Gas density measurement for compressed gas cylinders

Abstract

The pressure vessel assembly (28) includes a pressure vessel (10) and a gas density meter (30). The pressure vessel includes a vessel wall (15) defining an interior cavity (17). The gas density meter includes a parallel plate capacitor (32) having a pair of plates (34 and 36) whose opposing surfaces (38 and 40) are separated by a distance (d) that is the width of an open gap (44). The capacitance of the capacitor is related to the density of the gas in the open gap.

Description

Detailed Description of the Invention

[0001] [background] Pressure vessels are commonly used to contain a variety of fluids under pressure, such as, for example, hydrogen, oxygen, nitrogen, propane, methane, and other fuels. Generally, pressure vessels can have any size or configuration. These vessels can be, for example, heavy or light, single-use (e.g., disposable), reusable, subjected to high pressures (e.g., greater than 50 pounds per square inch (psi) (344,738 Pascals)), subjected to low pressures (e.g., less than 50 psi or 344,738 Pascals), or used to store fluids at high or cryogenic temperatures.

[0002] Suitable pressure vessel shell materials include metals such as steel, or composites, which may include layers of wound fiberglass filaments or other synthetic filaments bonded by a thermosetting or thermoplastic resin. The fibers may be fiberglass, aramid, carbon, graphite, or any other commonly known fibrous reinforcing material. The resin materials used may be epoxy, polyester, vinyl ester, thermoplastic, or any other suitable resinous material capable of providing fiber-to-fiber adhesion, fiber layer-to-layer adhesion, and crush resistance required for the particular application in which the vessel is to be used. The composite construction of these vessels offers numerous advantages, including light weight and resistance to corrosion, fatigue, and catastrophic failure. These properties are due to the high specific strength of the reinforcing fibers or filaments.

[0003] A polymeric or other non-metallic elastic liner or bladder is often disposed within the composite shell to seal the container and prevent the internal fluid from contacting the composite. The liner can be manufactured by compression molding, blow molding, injection molding, or any other commonly known technique. Alternatively, the liner can be made of other materials, including steel, aluminum, nickel, titanium, platinum, gold, silver, stainless steel, and any alloys thereof. Such materials can generally be characterized as having a high modulus of elasticity. In one embodiment, the liner is formed of blown high density polyethylene (HDPE).

[0004] Figure 1 illustrates an elongated pressure vessel 10 according to the prior art, as disclosed in U.S. Patent No. 5,476,189, entitled "Pressure vessel with damage mitigating system," and International Application Publication No. WO2019 / 070409, entitled "Pressure Indicator," each of which is incorporated herein by reference. Figure 2 illustrates a partial cross-sectional view taken along line 2-2 of Figure 1, illustrating features as disclosed in U.S. Patent No. 5,429,845, entitled "Boss for a filament wound pressure vessel," which is incorporated herein by reference.

[0005] The pressure vessel 10 has a body portion 12 and a substantially hemispherical or dome-shaped end portion 14. A boss 16, typically constructed of aluminum, is provided at one or both ends of the pressure vessel 10 to provide a port for communication between an interior environment or cavity 17 of the pressure vessel 10 and an exterior environment 19. As shown in FIG. 2, the pressure vessel 10 has a vessel wall 15 formed with a liner 20 (such as an inner polymeric liner) that is covered by a shell 18. In one example, the shell 18 can be a filament wound composite shell. The shell 18 relieves structural loads on the pressure vessel 10, while the liner 20 provides a gas barrier.

[0006] The boss 16 may include a neck 22 having an exterior surface 23 and a port 26. The port 26 generally spans the exterior surface 23 of the boss 16 to allow fluid communication between the exterior environment 19 and the interior environment 17 of the pressure vessel 10. The boss 16 may also include a flange 24 (depicted as an annular flange) extending radially outward from the neck 22. The flange 24 may be received between portions of the liner 20 and / or may be sandwiched between the liner 20 and the shell 18. This structure secures the boss 16 to the pressure vessel 10 and provides a seal at the interface between the boss 16 and the liner 20.

[0007] It is often desirable to know the amount of gas (e.g., specific density or specific weight) contained within the interior void 17 of a known volume of a pressure vessel, such as when attempting to fill a pressure vessel with a particular volume of gas or when estimating when the pressure vessel will need refilling.

[0008] One technique that can be used to measure the amount of gas contained within a pressure vessel is based on the internal pressure. Pressure measuring devices suitable for this purpose are widely available.

[0009] One challenge with estimating the amount of gas in a pressure vessel based on pressure measurements is that the pressure measurements are temperature dependent. This can be easy to address when the temperature of the gas in the pressure vessel is at a steady temperature. However, during filling of the pressure vessel, the temperature inside the vessel changes significantly due to stratification and turbulent mixing of the gas, making it extremely difficult to accurately measure the amount of gas in the pressure vessel from time to time.

[0010] [overview] Embodiments of the present disclosure relate to devices, pressure vessel assemblies, systems, and methods for measuring the density of a gas contained within an internal cavity of a pressure vessel non-responsive to the temperature of the gas. One device is a pressure vessel plug configured to seal a port in a boss of the pressure vessel. The plug includes a plug body configured to be received within the port and a gas density meter. The gas density meter includes a parallel plate capacitor supported by the plug body. The capacitor includes a pair of plates having opposing surfaces separated by a distance the width of the open gap. The capacitance of the capacitor is related to the density of the gas in the open gap.

[0011] One example of a pressure vessel assembly includes a pressure vessel and a gas density meter. The pressure vessel includes a vessel wall defining an interior cavity. The gas density meter includes a parallel plate capacitor having a pair of plates, opposing surfaces of the plates separated by a distance equal to the width of an open gap. The capacitance of the capacitor is related to the density of the gas in the open gap.

[0012] One example of a system for measuring the density of a gas in a pressure vessel includes a pressure vessel assembly and a capacitance measurement circuit. The assembly includes a pressure vessel and a gas density meter. The pressure vessel includes a vessel wall defining an interior cavity. The gas density meter includes a parallel plate capacitor having a pair of plates separated by a distance a defined width of an open gap. The opposing surfaces and the gap are exposed to the gas in the interior cavity. The capacitance measurement circuit is configured to generate a capacitance signal related to the capacitance of the capacitor and the density of the gas in the interior cavity.

[0013] In one example of a method for measuring the density of a gas in a pressure vessel, the gas is contained within an interior cavity of the pressure vessel defined by the vessel walls. A gap between a pair of plates of a parallel plate capacitor is exposed to the interior cavity. A capacitance measurement circuit is used to generate a capacitance signal indicative of the capacitance of the capacitor. The capacitance signal is related to the density of the gas.

[0014] This disclosure, whether in apparatus or method form, may also be characterized by the following items in various combinations thereof: 1. A pressure vessel plug configured to seal a port of a boss of a pressure vessel, comprising: a plug body configured to be received within the port; a parallel plate capacitor including a pair of plates having opposing surfaces supported by the plug body and spaced apart a distance equal to the width of the open gap; Equipped with The capacitance of the capacitor is related to the density of the gas in the open gap. Pressure vessel plug. 2. The plug body includes a proximal end and a distal end that is exposed to an interior cavity of the pressure vessel when the plug body is received within the port; the capacitor being supported at the distal end; The plug according to claim 1. 3. The plug of claim 2, wherein the gas density meter includes a terminal block at the proximal end having a pair of terminals, each of the terminals connected to one of the plates via a conductive path. 4. A plug as claimed in any one of claims 1 to 3, wherein the plug includes a valve system through which gas can be injected into or exhausted from the internal void. 5. A pressure vessel assembly comprising: a pressure vessel including a vessel wall defining an interior cavity; a gas density meter comprising a parallel plate capacitor including a pair of plates separated by a distance equal to the width of an open gap; Equipped with the opposing surface and the gap are exposed to gas within the internal gap; The capacitance of the capacitor is related to the density of the gas. Pressure vessel assembly. 6. A boss supported by the vessel wall and including a port into the interior cavity; a plug including a plug body received within the port; Including, the parallel plate capacitor being supported by the plug; 6. The assembly of claim 5. 7. The plug body includes a proximal end and a distal end exposed to the interior cavity; the capacitor being supported at the distal end; 7. The assembly of claim 6. 8. An assembly according to any of claims 5 to 7, wherein the plug includes a valve system through which gas can be injected into or exhausted from the internal cavity. 9. The assembly of any of claims 5 to 7, including a capacitance measurement circuit configured to generate a capacitance signal related to the capacitance of the capacitor and the density of the gas. 10. A first one of the plates is connected to electrical ground; the capacitance measurement circuit includes a timer circuit connected to a second one of the plates and configured to output the capacitance signal having a frequency related to the capacitance. 10. The assembly of claim 9. 11. The capacitance measurement circuit includes a controller, the controller comprising: determining the capacitance of the capacitor based on the capacitance signal; calculating a relative dielectric constant of the gas based on the capacitance, the distance between the opposing surfaces, the surface area of ​​the opposing surfaces, and the dielectric constant of free space; determining the density of the gas based on the dielectric constant and a relationship between the dielectric constant of the gas and a density of the gas; generating a signal representative of a value related to the density of the gas; It is configured as follows: 10. The assembly of claim 9. 12. A method for measuring the density of a gas in a pressure vessel, comprising: Containing a gas within an interior cavity of the pressure vessel defined by a vessel wall; exposing a gap between a pair of plates of a parallel plate capacitor to the interior gap; generating a capacitance signal indicative of the capacitance of the capacitor using a capacitance measurement circuit; Including, the capacitance signal being related to the density of the gas; method. 13. The pressure vessel includes a boss supported by the vessel wall and having a port into the interior cavity; The method comprises: providing a plug having a plug body; sealing the port, the receiving member including receiving the plug body within the port; Including, the parallel plate capacitor being supported by the plug body; The method of claim 12. 14. Connecting a first one of the plates to electrical ground; connecting a second one of the plates to a timer circuit of the capacitance measurement circuit; generating the capacitance signal using the timer circuit; Including, the capacitance signal has a frequency related to the capacitance; The method of claim 12. 15. A processing step performed using a controller, the processing step comprising: determining the capacitance of the capacitor based on the capacitance signal; calculating a dielectric constant of the gas based on the capacitance, the distance between the opposing surfaces, the surface area of ​​the opposing surfaces, and the dielectric constant of free space; determining the density of the gas based on the dielectric constant and a relationship between the dielectric constant of the gas and a density of the gas; generating a signal representative of a value related to the density of the gas; Including, 15. The method according to any one of claims 12 to 14.

[0015] This Summary is provided to introduce in a simplified form a selection of concepts that are further described below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. In particular, features disclosed herein with respect to one embodiment may be equally applicable to other embodiments. Moreover, this Summary is not intended to be used as an aid in determining the scope of the claimed subject matter. Many other novel advantages, features, and relationships will become apparent as this description progresses. The figures and description that follow more particularly exemplify exemplary embodiments. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a side view of an example of a conventional pressure vessel.

[0017] [Diagram 2] 2 is a partial cross-sectional view of one end of the pressure vessel of FIG. 1 taken along line 2-2 of FIG. 1, showing an example of a typical boss, liner, and shell.

[0018] [Diagram 3] FIG. 1 is a simplified cross-sectional view of a pressure vessel assembly according to an embodiment of the present disclosure.

[0019] [Figure 4] FIG. 1 is a schematic diagram of a system for measuring the density of a gas in a pressure vessel according to an embodiment of the present disclosure.

[0020] [Diagram 5]1 is a chart illustrating the relationship between the dielectric constant of hydrogen gas and the density of the gas obtained at various temperatures.

[0021] [Figure 6] FIG. 2 is a diagram of a capacitance measurement circuit according to an embodiment of the present disclosure.

[0022] [Figure 7] 1 is a flow chart illustrating a method for measuring the density of a gas in a pressure vessel according to an embodiment of the present disclosure.

[0023] While the above figures set forth one or more embodiments of the disclosed subject matter, other embodiments are contemplated, as noted in this disclosure. In all cases, this disclosure presents the disclosed subject matter by way of representation and not by way of limitation. It should be understood that numerous other variations and embodiments can be devised by those skilled in the art which fall within the scope of the principles of this disclosure.

[0024] The figures may not be drawn to scale. In particular, some features may be enlarged relative to other features for clarity. Also, where terms such as above, below, above, below, top, bottom, side, right, left, vertical, horizontal, etc. are used, it will be understood that they are used only for ease of understanding of the description. It is contemplated that structures may be oriented in other manners. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS The embodiments of the present disclosure are described more fully hereinafter with reference to the accompanying drawings. Elements identified using the same or similar reference characters refer to the same or similar elements. However, various embodiments of the present disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be comprehensive and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0026] In the compressed gas industry, the temperature pressure dependence that exists for compressed gas creates challenges in the ability to completely fill a cylinder or pressure vessel. Most jurisdictions around the world allow high pressure cylinders or vessels to be filled to a given pressure at a specified temperature. A more efficient means of filling the pressure vessel would be to fill the cylinder to a weight. For many pressure vessel applications (e.g., vessels mounted in automobiles, trailers, etc.), filling the vessel to a weight is not feasible or cost effective.

[0027] The embodiments of the present disclosure relate to devices, pressure vessel assemblies, systems, and methods for measuring the density of a gas contained within the interior volume 17 of the pressure vessel 10 in a manner that is substantially insensitive to the temperature of the gas. These embodiments can be used to fill a pressure vessel in a manner that would otherwise not be possible to fill the pressure vessel to a certain weight. Thus, the embodiments of the present disclosure provide a wide range of benefits, including allowing an automobile fuel gauge to more accurately know the amount of energy available in a fuel container (pressure vessel) as opposed to simply relying on a pressure estimate, and allowing for the safe and accurate filling of a pressure vessel to its full volume.

[0028] 3 is a simplified cross-sectional view of a pressure vessel assembly 28 according to an embodiment of the present disclosure. In general, the assembly 28 includes a gas density meter 30 and a pressure vessel 10. The meter 30 includes a parallel plate capacitor 32 having a pair of parallel plates 34 and 36. The plates 34 and 36 have opposing surfaces 38 and 40 spaced apart from one another by a distance d and having generally coincident surface areas. The distance d may be substantially fixed. However, the distance d may vary due to deformation of the plates 34 and 36. In general, the distance d is a known parameter that may be determined using suitable techniques. The plates 34 and 36 may be supported in a suitable electrically insulating material 42, e.g., polyoxymethylene (POM), polyetheretherketone (PEEK), and / or another suitable dielectric or electrically insulating material, that electrically insulates the unexposed portions of the plates 34 and 36 from surrounding materials and fixes the relative positions of the plates 34 and 36.

[0029] A gap 44, corresponding to separation distance d, extends between surfaces 38 and 40 of plates 34 and 36. Capacitor 32 is supported such that gap 44 is open to interior cavity 17 of pressure vessel 10. Gas within interior cavity 17 of pressure vessel 10 thus fills gap 44 and acts as a dielectric for capacitor 32. As discussed below, a relationship exists between the dielectric constant (and thus the capacitance of capacitor 32) and the density of the gas contained within interior cavity 17. Thus, the capacitance measured using meter 30 can be used to determine the density and amount of gas contained within interior cavity 17.

[0030] The gauge 30 of the assembly 28 may be supported in any suitable manner such that the gap 44 is open to the interior cavity 17 of the pressure vessel 10. In one embodiment, the gauge 30 is supported by the wall 15 of the pressure vessel 10 (FIG. 2). In one more specific example, the capacitor 32 is attached to a plug 50, which may be supported by the wall of the pressure vessel 10 via a boss 16, as shown in FIG.

[0031] The plug 50 may take on any suitable shape and may be operable to seal against the boss 16 of the pressure vessel, as shown in FIG. 3. Conventional sealing components, such as O-rings, are not shown to simplify the drawing. The plug 50 may include a plug body 52 having a shoulder portion 54 at a proximal end 56 and a cylindrical portion 58 extending toward a distal end 60. The cylindrical portion 58 may include a threaded outer surface 62 configured to be received within the boss 16 and cooperate with a threaded inner surface 64 of the boss 16 to secure the plug 50 to the boss 16. Other conventional techniques for securing the plug 50 to the boss 16 may also be used.

[0032] In some embodiments, the plug 50 includes a conventional valve system 66 that may facilitate the exit of gas from and the entry of gas into the interior cavity 17 of the pressure vessel 10 .

[0033] 3, capacitor 32 is supported at a distal end 60 of plug body 52 with gap 44 exposed to interior cavity 17. Alternatively, capacitor 32 may be supported at a more interior location in plug body 52, provided that an open passage to interior cavity 17 extends toward gap 44.

[0034] In one embodiment, the meter 30 includes a terminal block 68 at the proximal end 56 of the plug body 52. ​​The terminal block 68 may include terminals 70 and 72 connected to the plates 34 and 36, respectively, via suitable conductors 74 and 76. In one alternative, the gas density meter 30 may be configured to wirelessly transmit the capacitance of the capacitor 32 to an external circuit for processing.

[0035] FIGURE 4 is a simplified block diagram of a system 80 for measuring the density of a gas within an interior of a pressure vessel according to an embodiment of the present disclosure. The system 80 may include a pressure vessel assembly 28 formed in accordance with one or more embodiments disclosed herein and a capacitance measurement circuit 82. As described above, and as shown in FIGURE 4, an embodiment of the assembly 28 generally includes a pressure vessel 10 and a gas density meter 30 supported by a wall 15 of the vessel 10. This includes the configuration of FIGURE 3 where the meter 30 is supported by a plug 50, which in turn is supported by a boss 16 attached to the wall 15 of the pressure vessel 10.

[0036] The capacitance measurement circuit 82 is connected to the capacitor 32 through the terminal block 68 of the meter 30 and measures the capacitance (C MEAS ) and generating a capacitance signal 84 indicative of or related to the measured capacitance of capacitor 32.

[0037] In one embodiment, circuitry 82 includes a controller 90 configured to calculate a dielectric constant of the gas in interior cavity 17 based on the capacitance signal, and to determine the density of the gas based on the dielectric constant. As described below, for some gases, this calculation of the amount of gas in vessel 10 can be performed independent of the temperature of the gas.

[0038] The controller 90 may represent one or more processors (e.g., central processing units) that control components of the circuit 82 and / or process signals, such as the capacitance signal 84, to perform one or more functions described herein, such as, for example, determining or calculating the density of the gas in the vessel 10. The controller 90 performs these control functions in response to the execution of instructions. The instructions may be stored in a memory 92, which represents a local and / or remote memory or computer-readable medium. Such memory 92 may include any suitable computer-readable medium eligible for patent subject matter that does not include a transitory wave or signal, such as, for example, a hard disk, a CD-ROM, an optical storage device, and / or a magnetic storage device. The one or more processors of the controller 90 may be components of one or more computer-based systems, which may include one or more control circuits, a microprocessor-based engine control system, and / or one or more programmable hardware components, such as, for example, a field programmable gate array (FPGA). One suitable controller 90 is an ARM7 32-bit reduced instruction set computer (RISC) microcontroller or similar controller.

[0039] The controller 90 may calculate the relative dielectric constant κ of the gas in the internal gap 17 using Equation 1 shown below.

number

[0040] Furthermore, there is a known relationship between the dielectric constant of a gas and its density. Thus, when the interior volume 17 of the pressure vessel 10 is filled with a known gas, the dielectric constant calculated by the controller 90 can be used to determine the corresponding density of the gas based on a mapping of the dielectric constant of the gas to density. The mapping may be contained in the memory 92 of the circuitry 82. The controller 90 or another computing device can then use the determined density of the gas to calculate the weight or mass of the gas in the interior volume 17 of the pressure vessel 10 based on the volume of the interior volume 17.

[0041] In some embodiments, hydrogen gas (H2) is contained within the internal volume 17 of the pressure vessel 10, and the memory 92 stores a mapping of the relationship of the dielectric constant of the hydrogen gas to density. FIG. 5 is a chart illustrating such a relationship, in which the dielectric constant of the hydrogen gas is obtained at various temperatures. As shown in the chart, the dielectric constant of the hydrogen gas is substantially independent of temperature over a wide range of operating temperatures of the process vessel 10 (e.g., −40° C. to 85° C.). As a result, density measurements according to the present disclosure may be useful in determining the density of the hydrogen gas contained within the volume 17 of the pressure vessel 10. Thus, the density of hydrogen or another gas contained within the internal volume 17 and having a dielectric constant that is substantially independent of temperature over the range of operating temperatures of the vessel 10 may be determined by the controller 90 without the need to provide temperature compensation through the use of a temperature sensor.

[0042] In one embodiment, the controller 90 outputs a signal 94 that represents a value related to the determined density of the gas within the internal void 17. Thus, the value represented by the signal 94 may directly represent the density of the gas. Alternatively, the controller 90 may process the determined density to generate a signal 94 that represents the mass or weight of the gas contained within the void 17 based on the determined density and the known volume of the internal void 17.

[0043] Circuit 82 may generate capacitance signal 84 using any suitable technique. In one example, as shown in FIG. 4, circuit 82 includes a timer circuit 86 that connects plate 36 to an electrical ground or electrical common 88 of circuit 82. In one embodiment, timer circuit 86 is configured to output capacitance signal 84 indicative of or having a frequency related to the capacitance of capacitor 32.

[0044] 6 is a schematic diagram of a capacitance measurement circuit 82 including an example timer circuit 86 according to an embodiment of the present disclosure. In one example, the timer circuit 86 includes a NE555 timer integrated circuit chip 96 or a similar component. The configuration of the timer circuit 86 shown in FIG. 6 includes a C MEAS may be calculated by the controller 90 using Equation 2, where f OUT is the frequency of the pulses of the capacitance signal 86 output from the tip 96 at pin 3.

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[0045] Additional processing circuitry 98, representing one or more processors or computing devices, may be used to perform further processing of signal 94, such as, for example, calculating the weight of the gas in internal void 17 based on the density value represented by density signal 94 and the known volume of internal void 17. In some embodiments, as shown in FIGURE 4, processing circuitry 98 presents the gas density value, the gas weight, and / or other information to a user of system 80 on display 100. The presented information may assist the user in filling internal void 17 of pressure vessel 10 with a selected gas to a desired density or weight.

[0046] 7 is a flow chart illustrating a method for measuring the density of a gas within an interior cavity 17 of a pressure vessel 10 according to an embodiment of the present disclosure. At 102, the method includes a gas being contained within the interior cavity 17 of the pressure vessel 10 defined by the vessel wall 15 (FIGS. 1 and 2). At 104, a gap 44 between a pair of plates 34 and 36 of a parallel plate capacitor 32 is exposed to the interior cavity in a manner as shown in FIGS. 3 and 4. At 106, the method includes a capacitance signal 84 indicative of or representative of a value of the capacitance of the capacitor 32 is generated using a capacitance measurement circuit 82 (FIG. 4) according to the techniques described herein.

[0047] As discussed above, the measured capacitance is related to the density of the gas. In some embodiments of the method, the controller 90 determines or calculates the capacitance of the capacitor 32 based on the capacitance signal 84, as shown at 108, in a manner such as using Equation 2 described above. In one embodiment, the controller 90 calculates the dielectric constant of the gas, as shown at 110, based on the capacitance, the distance d separating the plates 34 and 36, the surface areas of the opposing surfaces 38 and 40 of the plates 34 and 36, and the dielectric constant of free space, as described above. The controller 90 may then determine the density of the gas, at 112, based on the dielectric constant and the relationship between the dielectric constant and the density of the gas, and may generate a signal 94, at 114, representing a value related to the density of the gas.

[0048] Although embodiments of the present disclosure have been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the present disclosure.

Claims

1. 1. A pressure vessel plug configured to seal a port of a boss of a pressure vessel, comprising: a plug body configured to be received within the port; a parallel plate capacitor including a pair of plates having opposing surfaces supported by the plug body and spaced apart a distance equal to the width of the open gap; a valve system configured to control the injection or evacuation of gas through the plug body; Equipped with a first passageway extending from a first opening in a distal surface of the plug body to the open gap; a second passageway extending from a second opening in the distal face to the valve system; The capacitance of the capacitor is related to the density of the gas in the open gap. Pressure vessel plug.

2. the plug body includes a proximal end and a distal end including the distal face and exposed to an interior cavity of the pressure vessel when the plug body is received within the port; the capacitor being supported at the distal end; The plug according to claim 1.

3. the gas density meter includes a terminal block at the proximal end having a pair of terminals; Each of the terminals is connected to one of the plates via a conductive path. The plug according to claim 2.

4. 1. A pressure vessel assembly comprising: a pressure vessel including a vessel wall defining an interior cavity; a boss supported by the vessel wall and including a port into the interior cavity; a plug including a plug body received within the port; a parallel plate capacitor supported by the plug body and including a pair of plates having opposing surfaces spaced apart a distance equal to the width of the open gap; a valve system through which gas can be injected into or exhausted from the internal cavity; and Equipped with a first passageway extending from a first opening in a distal surface of the plug body to the open gap; a second passageway extending from a second opening in the distal face to the valve system; the opposing surfaces and the open gap are exposed to gas within the interior gap; The capacitance of the capacitor is related to the density of the gas. Pressure vessel assembly.

5. the plug body includes a proximal end and a distal end including the distal surface and exposed to the interior cavity; the capacitor being supported at the distal end; 5. The assembly of claim 4.

6. The gas density instrument includes a terminal block having a pair of terminals at the proximal end; Each of the terminals is connected to one of the plates.

6. The assembly of claim 5.

7. a capacitance measurement circuit configured to generate a capacitance signal related to the capacitance of the capacitor and the density of the gas.

6. An assembly according to claim 4 or 5.

8. a first one of the plates is connected to electrical ground; the capacitance measurement circuit includes a timer circuit connected to a second one of the plates and configured to output the capacitance signal having a frequency related to the capacitance; 8. The assembly of claim 7.

9. The capacitance measurement circuit includes a controller, the controller comprising: determining the capacitance of the capacitor based on the capacitance signal; calculating a relative dielectric constant of the gas based on the capacitance, the distance between the opposing surfaces, the surface area of ​​the opposing surfaces, and the dielectric constant of free space; determining the density of the gas based on the dielectric constant and a relationship between the dielectric constant of the gas and a density of the gas; generating a signal representative of a value related to the density of the gas; It is configured as follows:

8. The assembly of claim 7.

10. 1. A method for measuring the density of a gas in a pressure vessel, comprising: The pressure vessel comprises: a vessel wall defining an interior cavity; a boss supported by the vessel wall and including a port into the interior cavity; a plug including a plug body received within the port; a parallel plate capacitor supported by the plug body and including a pair of plates having opposing surfaces spaced apart a distance equal to the width of the open gap; a valve system through which gas can be injected into or exhausted from the internal cavity; and a first passageway extending from a first opening in a distal surface of the plug body to the open gap; a second passageway extending from a second opening in the distal surface to the valve system; The method comprises: containing a gas within the interior volume of the pressure vessel; exposing the open gap to the interior cavity via the first path; exposing the valve system to the interior cavity via the second path; generating a capacitance signal indicative of the capacitance of the capacitor using a capacitance measurement circuit; Including, the capacitance signal being related to the density of the gas; method.

11. connecting a first one of the plates to an electrical ground; connecting a second one of the plates to a timer circuit of the capacitance measurement circuit; generating the capacitance signal using the timer circuit; Including, the capacitance signal has a frequency related to the capacitance; The method of claim 10.

12. The processing steps include a process performed using a controller, the process steps comprising: determining the capacitance of the capacitor based on the capacitance signal; calculating a dielectric constant of the gas based on the capacitance, the distance between the opposing surfaces, the surface area of ​​the opposing surfaces, and the dielectric constant of free space; determining the density of the gas based on the dielectric constant and a relationship between the dielectric constant of the gas and a density of the gas; generating a signal representative of a value related to the density of the gas; Including, 12. The method according to claim 10 or 11.

13. The plug body includes a proximal end and a distal end exposed to the internal cavity, the capacitor being supported at the distal end; The method of claim 10.

14. The gas density instrument includes a terminal block having a pair of terminals at the proximal end; Each of the terminals is connected to one of the plates. The method of claim 13.

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