a chemical conversion flow and a bypass flow. Hydrogen in the chemical conversion flow is converted to water vapor (e.g., in a catalytic oven). Water vapor measurements are alternated between the converted chemical conversion flow or the bypass flow to produce a water vapor signal. The alternating actively modulates the hydrogen-derived component of the water vapor signal facilitating separation of the hydrogen-derived water vapor signal from the water vapor signal. 
     However, additional challenges may be encountered in embodiments that divide sample gas into a chemical conversion flow and a bypass flow. One challenge that may be encountered is minute differences in the paths of the chemical conversion flow and the bypass flow, caused by leaks, differing amounts of water adsorption or desorption on surfaces, or other factors. If the paths were absolutely identical, the water vapor signal resulting from flow through each path should be identical, such that in the absence of hydrogen in the sample gas a differential signal measuring difference in water vapor concentration of flow through each path should be zero. However, in practice there is often a non-zero differential signal (i.e., an offset). 
     In one embodiment, such offset may be addressed by dividing the sample gas into a chemical conversion flow and a bypass flow before dampening variation (e.g., by passing through a gas dryer) and converting hydrogen to water vapor at two different points (e.g., using two catalytic ovens), one disposed before the dampening and one after. In such an arrangement, any non-zero differential signal (i.e., offset) may be erased by the first conversion of hydrogen to water vapor (e.g., in a first catalytic oven) and dampening (e.g., passing through the gas dryer). This may also erase any hydrogen-derived water vapor signal produced by the first chemical conversion. A hydrogen-derived water vapor signal may be recovered by performing a second conversion of hydrogen to water vapor (e.g., in a second catalytic oven). Measurement of water vapor from this second conversion may be used to indirectly detect hydrogen in the sample gas. 
     It should be understood that a variety of additional features and embodiments may be implemented other than those discussed in this Summary. This Summary is intended simply as a brief introduction to the reader for the further description that follows, and does not indicate or imply that the features and embodiments mentioned herein cover all aspects of the disclosure, or are necessary or essential parts of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description refers to the accompanying drawings of example embodiments, of which: 
         FIG.  1 A  is a flow diagram of an example sequence of steps for detecting molecular hydrogen according to a first embodiment; 
         FIG.  1 B  is a block diagram of an example hydrogen detector with components that may implement the sequence of steps in  FIG.  1 A ; 
         FIG.  1 C  is a set of graphs  154 - 158  illustrating an example water vapor signal, ambient water vapor signal and hydrogen-derived water vapor signal as a function of time; 
         FIG.  2 A  is a flow diagram of an example sequence of steps for detecting molecular hydrogen according to a second embodiment; 
         FIG.  2 B  is a block diagram of an example hydrogen detector with components that may implement the sequence of steps in  FIG.  2 A ; 
         FIG.  2 C  is a graph illustrating an example water vapor signal having a hydrogen-derived water vapor component that has been rapidly modulated; 
         FIG.  3 A  is a flow diagram of an example sequence of steps for detecting molecular hydrogen according to a third embodiment; and 
         FIG.  3 B  is a block diagram of an example hydrogen detector with components that may implement the sequence of steps in  FIG.  3 A . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG.  1 A  is a flow diagram of an example sequence of steps  100  for detecting molecular hydrogen according to a first embodiment.  FIG.  1 B  is a block diagram of an example hydrogen detector  102  with components that may implement the sequence of steps  100  in  FIG.  1 A . At step  110 , an inlet  112  of the hydrogen detector  102  receives sample gas (e.g., atmospheric air) that includes ambient water vapor, molecular hydrogen and potentially hydrogen bearing molecules (including methane and non-methane hydrocarbons (NMHCs)). Ambient water vapor in the sample gas may vary, typically falling between 10,000 and 20,000 ppm for atmospheric air. Hydrogen bearing molecules in the sample gas are typically present in far smaller quantities, typically being about 3 ppm for atmospheric air (methane usually accounting for about 2 ppm and NMHCs accounting for the remaining 1 ppm). Absent a hydrogen source (e.g., a hydrogen leak), hydrogen typically is found at about 0.5 ppm in atmospheric air. A simple implementation of the hydrogen detector  102  may be suited for detecting hydrogen concentrations of about 1 ppm to about 40,000 ppm (i.e., the lower explosive limit of hydrogen). More complicated implementations of the hydrogen detector (e.g., that account for the potential presence of hydrogen bearing molecules in the sample gas) may be capable of specifically detecting sub-1 ppm concentrations. 
     At step  120 , the sample gas from the inlet  112  is passed through a gas dryer  122 , or more specifically a high-pressure segment thereof. In one implementation the gas dryer  122  is a Nafion® sulfonated tetrafluoroethylene based fluoropolymer-copolymer membrane gas dryer. Alternatively, a variety of different types of gas dryer may be employed. The gas dryer  122  may remove some ambient water vapor from the sample gas. However, the primary purpose of the gas dryer  122  is not to remove ambient water vapor, but to instead dampen time response in ambient water vapor, while having little effect on hydrogen. As explained further below, the gas dryer  122  serves a role similar to a low pass filter in the field of electronics. 
     At step  130 , sample gas from the gas dryer  122  (or more specifically the high-pressure segment thereof) is received at a catalytic oven  132 , which chemically converts hydrogen in the sample gas to water vapor (i.e., oxidizes the hydrogen). The catalytic oven  132  may include a hot (e.g., 100 to 200° Celsius (C.)) catalytic surface (e.g., a platinum (Pt) surface) that rapidly and quantitatively converts hydrogen to water vapor. 
     The gas dryer  122  and catalytic oven  132  may operate near atmospheric pressure. At step  140 , the converted sample gas from the catalytic oven  132  is passed through a flow controller  142  that reduces pressure. The flow controller  142  may cause a pressure drop by limiting flow in various manners. In one implementation, the flow controller is a critical orifice that limits flow. In some cases (e.g., cases with 40,000 ppm or greater water vapor concentrations) condensation may be prone to occur between the catalytic oven  132  and the flow controller  142 . To avoid condensation, this region may be maintained at an elevated temperature (e.g., 30° C. or greater). In one implementation, this elevated temperature may be achieved by placing the flow controller  142  very close to the catalytic oven  132  so that byproduct heat from the catalytic oven  132  maintains the elevated temperature. 
     At step  150 , the converted sample gas from the flow controller  142  is received by a water vapor monitoring cell  152 , which measures water vapor therein to produce a water vapor signal. The water vapor monitoring cell  152  may employ optical detection, for example, laser spectroscopy or non-dispersive infrared (NDIR) absorption spectroscopy. It should be understood, however, that other types of detection, including other types of optical detection, may be employed, and the water vapor monitoring cell  152  may include various types of devices, including other types of spectrometers. 
     The water vapor signal produced by the water vapor monitoring cell  152  includes two components in the time domain: a component derived from ambient water vapor referred to herein as the “ambient water vapor signal” and a component derived from converted hydrogen referred to herein as the “hydrogen-derived water vapor signal.”  FIG.  1 C  is a set of graphs  154 - 158  illustrating an example water vapor signal, ambient water vapor signal and hydrogen-derived water vapor signal as a function of time. The ambient water vapor signal often has a significant offset (e.g., 100 to 1000 pm) even after reduction by the gas dryer  122  due to the typically large (e.g., 10,000, 20,000 ppm, etc.) concentrations of ambient water vapor in atmospheric air, and typically only varies over long time periods (e.g., time periods greater than a minute). The ambient water vapor signal typically is relatively constant over short time periods (e.g., time periods of less than 1 second (s)) due to the dampening effects of the gas dryer  122 . The hydrogen-derived water vapor signal typically has no significant offset due to the typically tiny (e.g., 0.5 ppm) concentrations of hydrogen in atmospheric air (absent a hydrogen source, such as a hydrogen leak). The hydrogen-derived water vapor signal may vary over short time periods (e.g., time periods of less than 1 s) since it is unaffected by the gas dryer  122 . 
     It may be noted that any leaks in the system between the gas dryer  122  (or more specifically the high-pressure segment thereof) and the water vapor monitoring cell  152  may introduce water vapor changes that would not be dampened. Accordingly, special measures may be taken to minimize and/or eliminate potential leaks in this region (e.g., using high quality vacuum plumbing components in this region). 
     At step  160 , the converted sample gas leaves the water vapor monitoring cell  152  and passes again through the gas dryer  122 , or more specifically the low-pressure segment thereof. An optional bypass path  162  may also be provided to avoid the low-pressure segment. 
     At step  170 , the converted sample gas passes through a vacuum pump  172  that pulls the sample gas through the hydrogen detector  102  and out an outlet  174 . 
     In parallel, at step  180 , the water vapor signal from the water vapor monitoring cell  152  is received by a processor  182  that separates the water vapor signal in the time domain into the ambient water vapor signal and the hydrogen-derived water vapor signal, for example, using digital signal processing (DSP) techniques. The processor  182  determines a hydrogen signal that describes molecular hydrogen in the sample gas based on the hydrogen-derived water vapor signal. In some implementations, the hydrogen-derived water vapor signal may be simply used as the hydrogen signal. In more complicated implementations, a conversion process may be employed to account for sources or errors or other factors. 
     Finally, at step  190 , the processor  182 , outputs the hydrogen signal, for example, storing it in a memory, passing it to another instrument, using it to generate a display in a user-interface of the hydrogen detector  102  itself, etc. 
     Performance of the example hydrogen detector  102  discussed in relation to  FIGS.  1 A- 1 B  may be improved by actively modulating the hydrogen-derived water vapor signal.  FIG.  2 A  is a flow diagram of an example sequence of steps  200  for detecting molecular hydrogen according to a second embodiment.  FIG.  2 B  is a block diagram of an example hydrogen detector  202  with components that may implement the sequence of steps  200  in  FIG.  2 A . Where the steps and components of  FIGS.  2 A- 2 B  are similar to those of  FIGS.  1 A- 1 B  they will be discussed again only briefly, and the reader is referred to the above discussion for more detail. 
     At step  210 , an inlet  212  of the hydrogen detector  202  receives sample gas (e.g., atmospheric air) that includes ambient water vapor, molecular hydrogen and potentially hydrogen bearing molecules (including methane and NMHCs). 
     At step  220 , the sample gas from the inlet  212  is passed through a gas dryer  222 , or more specifically a high-pressure segment thereof. Again, the gas dryer  222  may be a Nafion® sulfonated tetrafluoroethylene based fluoropolymer-copolymer membrane gas dryer or another type of dryer, and its primary purpose may be to dampen time response of ambient water vapor, while having little effect on hydrogen. 
     At step  230 , sample gas from the gas dryer  222  (or more specifically the high-pressure segment thereof) is passed to a flow divider  224  that divides the sample gas in half, creating a first flow that is passed to the catalytic oven  232  and that is referred to herein as the “chemical conversion flow,” and a second flow that bypasses the catalytic oven  232  and that is referred to herein as the “bypass flow.” 
     At step  240 , the catalytic oven  232  chemically converts hydrogen in the chemical conversion flow to water vapor. Again, the catalytic oven  232  may include a hot (e.g., 200° C.) catalytic surface (e.g., a Pt surface) that rapidly and quantitatively converts hydrogen to water vapor. The gas dryer  222  and catalytic oven  232  may operate near atmospheric pressure. 
     At step  250 , the chemical conversion flow from the catalytic oven  232  is passed through a first flow controller  242  that reduces its pressure, and the bypass flow is passed through a second flow controller  244  that reduces its pressure. Again, to avoid water condensation these regions may be maintained at an elevated temperature (e.g., 30° C. or greater). 
     At step  260 , the water vapor monitoring cell  252  alternates between measuring water vapor in just the chemical conversion flow and in just the bypass flow to produce the water vapor signal. Again, the water vapor monitoring cell  252  may employ optical detection, for example, laser spectroscopy or NDIR absorption spectroscopy. To alternate measurement, first and second valves (e.g., electronically-controlled three-way valves)  254 ,  256  may be employed. In one implementation, the first valve  254  receives the chemical conversion flow from the first flow controller  242  and directs it either to the water vapor monitoring cell  252  (in its resting state) or to a bypass  258  around the water vapor monitoring cell  252  (in its activated state). The second valve  256  receives the bypass flow from the second flow controller  244  and directs it either around the water vapor monitoring cell  252  (in its resting state) or to the water vapor monitoring cell  252  (in its activated state). 
     In parallel steps, at step  290 , a processor  282  is configured to issue a switching signal (e.g., a square wave) to the first and second valves  254 ,  256  to activate and deactivate them. The arrangement may maintain flow through the water vapor monitoring cell  252  and the bypass  258  around the water vapor monitoring cell  252  at all times, while alternating between the vapor monitoring cell  252  measuring water vapor in the chemical conversion flow or in the bypass flow. Provided the first and second valves  254 ,  256  are switched simultaneously, pressure in the vapor monitoring cell  252  will not significantly change, avoiding transient signals from the switching process itself. Further, flow rates through the water vapor monitoring cell  252  and the bypass  258  around the water vapor monitoring cell  252  need not be perfectly matched since their sum is constant and that will guarantee a nearly constant pressure in the water vapor monitoring cell  252 . 
     Again, the water vapor signal produced by the water vapor monitoring cell  252  includes two components in the time domain: an ambient water vapor signal and a hydrogen-derived water vapor signal. However, the above discussed flow switching will modulate the hydrogen-derived water vapor signal making it easier to separate. The more rapid the switching signal to the valves  254 ,  256 , the more rapid the modulation and the easier the separation (provided the switching signal is still less (e.g., 10× less) than a measurement rate of the water vapor monitoring cell  252  to ensure resolution). In one embodiment, the switching signal is greater than 1 Hertz (Hz) (e.g., a 2 Hz square wave). 
       FIG.  2 C  is a graph  258  illustrating an example water vapor signal having a hydrogen-derived water vapor component that has been rapidly modulated. In this example, the rapidly modulated hydrogen-derived water vapor component corresponds to about 50 ppm of hydrogen and rides upon an ambient water vapor component that begins around 500 ppm and slowly decreases to about 400 ppm. The hydrogen-derived water vapor signal component is well separated in the time domain from the ambient water vapor component. In an idealized system, one would expect a 50 ppm square wave riding upon the ambient water vapor component. However, due to mixing and surface effects in real-world implementations, a damped square wave is more typical. 
     At step  270 , the flows are reunited and both again pass through the gas dryer  222  (or more specifically the low-pressure segment thereof). An optional bypass path  262  may also be provided to avoid the low-pressure segment. 
     At step  280 , the reunited flows pass through a vacuum pump  272  that pulls everything through the hydrogen detector  202  and out an outlet  274 . 
     In parallel, at step  295 , the water vapor signal from the water vapor monitoring cell  252  is received by the processor  282 , which separates the water vapor signal in the time domain into the ambient water vapor signal and the modulated hydrogen-derived water vapor signal. The processor  282  determines a hydrogen signal that describes molecular hydrogen in the sample gas based on the modulated hydrogen-derived water vapor signal. In a simple implementation, while ignoring dampening of the modulation, the hydrogen signal can be determined as: 
       [H2]=[H2O] cc −[H2O] bp  
 
     where [H2O] cc  is water vapor concentration measured when monitoring the chemical conversion flow and [H2O] bp  is water vapor concentration measured when monitoring the bypass flow. In more complicated implementations, DSP techniques may be used. For example, the processor  282  may implement a digital lock-in amplifier to account for distortion caused by dampening effects. Any ambient water vapor fluctuations that are not completely suppressed by the gas dryer  222  will not be phase coherent with the modulation frequency and thereby can readily be suppressed. 
     Finally, at step  297 , the processor  282  outputs the hydrogen signal, for example, storing it in a memory, passing it to another instrument, using it to generate a display in a user-interface of the hydrogen detector  202  itself, etc. 
     Despite diligent efforts to maintain the paths of the chemical conversion flow and the bypass flow identical in the example hydrogen detector  202  discussed in relation to  FIGS.  2 A- 2 B , minute differences caused by leaks, differing amounts of water adsorption or desorption on surfaces, or other factors may affect accuracy, such that in the absence of hydrogen in the sample gas there may be a non-zero differential water vapor signal (i.e., an offset). That is, in the absence of hydrogen in the sample gas a differential water vapor signal should be exactly zero, but in practice it may be a non-zero quantity erroneously indicating presence of a small amount of hydrogen (e.g., &lt;1 ppm). 
     Typically, such a non-zero differential water vapor signal (i.e., offset) is stable. Accordingly, it could be measured for the given hydrogen detector, and subtracted from the water vapor signal during subsequent measurements to provide correction. While workable, such an instrument-specific approach may be inconvenient and it may be desirable to design a hydrogen detector in a manner that ensures a differential water vapor signal of zero. 
     To such goal, the example hydrogen detector  202  discussed in relation to  FIGS.  2 A- 2 B  may be modified by dividing the sample gas into the chemical conversion flow and the bypass flow before passing through the gas dryer, rather than after, and employing two catalytic ovens, rather than one.  FIG.  3 A  is a flow diagram of an example sequence of steps  200  for detecting molecular hydrogen according to a third embodiment.  FIG.  3 B  is a block diagram of an example hydrogen detector  302  with components that may implement the sequence of steps  300  in  FIG.  3 A . Where the steps and components of  FIGS.  3 A- 3 B  are similar to those of  FIGS.  1 A- 1 B and  2 A- 2 B  they will be discussed again only briefly, and the reader is referred to the above discussion for more detail. 
     At step  310 , an inlet  312  of the hydrogen detector  302  receives sample gas (e.g., atmospheric air) that includes ambient water vapor, molecular hydrogen and potentially hydrogen bearing molecules (including methane and NMHCs). 
     At step  320 , sample gas from the inlet  312  is passed to a flow divider  324  that divides the sample gas in half, creating a first flow that is referred to herein as the “chemical conversion flow” and a second flow that is referred to herein as the “bypass flow.” It should be noted that in contrast to the example hydrogen detector  202  discussed in relation to  FIGS.  2 A- 2 B , here the flow divider  324  is positioned before the gas dryer, or more specifically before a high-pressure segment thereof. 
     At step  330 , the chemical conversion flow is passed to a first catalytic oven  332  is that chemically converts hydrogen in the chemical conversion flow to water vapor. Like previously described catalytic ovens, the first catalytic oven  332  may include a hot (e.g., 200° C.) catalytic surface (e.g., a Pt surface) that rapidly and quantitatively converts hydrogen to water vapor, operating near atmospheric pressure. 
     At step  340 , a gas dryer  322 , or more specifically a high-pressure segment thereof, alternates between drying the converted chemical conversion flow or the bypass flow to produce a combined flow having alternating chemical conversion phases and bypass phases that is referred to herein as the “modulated flow.” Again, the gas dryer  222  may be a Nafion® sulfonated tetrafluoroethylene based fluoropolymer-copolymer membrane gas dryer or another type of dryer. To cause such alternation, first and second valves (e.g., electronically-controlled three-way valves)  354 ,  356  may be employed. In one implementation, the first valve  354  receives the chemical conversion flow from the first catalytic oven  332  and directs it either to the gas dryer  322  (in its resting state) or to a bypass  358  around the gas dryer  322  and other downstream components (in its activated state). The second valve  356  receives the bypass flow from the inlet  312  and directs it either around the gas dryer  322  and other downstream components (in its resting state) or to the gas dryer  322  (in its activated state). The first and second valves  354 ,  356  may operate in response to a switching signal (e.g., a square wave). 
     In parallel steps, at step  390 , a processor  382  is configured to issue the switching signal (e.g., a square wave) to the first and second valves  354 ,  356  to activate and deactivate them at a switching rate. In one embodiment, the switching rate is greater than 1 Hz (e.g., a 2 Hz), such that modulated flow has alternating chemical conversion phases and bypass phases with periods of less than 1 second (s) (e.g., less than 0.5 s). The switching signal may maintain constant flow and pressure through downstream components, such as the vapor monitoring cell  352 . 
     As discussed above, the gas dryer  122  serves a role similar to a low pass filter in the field of electronics. Because of its placement after the first catalytic oven  332  it may serve to erase a non-zero differential water vapor signal. This is because any variations or drift resulting from leaks, differing amounts of water adsorption or desorption on surfaces, or other factors, typically will be smeared (i.e., dispersed) over a lengthy period of time (referred to herein as a “drift time”), for example, a period of time greater than 1 minute (min) (e.g., &gt;2 mins), while the period of the chemical conversion phases and bypass phases of the modulated flow are typically short, for example, less than 1 second (s) (e.g., &lt;0.5 s). Said differently, the switching rate will typically be significantly faster, for example, at least 5 times faster (e.g., &gt;60 times faster) than the drift time. As such, when time domain separation is performed in later operations the variation or drift will be separated out, and any differences in water vapor concentration between the chemical conversion flow and the bypass flow erased. 
     One issue, however, is that placement of the gas dryer  322  in this location will also erase any hydrogen-derived water vapor signal produced by the first chemical conversion, such that without something more the detector  302  would not operate. To recover a hydrogen-derived water vapor signal, a second conversion of hydrogen to water vapor may be performed that produces hydrogen-derived water vapor from hydrogen in the bypass phases of the modulated flow. 
     At step  350 , the modulated flow is passed to a second catalytic oven  334  that chemically converts hydrogen in the modulated flow to water vapor, producing a converted modulated flow. While hydrogen in the chemical conversion phases of the modulated flow was previously converted to water vapor (and any hydrogen-derived water vapor signal therefrom erased), hydrogen in the bypass phases of the modulated flow is still available for conversion in the second catalytic oven  334 . Like previously described catalytic ovens, the second catalytic oven  334  may include a hot (e.g., 200° C.) catalytic surface (e.g., a Pt surface) that rapidly and quantitatively converts hydrogen to water vapor, operating near atmospheric pressure. 
     At step  360 , the converted modulated flow from the second catalytic oven  334  is passed through a first flow controller  342  that reduces its pressure, and the bypass flow is passed through a second flow controller  344  that reduces its pressure. Again, to avoid water condensation these regions may be maintained at an elevated temperature (e.g., 30° C. or greater). 
     At step  370 , the converted modulated flow is passed to a water vapor monitoring cell  352  that measures water vapor in the converted modulated flow to produce a water vapor signal. Again, the water vapor monitoring cell  352  may employ optical detection, for example, laser spectroscopy or NDIR absorption spectroscopy. 
     At step  380 , the converted modulated flow is passed to the gas dryer  322 , or more specifically the low-pressure segment thereof. An optional bypass path  362  may also be provided to avoid the low-pressure segment. 
     At step  385 , the flows are reunited and passed through a vacuum pump  372  that pulls everything through the hydrogen detector  302  and out an outlet  374 . 
     In parallel steps, at step  395 , the water vapor signal from the water vapor monitoring cell  252  is received by a processor  382 , which separates the water vapor signal in the time domain to extract the hydrogen-derived water vapor signal. As in the detector  202  of  FIGS.  2 A- 2 B , the active modulation produced by operation of the valves  354 ,  356  facilitates separation. A measurement rate that is significantly greater than the switching rate (e.g., a measurement rate at least 10× greater than the switching rate) and a switching rate that is significantly faster than the drift time (e.g., a switching rate at least 60× faster than the drift time) may be employed. 
     The processor  382  determines a hydrogen signal that describes molecular hydrogen in the sample gas based on the hydrogen-derived water vapor signal. In a simple implementation, while ignoring dampening, the hydrogen signal can be determined as: 
       [H2]=[H2O] bpmf −[H2O] ccmf  
 
     where [H2O] bpmf  is water vapor concentration measured during the bypass flow phase of the converted modulated flow, and [H2O] ccmf  is water vapor concentration measured during the chemical conversion phase of the converted modulated flow. In more complicated implementations, DSP techniques may be used. For example, the processor  382  may implement a digital lock-in amplifier to account for distortion caused by dampening effects. 
     Finally, at step  397 , the processor  382  outputs the hydrogen signal, for example, storing it in a memory, passing it to another instrument, using it to generate a display in a user-interface of the hydrogen detector  302  itself, etc. 
     Operation of the various embodiments may be improved with various enhancements. In some enhanced embodiments, ambient water vapor fluctuations may be further suppressed by humidifying the sample gas to a predetermined relative humidity (e.g., 99%) using a humidifier (not shown in  FIGS.  1 B,  2 B or  3 B ) prior to passing the sample gas through the gas dryer  122 ,  222 ,  322 . Humidifying the sample gas to a predetermined relative humidity will erase memory of the actual ambient water vapor concentration. The gas dryer  122 ,  222 ,  322  will reduce water vapor to a constant, stable level such that the ambient water vapor signal will not significantly vary even if actual ambient water vapor varies. 
     In further enhanced embodiments, hydrogen bearing molecules (including methane and NMHCs) in the sample gas that could be converted to water vapor and thereby produce an interfering signal are addressed. In some enhanced embodiments, hydrogen bearing molecules are suppressed by trapping using filter materials, membranes, molecular sieves, cryogenic traps and/or other trapping components. The trapping components may include pre-filters that reduce water vapor and carbon dioxide concentrations (e.g., to &lt;1 ppm) to avoid saturation of the components intended to capture hydrogen bearing molecules. In some implementations, trapping components may be disposed between the gas dryer  122 ,  222 ,  322  and the water vapor monitoring cell  152 ,  252 ,  352 . While trapping may be used with methane, trapping may be particularly well suited for suppressing hydrogen NMHCs since they are much stickier than hydrogen and are usually present in low concentrations. 
     In further enhanced embodiments, the catalytic oven  132 ,  232 ,  332 ,  334  may be tuned to selectively oxidize hydrogen while avoiding conversion of hydrogen bearing molecules to water vapor. Selective oxidation may be particularly well suited for suppressing methane since methane is relatively more difficult to oxidize than hydrogen, and thereby parameters may be selected for the catalytic oven  132 ,  232 ,  223 ,  334  that is substantially oxidize one but not the other. 
     In still further enhanced embodiments, hydrogen bearing molecules in the sample gas may be compensated for by measuring hydrogen bearing molecule concentrations in the sample gas, calculating an amount of hydrogen bearing molecule-derived water vapor based on the measured hydrogen bearing molecule concentrations, and subtracting out hydrogen bearing molecule-derived water vapor from the water vapor signal. Compensation-based techniques may be particularly well suited for addressing methane. Such measurements used in compensation-based techniques may be collected, for example, by adapting the techniques of  FIGS.  2 A- 2 B  or  FIGS.  3 A- 3 B . Total methane concentration may be measured in the bypass flow (e.g., using a methane gas detector (not shown in  FIG.  2 B )). In the chemical conversion flow, methane concentration that survives passage through the catalytic oven(s)  232 ,  332 ,  334  is also measured (e.g., again using a methane gas detector (not shown in  FIG.  2 B  or  FIG.  3 B )). The difference between the total methane concentration and the surviving methane concentration indicates the quantity of methane oxidized in the catalytic oven(s)  232 ,  332 ,  334 . The processor  282 ,  382  may determine a hydrogen signal that describes molecular hydrogen in the sample gas by subtracting out water vapor derived from methane oxidization. Each oxidized methane molecule will produce two water molecules, whereas oxidized hydrogen molecules will produce one water molecule. In more complicated implementations, the hydrogen signal can be determined using DSP techniques (e.g., a digital lock-in amplifier) to account for distortion caused by dampening effects. 
     In summary, the above description provide example techniques for rapid, sensitive detection of molecular hydrogen. It should be understood that various adaptations, modifications, and extensions may be made to suit various design requirements and parameters. For example, the techniques may be extended to determine hydrogen eddy covariance flux. An anemometer may be added to the hydrogen detector  102 ,  202 ,  302  to measure air movement. The processor  182 ,  282 ,  382  may be adapted to determine hydrogen eddy covariance flux based on correlation between the measurement of air movement and the water vapor signal. 
     Above all it should be understood that the above descriptions are meant to be taken only by way of example and the invention is not limited to the specific details of the example embodiments disclosed. What is claimed is: