Abstract:
A hydrogen gas storage container has a canister with at least one outlet opening for charging and discharging gas, the canister enclosing a metal hydride capable of absorbing and desorbing hydrogen gas and a gauge for measuring the capacity of hydrogen remaining within the hydride material. To further permit distribution of the hydrogen throughout the hydride material, a porous matrix may be disposed within said metal hydride material for providing efficient distribution of hydrogen gas to said metal hydride material. The fuel gauge may further comprise subassemblies for determining the hydrogen capacity, each of which depends for operation on a different property of the metal hydride material. For example, a pressure gauge sensitive to the plateau pressure indicative of the hydrogen capacity, a piezoelectric sensor that in combination with a rigid chamber in which hydride material is closely packed provides a pressure differential indicative of hydrogen capacity, or a resistivity sensor that in combination with a chamber in which hydride material is packed provides a resistance differential indicative of hydrogen capacity.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to hydrogen storage devices and more particularly relates to such devices utilizing metal hydrides for storage of high volume of hydrogen at lower pressures. 
   2. Background Art 
   With the increasing use of hydrogen gas in industrial and commercial applications, conventional methods of storing hydrogen as a gas under pressure are seen to present undesirable risks. Hydrogen storage as a gas is typically done in large, bulky steel cylinders at very high pressures (e.g., 2,000 psi). 
   Hydrogen may be stored in liquid form, typically in insulated containers at very low temperatures. Energy must be used to keep the temperature low to prevent the liquid hydrogen from evaporating or boiling off. Hence, cryogenic hydrogen production and storage is highly inefficient because of the extremely cold temperatures at which the storage vessels must be maintained. 
   More recently, attention has been directed to the storage of hydrogen in metallic compounds, known as hydrides. Metal hydrides can store large amounts of hydrogen at low pressures and in relatively small volumes. Low pressure storage of hydrogen in containers containing hydrides is relatively safe and allows the construction of hydrogen containers having forms significantly different than those storing gaseous hydrogen. Examples of low pressure hydride storage containers can be found in commonly assigned U.S. Pat. Nos. 5,250,368, 5,532,074 and 5,623,987, which are incorporated by reference herein. 
   Additionally, hydrogen storage containers are sold by the assignee of the invention under the trade name Ergenics ST-Series hydrogen storage units utilizing hydrides manufactured by the assignee hereof, for example, HY-STOR® 208 hydride alloys. Additional information about these storage units and hydride alloys is available on the website of the assignee of this invention, found at www.ergenics.com. The use of solid hydridable materials to store hydrogen is also disclosed in numerous patents, such as U.S. Pat. Nos. 3,508,514, 3,516,263, 4,036,944, 5,518,528, 5,697,221 and 5,906,792. 
   Such metal hydride storage units have many uses and applications in a variety of industrial and laboratory applications and environments. The diversity of applications requires a storage system that can provide the user with a reliable source of hydrogen at a variety of capacities. Small storage units are commonly used as hydrogen sources for laboratory experimentation, as in the examples described above. Much larger units are needed to provide the quantity of hydrogen necessary for vehicular transportation systems or in utility applications. Hydrogen storage systems are needed that will allow for quick and easy use of the hydrogen gas, and the storage system, regardless of the size and capacity of the container, must be robust enough to perform properly and reliably under a wide range of environmental conditions. 
   Another major consideration is the ability of a user of a hydrogen storage device to measure or gauge the amount of hydrogen gas in the hydride container available for further utilization, as and when needed. An accurate gauge is ideal, but even a gauge of the amount of hydrogen gas availability is desirable that is accurate to within 15-30%. 
   The construction and normal operation of known metal hydride hydrogen storage systems make it especially difficult to accurately gauge the amount of hydrogen gas available. It is possible to accurately estimate the amount of hydrogen gas available in the above described gas containers which store hydrogen in a gas state from the ideal gas law (PV=nRT), because the amount of hydrogen is directly proportional to the pressure, when temperature is maintained at a constant value. This is possible because in those types of storage systems, the volume, V, and temperature, T, remain essentially constant as an amount of hydrogen gas (n) is utilized and being withdrawn from the system. The utilization of hydrogen gas, as it is withdrawn from the container is indicated by a reduction in the amount of hydrogen (n), and produces a directly proportional reduction in pressure, P. Thus, it is possible to estimate to a very accurate degree, the amount of hydrogen gas available in the container by an accurate reading of the pressure, P. 
   However, because the hydrogen gas is incorporated into the lattice structure of the metal hydride, storage containers using hydrides cannot rely on the pressure P as a directly proportional measure of the amount of hydrogen gas (n) remaining in the container. It is thus necessary to provide a fairly accurate measure of hydrogen gas contained within a container, similar to a fuel gauge in a standard internal combustion automobile. 
   SUMMARY OF THE INVENTION 
   Accordingly what is described and claimed as Applicants&#39; invention is a hydrogen gas storage container having a canister with at least one outlet opening for charging and discharging gas, the canister enclosing a metal hydride capable of absorbing and desorbing hydrogen gas and a gauge for measuring the capacity of hydrogen remaining within the hydride material. To further permit distribution of the hydrogen throughout the hydride, a porous matrix may be disposed within said metal hydride material for providing efficient distribution of hydrogen gas to said metal hydride material. The fuel gauge may further comprise subassemblies for determining the hydrogen capacity, and may depend on different principles, for example, a pressure gauge sensitive to the plateau pressure indicative of the hydrogen capacity, a piezoelectric sensor that in combination with a rigid chamber in which hydride material is closely packed provides a pressure differential indicative of hydrogen capacity, or a resistivity sensor that in combination with a chamber in which hydride material is packed provides a resistance differential indicative of hydrogen capacity. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       FIG. 1  illustrates in an elevational view a hydrogen storage device according to the present invention. 
       FIG. 2  is a graph showing the relationship between hydrogen capacity and hydrogen pressure at different temperatures. 
       FIG. 3  illustrates in a perspective view a first embodiment of the fuel gauge according to the present invention. 
       FIG. 4  illustrates a second embodiment of the fuel gauge indicator according to the present invention. 
       FIG. 5  illustrates yet another embodiment of the present invention. 
       FIG. 6  is a detailed view of the indicator portion of the inventive hydride storage container shown in FIG.  5 . 
       FIG. 7  illustrates another embodiment of the invention modifying the embodiment illustrated in FIG.  5 . 
       FIG. 8  illustrates yet another embodiment of a fuel gauge system according to the present invention. 
       FIG. 9  illustrates yet another embodiment of the present invention. 
       FIG. 10  is a detailed cross-sectional view of the embodiment shown in FIG.  9 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A hydrogen gas storage device  10 , according to one embodiment of the present invention is shown in FIG.  1 . The storage device  10  comprises a storage canister  12 , normally steel or aluminum bottles used for containing a hydride material for storing hydrogen, such as that disclosed in commonly assigned U.S. Pat. No. 4,396,114. The device  10  further includes an opening  14  through which the hydrogen is charged or discharged from the canister  12 . The opening  14  is closed off by a hermetically sealed valve  16 , controlled by a knob  15 , such as are known in the prior art. Turning the knob  15  to a closed position closes off the valve  16 , which should remain closed as long as the canister is not connected to another connected device (not shown) that utilizes the hydrogen stored within the canister  12 , for example, a fuel cell, a nickel hydrogen electrochemical cell or a hydrogen combustion engine (not shown). The valve  16  includes a fluid outlet  18  through which the hydrogen is charged or discharged only when the fluid outlet  18  is directly connected to either a hydrogen charging or discharging connected device. In this embodiment, a fuel gauge  20  is attached to the valve  16  by a fluid communication member  22 . The fuel gauge  20  may take one of several forms according to one of the several embodiments of the invention described below. 
   Referring now to  FIGS. 1 and 3 ,  FIG. 3  illustrates a detailed section of the fuel gauge  20  shown in a perspective view. The fuel gauge  20  comprises a fluid communication member  22  for connection to the valve  16 . Fluid communication member  22  provides fluid communication between a fuel gauge body  24  and the valve  16  so as to indicate the pressure of the hydrogen contained within the canister  12 . The body of the fuel gauge  24  may comprise a standard hydrogen pressure gauge, as is known for other gases. The fuel gauge/pressure gauge also has a front face  32  in which an indicator indicates the pressure of the hydrogen within the canister  12 . The indicator may comprise a needle indicator  30 , as shown. 
   The face  32  of the gauge  20  is different from other pressure gauges in that it has a plurality of scales, as will be explained below with reference to the graph in FIG.  2 . Several scales are necessary in order to be able to accurately gauge the amount of hydrogen capacity left within the canister  12 . As is known in the hydriding field, the pressure of hydrogen gas being stored within a hydride does not correlate directly with the pressure of the hydrogen within the canister, as would be expected with a general non-hydride type storage device. Furthermore, because of the peculiar characteristics of the hydriding/dehydriding process, the temperature of the hydride and the hydrogen gas within the canister also generates variability in the amount of hydrogen gas indicated as being available for use within the canister  12 . 
   Referring now to  FIG. 2 , a graph of the hydrogen capacity of a hydride M is correlated with the pressure within the canister  12  of the hydrogen gas labeled P H2 . The characteristics of hydrides require an increased amount of hydrogen to be absorbed by the hydride upon initial charging of the canister  12  with a hydrogen gas. As the hydrogen gas is absorbed by the hydride, an equilibrium pressure is reached in the canister  12  between the hydrogen that is absorbed within the hydride and the hydrogen gas freely floating within the hydrogen gas canister  12 . As additional pressurized hydrogen continues to be charged into the canister  12 , the hydrogen continues to be absorbed by the hydride at a slow rate until a point is reached within the pressure equilibrium curve, which is known as the plateau pressure. 
   The plateau pressures of the isotherms  40 ,  42 ,  44  are shown in FIG.  2 . The plateau pressure of isotherm  40 , indicating the pressure taken at a constant temperature of 20° C., begins at a point approximately around  50  and ends at a point approximately around  52 . Isotherm  40  is described herein in an example to indicate how the amount of hydrogen being absorbed as the pressure of the hydrogen gas is increased varies with the pressure as additional pressurized hydrogen gas is introduced into canister  12 . The flattening of the isotherm curve between points  50  and  52 , generally recognized in the hydride field as the plateau pressure, provides the means through which this embodiment of the invention is made effective. 
   The plateau pressure of isotherm  40  indicates that absorption of the hydrogen gas into the hydride is most rapid during the slow pressure increase of the plateau, between points  50  and  52 . As can be seen, isotherm  40  provides an increased capacity at the plateau pressure, as do the other isotherms  42  and  44 . Thus, a fuel gauge that is calibrated to render it sensitive to slight variations in pressure at or around the plateau pressures would be able to provide a reasonable indication of the hydrogen capacity contained within the hydride disposed within the canister  12 . 
   The phenomenon of hydrogen gas absorbed by the hydride over varying temperatures is a result of the isotherm of an equilibrium pressure being measured while the temperature is maintained at a constant temperature, as shown in FIG.  2 . On the other hand, if the pressure of the hydrogen gas in the canister  12  is maintained at a constant value, for example, as shown in  FIG. 2 , the hydrogen capacity M of the hydride increases with decreasing temperature at constant pressure P 1 . That is, as the temperature is decreased from the isotherm  44  (30° C.) to isotherm  42  (25° C.), the capacity increases from M 1  to M 2 , and for isotherm  40 , (20° C.), the capacity increases to M 3 . Similarly, for a constant capacity, for example, M 1 , a higher equilibrium pressure of the hydride material can be expected with increased temperature because maintaining capacity constant and increasing the temperature heats up the hydride and causes the release of hydrogen through desorption thereby increasing the pressure within the canister  12 . Moreover, as can be seen from the graphs, calibration and increased sensitivity of the pressure measurement apparatus to pressures immediately below P 1  in the region of the plateau pressures of each of the isotherms  40 ,  42 ,  44 , would more accurately gauge the remaining capacity, even though the capacity varies more steeply on the basis of smaller pressure differentials. 
   However, the plateau pressures as shown in  FIG. 2  can only indicate a specific pressure without reference to the underlying amount of hydrogen gas and the equilibrium point which is achieved at any specific temperature. A constant pressure P 1 , as shown in  FIG. 2 , would be indicative of different hydrogen capacities, i.e., M 1 , M 2 , M 3 , each depending on the temperature experienced in the hydrogen gas canister  12  and the hydride material within the canister. In other words, the same pressure P 1  may be indicative of a greater or lesser amount of hydrogen stored within the hydride, as shown by the values M 1 , M 2 , and M 3 , which is the intersection of the value P 1  with each of the different isotherms  44 ,  42 ,  40  respectively. 
   A correlation is necessary between the temperature of the contents of canister  12  and the pressure that is indicated by the fuel gauge  20 , which accounts for the pressure that is indicated indicative is dependent on the temperature to accurately indicate hydrogen capacity M. Thus, referring again to  FIG. 3 , the face  32  of the fuel gauge  20  includes a plurality of scales  34 ,  36 ,  38 , each of which indicate the temperature at which that the pressure should be gauged. A separate temperature dependent strip  39  may indicate by a varying color what temperature is being experienced by the system  10 . The strip  39  can show by a color change from blue to green to red the difference in temperatures between 20° C., 25° C., and 30° C., respectively. For example, again using the isotherm  40  as an example, a pressure P 1  as shown by the indicator  30  is indicative that at 20° C., the equilibrium pressure isotherm indicates a capacity M 3  of the hydrogen within the canister  12  and by a calibration of the amount of pressure, the scale  34  indicates that the value of M 3  correlates to full capacity of the hydrogen in the hydride. Thus, by an indication of a blue strip  39  an observer knows that the temperature is at 20° C. The observer, who is reading the gauge  20  knows, to refer to the fuel gauge at the 20° C. temperature, i.e., to scale  34 , for an indication of the capacity of the hydrogen left in the canister. 
   Conversely, if the strip  39  shows a green color, correlating to the 25° C. isotherm the observer will note that the scale  36  is indicative that the amount of fuel present within the canister  12  is at one half capacity. Finally, if the temperature is indicated as being red by the strip  39 , i.e., that the 30° C. scale is appropriate, then the observer knows to observe the scale  38 , correlating to 30° C., and would know that the fuel capacity is approaching empty. 
   Referring now to  FIG. 4 , therein is indicated a face  132  on a gauge  130  of another embodiment of the invention, in which rather than having several different scales, as does the embodiment  20  of  FIG. 3 , the face  132  has only one scale that is visible at any one time, which scale being visible is dependent on the temperature. Strips of temperature sensitive material, such as that used for the strip  39  (FIG.  3 ), and that are known and available from Omega Engineering, Inc. located at Stamford, Connecticut, are placed at appropriate parts of the face  132  of this embodiment of the fuel gauge. Each scale  134 ,  136 ,  138  and  140  appear and become easily visible for viewing by an observer only at the appropriate temperature. For example, if the temperature is 20° C., the blue scale  134  appears while the other scales are dark or indicate a darker color. Thus, only the scale  134  is easily visible because it is brighter than the other scales, as a result of the temperature sensitivity of the scale  132  at the temperature 20° C., and the observer knows to view only that portion of the face  132  that pertains to the isotherm scale  134 , appropriate for the temperature of 20° C. Similarly, on the indicator face  132 , other scales  136 ,  138 ,  140  would light up at the associated temperatures, for example, 25°, 30° or 35°, respectively. Each of these other scales is indicated by the dotted lines in the face  132  of FIG.  4 . Thus, use of this particular embodiment would ensure that the correct temperature would be read because only that part of the fuel gauge face  132  would be visible to the observer at the appropriate temperature. 
   In another aspect of the invention, as shown in  FIG. 5 , the storage canister  112  has a portion  114  which provides an inset for a fuel gauge  116 . Preferably, the fuel gauge  116  is inset into portion  114  disposed adjacent the bottom of canister  112 , as shown in FIG.  5 . Alternatively, the fuel gauge may be disposed on a side wall of a canister (not shown). 
     FIG. 6  is a detailed view of the fuel gauge according to this embodiment of the invention. The fuel gauge  116  comprises a container  118 , defining a chamber  122 , which rigidly retains a hydride alloy  120  within the chamber  122 . The container  118  preferably comprises a rigid material, such as steel or similar type of material that does not expand greatly as a result of an increase in temperature. The hydride alloy material  120  is held within chamber  122  in such a way that any expansion of the hydride alloy  120  must take place within the chamber  122  along a longitudinal direction as shown by the arrows  121 . The container  118  specifically comprises a longitudinally oriented chamber  122  containing the hydride alloy material  120 . Preferably, the hydride alloy material within the chamber  122  is the same alloy material that is present within the remainder of the canister  112 . 
   It is a known phenomenon that hydriding/dehydriding alloys exhibit considerable expansion during the hydrogen absorption process and may expand as much as 10 to 25% in volume as the alloy absorbed from zero capacity to full capacity hydrogen. Preferably, the hydride alloy is disposed within a longitudinally extending container so as to magnify the effects of the expansion process by the shape of the container. Thus, the increase in hydrogen capacity within the specific alloy can provide greater or lesser expansion pressure depending on the amount of hydrogen that has been absorbed within that alloy. 
   In order for the hydrogen to be absorbed and to be indicative of the amount of hydrogen that has been absorbed within the alloy in the canister  112 , there is preferably a means for communicating and providing the hydrogen in the same amount of pressure within the container  118  that is present in the remainder of the canister  112 . Thus a communication means is disposed between the canister  112  and the chamber  122 , such as a perforated screen (not shown) or aperture  126 , as indicated in FIG.  6 . The following embodiments of the invention each depend on the specific structure of the inset  114 , as shown in  FIG. 5 , so that the different pressure sensitive means of showing the amount of hydrogen capacity left within the canister  112 , according to each of the several embodiments described below, will be indicative of the invention as described. 
   In one embodiment of the invention utilizing the chamber illustrated in  FIG. 6 , the fuel gauge is connected to and works directly with a diaphragm  150 . The fuel gauge  140  may be a standard gauge showing the amount of expansion of the alloy in the chamber  122  by means of a single scale on the face  142  of the gauge  140  and a needle indicator  144 , as shown. The face  142  may have a shape and dimensions that match the size of the aperture  116  in the wall of canister  112 . Thus a person desiring to gauge the capacity of hydrogen left in the canister  112  need simply to view the face  142  to see the needle indication of metal hydride material expansion, which roughly correlates to the amount of remaining hydrogen capacity. 
   A single scale is shown on face  142  because the physical volumetric expansion of the contained metal hydride material depends to a much greater extent on the amount of hydrogen that is contained in the hydride material than it depends on the temperature of the hydride material. The correlation of volumetric expansion is so much more dependent on the hydrogen absorption amount that the temperature expansion effect can be effectively disregarded. However, should a more accurate reading be desired, so as to take into account the volumetric expansion of the hydride material which is contributed by an increase in temperature, a calibrated scale can also be provided to the face  142 , similar to those of the faces  32 ,  132  shown respectively in  FIG. 3  or  4 . 
   The diaphragm  150  ( FIG. 6 ) of the container  118  closes off fluid or gas communication so as to provide a hermetic seal of the canister  112  at aperture  116 . The diaphragm  150 , either directly or indirectly is in contact with the portion of the hydride material  120 , which is capable of expansion in the direction of arrow  121 , expansion in the other directions being constrained by the rigid nature of the walls of container  118 . Thus, with correct packing of the hydride material  120 , all of the volumetric expansion is translated into the longitudinal direction, shown by arrow  121 . Utilizing the principles of hydraulic motion, a small aperture size of the opening  152 , covered by the diaphragm  150 , translates a 10-25% volumetric expansion of the hydride material  120  into a strong, longitudinal actuation pressure acting on the diaphragm  150 . A post  156  connects the diaphragm  150  to the gauge  140 , so that the longitudinal variation in the post  156  turns the needle  144  to the appropriate indication of hydrogen capacity in the metal hydride material  120  within the chamber  118 . 
   The metal hydride material  120  is preferably identical to that of the metal hydride material in the canister  112 , so that the hydrogen absorption characteristics of the hydride material contained are the same. That is, good packing of the hydride material  120  and effective distribution of the hydrogen gas introduced through the fluid communication conduit  126  will cause identical hydrogen absorption, and consequently, identical volumetric expansion of the hydride material, thus providing a reasonably accurate reading of the hydrogen capacity of the canister  112 . 
   Alternatively, a different alloy material which is more susceptible to volumetric expansion, may be utilized within the chamber  122 , so as to further magnify the effect of the hydrogen expansion. That is, metal hydride alloy material, which expands more than the general hydride material in the canister  112 , will provide greater relative fluctuation in the volume of hydride material within the chamber  122 , thus permitting greater precision in the reading of the scale on face  142 . If two different metal hydrides are utilized, care must be taken, so that the plateau pressures of each of the hydrides can be correlated so as to accurately calibrate the scale. 
   Referring now to  FIG. 7 , another embodiment of the container  218  is disclosed, which has dimensions and a shape permitting its insertion within the aperture  116  of canister  112  (FIG.  5 ). Container  218  comprises rigid walls  219 , an aperture  222 , and a metal hydride material  220 , which may be identical to the hydride material in the canister  112 . A fluid communication conduit (not shown) may not be necessary in this or any of the other above described embodiments, if the rigid constraining walls  219  of container  218  comprise a material that retains its rigidity, but is also porous to hydrogen gas. This arrangement is preferable, in that more efficient and even distribution of hydrogen within the chamber  218  is possible, permitting the hydrogen gas to be absorbed by the metal hydride material  220  evenly, thus providing continuous expansion characteristics. 
   Of course, other means of display are possible with the inventive fuel gauge. For example, the diaphragm may be coupled with an electronic pressure indicator or other type of indicator that provides the required information to the operator. For example, as shown in  FIG. 7 , the rigid container  218  provides a diaphragm  240  comprising a pressure sensitive plastic, that changes color, for example, from blue to red, depending on the amount of pressure it is experiencing. The pressure sensitive plastic diaphragm  240  may be retained in place snugly against the metal hydride material  220  by a glass cover  242  and a threaded bezel  250  that, when screwed into place, impinges on the glass cover  242  so as to seal the aperture  222  and prevent hydrogen gas leakage outside of canister  112 . 
   The pressure sensitive plastic diaphragm  240  would have characteristics that produce a change to a different color, depending on the amount of longitudinal pressure exerted on the diaphragm  240  by the hydride material  220  contained within the chamber  222 . For example, the diaphragm may be set to show a blue color when the hydrogen has been depleted from the hydride material so that no expansion of the hydride material is evident, it could show a green color when there is about ½ capacity, when measured at or near the plateau pressure, as shown in  FIG. 2 , and could be a red color when the hydrogen is fully charged in the hydride material and no further hydrogen can be absorbed by the hydride material  220 . Of course, the pressure exerted by the hydride material acting on the diaphragm  240  would be affected by the longitudinal pressure exerted by the hydride material  220 . It is contemplated that the volumetric expansion pressure would be greater than the gas pressure exerted by the hydrogen gas within the chamber  222 . Appropriate means, such as a screwable adjustment of the bezel  250 , may be necessary to further calibrate the color code of the diaphragm  240  against the hydrogen capacity of the hydride material in the canister  112 . 
   Referring now to  FIG. 8 , another embodiment of the inventive fuel gauge  310  is shown, comprising a canister  312 , and including a port  314  through which lead lines  316 ,  318  extend into the canister  312 . The two lead lines  316 ,  318  are each connected to condition sensors that sense conditions within the canister  312 . For example, lead lines  316  are connected to a pressure transducer (not shown) within the canister that can sense either the hydrogen gas pressure within the canister  312  or preferably, on the pressure sensed by a piezoelectric transducer  320  within the canister  312 . The pressure sensor transducer may provide for a direct reading of the hydrogen gas pressure, or may have a container (not shown), similar to container  218  (FIG.  6 ), which is sensitive to the pressure exerted by a self contained hydride material unit. In either case, the transducer  320  generates an electric signal which corresponds to a pressure value and transmits it to a programmable logic controller (PLC) or micro processor  330 , preferably disposed outside of the canister  312 , as shown by dotted lines  326 . 
   A second set of lead lines  318  is connected to a temperature sensor  322  also disposed within the canister  312 , which may comprise, for example, a thermocouple or a resistance temperature device (RTD). The temperature sensor  322  also generates a signal through lead lines  318 , which is also transmitted to the central processor  330  through dotted lead lines  328 . 
   The processor  330  receives the signals transmitted by the sensors  320 ,  322 , and through electronic manipulation using a predetermined algorithm, can compute a value for the hydrogen capacity within the canister  312 . That value may be an analog or, preferably, a digital value, which may be displayed by a display  332  connected to the central processor  330 . Alternatively, or in conjunction with the central processor  330 , the values of the pressure and temperature sensed by sensors  320 ,  322  may independently corroborate that the hydrogen capacity display  332  is providing reasonable data, or the operator may utilize the data on displays  336 ,  338 , disposed in line with the lead lines  316 ,  318 , respectively, to independently determine the hydrogen capacity for example, by reference to a table. 
   Alternatively, the displays may themselves be displayed, for example, by displays  336 ,  338 , respectively, from which values an operator either the hydrogen gas pressure within the canister  312  or alternatively, on the piezoelectric pressure sensed by a transducer  320  within the canister  312 . The lead lines  316 ,  318  may be attached permanently to the sensor ports  314 , or the lead lines may be detachable and provided with a potable microprocessor unit  330  that may be attached to the ports  314  when information about the conditions in the canister  312  is desired. 
   Yet another embodiment of this invention is illustrated in  FIGS. 9 and 10 . A hydrogen storage device  410  includes a storage canister  412  having an inset portion  414  disposed in the side of the wall of canister  12 , although the disposition of the inset portion  414  may be in the bottom wall of the canister  412 , as shown in the embodiment of  FIG. 5 , or even in the valve  416  (not shown). The inset portion  414  comprises a port for a set of insulated lead wires  418  to provide an electrical connection to a resistance member  420  within the canister  412  and will be described in greater detail below with reference to FIG.  10 . 
   External to the canister  412 , the lead lines  418  are electrically connected to a device  440  for measuring electrical resistance. The device  440  may include a display  442 , as shown, that indicates resistance or, alternatively, indicates the hydrogen capacity from the electric resistance signal received from the lead wires  418  and a calibrated algorithm may be accessed on a logical control circuit  444  within the device  440  so as to calculate the hydrogen capacity from the resistance signal received from within the canister  412 . The lead lines  418  may also be usable with a portable stand alone resistance measuring device  440 , the lead lines  418  being temporarily attachable to electrical inlet plugs (not shown) disposed within the inset portion  414 , for example, similar to an electrical outlet. This feature would permit an operator to carry the measurement device  440 , plug in the lead lines  418 , measure the hydrogen capacity, withdraw the lead lines  418  from the inset plug and continue to monitor other devices  410 . 
   The principle, upon which device  410  operates, depends on another characteristic of hydrides, which correlates increasing resistivity with increasing hydrogen capacity. While a direct correlation has not been definitively established, it is a smooth enough correlation to provide an accurate gauge of hydrogen capacity, after proper calibration. That is, since the increase in resistivity provides a known correlation with the hydrogen capacity contained within the hydride, knowledge of the local electrical resistance of the hydride can be translated directly to a value of hydrogen capacity. Within the context of the metal hydride alloy materials contemplated for use in this invention, it is known that the resistance may vary up to 200% between a value when no hydrogen is contained by the hydride and at or near the full saturation point of the hydride material by pressurized hydrogen. 
   Referring now to  FIG. 10 , the resistance member  420  of  FIG. 9  is shown in a detailed cross-sectional view. Resistance member  420  is internally disposed within the canister  412 , but communicates by means of electrical signals transmitted through the electrical leads  418  at the inset  414 , as described above. Electrical lead lines  418  each terminate at terminals  422  that are embedded within a tightly packed internal chamber  424 . It is preferable that the chamber  424  has cylindrical walls  426  and end walls  428  defining a cylinder and the terminals  422  are spaced from each other and each disposed adjacent either of the two end walls  428  of the cylinder. Alternatively, other shapes may be utilized, for example, hexagonal or square shapes as viewed in cross-section. 
   The walls  426 ,  428  of the chamber  424  comprise an electrically insulating material that is permeable to hydrogen gas, for example Teflon® available from E. I. du Pont de Nemours and Company, of Wilmington, Del. The permeability of the material comprising walls  426 ,  428  permits the hydrogen to be absorbed and desorbed freely and equally by hydride alloy material  430  within the chamber  424 . The spatial separation between the terminals  422  may be predetermined so as to provide a desired resistance between the terminals for a chamber  424  of specified size. 
   The chamber  424  is packed tightly with the metal hydride material  430 , in powder form, to a predetermined density to provide a predetermined resistance level, which varies depending on hydrogen capacity of the hydride material  430 . Preferably, the hydride material is identical in both the chamber  424  and in the remainder of canister  412 , so that the hydrogen capacity of the hydride material matches throughout the device, thus providing an accurate reading of the hydrogen capacity of the hydride material within the canister  412 . 
   During operation, a signal is transmitted from the resistance member  420  that is indicative of the resistance between the terminals  422 , and is also essentially identical to the resistance of the hydride material within the canister  412 . This signal is sent to a resistance measuring device  440 , which analyzes the signal and utilizing calibrated algorithm accessed in the logical control circuit  444  determines the hydrogen capacity in the metal alloy hydride material in the canister  412 . This embodiment provides some advantage in that the resistance measuring device  440  also absorbs hydrogen, and thus the space taken up by device  440  is not underutilized, but is utilized in the storage of the hydrogen. 
   Other modifications and alterations may become apparent to a person having skill in the art from an understanding of the present invention. For example, different combinations of sensors and/or colored indicators may be modified or substituted for those disclosed. Alternatively, other types of conditions may be sensed by other types of sensors not described in detail herein. For example, a weight sensor may be provided, either internally within the canister or outside, that measures the weight of the hydride material, using a tare weight for the bottle and valves, if externally measured, to provide an alternative or additional measure of hydrogen capacity. Also, Hall effect measurements of a material, such as a hydride material, are known to change the material&#39;s electromagnetic properties with increasing hydrogen capacity. It is contemplated that such a sensor may gauge the hydrogen capacity of an enclosed hydride material by inducing a magnetic field and measuring the electrical properties, such as voltage, of the material. Calibration of the material against known capacities would be required to provide accurate readings. 
   The above-described embodiments of the inventive hydrogen fuel gauge are to be considered as illustrative only and not limiting, the invention only being limited by the following claims.