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 various fluids, such as hydrogen, oxygen, nitrogen, propane, methane, and other fuels, under pressure. Generally, pressure vessels can have any size or configuration. These containers can be, for example, heavy or light, single-use (e.g., disposable), reusable, withstand high pressures (e.g., greater than 50 pounds per square inch (psi) (344,738 pascals)), withstand low pressures (e.g., less than 50 psi or 344,738 pascals), or be used to store fluids at high or extremely low temperatures.

[0002] Suitable pressure vessel shell materials include metals such as steel, or composites, which may include laminates of wound fiber glass filaments or other synthetic filaments bonded by a thermosetting resin or a thermoplastic resin. The fibers can be fiber glass, aramid, carbon, graphite, or any other commonly known fibrous reinforcing material. The resin material used can be an epoxy, polyester, vinyl ester, thermoplastic, or any other suitable resinous material that can provide adhesion between the fibers, adhesion between the fiber layers, and the crush resistance required for the specific application for which the container is to be used. The construction of these containers from composites provides a number of advantages, such as light weight and resistance to corrosion, fatigue, and damage. These properties are due to the high specific strength of the reinforcing fibers or filaments.

[0003] To seal the container and prevent the internal fluid from contacting the composite material, an elastic liner or inner bag made of polymeric or other non-metallic materials is often disposed within the composite shell. 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 blow-molded high-density polyethylene (HDPE).

[0004] FIG. 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. FIG. 2 illustrates a partial cross-sectional view taken along line 2-2 of FIG. 1 and illustrates 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 substantially hemispherical or dome-shaped ends 14. The boss 16 is typically constructed of aluminum and is provided at one or both ends of the pressure vessel 10 to provide a port for communication between the internal environment or internal void 17 of the pressure vessel 10 and the external environment 19. As shown in FIG. 2, the pressure vessel 10 has a container wall 15 formed using a liner 20 (such as an inner polymer liner) covered by a shell 18. In one example, the shell 18 can be a filament-wound composite shell. While the shell 18 relieves the structural load applied to the pressure vessel 10, the liner 20 provides a gas barrier.

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

[0007] When attempting to fill a pressure vessel with a specific volume of gas or when estimating when the pressure vessel needs replenishment, it is often desirable to know the amount (e.g., specific density or specific weight) of gas contained within the known volume of the internal void 17 of the pressure vessel.

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

[0009] One problem with estimating the amount of gas within a pressure vessel based on pressure measurement is that this pressure measurement is temperature dependent. When the temperature of the gas within the pressure vessel is at a steady temperature, it may be easy to address this. However, during the filling of the pressure vessel with gas, the temperature within the vessel changes significantly due to gas stratification and turbulent mixing, making it extremely difficult to accurately measure the amount of gas within the pressure vessel at any given time.

[0010] [Summary] Embodiments of the present disclosure relate to a device, a pressure vessel assembly, a system, and a method for non-reactively measuring the density of a gas contained within an internal void of a pressure vessel with respect to the temperature of the gas. One device is a pressure vessel plug configured to seal a port of a boss of the pressure vessel. The plug includes a plug body configured to be received within the port and a gas density instrument. The gas density instrument includes a parallel plate capacitor supported by the plug body. The capacitor includes a pair of plates having opposing surfaces 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 within the open gap.

[0011] One example of a pressure vessel assembly includes a pressure vessel and a gas density instrument. The pressure vessel includes a vessel wall that defines an internal void. The gas density instrument includes a parallel plate capacitor having a pair of plates. The opposing surfaces of the plates are 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 within the open gap.

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

[0013] In one example of a method for measuring the density of a gas within a pressure vessel, the gas is contained within an internal void of the pressure vessel defined by a vessel wall. A gap between a pair of plates of a parallel plate capacitor is exposed to the internal void. A capacitance signal indicative of the capacitance of the capacitor is generated using a capacitance measurement circuit. The capacitance signal is related to the density of the gas.

[0014] This disclosure may be characterized by the items listed below, whether in the form of an apparatus or a method, or in various combinations thereof. 1. A pressure vessel plug configured to seal a port of a boss of a pressure vessel, a plug body configured to be received within the port, and a gas density instrument comprising a parallel plate capacitor including a pair of plates having opposing surfaces supported by the plug body and separated by a distance of the width of an open gap, comprising, wherein the capacitance of the capacitor is related to the density of the gas within the open gap, the pressure vessel plug. 2. The plug according to claim 1, wherein the plug body includes a proximal end and a distal end that is exposed to an internal void of the pressure vessel when the plug body is received within the port, and the capacitor is supported at the distal end. The plug according to claim 1. 3. The plug according to claim 2, wherein the gas density instrument includes, at the proximal end, a terminal block having a pair of terminals, each of the terminals being connected to one of the plates via a conductive path. 4. The plug according to any one of claims 1 to 3, wherein the plug includes a valve system through which gas can be injected into or discharged from the internal void. 5. A pressure vessel assembly, comprising a pressure vessel including a vessel wall defining an internal void, and a gas density instrument comprising a parallel plate capacitor including a pair of plates separated by a distance of the width of an open gap, comprising, wherein the opposing surfaces and the gap are exposed to the gas within the internal void, and the capacitance of the capacitor is related to the density of the gas. the pressure vessel assembly. 6. A boss supported by the vessel wall and including a port to the internal void, A plug comprising a plug body received within the port, including the parallel plate capacitor being supported by the plug, The assembly according to claim 5. 7. The plug body includes a proximal end and a distal end exposed to the internal void, the capacitor being supported at the distal end, The assembly according to claim 6. 8. The plug includes a valve system through which gas can be injected into or discharged from the internal void, the assembly according to any one of claims 5 to 7. 9. The assembly according to any one 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. The first of the plates is connected to electrical ground, the capacitance measurement circuit being connected to the second of the plates and comprising a timer circuit configured to output a capacitance signal having a frequency related to the capacitance. The assembly according to claim 9. 11. The capacitance measurement circuit includes a controller that determines the capacitance of the capacitor based on the capacitance signal, calculates the relative permittivity of the gas based on the capacitance, the distance between the opposing surfaces, the surface area of the opposing surfaces, and the permittivity of free space, determines the density of the gas based on the relative permittivity and the relationship between the relative permittivity of the gas and the density of the gas, and generates a signal representative of a value related to the density of the gas, being configured as such. The assembly according to claim 9. 12. A method for measuring the density of a gas within a pressure vessel, accommodating a gas within an internal void of the pressure vessel defined by a vessel wall; exposing a gap between a pair of plates of a parallel plate capacitor to the internal void; generating a capacitance signal indicative of the capacitance of the capacitor using a capacitance measurement circuit; comprising; wherein the capacitance signal is related to the density of the gas; a method. 13. The pressure vessel includes a boss supported by the vessel wall and having a port to the internal void, the method comprising: providing a plug having a plug body; sealing the port by receiving the plug body within the port; comprising; wherein the parallel plate capacitor is supported by the plug body; The method according to 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; comprising; wherein the capacitance signal has a frequency related to the capacitance; The method according to claim 12. 15. comprising processing steps executed using a controller, the processing steps comprising: determining the capacitance of the capacitor based on the capacitance signal; calculating a relative permittivity of the gas based on the capacitance, a distance between the opposing surfaces, a surface area of the opposing surfaces, and a permittivity of free space; determining the density of the gas based on the relative permittivity and a relationship between the relative permittivity of the gas and the density of the gas; The step of generating a signal representing a value related to the density of the gas; including 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 features or essential features of the claimed subject matter, nor is it intended to be used as an aid in defining the scope of the claimed subject matter. Specifically, the features disclosed herein with respect to one embodiment may be equally applicable to another embodiment. Further, this summary is not intended to be used as an aid in defining the scope of the claimed subject matter. As the description proceeds, many other novel advantages, features, and relationships will become apparent. The figures and description that follow illustrate exemplary embodiments in more specific detail.

Brief Description of the Drawings

[0016]

Figure 1

[0017]

Figure 2

[0018]

Figure 3

[0019]

Figure 4

[0020]

Figure 5

[0021]

Figure 6

[0022]

Figure 7

[0023] The above figures specify one or more embodiments of the disclosed subject matter, but as noted in this disclosure, other embodiments are also contemplated. In all cases, this disclosure presents the disclosed subject matter by way of illustration and not limitation. It should be understood that numerous other variations and embodiments within the scope of the principles of this disclosure are possible to be devised by those skilled in the art.

[0024] These figures may not be drawn to scale. In particular, some features may be enlarged relative to other features for clarity. Also, when terms such as above, below, over, under, top, bottom, side, right, left, vertical, horizontal, etc. are used, it is to be understood that they are used only to facilitate understanding of the description. It is contemplated that the structure may be oriented in other ways.

Mode for Carrying Out the Invention

[0025] [Detailed Description of Exemplary Embodiments] Embodiments of the present disclosure are more fully described hereinafter with reference to the accompanying drawings. Elements identified by the use of the same or similar reference numerals refer to the same or similar elements. However, the various embodiments of the present disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0026] In the compressed gas industry, due to the existing temperature-pressure dependence regarding compressed gas, there are challenges in the ability to fully fill a cylinder or pressure vessel. Most administrative regions around the world allow high-pressure cylinders or vessels to be filled to a given pressure at a specified temperature. A more effective means of filling a pressure vessel would be to fill the cylinder to a certain weight. For many pressure vessel applications (e.g., vessels installed in automobiles, trailers, etc.), filling the vessel to a certain weight is not feasible or cost-effective.

[0027] 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 void 17 of a pressure vessel 10 in a manner that is substantially non-responsive to the temperature of the gas. These embodiments can be used to fill a pressure vessel in such a way when filling the pressure vessel in a method such as filling it to a certain weight is otherwise not possible. Thus, embodiments of the present disclosure provide a wide range of benefits, including enabling a more accurate knowledge of the amount of energy available within a fuel container (pressure vessel) as opposed to an automotive fuel gauge simply relying on a pressure estimate, and enabling a safe and accurate fill to the full volume of the pressure vessel.

[0028] FIG. 3 is a simplified cross-sectional view of a pressure vessel assembly 28 according to an embodiment of the present disclosure. Generally, the assembly 28 includes a gas density instrument 30 and a pressure vessel 10. The instrument 30 includes a parallel plate capacitor 32 having a pair of parallel plates 34 and 36. The plates 34 and 36 are separated from each other by a distance d and have opposing surfaces 38 and 40 with generally matching surface areas. The distance d may be substantially fixed. However, the distance d may vary due to deformation of the plates 34 and 36. Generally, the distance d is a known parameter that can be determined using suitable techniques. The plates 34 and 36 are supported within a suitable electrical insulating material 42, such as polyoxymethylene (POM) or polyetheretherketone (PEEK), and / or within another suitable dielectric or electrical insulating material that electrically insulates the non-exposed portions of the plates 34 and 36 from the surrounding material and fixes the relative positions of the plates 34 and 36.

[0029] A gap 44 corresponding to the separation distance d extends between the surfaces 38 and 40 of the plates 34 and 36. The capacitor 32 is supported such that the gap 44 opens into the internal void 17 of the pressure vessel 10. Thus, the gas within the internal void 17 of the pressure vessel 10 fills the gap 44 and functions as a dielectric for the capacitor 32. As discussed below, there is a relationship between the relative permittivity (and thus the capacitance of the capacitor 32) and the density of the gas contained within the internal void 17. Thus, the capacitance measured using the instrument 30 can be used to determine the density and amount of gas contained within the internal void 17.

[0030] The instrument 30 of the assembly 28 can be supported in any suitable manner such that the gap 44 opens into the internal void 17 of the pressure vessel 10. In one embodiment, the instrument 30 is supported by the wall 15 (FIG. 2) of the pressure vessel 10. In a more specific example, the capacitor 32 is attached to a plug 50 that can be supported by the wall of the pressure vessel 10 via a boss 16 as shown in FIG. 3.

[0031] The plug 50 may assume any suitable form and, as shown in FIG. 3, may operate to seal the boss 16 of the pressure vessel. 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 the proximal end 56 and a cylinder portion 58 extending toward the distal end 60. The cylinder portion 58 is configured to be received within the boss 16 and may include a threaded outer surface 62 that cooperates with the 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 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 internal void 17 of the pressure vessel 10.

[0033] In one embodiment, as shown in FIG. 3, the capacitor 32 is supported at the distal end 60 of the plug body 52 with the gap 44 exposed to the internal void 17. Alternatively, the capacitor 32 may be supported at a more internal location of the plug body 52 on the premise that an open passage to the internal void 17 extends toward the gap 44.

[0034] In one embodiment, the instrument 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 instrument 30 may be configured to wirelessly transmit the capacitance of the capacitor 32 to an external circuit for processing.

[0035] FIG. 4 is a simplified block diagram of a system 80 for measuring the density of a gas inside a pressure vessel according to an embodiment of the present disclosure. The system 80 may include a pressure vessel assembly 28 formed according to one or more embodiments disclosed herein and a capacitance measurement circuit 82. As described above and as shown in FIG. 4, generally, an embodiment of the assembly 28 includes a pressure vessel 10 and a gas density instrument 30 supported by a wall 15 of the vessel 10. This includes the configuration of FIG. 3, in which the instrument 30 is supported by a plug 50, and the plug 50 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 a capacitor 32 so as to pass through a terminal block 68 of the instrument 30, and is configured to measure (e.g., detect or sense) the capacitance (C MEAS ) of the capacitor 32 and generate a capacitance signal 84 indicative of or related to the measured capacitance of the capacitor 32.

[0037] In one embodiment, the circuit 82 includes a controller 90 configured to calculate the relative permittivity of the gas in the internal void 17 based on the capacitance signal and determine the density of the gas based on the relative permittivity. As described below, for some gases, this calculation of the amount of gas in the vessel 10 can be performed independently of the temperature of the gas.

[0038] Controller 90 may represent one or more processors (e.g., a central processing unit) that control the components of circuit 82 and / or process signals such as capacitance signal 84 to perform one or more functions described herein, such as determining or calculating the density of the gas within container 10. Controller 90 performs these control functions in response to the execution of instructions. Such instructions may be stored within memory 92, which represents local and / or remote memory or a computer-readable medium. Such memory 92 includes any suitable computer-readable medium eligible for patent subject matter that does not include transient fluctuations or signals, such as, for example, a hard disk, a CD-ROM, an optical storage device, and / or a magnetic storage device. One or more processors of controller 90 may be components of one or more computer-based systems and 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 a similar controller.

[0039] Controller 90 may calculate the relative permittivity κ of the gas within internal void 17 using Equation 1 shown below.

Equation

[0040] Furthermore, there is a known relationship between the dielectric of a gas and its density. Thus, when the internal void 17 of the pressure vessel 10 is filled with a known gas, the corresponding density of the gas can be determined using the relative permittivity calculated by the controller 90 and based on the mapping of the relative permittivity of the gas to density. The mapping may be stored in the memory 92 of the circuit 82. Thereafter, using the controller 90 or another computing device, the determined density of the gas can be used to calculate the weight or mass of the gas within the internal void 17 of the pressure vessel 10 based on the volume of the internal void 17.

[0041] In some embodiments, hydrogen gas (H2) is contained within the internal void 17 of the pressure vessel 10, and the memory 92 stores a mapping of the relationship of the relative permittivity of hydrogen gas to density. FIG. 5 is a chart illustrating such a relationship, where the relative permittivity of hydrogen gas is obtained at various temperatures. As shown in this chart, the relative permittivity of hydrogen gas is substantially independent of temperature over a wide range of operating temperatures of the processing vessel 10 (e.g., -40°C to 85°C). As a result, the density measurement according to the present disclosure can be useful for determining the density of the hydrogen gas contained within the void 17 of the pressure vessel 10. Thus, the density of hydrogen or another gas contained within the internal void 17 and having a relative permittivity that is substantially independent of temperature over the range of operating temperatures of the vessel 10 can be determined by the controller 90 without the need to provide temperature compensation via the use of a temperature sensor.

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

[0043] Circuit 82 may use any suitable technique to generate capacitance signal 84. In one example, as shown in FIG. 4 , circuit 82 includes a timer circuit 86 that connects plate 36 to an electrical ground or common 88 of circuit 82. In one embodiment, timer circuit 86 is configured to output capacitance signal 84 that is indicative of or has 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 an NE555 timer integrated circuit chip 96 or a similar component. For the configuration of the timer circuit 86 shown in FIG. 6, 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.

number

[0045] For example, based on the density value represented by density signal 94 and the known volume of internal void 17, additional processing circuitry 98 representing one or more processors or computing devices may be used to perform further processing of signal 94, such as calculating the weight of the gas within internal void 17. In some embodiments, as shown in FIG. 4, processing circuitry 98 presents to a user of system 80, on display 100, the density value of the gas, the weight of the gas, and / or other information. 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] FIG. 7 is a flowchart illustrating a method of measuring the density of a gas within internal void 17 of pressure vessel 10 according to an embodiment of the present disclosure. In 102 of this method, a gas is contained within internal void 17 of pressure vessel 10 defined by vessel wall 15 (FIGS. 1 and 2). In 104, in a manner as shown in FIGS. 3 and 4, gap 44 between a pair of plates 34 and 36 of parallel plate capacitor 32 is exposed to the internal void. In 106 of this method, a capacitance signal 84 indicative of or representing the value of the capacitance of capacitor 32 is generated using capacitance measurement circuit 82 (FIG. 4) according to the techniques described herein.

[0047] As described above, the measured capacitance is related to the density of the gas. In some embodiments of this method, controller 90 determines or calculates the capacitance of capacitor 32 based on capacitance signal 84, in a manner as shown in 108, using equation 2 as described above. In one embodiment, controller 90 calculates the relative permittivity of the gas, as shown in 110 of this method, based on the capacitance, the distance d by which plates 34 and 36 are separated, the surface area of opposing surfaces 38 and 40 of plates 34 and 36, and the permittivity of free space, as described above. Controller 90 may then determine the density of the gas in 112, based on the relative permittivity and the relationship between the relative permittivity and the density of the gas, and may generate a signal 94 representative of a value related to the density of the gas in 114.

[0048] While embodiments of the present disclosure have been described with reference to preferred embodiments, those 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. 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 gas density instrument comprising a parallel plate capacitor including a pair of plates having opposing surfaces supported by the plug body and spaced apart by a distance equal to the width of an open gap; a valve system configured to control injection or discharge of gas through the plug body; wherein: a first path extends from a first opening at a distal surface of the plug body to the open gap; a second path extends from a second opening at the distal surface to the valve system; " the capacitance of the capacitor is related to the density of the gas within the open gap; a pressure vessel plug.

2. The plug body includes a proximal end and a distal end including the distal surface and exposed to an internal void of the pressure vessel when the plug body is received within the port, wherein the capacitor is supported at the distal end. The plug according to claim 1.

3. The gas density instrument includes, at the proximal end, a terminal block having a pair of terminals, each of the terminals being connected to one of the plates via a conductive path. The plug according to claim 2.

4. A pressure vessel assembly comprising: a pressure vessel including a vessel wall defining an internal void; a boss supported by the vessel wall and including a port to the internal void; a plug including a plug body received within the port; a gas density instrument comprising a parallel plate capacitor including a pair of plates having opposing surfaces supported by the plug body and spaced apart by a distance equal to the width of an open gap; a valve system through which gas can be injected into or discharged from the internal void; wherein: a first path extends from a first opening at a distal surface of the plug body to the open gap; a second path extends from a second opening at the distal surface to the valve system; the opposing surfaces and the open gap are exposed to the gas within the internal void; the capacitance of the capacitor is related to the density of the gas; a pressure vessel assembly.

5. The plug body includes a proximal end and a distal end including the distal surface and exposed to the internal void, wherein the capacitor is supported at the distal end. The assembly according to claim 4. **Claim 6**: The gas density meter includes, at the proximal end, a terminal block having a pair of terminals, each of the terminals being connected to one of the plates, The assembly according to claim 5. **Claim 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, The assembly according to claim 4 or 5. **Claim 8** a first one of the plates is connected to an electrical ground, the capacitance measurement circuit is connected to a second one of the plates and includes a timer circuit configured to output the capacitance signal having a frequency related to the capacitance, The assembly according to claim 7. **Claim 9** the capacitance measurement circuit includes a controller, and the controller determines the capacitance of the capacitor based on the capacitance signal, calculates the relative permittivity of the gas based on the capacitance, the distance between the opposing surfaces, the surface area of the opposing surfaces, and the permittivity of free space, determines the density of the gas based on the relative permittivity and the relationship between the relative permittivity of the gas and the density of the gas, and generates a signal representing a value related to the density of the gas, and is configured as such. The assembly according to claim 7. **Claim 10** A method for measuring the density of a gas in a pressure vessel, the pressure vessel including a vessel wall defining an internal void, a boss supported by the vessel wall and including a port to the internal void, a plug including a plug body received in the port, a gas density meter including a parallel plate capacitor supported by the plug body and including a pair of plates having opposing surfaces separated by a distance of the width of an open gap, a valve system through which gas can be injected into or discharged from the internal void, a first path extending from a first opening at a distal surface of the plug body to the open gap, and a second path extending from a second opening at the distal surface to the valve system, the method including accommodating gas in the internal void of the pressure vessel, exposing the open gap to the internal void through the first path, Exposing the valve system to the internal void via the second path; Generating a capacitance signal indicative of the capacitance of the capacitor using a capacitance measurement circuit; comprising; the capacitance signal being related to the density of the gas; method.

11. 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; comprising; the capacitance signal having a frequency related to the capacitance; The method according to claim 10.

12. Including processing steps executed using a controller, the processing steps comprising: Determining the capacitance of the capacitor based on the capacitance signal; Calculating the relative permittivity of the gas based on the capacitance, the distance between the opposing surfaces, the surface area of the opposing surfaces, and the permittivity of free space; Determining the density of the gas based on the relative permittivity and the relationship between the relative permittivity of the gas and the density of the gas; Generating a signal representing a value related to the density of the gas; including; The method according to claim 10 or 11.

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

14. The gas density instrument includes a terminal block having a pair of terminals at the proximal end, each of the terminals being connected to one of the plates. The method according to claim 13.

Images