BACKGROUND 
       [0001]    This application relates to a leak test apparatus and method that does not require helium, and in particular to a leak test apparatus and method that takes advantage of the small molecular size of hydrogen while providing appropriate safeguards against combustion or explosion. 
         [0002]    Composite tanks (also known as composite overwrapped pressure vessels) are commonly used as fuel tanks for natural gas and hydrogen powered vehicles, as well as for storage of other potentially hazardous materials. Such tanks must be tested to verify the structural integrity of the tank. Conventionally, structural testing is performed using a hydro test, in which the tank is filled with liquid and then pressurized. An advantage of hydro testing is that in the event of a catastrophic failure or rupture of the tank, liquid does not expand in volume upon relief of pressure in the way that gas would expand, thereby significantly alleviating safety concerns from such a failure. Hydro testing is quite common practice, is safe, and equipment is readily available. 
         [0003]    In addition, recent industry standards such as NGV2 now require tank manufacturers to perform leak testing to identify smaller/finer leaks. Leak testing such large vessels is challenging due to lack of equipment and methods. Present industry standards specify certain test requirements but do not provide much detail regarding how to perform the test or what equipment to use. Some present leak testing methods are targeted to specific portions of a tank where leaks are likely to occur, but are not capable of comprehensive leak testing of an entire large tank. 
         [0004]    Helium is often used for leak testing, due to its small size and ability to find small leaks. However, helium is expensive due to limited supply. Also, composite tanks cannot typically hold vacuum pressures due to risk of buckling/collapse, so inboard leak testing is not an option. Therefore, it would be desirable to have an outbound leak test apparatus and method capable of safely leak testing large tanks. 
       SUMMARY 
       [0005]    A leak test apparatus and method are described herein in which a blend of hydrogen (H2) gas and an inert gas such as nitrogen (N2) gas is used to internally pressurize a tank to be leak tested. During the test, the tank is positioned in a test chamber, and the internal pressure is maintained in the tank for a predetermined time. Small leaks are detected by measuring an amount or an increase of hydrogen in the accumulation chamber. 
         [0006]    Various embodiments of a leak test apparatus and method are described. 
         [0007]    In one embodiment, a leak test apparatus is described for leak testing a tank. The apparatus includes a gas supply system configured to provide to the tank a leak detection gas mixture including hydrogen, a test chamber having a cavity for sealably enclosing the tank such that dead space exists outside the tank and within the cavity, and a gas conduit configured to interconnect the gas supply system and the tank, a control system including a controller programmed to pressurize the tank with the leak detection mixture by flowing gas from the gas supply system through the gas conduit, and a leak detector to determine the presence or absence of a leak in the tank by measuring a concentration of hydrogen in the dead space. 
         [0008]    In one aspect of the leak test apparatus, the test chamber is changeable between an open configuration to enable insertion and removal of the tank and a closed configuration in which the tank is sealed within and surrounded by the cavity. 
         [0009]    In another aspect of the leak test apparatus, the leak detection gas mixture further includes an inert gas, and the gas supply system includes a hydrogen valve for regulating flow of hydrogen into the tank and an inert gas valve for regulating flow of inert gas into the tank, including for purging and filling to a test pressure. In another aspect of the leak test apparatus, the gas supply system further includes a pump for boosting the pressure of the inert gas. 
         [0010]    In another aspect of the leak test apparatus, the gas supply system further includes a vent valve for venting the leak detection mixture from the tank. 
         [0011]    In another aspect of the leak test apparatus, the test chamber further includes at least one recirculating fan in the cavity for homogenizing gas in the cavity surrounding the tank. 
         [0012]    In another aspect of the leak test apparatus, the test chamber further includes a safety relief device. 
         [0013]    In another aspect of the leak test apparatus, the test chamber further includes a vent valve for exhausting gas from the cavity. In another aspect of the leak test apparatus, the test chamber further includes a fan configured to draw flow from the cavity through the test chamber vent valve. In another aspect of the leak test apparatus, the test chamber further includes a hydrogen safety sensor positioned within the cavity, and the controller is programmed to activate the fan and open the vent valve when the hydrogen safety sensor detects an unsafe concentration of hydrogen in the cavity. 
         [0014]    In another aspect of the leak test apparatus, the leak detector includes a hydrogen sensor. In another aspect of the leak test apparatus, the leak detector further includes a sampling pump configured to draw a sample from the cavity to the hydrogen sensor. 
         [0015]    In another aspect of the leak test apparatus, the leak detector further includes an indicator to indicate detection of a leak. 
         [0016]    In another aspect of the leak test apparatus, the test chamber further includes filler material positioned outside the tank and within the cavity to reduce the volume of the dead space. In another aspect of the leak test apparatus, the filler material includes an inflatable bladder, water, or both. 
         [0017]    In another embodiment, a method of leak testing a tank is described. The method includes enclosing the tank in a cavity of a test chamber such that a dead space is present outside the tank and within the cavity, purging the tank with an inert gas to reduce the concentration of O2 to a safe level, flowing hydrogen into the tank to achieve a fill pressure, flowing an inert gas into the tank to increase the pressure in the tank from the fill pressure to a test pressure and to dilute the concentration of hydrogen to a test concentration, holding pressure in the tank for a predetermined hold time, measuring the concentration of hydrogen in the dead space, and detecting a leak if the concentration of hydrogen in the dead space exceeds a predetermined threshold. 
         [0018]    In one aspect of the method, the inert gas is nitrogen. 
         [0019]    In another aspect of the method, the test pressure is from about 3600 PSIG to about 5000 PSIG. 
         [0020]    In another aspect of the method, the test concentration of hydrogen is from about 4% to about 6%. 
         [0021]    In another aspect of the method, the predetermined hold time is from about 5 minutes to about 60 minutes. 
         [0022]    In another aspect of the method, the predetermined threshold is from about 1 ppm to about 100 ppm. 
         [0023]    In yet another embodiment, a leak test apparatus for leak testing a tank is described. The apparatus includes a test chamber having a cavity and a gas conduit configured to provide a leak detection gas mixture to the tank, the test chamber being changeable between an open configuration to enable insertion and removal of the tank and a closed configuration in which the tank is sealed within the cavity such that dead space exists outside the tank and within the cavity, the leak detection gas mixture including hydrogen and nitrogen. The apparatus further includes a gas supply system including a low pressure nitrogen valve to provide purge gas to the tank via the gas conduit, an hydrogen valve to provide hydrogen to the tank via the gas conduit, a booster pump and high pressure nitrogen valve to provide nitrogen to the tank and to pressurize the tank to a test pressure via the gas conduit, and a vent valve for venting the tank via the gas conduit. The apparatus further includes a control system including a controller programmed to purge the tank by actuating the low pressure nitrogen valve, to provide hydrogen to the tank by actuating the hydrogen valve, to pressurize the tank to a test pressure with the leak detection gas mixture by providing nitrogen to the tank by actuating the booster pump and high pressure nitrogen valve, and to vent the tank via the gas conduit. The apparatus further includes a leak detector configured to determine the presence or absence of a leak in the tank by measuring a concentration of hydrogen in the cavity dead space, the leak detector including a hydrogen sensor, a sampling pump configured to draw a sample form the cavity to the hydrogen sensor, and an indicator to indicate detection of a leak. 
         [0024]    In yet another embodiment, a method of leak testing a tank is described. The method includes enclosing the tank in a cavity of a test chamber such that a dead space is present outside the tank and within the cavity, purging the tank with nitrogen to reduce the concentration of O2 to less than or equal to about 1%, flowing hydrogen into the tank to achieve a fill pressure, flowing nitrogen into the tank to increase the pressure in the tank from the fill pressure to a test pressure and to dilute the concentration of hydrogen to a test concentration, holding pressure in the tank for a predetermined hold time, measuring the concentration of hydrogen in the dead space, and detecting a leak if the concentration of hydrogen in the dead space exceeds a predetermined threshold, wherein the test parameters include, either separately or in any combination with each other, the test pressure being from about 3600 PSIG to about 5000 PSIG, the test concentration of hydrogen being from about 4% to about 6%, the predetermined hold time being from about 5 minutes to about 60 minutes, and the predetermined threshold being from about 1 ppm to about 100 ppm. 
         [0025]    It is understood that any of the foregoing aspects may be used separately or in combination with any one or more of the other foregoing aspects. 
         [0026]    Other aspects of the invention are described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a schematic showing an embodiment of a leak test apparatus. 
           [0028]      FIG. 2  is a flow chart showing an embodiment of a leak test method. 
           [0029]      FIG. 3  is a schematic showing another embodiment of a leak test apparatus. 
           [0030]      FIG. 4  is a schematic showing an embodiment of a leak test system including stations for leak testing multiple tanks of different sizes. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 1  depicts an embodiment of a leak test apparatus  10 . The apparatus  10  includes a gas supply system  12 , a test chamber  14 , and a control system  16 . 
         [0032]    The gas supply system  12  is configured to selectively supply low pressure hydrogen, low pressure nitrogen or other inert gas, and high pressure nitrogen or other inert gas, separately or in combination, to the test chamber  14 . As used herein, the term “inert” gas means any gas that is essentially non-reactive with hydrogen at ambient temperatures. In the depicted embodiment, the gas supply system  12  includes a hydrogen source  20  that feeds a hydrogen conduit  22 , and a nitrogen source  30  that feeds a low pressure nitrogen conduit  32  and a high pressure nitrogen conduit  42 . A gas manifold  50  joins the downstream ends of the hydrogen conduit  22 , the low pressure nitrogen conduit  32 , and the high pressure nitrogen conduit  42  into a single gas conduit  52 . 
         [0033]    A hydrogen control valve  24  is positioned in the hydrogen conduit  22  for regulating the flow of hydrogen through the hydrogen conduit  22 . A low pressure nitrogen control valve  34  is positioned in the low pressure nitrogen conduit  32  for regulating the flow of nitrogen through the low pressure nitrogen conduit  32 . A booster pump  46  is positioned in the high pressure nitrogen conduit  42  for increasing the pressure of the nitrogen supplied by the nitrogen source  30 , and a high pressure nitrogen control valve  44  is positioned in the high pressure nitrogen conduit  42  downstream of the booster pump  46  for regulating the flow of nitrogen through the high pressure nitrogen conduit  42 . Each of the control valves  24 ,  34 , and  44  is independently controlled by a controller  100  in the control system  16 . 
         [0034]    The test chamber  14  includes an enclosure  60  having an internal cavity  70  that is configured to receive and enclose a tank  200 . The tank includes a first boss port  202  and a second boss port  204 . The enclosure  60  is changeable from a first or open position in which the tank  200  can be inserted into or removed from the cavity  70  and a second or closed position in which the tank  200  is sealed within the cavity  70 . For example, the enclosure  60  may be constructed in a clamshell or coffin configuration, hinged on one edge, with a perimeter seal between two halves or portions of the enclosure  60 . 
         [0035]    In the closed position of the enclosure  60 , when the tank  200  is sealed within the cavity  70 , a dead space exists between the tank  200  and the enclosure  60  that is filled with air. As discussed below in detail, leak detection will be performed by measuring the concentration of hydrogen in the dead space. Therefore, it is preferable to reduce or minimize the dead space volume (i.e., the sampled space) to thereby increase the test accuracy and/or quicken the response time of leak testing. In other words, a smaller dead space volume may enable one or more of higher detection thresholds, faster leak detection, accurate detection of leaks in smaller tanks for the same size enclosure, and detection of smaller leaks. In one example, for a given size tank and a given size leak, decreasing the dead space volume will cause the measured hydrogen concentration in the dead space to reach a detection threshold sooner. In another example, for a given leak detection time and leak size, a higher threshold (i.e., a less sensitive detector) can be used for detection when the dead space volume is smaller. 
         [0036]    A filler material  72  may be used to reduce the dead space volume between the tank  200  and the enclosure  60 . Using filler material also enables the enclosure  60  to accommodate various size tanks  200  and to adjust the dead space volume for each tank to maintain a reasonable response time and hydrogen concentration threshold. In one embodiment the filler material is an inflatable bladder. In another embodiment, the filler material is water. However, it is understood that the filler material may be any material or structural element, or combinations of materials and/or structural elements, that are capable of filling a portion of the dead space and do not affect the hydrogen concentration of the dead space (i.e., the filler material should not emit or absorb hydrogen). 
         [0037]    The enclosure may include one or more cradles  62 , as shown, to support the tank  200  within the cavity  70 . A fitting  63  mounted in a wall of the enclosure  60  is configured to mate with the gas conduit  52  of the gas manifold  52 . Another gas conduit  64  is connected to the fitting  63  and extends into the cavity  70  for connection with a fitting  65  that is configured to mate with a gas inlet in the boss port  204  on the tank  200 . In one embodiment, the gas conduit  64  is a flexible conduit or hose to accommodate connection to various size tanks  200  that may be leak tested in the test chamber  14 . 
         [0038]    A vent valve  54  is mounted to the gas conduit  52  in the gas manifold  50  for exhausting pressurized gas from the tank  200  during pre-test purging of the tank  200  and after the completion of a leak test. A pressure transmitter  56  mounted to the gas conduit  52  is used to measure and transmit the fill pressure of the tank  200  as it is being filled and while pressure is being maintained during a leak test. The pressure transmitter  56  sends a pressure signal to the controller  100 , which uses that pressure signal to modulate the control valves  24 ,  34 , and  44 , as well as the vent valve  54 , depending on the desired pressure setpoint and the state of operation of a leak test. 
         [0039]    One or more recirculating fans  66  may be mounted within the cavity  70  of the enclosure  60 , to improve homogeneity of the gas in the dead space surrounding the tank  200 . In the depicted embodiment, two recirculating fans  66  are shown, it being understood that a different number of fans  66  may be used depending on the size of the enclosure  60  and the size of the tank  200 , and it being further understood that not all fans  66  need be operated at the same time or at the same speed. For safety, a blow-out plate  68  or other pressure relief device is mounted in the wall of the enclosure  60  to prevent overpressurization of the enclosure  60 . 
         [0040]    A vent valve  74  is mounted to the enclosure  60  for venting the cavity  70 , and an exhaust fan  76  may be provided downstream of the vent valve  74  to accelerate the exhausting of gas from the cavity  70 , particularly in the event of a high hydrogen level is detected in the dead space between the tank  200  and the enclosure  60 . 
         [0041]    The control system  16  includes the controller  100 , as well as a hydrogen sensor  80  for detecting the concentration of hydrogen in the dead space in the cavity  70  surrounding the tank  200  being leak tested. Preferably, a sampling pump  82  draws a gaseous sample through a conduit  84  from the cavity  70  into the hydrogen sensor  80 , which then determines whether the hydrogen concentration is at or above a threshold hydrogen level that indicates a leak in the tank  200 . The sampling pump  82  is controlled by the controller  100 , and may be operated to draw a continuous sample or periodic samples, depending on the desired detection result and methodology. An indicator, such as a beacon, a light, or an audible alarm, may be connected to the hydrogen sensor  80  to indicate the presence of a leak in the tank  200 , if the hydrogen sensor  80  detects a hydrogen concentration above the threshold level. The controller  100  is operatively connected to be able to independently regulate each of the control valves  24 , 34 , and  44 , the vent valves  54  and  74 , the sampling pump  82 , the hydrogen sensor  80 , optionally the indicator  86 , the exhaust fan  76 , and the recirculating fans  66 , and to receive signal inputs from the pressure transducer  56  and the hydrogen sensor  80 . 
         [0042]    The apparatus  10  uses a blend of hydrogen gas and nitrogen gas to pressurize a tank  200  in the enclosure  60 , and holds the pressure for a predetermined time to check for small leaks in the tank  200  using a hydrogen leak detector. In one embodiment, the tank  200  is pressurized with a gas mixture of hydrogen and nitrogen (or other inert gas) having a hydrogen concentration about at the hydrogen lower explosive limit (LEL) of 4%. In another embodiment, the tank  200  is pressurized with a gas mixture of hydrogen and nitrogen (or other inert gas) having a hydrogen concentration somewhat above the LEL, for example up to about 5% or up to about 6%. A higher hydrogen concentration enables faster leak detection. It is understood that any leak in the tank  200  will reduce the hydrogen concentration in the tank  200 , and the concentration of leaked hydrogen outside the tank  200  and inside the enclosure  60  will be at the ppm level in any event, so the risk of having a hydrogen concentration above the LEL in the tank  200  is minimal. In addition the purge and test procedures ensure that the tank  200  has low levels of oxygen prior to being filled with the hydrogen-containing gas mixture. Further, no sources of ignition are present in the tank  200 . 
         [0043]    The enclosure  60  further includes a hydrogen safety sensor  88  positioned in the dead space of the cavity  70  outside the tank  200 . In contrast to the hydrogen sensor  80 , the hydrogen safety sensor  88  is set to alarm if a potentially unsafe level of hydrogen is detected in the dead space. The hydrogen safety sensor  88  may be set to alarm at a threshold of less than 4% hydrogen in the cavity  70 . For example, the hydrogen safety sensor  88  may be set to a threshold of about 1% or about 2% or about 3% hydrogen in the dead space. The hydrogen safety sensor  88  sends a signal to the controller  100  if the hydrogen in the dead space reaches or exceeds the alarm threshold. In response to such a signal, the controller  100  acts to decrease the hydrogen concentration in the cavity  70  and prevent further hydrogen accumulation in the cavity  70 . These actions include one or more of: opening the vent valve  74  and activating the exhaust fan  76  to quickly evacuate the cavity  70 , and stopping flow of hydrogen and/or nitrogen into the vessel  200  and opening the vent valve  54  to vent the hydrogen-containing gas mixture from the vessel  200 . 
         [0044]    The hydrogen source  20  can be a low pressure hydrogen source. In one embodiment, the hydrogen source can deliver hydrogen up to about 200 PSIG. The nitrogen source  30  can be a low pressure nitrogen source. In one embodiment, the nitrogen source  30  can deliver nitrogen up to about 200 PSIG, and in another embodiment at up to about 300 PSIG. The nitrogen booster pump  46  is capable of delivering nitrogen at much higher pressures desirable for leak testing. The apparatus  10  may be configured to leak test a tank at any desired pressure, depending in part on the rated pressure of the tank. The apparatus  10  will typically be operated using a test pressure from about 2500 PSIG to about 6000 PSIG, preferably from about 3000 PSIG to about 5500 PSIG, and more preferably from about 3600 PSIG to about 5000 PSIG. In one embodiment, nitrogen is delivered at a pressure of at least about 3600 PSIG and in another embodiment up to about 5000 PSIG. 
         [0045]    The enclosure  60  can be configured to accommodate any size tank, and in one embodiment is configured to accommodate common sizes of composite tanks being tested (i.e., tanks typically from 18″ to 26″ in diameter and from 60″ to 144″ in length). As described above, filler material or spacer pieces may be used to occupy unused volume or dead space in the cavity  70 , depending on size of the tank  200  being tested, to minimize the air volume in the cavity  70 . This could also be accomplished by using an inflatable bladder or filling the cavity dead space with water. 
         [0046]      FIG. 3  depicts another embodiment of a leak detection apparatus  10 . As indicate by the like reference numerals, this embodiment is nearly identical to the embodiment of  FIG. 1 , except for the boundaries of the sealed enclosure  160  in  FIG. 3  relative to the tank  200  (as compared with the boundaries of the sealed enclosure  60  in  FIG. 1  relative to the tank  200 ). The features common between the two embodiments will not be described again; instead the following description addresses the differences between the embodiments of  FIG. 3  and  FIG. 1 . 
         [0047]    As shown in  FIG. 3 , the boss ports  202  and  204  protrude outside the ends of the enclosure  160 , and the enclosure  160  is sealed around the boss ports  202 ,  204 . There are several benefits for this arrangement. Because the boss ports  202 ,  204  have a small diameter relative to the tank  200  and extend axially, positioning the boss ports  202 ,  204  within the enclosure  60  significantly increases the dead space that either must be filled with filler material  72  or must be accounted for as part of the sampled volume when detecting leak. Further, the boss ports  202 ,  204 , as well as the fittings  63 ,  65  and the hose  64 , are the most likely source of leaks in the system, so by positioning these elements outside the enclosure  160 , a leak test can be focused solely on the integrity of the tank  200 . Note that an additional hydrogen safety sensor (not shown, but similar to the hydrogen safety sensor  88 ) can be positioned outside the enclosure  160  to detect any substantial leaks that may occur in the boss ports  202 ,  204 , the fittings  63 ,  65 , and/or the hose  64 . 
         [0048]      FIG. 4  shows a leak test system  11  configured with multiple test stations  110 . As depicted, the system  11  has two test stations  110   a  and  110   b,  it being understood that any number of test stations  110  could be supported. A common gas supply system  112  performs all the functions needed to supply the test stations  110 , and a common control system  116  performs all the functions needed to control leak testing and safety for the test stations  110 , as described above with regard to the gas supply system  12  and the control system  16  in the analogous single-tank leak test apparatus  10 . Each test station  110  includes its own test chamber  14  and enclosure  160 , and can be connected via a hose  64  to the common gas supply system  112 . A primary advantage of having multiple test stations  110   a,    110   b  is that the separate enclosures  160   a,    160   b  can be sized to accommodate differently sized tanks with minimal dead space, thereby enhancing test accuracy and/or decreasing test times, as discussed in more detail above, and may avoid the need for any filler material. Although the depicted leak test system  11  indicates that only one test chamber  14  can be connected at a time to the common gas supply system  112 , the leak test system  11  could alternately be arranged, and the common gas supply system  112  and common control system  116  configured, to conduct multiple leak tests in parallel. One embodiment of an operation sequence  300  for leak testing a tank  200  is shown in  FIG. 2 . Other sequences may be used with equal effectiveness, as would be understood by a person of ordinary skill in the art. 
         [0049]    In step  305 , the enclosure  60  is placed in the open position and a tank  200  is loaded into the cavity  70 . Loading of the tank  200  may be done manually or by an automated process. In step  310 , the conduit  64 , via the fitting  65 , is connected to the tank  200  to enable the tank to be filled with a test gas. In step  315 , if desired or necessary, filler material  72  is positioned in the dead space in the cavity  70 . It is understood that steps  310  and  315  may be interchanged in order without negative impact. In step  320 , the enclosure  60  is placed in the closed position and the cavity  70  is sealed. 
         [0050]    In step  325 , the controller  100  opens the low pressure nitrogen control valve  34  to flow low pressure nitrogen into the tank  200  for purging the tank of oxygen. The flow rate and time of nitrogen purge is set so as to reduce the oxygen concentration in the tank  200  to a safe level. As used herein, a “safe level” is an oxygen concentration insufficient to support ignition and/or combustion of a hydrogen-containing gas with up to about 4% or up to about 5% or up to about 6% hydrogen in an inert gas. In one embodiment, a safe level of oxygen concentration means less than or equal to about 1%. This calculation can be performed as a volumetric dilution based on the size of the tank  200 , which may be inputted into the controller  100 . For example, in one embodiment, since oxygen is about 21% of atmospheric air, about 20 tank volumes of nitrogen is flowed into the tank  200 . This could be done by filling the tank  200  with nitrogen up to about 300 PSIG, as measured by the pressure transducer  56 , and then venting the tank  20  through the vent valve  54  back to about atmospheric pressure (0 PSIG or 14.7 PSIA), or by twice successively filling the tank  200  with nitrogen up to about 150 PSIG and then venting the tank  200  through the vent valve  54  back to about atmospheric pressure, or another approach with a different number of fillings and ventings. In step  330 , once the oxygen concentration in the tank  200  has been diluted to less than or equal to about 1%, the controller closes the control valve  34  and then opens the vent valve  54  to vent the tank  200  back to about atmospheric pressure containing a mixture of less than or equal to about 1% oxygen in primary nitrogen. 
         [0051]    In step  330 , the controller  100  opens the hydrogen control valve  24  to flow hydrogen into the tank  200  to achieve a desired fill pressure. In one embodiment, the tank  200  is filled to a fill pressure of about 130 PSIG, as measured by the pressure transducer  56 , at which point the control valve  24  is closed. In step  335 , the controller  100  activates the booster pump  46  and opens the high pressure nitrogen control valve  44  to flow nitrogen into the tank  200 . Nitrogen is added to the tank  200  until the a desired test pressure is reached and the hydrogen concentration in the tank  200  is below the LEL. In one embodiment, the tank  200  is filled to a test pressure of about 3600 PSIG (about 3615 PSIA). Based on a hydrogen fill pressure of about 130 PSIG (about 145 PSIA), the final hydrogen concentration would be about 4%, or at about the LEL threshold. As noted above, in other embodiments, the tank  200  may be filled by a combination of hydrogen (to the fill pressure) and nitrogen (to the test pressure) to achieve a hydrogen concentration that is higher than the LEL, such as up to about 5% or up to about 6%. For example, to achieve a hydrogen concentration of about 6%, hydrogen would be filled into the tank to a fill pressure of about 202 PSIG, followed by nitrogen up to a test pressure of about 3600 PSIG. It is understood that if a higher test pressure is desired, then a correspondingly higher hydrogen fill pressure can be used to achieve about the same final hydrogen concentration. It is preferable to use a hydrogen concentration as high as safely possible. For example, if a test pressure of 5000 PSIG is desired, a hydrogen fill pressure of about 185 PSIG can be used to achieve a test hydrogen concentration of about 4%, or a hydrogen fill pressure of about 285 PSIG can be used to achieve a test hydrogen concentration of about 6%, followed by the addition of high pressure nitrogen to boost the test pressure to about 5000 PSIG. 
         [0052]    In step  340 , pressure is maintained in the tank  200  and the concentration of hydrogen in the dead space  70  is detected, continuously or periodically, by the hydrogen sensor  80 . During this hold time, the control valves  24 ,  34 , and  44  are all closed, the tank vent valve  54  is closed, and the cavity vent valve  74  is closed. Continuously or periodically during the hold time, the sampling pump  82  is activated to draw gas from the dead space in the cavity  70  to the hydrogen sensor  80 , and the hydrogen concentration in the dead space is detected. If the tank  200  is leaking, hydrogen will escape from the tank  200  via the leak into the dead space of the cavity  70 , and the hydrogen concentration in the dead space will increase. If the leak is sufficiently large, the hydrogen concentration in the dead space will increase to a detectable level above a threshold level during the hold time. The threshold level will depend on the dead space volume and the size of the tank  200 , as well as the test pressure, all of which can be readily calculated by a person skilled in the art. 
         [0053]    In step  342 , if the hydrogen sensor  80  detects a hydrogen concentration of less than the threshold level, the process proceeds to step  346 . In step  346 , if the predetermined hold time has elapsed and the hydrogen sensor  80  does not detect a hydrogen concentration at or above the threshold level, then the tank  200  is deemed to have successfully passed the leak test, and operation proceeds to step  350 . In step  346 , if the predetermined time has not elapsed, operation returns to step  340  to continue retesting until the hold time elapses (testing loop). 
         [0054]    In step  342 , if the hydrogen sensor  80  detects a hydrogen concentration of equal to or greater than the threshold level during the hold time, the tank  200  is deemed to have failed the leak test. In step  344 , the indicator  86  is activated to indicate a leak and thus failure of the test. Failure of the leak test concludes the testing loop and sends operation to step  350 . 
         [0055]    The hydrogen sensor  80  may be set to have a threshold from about 1 ppm to about 100 ppm, preferably from about 2 ppm to about 50 ppm, and more preferably from about 5 ppm to about 25 ppm, depending upon the tank size, test pressure, and dead space volume, as well as the hold time. Similarly, the hold time may be set to be from about 5 minutes to about 60 minutes, preferably from about 10 minutes to 45 minutes, and more preferably from about 15 minutes to 30 minutes, again depending upon the tank size, test pressure, and dead space volume, as well as the hydrogen detection threshold. In one exemplary embodiment, the hydrogen sensor  80  is set to a threshold of 5 ppm and the hold time is set to 30 minutes, and the apparatus is capable of detecting a leak rate on the order of 0.005 cubic centimeters per second (cc/sec). In other embodiments, other combinations of thresholds and hold times can be used to adjust the minimum detectable leak rate. Elapse of the hold time and successful completion of the leak test concludes step  345  and sends operation to step  350 . 
         [0056]    At any time concurrently with or after step  325 , the controller  100  ensures that vent valve  74  is closed. Similarly, at any time concurrently with or after step  325 , the controller  100  can turn on one or more of the recirculating fans  66 . The recirculating fans  66  can remain active, or can operate intermittently or selectively, through steps  330 ,  335 ,  340 , and  345 . 
         [0057]    In step  355 , the tank  200  is vented by opening the vent valve  54 . Venting of the tank  200  continues until the pressure transducer  56  detects a pressure of about atmospheric pressure. Because of the hydrogen content in the gas being vented, the vent outlet is outside and preferably at a safe distance above the ground. In step  360 , the cavity  70  is vented by opening the cavity vent valve  74 , and if deemed necessary, by activating the exhaust fan  76 . The exhaust fan  76  is also helpful to evacuate even small amounts of hydrogen from the dead space in the cavity  70  so as to avoid any contamination (i.e., false positives) in each subsequent test of another tank after a previous tank (with or without a detected leak) has been removed. Further, if a large leak was detected, and in particular a leak sufficiently large to reach the detection threshold of the hydrogen safety sensor  88 , then the exhaust fan  76  is activated for safety purposes to quickly vent the cavity  70 . 
         [0058]    In step  365 , the enclosure  60  is unsealed and moved from the closed position to the open position. In step  370 , the gas conduit  64  is disconnected from the tank  200  and the tank  200  is removed from the enclosure  60 . 
         [0059]    The apparatus  10  enables tank manufacturers, and in particular composite tank manufactures, to safely perform leak testing with hydrogen gas, which has a small molecular size similar to that of helium (He), but is much less expensive and readily available. In particular, the apparatus  10  enables detection of low leak rates (small leaks) in large tanks of various sizes. 
         [0060]    In addition, the leak test performed by the apparatus  10  and corresponding method described herein is superior to a conventional pressure decay test for at least two reasons. First, the method described herein is not susceptible to temperature changes (e.g., due to gas heating during tank fill and due to ambient fluctuations during test hold), but rather measures only hydrogen leakage out of a tank. Second, pressure indicating instruments with the resolution/accuracy required to detect such small leak rates (i.e., changes in pressure) at such high test pressures are generally not available, so the present apparatus and method can detect leaks smaller than currently available technology used in convention pressure decay testing. 
         [0061]    The present invention is not to be limited in scope by the specific aspects or embodiments disclosed in the examples which are intended as illustrations of a few aspects of the invention and any embodiments that are functionally equivalent are within the scope of this invention. Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art and are intended to fall within the scope of the appended claims.
A leak test apparatus for leak testing a tank including a gas supply system configured to provide to the tank a leak detection gas mixture including hydrogen, a test chamber having a cavity for sealably enclosing the tank such that dead space exists outside the tank and within the cavity, and a gas conduit configured to interconnect the gas supply system and the tank, a control system including a controller programmed to pressurize the tank with the leak detection mixture by flowing gas from the gas supply system through the gas conduit, and a leak detector to determine the presence or absence of a leak in the tank by measuring a concentration of hydrogen in the dead space.