CA2112458A1 - Improved method and apparatus for dispensing natural gas - Google Patents

Abstract

A supply plenum and valve body assembly (40) connected to a source of compressed natural gas (CNG) selectively turns on the flow of CNG through either a first sonic nozzle (52) or a second sonic nozzle (54) and out through respective dispensing hose assemblies (28, 30). A pressure transducer (96), a supply plenum temperature transducer (118), and an ambient temperature transducer (91) measure the stagnation pressure and temperature of the CNG and the ambient temperature, respectively. A pressure transducer (92) fluidically connected to the vehicle tank via the dispensing hose assembly monitors the pressure of the CNG in the vehicle tank. An electronic control system (13) connected to the pressure and temperature transducers and to the supply plenum and control valve assembly (40) calculates a vehicle tank cut-off pressure based on the ambient temperature and on the pressure rating of the vehicle tank that has been pre-programmed into the electronic control system (13), calculates the volume of the vehicle tank and the additional mass of CNG required to increase the tank pressure to the cut-off pressure, and automatically turns off the CNG flow when the additional mass has been dispensed into the vehicle tank. The electronic control system (13) also determines the amount of CNG
dispensed through the sonic nozzles (52, 54).

Description

W0 93J00264 2 ~ 5 ~ Pcr/usg2/0~s38 , . . .

- IMPltOVED M~IHOD AND APPARATUS EY)R DISPENSING NATURAL GAS

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-` This invention relates generally to methods and apparatus for measuring and . :
: ` controlling fluid flow rates and, more particularly, to a me~od and app~ratus for dispensing S na~ral gas.

~i Ba*,eround An:
Over t~e past few years, ~ere has been a steadily increasing interest in developing alternative ~uels for automobiles in an effort to reduce the hannful emissions produced by ` ~onveDtional gasoline and diesel powered vehicles. One such alternative fuel that has already been used with favorable results is compressed natural gas (CNG). Besides being much - cleaner burning than gasoline or diesel fuel, most modern automobiles can be conYerted to ~ operate on compressed na~ural gas (CNG). Typically, such a conversion may include various . .
minor modifications to the engine and ~uel delivery system and, of course, ~he installation of ~;~, a natural gas fuel tanl~ capable of stormg a sufficient amount of CNG to provide the vehicle with ra~ge and endurance comparable to that of a conventionally fueled vebicle. In order to ~., ''''4^~,'j provide a reasonably sized storage tank, the CNG is usually stored under relatively high , ~ j pressures, such as 3,000 to 4,0QO pounds per square inch gauge (psig).
While the conversion process described above is relatively simple, the relatively high pressure under which ~e CNG is stored cseates certain refueling problems that do not exist 4~ 20 for conventional vehicles powered by liquid fuels, such as gasoline. Obviously, since the gas is transfe~red and stored under high pressure, special fittings, seals, and valves have to be ~,ed when ~e CNG is ~ansferred into the CNG storage tan~ on the vehicle to prevent loss of CNG into the atmosphere. Also, special precautions must be taken to minimize the danger ~ of fire or e~cplosion that could result ~rom the unwanted escape of the high pressure CNG.
'1~; 25 Accurate, yet eonvenie~t and easy to use measurement of ~e amount of CNG delivered into the vehicle's storage tank is also a problem. Consequently, most currently available natural gas refueling systems require that several relatively complex steps be performed during the ,J re~ueling process to prevent leakage, minimize ~he risk of fire or e~cplosion, and to measure s~ the amount of fuel delhered. Unfortunately, because such processes tend to be relatively 30 comple~c, ~ey cannot be carried out very easily by most members of the general public or si even by unslcilled workers. Therefore, most CNG dispensing systems usually require trained personnel to perform the refueling process. As of date, providing trained operators to ~t ,.,~
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- WO 93/00264 PCI/US92/0~538 . 21 ~ ~ 4 3 ~3 . 2 perform the refueling operation has not yet posed a significant problem, because natural gas ~:, re&eling stations are generally lirnited to fleet operators of vehicles who can afford to have -~? trained person~el to perform the refueling operations and who either do not care to keep . accurate measurements of each vehicle fill-up or who can afford comple~c flow measuring equipment to do it. However, because the interest in natural gas powered automobiles is increasiDg rapidly, and ~here is a growing need to develop a natural g~s~efueling system that ,~; .
iS highly automated and has sufficient fail-safe systems to minin~ize the danger of fire or e~plosio~, while at the same tirne being capable of accurate measurements and being used safely by the general public. Ideally, such a nah~ral gas dispensing system should be as ; j 10 familiar to the customer and as easy to use as a conventional gasoline pump and refueling station.
As mentioned above, there are several natural gas dispensing "pumps" currently , ~
`~ available. One such system is disclosed in the patent to Fisher e~ al., U.S. Patent No.
4,527,600. While the dispensing system disclosed by Fisher a al., is relatively easy to use, ~` 15 it requires certain relatively e~pensive components. For e~ca¢nple, Fisher's dispensing system u~ilizes differential pressure transducers to determine ~e amount of CNG that is dispensed into ~e vehicle tanlc. Disadvantageously, however, such differential pressure transducers are e~pensive, and have a rather limited range of pressure ratios of about 3 to 1.
Another significant problem associated with the dispensing systems currently ~ 7~ ~
available, such as the system disclosed by Fisher, is that such systems cannot determine accurately when the natural gas storage tank in the vehicle is filled to rated capacity, yet not overfilled. That is, since natural gas storage tanlcs in vehicles have to be rated to safely contain CNC; under a gi~ren pressure at a given t~nperah~re (e.g., 3000 psig at a temperature of 70 F, the ~standard temperature~), it is important to determine the correct pressure to which the tanlc should be filled when the ambient temperature is not e~cactly 70 F. For e~ample, if the ambient temperature is wanner than the standard temperature of 70 F, the tanlc can be filled safely to a pressure higher than the rated pressure. In fact, the tank will not be completely filled under such circumstances until it is at such a higher pressure.
Conversely, if the ambient temperature is below standard temperature, the tank cannot be filled safely to the rated pressure, because as the CNG warms to the standard temperature, the pressure will exceed the rated pressure. In this situation, the tank is overfilled, and there is a significant danger of the safety relief valve on the tanlc vendng the excess CNG to the atmosphere, thereby losing the CNG and possibly even creadng an explosion hazard. Worse i~ yet, the tanlc may actually rupture if the safety valve malfuncdons.
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2 ~ Pcrf us92~0ss38 `~ Unfortun~ely, however, none of the currently available natural gas dispensing pumps compersate for changes in ambient ternperature. Accordingly, these cur,rently available . ~, dispensing systems are usually configured to turn off the flow of natural gas at pressures well :
below the rated pressure of the tanlc to avoid the dangerous overfillmg and consequent over-S pressurization the vehicle storage tanlc described above. Consequently, if the ambient . .
J!' tempera~re is higher ~Ihan the standard temperature of 70 F, the t~will be substantially ; underfilled.
, . , Another problem relates to accura~ely sensing the vehicle tank pressure while the ve~icle tank is being filled. For e~ample, it is impossible to sense the vehicle tank pressure wi~h a remotely located pressure se~sor if the CNG flow th~rough the dispensing hose reaches ` . sonic velocity (a choke point) at some point between the pressure sersor and the vehicle tank ~; itself. Typically, such a cholce point occurs in ~e safety checlc valve located in the vehicle tan~ coupler assembly. Accordingly, such dispensing pumps are usually designed to ensure that the flow of CNG between the remote pressure sensor for sensing the vehicle tanlc pressure and the vehisle tan~ itself remains subsonic at all times and under all flow s ~ conditions, which, of course, limits the ma~imum delivery rate of the pump. Unfortunately, even if the dispeosing pump is designed to ensure that a sonic choke point does not occur '~ between the pressure sensor and the tanlc, it is still necessa~ to compensate for pressure ~i erro~s due to the pressure drop in ~e hose and coupler/checlc valve assembly, which is ;"~. ~.3 difficult, since ~he pressure drop in the ve~icle check valve may vary depending on ~e characteristics of particular valve.
~ . Therefore, there is a need for a ~a~ral gas dispensing system that provides ~e ;~. desir~d degree of safet~r for dispensing ~a~ral gas under high pressures that is preferabl~ as ~asy to use a conventional gasoline pump. Su~h a dispensing system should be relatively simple and reliable and ideally would not require e~ensive and comple~ differential pressure trans~uce~s. Most importan~dy, wch a dispensing system should be capable of automatically determining a temperah~re co~rected cut-off pressure to ensure that the vehicle storage tank is completely filled regardless of the ambient temperature and regardless of whelher the CNG
i,~
flows through 8 sonic cholce point in the disperlsing hose or coupler/check valve assembly.
Finally, it would be desirable for such a dispensing system to accornmodate two or more dispensing hoses from a single supply plenum to reduce the number of pressure and temperature sensors to a minimum, thus providing better overall system reliability and lower , . .
. cost.
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WO 93/00264 PCr/US92/05~38 Diselosure of ~ç Invention:
Accordingly, it is a general object of this invention to provide a pressurized fluid dispensing syseem that can automatically compensate for non-standard ambient gastemperature to promote com~lete filling of a pressurized storage tanlc.
It is a further general obje~ of ~is invention to provide a pressurized fluid dispensing system that can aceura~ely fill a pressurized storage tanl~ to its rat~d eapa~ity even though the ~low of CNG through the dispensing hose passes through a sonic cholce point.
It is another general object of this invention to provide a pressurized fluid dispensing system that can accurately measure the amount of fluid transferred into a pressurized storage tan~ without the need to resort to e~pensive and performance limiting differential pressure transducers and regardless of whether ~e CNG in the dispensing hose flows ~rough a sonic choke point.
It is another obje~ of this invention to provide a pressurized fluid dispensing system that uses sonic nozzles to measure the amount of fluid dispensed.
~5 It is a more specific object of this invention to provide a natural gas dispeDsiDg system ~a~ is highly automated and 8imple to use while pro~iding a high degree of safety.
It is yet another more specific object of this invention to provide a natural gas dispensiIIg system ~at ca~ easily support multiple dispensing hoses from a single supply plenum.
Additional objects, advantages, and novel fea~ures of the invention shall be set forth in part in the description ~at follows, and in part will become apparent to those s~illed in the art upon e~camiDation of the foregoing or may be learned by the practice of this invention.
The objects and advantages of the i~ tion may be realized and attained by means of the instrume~talities and in combinatioDs particularly pointed out in the appended claims.
To achieve ~e foregoing and other objects, and in accordance with the purpose ofthe present invention, as embodied and broadly described herein, the natural gas dispensing system according t~ this invention may comprise a supply plenum connected to a CNG source and a control valve assembly for selectively turning on the flow of CNG through a sonic nozzle and out through a dispensing hose wembly. Pressure and temperature transducers connected to the supply plenum measure the stagnation pressure and temperature of the CNG
and a pressure transducer fluidically connected to the vehicle tank via the dispensing hose assembly is used to determine the discharge pressure. A second temperature transducer is used to measure the ambient temperature. An electronic control system connected to the pressure and temperature transducers and to the control valve assembly calculates a vehicle WO 93/0~264 2 1 1 2 4 5 ~ P~/US92/05538 tank cut~ff pressure based on the ambient temperature and on the pressure rating of the ve~icle tan~ that has been pre-programmed into ~he electronic control system, calculates the volume of the vehicle tank and ehe additional mass of CNG required to increase the tank pressure to the cut~ffpressure, and autornatically hlrns offthe CNG flow when the additional mass has 'been dispensed into ~e ve~icle tanlc. The electronic control system also determines ~he amount of CNG dispensed through the sonic nozzle based on the~upstream stagnation temperature and pressure of ~e CNG and the leng~h of time the CNG was flowing through the sonic nozzle.
The method of this invention includes the steps of cormecting a CNG supply tank and the vehicle tanlc with a pressure tight dispensing hose, sensing the ambient temperature before initiating ~e dispensing cycle, and calculating a cut~ff pressure for the vehicle tank based on the ambient tempera~re and based on the pressure radng for the vehicle tank. The dispensing cycle is then initiated by bAefly cycli g the valve tu pop open the vehicle tank check valve and equalize the pressure in the dispensing hose and the vehicle tank and sensing the initial vehicle ~ank pressure. Neott, a predetelmined mass of CNG is dispensed into the vehicle st4rage tan~ to increase the tanlc pressure to an intermediate pressure. The initial and intermediate tanlc pressures are then used to determine the volume of the vehicle tank. Well-~nown gas relations are ~en used to calculate the mass of CNG required to fill the vehicle tanlc to the temperature compensated cut-offpressure and the dispenser then fills the tank with the calculated mass of CNG.

Brief Descrjpt~n of ~e Drawings:
The accompanging drawings, which are incorporat~d herein and form a part of thisspe~ification, illus~ate ~e preferred e~bodiment of the present invention, and together with the description, serve to e~plain the principles of the invention. In the drawings:
Pig~re I is a front view in elevation of the natural gas dispensing system according to the present invention with the &on~ panel of the lower housing brolcen away to show the details of ~lhe ~atural gas supply plenum and valve body assembly, and with the front cover of an electrical junction bo~ brolcen away to show the location of the ambient temperature - se~sor;
Figure 2 is a perspective view of the supply plenum and valve body assembly shown in Figure 1 with the supply plenum cover removed and with a corner section broken away tl~ reveal the details of the sonic noz~le, the control valve assembly, and the pneumatic air reservoir;

WO 93/0û264 2 1 1 ~ 4 S 8 PCr/US92/0~538 Figure 3 is a plan view of ~e supply plenum and valve body assembly of the present invention showing the plenum chamber coYer in position, the various inlet and outlet connections, and the various pressure and temperature transducers used to sense the pressures and tempera~res of the CNG at various po~nts in the pleMlm and valve body assembly;
Figure 4 is a sectional view in elevation of the supply plenum and valve body assembly talcen along the line 4~4 of Figure 3 more clearly showing ~h2~d~ails of the plenum chamber cover, ~e supply plenum, one of the sonic nozzles, ~e corresponding control valve assembly, and ~e positioning of the various pressure and temperature transducers;
Figure S is a schematic view of the pneumatic system of the present invention showing ~e pneumatic connections to the con~ol valve assemblies, the locations of the various pressure and temperature transducers, and the path of the natural gas from the supply plenum ~Irough the sonic nozzles and ultimately through the hose connections;
Figure 6 is a block diagram of the elec~onic control system used to control the function ~ nd operation of ~e n~tural gas dispensing system according to the present invention;
Figure 7 is a graph of vehicle tanlc pressure vs. mass of CNG;
F'igure 8 is a flow chart showing the steps e~ecuted by the electronic control system of the present invention;
Figure 9 is a flow chart showing the detailed steps of the Start Sequence shown in Figure 8;
Pigure lO(a) is a flow chart showing the detailed steps of the Fill Sequence of Figure 8;
Pigure 10~o) is a continuation of Figure lO(a);
Figure 11 is a detailed flow chart showing the ste~s of the End Sequence of Figure 8; a~d Figure 12 is a flow chart showing an alternative Fi11 Seguence process that could be substituted for the Fill Sequence process shown 1n Figures 10(a) and lO(b).

Best Mode for~ = j~L~ l= ention:
11he major components of the natural gas dispensing system 10 according to the present invention are best seen in Figure 1 and comprise a lower housing 12 which houses the sllpply plenum and valve body assembly 40, along with numerous associated components, as will be described in detail below. The electrical wires from the various pressure and temperature transducors 92, 94, 96, and 58 u well as from t~e solenoid valve assombly 42 are routed to two sealed electrical junction bo~es 17 and 19, respectively, via pressur~tight WO 93/00264 2 1 1 2 4 ~ ~ PCI`/US92/05538 conduit 1~0 reduce the chances of fire or e~plosion. Electrical wires from these tWO junction bo~es 17, 19 are tbLen routed in pressure-tight conduit to anL elevated aDLd pressurized pentbLou;se 14 which houses thLe electronic control system 13 (t shown iDL F:igure 1, but showDL in Figure 6) and various display windows 136, 138, and l40. Penthouse 14 is S mounted to the lower housing 12 by left and righLt pentbLouse support members 16, 18. Two vertical lhose supports 20, 22 are attaLched to eitlher side of lower h~u~ing 12 and pentbLouse 14 aIId support tlhe two retrieving cable assemblies 24, 26, as well as a ffrst dispensing hose 28 and a second dispensing hose 30, respectively. ThLese first and second dispensing hoses 28, 30 are connected to ~e supply plenum and valve body assembly 40 via two breakaway connectors 36, 38 and two natural gas output 1ines 88 and 89. Finally, each dispensing hose 28, 30 terminates in respective thre~way valve assemUies 44, 46 and pressure-tight hose couplers 48, 50. Each pressure-tight hose coupler is adapted to connect its respective dispensing hose to the natural gas storage tank coupler on the vehic1e being refueled (not shown).
'rhe high degree of automation of the disperlsing system lO allows it to be easily and safely used o~ a "self serve" basis, much lilce conventional gasoline pumps, although the system could also be operated by a full-time attendant. In order to dispense the CNG into the vehic1e tanl~, a dispensing hose, such as dispensing hose 28, is connected to the vehicle tank being refueled via pressure-tight coupler 48, which is adapted to fit with the standardized connecter on the vehicle. The customer or attendant would then move the three way valve 44 to ~e "fill~ position and move the transaction switch 32 to the ~on~ position. The natural gas dispensing pump 10 then begins dispensing CNG into the vehicle tanlc, continuously indicating the amount of nab~ral gas being dispensed on display 140, theprice on display 136, and tihe pressure in ~he vehicle tank on display 138 (after a finite delay and under no flow conditions), much lilce a convendonal gasoline pump. After the vehicle tanl~ has been filled t~ the proper pressure, as determined by the ambient temperature sensed by temperature transducer 9l located within juncdon bo~ 17, the dispensing system lO automatically shuts off the flow of CNG into the vehicle, as will be described in detail below. The customer or attendant would then move transaction switch 32 to the ~ofP posidon, and turn the tbree-way valve to the ~vent" position to vent the natural gas ~apped in tbe space between the hose coupla 48 and the vehicle coupler (not shown) into a vent recovery system (not shown), where it is re-compressed and pumped back into the CNG storage tanlc (also not shown).
Tbe supply plenum and valve body uscmbly 40, along witb lbe electronic control system (not shown in Figure 1, but sbown in Figute 6 and fully described below) forms the WO 93/00264 ~ PCr/US92/05538 211245~ 8 ~`

heart of the natural gas dispe~sing system 10 and includes sonic nozzles, digital control valves, and various pressure and temperature transducers to automatically dispense the exact amolmt of na~ral gas required to completely fill the vehicle storage tank as well æ to autorna~ically calcula~ and display ~lhe total amount of natural gas dispensed into the storage tan~. Since the CNG dispensed by the system 10 is under considerable pressure (about 4,000 psig), the dispensing system 10 also includes a number of fail-safe jlh~emergency shut~ff features to minimize or eliminate any chance of fire or e~cplosion, as will be described below.
A significant advantage of the r~ural gas dispensing system 10 is that it does not require performance limiting and complex differendal pressure transducers to detennine the amount of CNG dispeD~ed into the ve~icle s~orage tank. Advantageously, the present invention can accurately measure the amount of CNG dispensed using relatively inexpensive and simple gauge pressure transducers, as will be discussed below.
Yet ano~er advantage of the na~ral gas dispensing system 10 is that a plurality of sonic no~zles can be connected to a single input or supply plenum, each of which may be ope~ated independently of the others without adversely affecting the metering accuracy or performance of the othe~ nozzles. Accordingly, the preferred embodiment u~illzes two sonic nozzles and two control valve assemblies, so that two dispensing hoses can be easily used in conjuncti~n with the single supply plenum and valve body assembly 40, thereby increasing utility and redl~cing cost. Furthermore~ because more than one sonic nozzle can be connected to the single supply plenum, only a single set of pressure and temperature transducers are required to sense the stagnation pressure and stagna~ion temperature of the CNG contained within the supply plenum, even though two or more hoses or "channels~ are connected to ~e singleplenum, there~y fur~er r~ducing the cost and comple~city of the dispensing system 10.
Perhaps lhe most significant feature of the present inve~tion is that the CNG
dispe~sing system 10 is tem~era~re compeDsated to automatically fill the vehicle natural gas tan3c to the correct pressure regardless of whether the ambient temperature is at the ~standard~
temperature of 70 F. For e~ample, if the ambient temperature is above standard, say 100 F, the vehicle tanlc will be automatically filled to a pressure greater than its rated pressure at standard tempera~re, since, when the CNG in the tanlc cools to the standard temperature, the pressure will decrease to the rated pressure of the tan~. The dispensing system 10 automatically determines the proper cut-off pressure for the ambient temperature and automatically terminates the refueling process when the calculated cut-offpressure is reached.
Such automatic temperature compensation, therefore, ensures tbat tbe vehicle storage tank is filled to capacity regardless of the ambient temperature.

WO 93~0026~ 2 1 1 2 ~ ~ ~ PCI/US92/05538 Another significant feature of the present invention is that it does not rely on a pressure sensor to determine the pressure of the vehicle tan~ during the filling process.
Instead, the CNG pump according to the pres~nt mvention first datermines the volume of the vehicle storage tanlc and then computes the mass of CNG required to fill the vehicle tank to S the previously determined, temperature compensated cut-off pressure. Therefore, ~e present inventio~ eliminates ~e need for continuously sensing the vebicle t,an~pressure, avoids the problems associated wi~lh pressure losses in the dispensing hose and coupler/check valve assembly, and is accurate even if a sonic cholce point e~ists in the interconnecting dispensing hose or coupler assembly.
The details of the supply plenum and valve body æsembly 40 are best seen and understood by referring to Figures 2, 3 and 4 simultaneously. As described above, the preferred embodiment includes two separate and independent noz~le, valve, and hose assemblies, which may be referred to hereinafter as channels. However, to simplify the descdption, only ~he f~rst channel i.e., the chamlel for hose 28 will be described in complete de~ail. The components utilized by the seeond charmel (hose 30) are identical in every respect, and, therefore, will not be described in detail.
In the preferred embodiment, the supply plenum and valve body assembly 40 is machined from a single bloclc of aluminum, although other materials could be used just as easily. Supply plenum a~d valve body assemblg 40 de~ines, in combination with the plenum chamber cover 112, a supply plenum 56, (see Pigures 3 and 4) and a pneumatic reservoir 86, and also houses the Iwo sonic nozzles 52, 54 and the corresponding control valve assemblies 64, 65. Various pressure ~ansducers 92, 94, and 96 and ~emperature transducer 118, as well as the solenoid v alve assembly 42 (shown in Figure 1, but not shown in Figures 2, 3, and 4 for clari~), are also mounted to the supply plenum and valve body assembly 40, as will be describ~d below.
As mentioned above, the na~ral gas dispensing system 10 of the present inventionutilizes sonic nozzles to accurately mete~ the flow of CNG through each dispensing hose.
Such sonic nozzles have been used for decades as flow regulators because the mass flow rate uf a gæ flowing through such a nozzle is irdependent of the back pressure at the nozzle exit, so long as the gas is flowing at sonic velocity in the throat section of the noz~le. Put in other words, ~e metering accuracy is not affected by variations in the vehicle tank pressure.
Therefore, sonic noz~les eliminate the need to measure both the upstream and downstream pressures of ~e gas in order to determine the gas flow rate.

WO 93/00264 PCr/US92/0553X
21:i2~ o Briefly, a sonic nozzle, such as sonic no~le 52, comprises a converging section 442 and a diverging section 444 separated by a throat section 446, which represents that portion of the nozzle having the smallest cross-sectional area, see Figures 2 and 4. Gas entering the converging or inlet section 442 of sonic nozzle 52 is accelerated until it is flowing at the S speed of sound in the throat, provided there is a sufficiently high pressure ratio between the "upstream" pressure (i.e., the pressure of the CNG in the supply ~lenum 56) and the ~downstream" pressure (i.e., the pressure in the intermediate chamber 62). If the diverging section 444 is properly designed in accordance with well-lcnown principles, ~e gas will decelerate in the diverging section 444 until nearly all of the velocity pressure has been converted back into static pressure before the gas enters the downstream or intermediate chamber 62. A significant feature of ~e sonic nozzle is that for a given set of stagnation pressures and temperatures of ~he fluid upstream of the nozzle, there is a maximum flow which can be forced ~hrough the nozzle that is governed by the throat area. No matter what happens downstream from the throat in the way of decreasing the pressure or increasing the flow area, the flow rate will remain the same, so long as sonic conditions are maintained at the throat. Accordingly, the mass flow rate through a sonic nozzle is governed by the following equation:
~n-~ (I) where m is the mass flow rate of the fluid flowing through the nozzle; Ct is the nozzle discharge coefficient for the panticular nozzle being used; lc is a constant depending on the ratio of specific heats and the gas consta~t of the fluid; p, is the stagnation pressure of the fluid in ~e supp1y plenum 56; A is the nozzle throat area; and T, is dle absolute temperature of the fluid in the supply plenum 56. See the te~ct, lhe D~namics and lhc~ mics of Com~ress~ble Fluid Flow, by Ascher H. Shapiro, Volume 1, page 85, equation (4.17), the Ronald Press Co., New York, 1953, for the e~act relationship between lc, the ratio of specific heats, and the gas constant. Ihe flow rate through a sonic nozzle is, therefore, proportional to the stagnation pressure p, in the supply plenum 56 divided by the square root of the stagnation temperature T, in supply plenum 56 times the effective throat area of the sonic nozzle. It follows that the fluid flow rate detenninative parameter is the stagnation pressure Pl divided by the square root of the stagnation temperature T,. This linear rela~ionship is maintained so long as the fluid flowing through the nozzle rernains sonic at the throat, which eliminates any dependence of flow rate upon the pressure in the downstrea n or intennediate WO 93/00264 2 1 ~ 2 ~1 ~ 8 PCr/U~g2/05538 chamber 62 (see Figure 2). Fur~er, proper design of such a sonic nozzle will allow ~e velocity in ~e throat to reach sonic velocity or "c~oke" at reasonably small pressure ratios of about 1.05 or 1.1. That is, the pressure of l:he fluid in the supply plenum 56 need only be about 5 to 10 percent higher tban sbe pressure in tbe intermediate or downstream chamber 62 to achieve and maintain sonic velocity in tbe throat section 446.
If the pressure ratio benveen the ssagnation pressure p, in su,~p~y ple~um 56 and the stagnation pressure P2 in the intelmediate (i.e. discharge) chamber 62, is not sufficient to sust~Ln sonic velocity through dle throat of the nozzle, then tbe flow rate througb the nozzle is dependent on the upstream stagnation temperature and pressure (T, and p,) as well as the downstream stagnation pressure P2. and the equation listed above becomes a function of tbe downstream stag~ation pressure, thus:

m=C ~Dl~pl-p2) (2) where m is the mass flow rate of the fluid passing through the nozzle; C~, is the nozzle discharge coefficient for ~e parsicular nozzle being used; k is a cc,nstant depending on the ratio of specific heats and ~e gas constant of the fluid; p, is the stagnation pressure of the fluid in ehe supp1y plenum 56; A is the nozzle thro~t area; Tl is the absolute temperature of ~e fluid in the supply plenum 56; and P2 is the stagnation discharge pressure.
Referring baclc to Figures 2, 3, and 4, si nultaneously, the flow of natural gas through the sonic nozzle 52 is con~olled by a "digital~ ~alve assembly 64 for the first dispensing hose 28 as shown in Figure l. The valve assembly 64 is referred to as a digital valve because it has oDly two positions: on and off. There are no intermediate positions typically associated with analog-type valves. As m~ioned above, there is an identical sonic noz~le 54 and digital valve assembly 65 for ~e second channel, i.e., hose 30, as shown in Figure 1.
Ihe digital valve assembly 64 for the first channel is oriented at right angles to the sonic nozzle 52 so ~at an intermediate or downstream chamber 62 is defined in the area between the downstream section of the wzzle and the valve body assembly 64. A vertical condensate leg 82 e~tends downward from the intermediate or downstream chamber 62 to collect any condensate from the CNG as it flows through sonic nozzle 52. A suitable plug 84 or, optiona11y, a valve assembly (not shown), can be attached to the bottom of the condensate leg 82 to allow the leg 82 to be drained at periodic intervals. The provision of a suitable valve assembly (not shown) would be obvious to persons having ordinary sl~ill in this art and therefore, is not shown ol described in further detail.

Wo 93/00264 Pcr/us92/O~
21:~ 2-~8 12 The digital valve assembly 64 comprises a pneumatically operated valve actuator assembly 69 that is secured to the supply plenum and valve body æsembly 40 via a plurality of bolts 70. The pneuma~ically operated valve actuator assembly 69 includes a piston 68 disposed within a cylinder 66 and suitable pneumatic ports 120 and 122. Air pressure applied to one side of the piston 68 via one such port 120 or 122 while the other side is vented by the other port allows the piston 68 to move in the preferred directlon~ as is well-known.
Therefore, valve ac~ator assembly 69 controls the position of the pressure balanced piston 76 wi~in sleeve 72 via piston rod 78 to selectively turn on or shut off the flow of natural gas from the intermediate chamber 62 through the outlet port B8. Note that pressure balanced piston 76 include~ a plurality of passageways 80 to equalize the pressure on both sides of the piston 76. This pressure equalization is necessary because the natural gas in the intennediate chamber 62 is under relatively high pressure of about 4,000 psig, whereas the compressed air used to actuate the valve ac~ator assembly 0 is in the range of about 100 psig. If the pressure were not equalized on both sides of piston 76, the high pressure of the natural gas ac~ing on the surface of piston 76 would force the piston and piston rod assembly 78 upward, and the relatively low pneumatic pres.sure acting on the actuator piston 68 would be unable to move ~e piston 68 back downward against the high pressure of the natural gas. As a result, the valve assembly comprising piston 76 and sleeve 72 could never be closed. Note also that sleeve 72 has a recessed area 73 e~CtendiDg circumferentially around the sleeve 7 in the area of outlet port 88 to allow na~ral gas flowing through several radial vent ports 74 in the sleeve 72 to e~cit throug~ outlet port 88. The pneumatic reservoir 86 contained within the supply plenum and valve body assembly 40 provides a reserve of pneumatic pressure in the eve~t of f~ilure of the pneumatic supply pressure to the valve actuator 69, as wDl be described in detail below.
Ihe supply plenum and valve body assembly 40 also houses the various pressure and temperature transducers required by the natural gas dispeDsing system 10 of the present invention. Essentially, the supply plenum 56 is fluidically coupled to a supply stagnation pressure transducer 96 via supply stagnation pressure port 60 (see Figures 2 and 4), which senses the supply stagnation pressure Pl- Similarly, a temperature probe 58 from a stagnation tempaature transducer 118 e~tends into the natural gas supply plenum S6 to measure the stagnation temperature T, of the CNG. The stagnation pressure P2 of the CNG in the interm~iate chamber 62 is measured by pressure transducer 92 via port 132 (Figure 4).
Finally, a vent pipe 98 and pressure relief valve assembly 99 fluidically coupled to the outlet port 88 via passageway 134 vents natural gas contaiDed within the dispensing hose 28 in the W0 93/0026~ 2 ~ i 5 ~ Pcr/us92/o5s38 event the pressure in the hose 28 e~ceeds a predetermined pressure. In the preferred embodiment, the pressure relief valve assembly 99 is set to about 3600 psig. Note also that a second veDt pipe 100 and corresponding pressure relief valve assembly 101 are connected to the i~tennediate chamber of the second channel.
S The details of the pneumatic system used to control the operation of the valve assemblies 64, 65, as well as ~e flow of the natural gas through the 6~em and through the hose assemblies 28 and 30 are best understood by referring to Figure 5. As was briefly described above, the natural gas control valve assemblies 64 and 65 are controlled by a conventional pneumatic system operating with instrument~uality pneumatic air under about 100 psig pressure supplied by a conventional compressor and regulator system (not shown).
lhis pneumatic supply air enters ~e system through check valve 124 and passes through inlet 110 into pneumatic reservoir 86. See also Figure 3. A srnall amou~t of air is talcen off this l~e 110 and passes through a check valve and purge regulator assembly 130 to maintain the penthouse 14 under a small posi~ive pressure, as will be described below. The pressurized air ne~t passes into storage reservoir 86 out through outlet 90 (Figure 2) and into the solenoid valve assembly 42, as seen in Figure 1. Essentially, solenoid valve assembly 42 comprises two con~entional ele~rically operated solenoid valves 41, 43, one for each channel or hose and which solenoid valves are controlled by the electronic control system, as will be described in detail below. Each solenoid valve 41, 43 in solenoid valve assembly 42 operates in a conven~ional rnanner. For example, a first solenoid valve 41 in valve assembly 42 is used to selectively reverse the flow to a valve body assembly 64 via inlet lines 120 and 122, therefore selectively opening or closing the digital valve assembly 64. An identical solenoid valve 43 in solenoid valve assembly 42 eonnected to valve assembly 65 operates the second ~channel" i.e., hose 30 of ~e ~a~ral gas dispensing system 10.
As was briefly mentioned above, a purge regulator and checlc valve assembly 130 is used to supply air under very low pressure, i.e., about one to five inches of water, to the pressur~tight penthouse 14 to ensure that a positive pressure is maintained in the penthouse compartment 14 (which houses all the electronics used by the natural gas dispensing system 10) to eliminate any possibility of Da~ral gas accumulation in the penthouse chamber, possibly leading to an e~plosion or fire bazard. Also in the preferred embodiment is a pressure relief valve 131, to vent e~ccess pressure from the penthouse in the event of a malfunction of the regulator and checlc valve assembly 130.
In operation, the supply of CNG connected to the supply plenum and valve assembly 40 enters the supply plenum 56 via input line 114 and inlet filter 116, and the stagnation WO 93/00264 PCr/US92/0~38 21124~ 14 pressure pl and the stagn~ion temperature Tl are sensed by pressure transducer 96 and temperature ~ransducer 118. See also Figure 4. During the idle loop process 212, described below, the system may be programmed to elin~inate the accumulated drift between the supply stagnation pressure ~ransducer 96 and pressure transducer 92. Essentially, the accumulated drift may be eliminated by moving the three way valve 44 to the "vent" position and opening valve 64 to e~ize the pressure between pressure transducers 96 and 92. The electronic contro1 system ~en re calibra~es transducer 92 to eliminate any systematic errors that would otherwise occur.
After the vehicle tan~ is coupled to the dispenser, the three-way valve 44 located at the end of the hose assembly 28 is moved to the "fill" position and the electronic control system then acluates solenoid valve 41, which opens valve 64, to dispense a small amount of CNG into ~he vehicle tanl~ to spen the checlc valve in the ve~icle and to ensure that the pressure in the hose 28 is equal to the pressure in the vehicle tank. This initial vehicle tank pressure p~0 is senf,ed by pressure transducer 92 and stored for later use. The cantrol system ne~ct opens the valve 64 and dispenses an initial known mass (m,) of CNG into the vehicle tank and determines the intermediate pressure Pvl of the vehicle tan~ after valve 64 is again closed. The change in pressure i.e., Pv, - p~O~ is then used to determine the volume of the vehicle tanlc V, according to the well-l~nown state equation: pV = (m/M)RT, or, when solved for tank volume V:

V = 1 ~1 ~b (3) vl ~ZO r O
20 where:
ml = the initial Icnown mass of the gas;
Z, = the gas compressibility factor at a point i;
R = ~e universal gas constant;
T.".b = the ambient tempera~re;
M = the molecular weight of the gas;
Pv~ = the pressure at a point i; and T; = gas temperature at a point i.
After ~he volume of the vehicle storage tanlc is determined, the control system then calculates the additional mass required (m2) to fill the tanlc to the previously calculated cutoff pressure WO 93/00~64 2 1 1 ~ ~ 5 ~ Pcr/US92/05s38 p~ ~" using th~ state equation solv~d for mass, thus:

nt2 = RT ~( Z J~ Z ) (4) ~ ~ I

The system then again opens valve 64 until an amount n~ has been dis~ensed into ~e vehicle tank, which will fill the tanlc to the cut-off pressure. After this filling process is complete, the operator then moves the thre~way val-~e 44 to the ~vent" position to allow the natural gas contai~ed in ~e section between the coupler 48 on the end of hose assembly 28 and ~e coupler a~ched to the vehicle tanlc to be evacuated from the system through check valve 126 and into ~e vent recovery system (not shown), where it is re compressed and pumped back into the CNG supply tanl~. If this pressunzed natural gas is not evacuated from the section between coupler 48 and ~e vehicle coupler, it would be impossible for the user to disconnect the hose 28 ~rom his vehicle, because the e~ctremely high pressure in the hose would prevent ~e couplers ~om disconnecting, which is a characteristic of the type of couplers used in this industry.
The electronic con~ol system used to control the operation of the solenoid valveassembly 42, monitor and de~mine ~e pressures and temperature measured by the various - 15 ~ansducers as descnbed a~ove, as wëïl as to perform the necessary computatio~s, is shown in Figure 6. Essentially, the output sigDals ~om the various pressure transducers 92, 94, and 96, ambien~ pressure transducer 91 ~Fig. 1) and stagnation temperature transducer 118 are received by analog multiple~cer 136, which multiple~es tbe signals and sends tbem to an analog to digital (A/D) com~erter 138. Ihe analog to digital converter 138 converts the an~og sig~als from the ~adous transducers into digital signals suitable for use by the micro-controller or microprocessor 140. In the preferred embodiment, microprocessor 140 is a MC68HC11 manufactured by the Motorola Corporation, although other micr~processors could be used wi~ egual effecthteness. Random access memory (RAM) 142 and read only memory (ROM) 144 are also conn~cted to ~e microprocessor 140 to allow the microprocessor 140 to e~cecute the desired routines at the desired times, as is well-known.
Microprocessor 140 also has hputs for receiviDg signals from tbe transaction switches 32 and 34 (see also Figure 1) for each respective dispensing hose assembly 28, 30.
Optiohally, a number of authorization switcbes, sucb as switches 31, 33, 35, and 37 could be connected in series witb switcbes 32 and 34 to provide addidonal autborization devices, such as a credit card readers, which must be activated before natural gas will be dispensed, WO 93/00264 PCr/US92/05538 2112~ 16 or to provide an emergellcy shut-off fea~re by means of a switch (such as 31, 33, 35, or 37) remotely located in the station building.
A series of communication ports 146, 148, 150, and 152 are also connected to microprocessor to 140 for ~he purposes of transmitting and receiving data, which data may S comprise new prograra information to modify the operation of the dispensing system 10 or may comprise specific authorization and coding d~a ~at could be ~ed~o a master control computer remotely located from the dispensing system 10. Since the details associated with such communication ports 146, 148, 150, and 152 are well-lcnown to persons having ordinary s~ill in the art, and since such persons could easily provide such communications ports depending on the desired configuradon and after becoming familiar with the details of this inventioII, these communications ports 146, 148, 150, and 152 will not be described in further detail. Similarly, two relay ou~uts 164, 166 are used to send pulse data to optionally connected card readers (also not shown), as is also well-lcnown.
A relay output latch 154 is also connected to ~he microprocessor 140 and multiplexes signals to relay outputs 156 and 158 which control the solenoid valves 41 and 43 for hose assemblies 28 and 30, respectively. Two spare relays 160 and 162 are also connected to relay output latch 154 and may be us~d to control other various functions not shown and described herein. Finally, in the preferred embodiment eight (8) rotary switches 145 are also connected to microprocessor 140 to allow the user to configure she microprocessor 140 to his particular requirements. Again, since such configuration-selectable features may vary depending on the particular use desired and microprocessor, and since it is well-known to provide for such user selectable features, the details of the rotary switches 145 will not be described in fufther detail.
The details processes e~ecuted by she microprocessor 140 during operation of thedi~peDsing system 10 are best seen by referring to she flow diagrams shown in Figures 8, 9, lO(a), lO(b), and 11. However, processes e3cecuted by the microprocessor 140 will be understood more easily by first describing the overall theory and operation of the rnass-based filling process used by ~e present invention.
As best seen in Figure 7, the stagnation pressure in the vehicle tank is linearly related to the mass of gas in the tanlt (neglectiDg compressibility and at constant temperature). As discussed above, many factors, such as frictional effects or sonic choke points, may make it difficult, if ~ot impossible, to use a remotely located ser upstream of the dispensing hose to accurately measure the pressure of the vehicle tanlc wbile it is beiDg fllled. To solve these problems the present invention f~rst determines the volurne V of the vehicle tank and then WO93/00264 211,? l 5~3 PCI'/US92/0ss38 ~Iculates the additional mass of CNG that is required to increase the pressure of the tank to ~e previously calculated cut~ff pressure py c,.~ The system then simply dispenses the addiffor al mass of CNG into ~e vehicle tanlc, thus insuring that the tanlc is always filled to the cut~ff pressure regardless of the pressure drop in the dispensing hose and regardless of whether a sonic choke point e~cists in the dispensing hose or coupler assem~ly between ~e vehicle tank and the pressure transducer 92. ,--Briefly, the fill method of the present invention first quicl~y cycles valve 64 to pop open ~e safety check valve and equalize the pressure in the dispensing hose and vehicle tank.
After valve 64 has closed, the hnitial vehicle tank pressure p~O can be accurately sensed by transducer 92, since there is no CNG flow through the dispensing hose. The initial tank pressure p~O corresponds to a~ initial mass mO of CNG already in the tank, as seen in Figure 7. The system the~ adds an initial known mass (m,) of gas to the tank, thus increasing the tank pressure to an initial pressure of p~,. Equation (3) above can now be used to determine the ~lume of the vehicle ~anlc V. Once the tan~ volume has been determined, Equation (4) is used to determine the additional mass (m2) required to increase the pressure in the tank to the previously calculated cut~ff pressure p~
Unfor~nately, there will always be a certain amount of uncertainty in the measured values of p~O and p~" as represented by the error bo~ces 183 and 185 tFigure 7), which will result in an error 187 in achieving the desired cut-off pressure p~ ~ by the addition of mass m2 of CNG. Therefore, ~e preser~ invention also includes suitable safeguards to ensure that t~e pressure error will never e~cceed p~ or fall below p~ More specifically, while ~e size of the error band 187 can be reduced by using precision pressure transducers to determine ~e pressure, even the best ttansducers will have some uncertain~r. Therefore, the method of the present i~vention limits the ma%imum e~trapolation pe~mitted in calculadng the additional mass (m~) required to reach the cut-off pressure. If the pressure of the vehicle storage tanlc is less than 1/4 of the final pressure, i.e., if Pvl / py~ 2 0.25, then ~e method of the present invention will reduce the calculated value of the additional rnass m2 t 75 % of its original value to avoid overshooting the cut-off pressure. Ihen, after the reduced mass m2 is added, ~e valve 64 is closed and a new tanlc pressure is determined. The new tank pressure is then used to recompute the additional mass required to fill the tank to the cut-off pressure.
Referring now to Figure 8, the steps performed by the microprocessor 140 are as follows. When power is initially applied to the natural gas dispensing system l0, the microprocessor 140 e~ecutes an initialization procedure 210, which serves to clear all faul~

wo g3,00264 2 1 1 2 ~ ~ ~ P~r/US92/0~538 lX
flags, blank out and turn on ~e displays, and perform various diagnostic tests on the microprocessor 140, the random access memory 142, and the read only memory 144. Since such initialization and diagnostic test procedures 210 are well-known in the art and are usually dependent on the particular hardware configuration being used, the precise details of these initialization and diagnostic procedures will not be e~plained in further detail.
After the initializationprocedure 210 has been comp1eted, ~e,~rogram flow continues to the idle loop and wait for start command process 212. Essentially, this process 212 places the dispensing system 10 in idle state, whereby the microprocessor 140 awaits input from one of ~e transaction switches 32 or 34 to signal that the operator wishes to begin dispensing natural gas. If the microprocessor 140 receives a signal from one of the transaction switches 32 or 34~ the microprocessor 140 will proceed to the start sequence procedure 214. In this start sequence procsdure 214, ttle micr~processor 140 execut&s a number of predetermined steps to measure and calibrate ~e pressures and temperatures received from ~e various pressure and temperature transducers connected to the supply plenum and valve assembly 140.
rne star~ sequence procedure 214 also calculates the vehicle tanlc cut-off pressure, Pv ~o~f~
based on the ambient tempera~re T.,", and data stored in the ROM relating to the rated pressure of the vehicle tanlc, as will be described below. After the start sequence procedure 214 is complete, the mic~oprocessor proceeds to the fill sequence process 216. The fill sequence 216 performs all of ~e necessary ~eps to completely fill the natural gas storage tank 2û in ~e ve~icle including the steps of initially cycling the valve to pressurize the dispensing hose, measuring the initial tanlc pressure, calculating ~e volu ne of the tank and the mass of CNG required to fill the tanlc to the cut~ff pressure, and, of course, autornatically shutting off the flow of nabural gas when the vehicle tanlc has been filled to the previously calculated cut-off pressure. Aher the fill process is complete, the microprocessor will next e~ecute the end sequence process 218 to complete the transaction process and return the system 10 to the idle state 212.
The details of the start sequence 214 are best seen in Figure 9. The process begins by e~cecuting 220 to clear all channel-specific fault flags and wait for an authorization code from a host computer system, if one is provided. The process ne~ct proceeds to 222 which begins by clearing any transaction totals from a previous filling operation and turns on the display segments to indicate to the user that the electronic control system is active and proceeding with the filling process. Also during the step 222, the computer measures and stores values for pt and P2 as sensed by transducer 96 and transducer 92, respectively. Since the valve 64 is not yet open, the pressures sensed by transducers 92 and 96 are identical, as WO 93/00264 2 1 1 ~ Pcr/usg2/o5s38 mentioned above. The ambient temperature T."," sensed by transducer 91 is also measured and stor~d at ~is time. Step 222 ne~t calculates a cut-off pressure limit Pva,~dT as a function of the previously measured T"~," and the predetermined tank pressure limit that was previously programmed into the microprocessor 140. Finally, the process 222 resets a state timer to zero seconds. Next, the rnicroprocessor 140 e~cecutes processes 224, 226 and 228.
EsseDtially, these processes measure and store the Yalues detec~ for- p, and P2 three (3) additio~al times, with at least a one second interval between measuring periods to insure that the pressures have stabilized and ItO compensate for the fact that the A/D converter 138 cannot convert the signals from the pressure transducers on a real time basis. Of course, if a real time AID converter were used, then it would not be necessary to wait one second between readings. In step 228, the pressure transduce~ 92 (P2) is calibrated by summing the earlier measurements for p, and subtracting the sum of the measurements from P2 and dividing by 4. This value is then stored by the microprocessor 140, which adds it to all subsequent pressure readings from transducer 92 to eliminate systematic errors. Finally, this process 228 sets a fault flag if the calibrated value for P2 (i.e., transducer 92) e~ceeds a predetermined limit, indicating a fault in the system or a defective transducer.
Start sequence step 214 ne~ct e~cecutes process 230 which re-checks the position of the transaction switch. If the transaction switch has been opened, the process will go baclc to the idle loop step 212. If the transaction switch is sffll closed, the microprocessor 140 will open the na~ral gas valve and set the transaction time to equal to ~e open valve response time.
The reason ~at the ~ansaction time is set to the open valve response time (in the preferred embodiment the open valve response time is about .1 second) is because in the preferred embodimellt i~ talces about .1 second before sonic flow is established in the sonic nozzle 52.
Note ~at this open val~re response time is dependent on the particular nozzle and valYe configuration employed and is determined for a particular set-up on an e~perimental basis, and the amount of natu~ral gas that flows through the nozzle during this time will be added to the total amount of CNC; dispensed. Finally, the start sequence process 214 e~cecutes step 232 which updates tbe transaction time and determines whether the transaction time is greater ~an the predetermined 8tabilizationtime. In the preferred embodiment, the stabilization time is appro~cimately 1 8econd and is used becau8e the analog to digital converter 138 connected to the microprocessor 140 does not operate on a real t me basis, i.e., the A/D converter 138 is relatively slow and is only capable of updating the data received from the various transducers about every 0.3 to 0.4 seconds. Therefore, the microprocessor 140 will wait until Wo 93/00264 2 1 1 2 ~ 5 ~ PCr/US92/0~538 the transaction time has e~cceeded the stabilization time before proceeding with the fill sequence process 216.
R~ferring now to Figures lO(a) and 10(b), fill sequence 216 begins with step 240which bri,efly cycles the valve 64 to dispense a s,mall amount of CNG into the vehicle tank and briefly pop open any check valves in the vehicle, thus equalizing ~e pressure in the dispensing hose with ~he pressure in the v~hicle tank. Step 240 al~rdëtermines the initial t~k press,ure p,O and estimates an initial fill mass m, ba~ed on the difference between the initial ~anL pressure p~0 and the previously calculated cut-off pressure Pv ~ to ensure ~at the cut-off pressure will not be exceeded by adding the initial mass ml.
Fill sequence 216 ne~t e~ecutes step 241, which controls the e~cact process by which the ve~icle tanlc is filled. For example, before the dispensing process is initiated, the user can input a total dollar amount of natural gas to be dispensed into his vehicle tank.
Alter~atively, the user can instruct the system to completely fill the vehicle tank. In any event, process 241 fo~ns one step in a loop that continuously determines whether the total lS dollar amount equals ~e preprogrammed fill limit or whether the tan~ is to be filled to the previously calibrated cut~Dff pressure p~, ~,~ calculated in step 222 (see Figure 9). Finally, step 241 also c~ntinually checks to insure that the transaction switch is still closed and that the computet has not re~eived any emergency shutdown commands from an outside host computer or a fault code g~erated within the microprocessor 140 itself. If none of these events occur, the process proceeds to step 242 which first updates the cycle time and then calculates ~e flow rate and total mass of CNG dispensed. Optionally, data output pulses may be sent to a card reader (not shown) and, in any event, the display will co~tinually be updated wi1h the total mass of CNG dispeDsed (or equivalent) and the total cost. Also during this process 242, the system continually mo~ors the time variation of the discharge pressure dp21dt. If dp2/dt e~ceeds a certain predetennined limit, indicating a sudden loss of outlet pressure, such as would result f~om a ruptured dispensing hose, the computer will automatically set a fault code and immediately turn off the flow of CNG. Finally, this process 242 continually checks the ratio of the discharge pressure P2 against the supply pressure p,. If this ratio e~ceeds the preprogrammed limit (0.82 in the preferred embodiment), the computer will also set a flag. The reason a flag is set in this case is that if ~e ,atio of P2 to Pl e~ceeds a certain limit, sonic flow will no longer be maintained and the sonic nozzle 52 and the mass flow calculations will DO longer be correct. In that case, the microprocessor will automatically calculate the mass flow rate for subsonic nozzle conditions, as e~plained a~ove. Process 242 is repeated until one of the conditions in step 243 is WO93/00264 2 ~ 45~ Pcr/VS92/0553g satisfied, in which case the process proceeds to step 244 which closes the valve, measures the initial pressure Pvl and T~" calculates the actual amount of initial mass (ml) dispensed, as well as ~e ~ volume V and additional mass (m2) required to fill the tank to the cut-off pr~,sure, as determined by Equations (3) and (4), respectively.
Process 245 ne~t determines whether the initial pressure Pvl is within 100 psig of the previously determined cut-off pressure p~ If so, the tanlc is fi~ n~he process executes the end sequence 218. If IlOt, the process proceeds to step 246 which determines whether the taDk is more than one quarter (1/4) full (on a pressure basis). If the tank is more than 114 filll, the process proceeds to step 248. However, if the tank is less then 1/4 full, then r~ is reduced to 75% of its original value before proceeding to step 248. As mentioned above, this process 246 minimiz~ ~e chances for tank overfilling due to uncertainties in the measured values for the tanlc pressure. Step 248 is identical to ste~ 242 and, therefore, will not be described again. Process 248 is repeated until one of the conditions in s~ep 249 is satisfied, in which case the process proceeds to step 2S1 which closes the valve, measures the tank pressure P"2, T~",", and calculates the actual amount of additional mass (m2) dispensed into the tank. Finally, s~ 253 checlcs to see whether ~e tanlc pressure is within 100 psig of the calculated cut~ff pressure. If it is, the ta~ is full and the process e~cecutes the end sequence process 2113. If not, 1he tanlc is ~till not filled, a new mass (m3) is calculated based on the tanlc pressure P~2 and the process 248 is repeated agaim The detailed steps of the end sequence process 218 are shown in Figure 11. This process 218 begins by e~ecuting step 250 whic~ sets the total cycle or transaction time to equal the ac~al measured cycle tirne plus the valve close response time, which, in the preferred embodiment is about 0.25 seconds. Here again, the valve close response time is added to the total cycle time because ~e A-D converter 138 cannot convert data on a real time basis. Ne3ct, ~e total amou~t of compressed natural gas dispensed is calculated based on ~e total cycle time and in accordance with the preprogrammed relation for mass flow through the sonic nozzle, both when the flow was cholced and when it was not choked (i.e., subsonic~, plus the srnall amount of natural gas that flows through the nozzle during the valve opening a~d closing times. Output pulses are again seDt to a card reader (not shown) and the total volume dispensed and the total cost are displayed on displays 140 and 136. Optionally, ~e discharge pressure p~ can be displayed on display 138. Process 250 then updates the grand total of the volume of compressed natural gas dispensed from the system for accounting purposes and the mass and volume flows are zeroed by the computer. If any fault flag was detected, the computer will set the specific cbannel iD which the fault flag was detec~ed to a WO 93/00264 PCr/US92/0~538 21~2~58 22 ,,.~/t., fault state. The process 218 next e~cecutes step 252 which continually monitors the condition of ~e ~ansaction switch. If the switch is closed, the process will remain at this step until the user opens, the switch indicating that the fill process is complete. Process 252 then sets the inter-transaction timer to zero seconds. Finally, the process 218 e~cecutes step 254 which S waits uDtil ~lhe inter-transaction time-out period has elapsed. Once the time-out period has elapsed, ~e process will return to the idle loop 212 and the disp,e~ing process can be initiated again by a new customer.
While the preferred embodiment of the natural gas dispensing system 1() according to the pre~ent invention utilizes the Fill Sequence 216 illustrated in Figures 10(a) and 10(b) and descri~bed above, other fill sequences could be substituted as alternates, depending on the particular configuration of the vehicle storage tank and the desired level of accuracy for measuring the amount of CNG dispensed into ~e vehicle tank. For example, if a check valve is not installed in the vehicle, or if it is ~own that the flow of CNG will not pass through a sonic ch~Dlce poin~ between ~e pressure sensor 92 and the vehicle tank, the Fill Sequence 216 illustrated in Figures 10(a) and 10(b) may be replaced with a simpler Fill Seqluence 1216 illustrated in Figure 12. 3Replacing the Fill Sequence 216 with Alternath e Fill Sequence 1216 would be accomplished by simply reprogramming the microprocessor 140 to perform the steps shovm in Figure 12. Essentially, Alte~native Fill Sequence 1216 monitors the vehicle tanlc presmre, essentially P2, via pressure transducer 92 until the pressure P2 reaches the previously determined cut~ffpressure P2,~ff. When the tank pressure P2 reaches the cut-off pressure, Ihe Alternative Fill Sequence 1216 is complete, and the system would then proceed to t~e End~ Seque~ce step 218 as shown in Figure 8 and described in detail above.
The de~ails of the Alternative F~l Sequence process 1216 are shown in Figure 12 and the process begins by sxecuting decision step 1240. ln this particular embodunent, the dispensing system 10 can be programmed to fill the vehicle nabmal gas tank according to a numbe~ of different options. For e~cample, before the Alternative Fill Sequence process 1216 is initiatedl, ~e user could input a total dollar amount of natural gas to be dispensed into his vehicle tank. Alternatively, the system could be programmed to completely fill the vehicle tanlc, without setting any particular dollar ma~cimum. In any event, process 1240 forrns a loop to continually check and deternine whether the total dollar amount equals the preprogramrned fill lirnit or whether the tanl~ pressure P2 is greater than the cut~off pressure P2,~d~ determined in step 222 (see Figure 9). Finally, step 1240 also continuaJly checks to ensure that the transaction switch is still closed and that the microprocessor 140 has not received any emergency shutdown commands from an outside host computer or a fault code WO 93/0~264 2 1 ~. 2 ~ ~ 3 PCI/US92/05s38 generated within the microprocessor 140 itself. If none of these eveDts occur, the process 1216 proceeds to step 1242 which first updates the cycle time and tben calculates the flow rate and total volume dispensed. Data output pulses will be sent ~lo a card reader (not shown) and the display will be co~tinually updated with the total volume of CNG dispensed, the total cost, as well as the discharge pressure P2 as measured by pressure transducer 92. Also during ~is process 1242, the system continually monitors the tirne variation of the discharge pressure dp21dt. If dpJdt e~ceeds a certain predetermined limit, indicating a sudden loss of ou~et pressure, such as would result from a ruptured dispensing hose, the computer will automatically set a fault code and immediately turn off the flow of CNG. Finally, this process 1242 continually checlts the ratio of the discharge P2 against the supply pressure p,.
If that ratio exceeds the preprogrammed lirnit, the computer will also set a flag. The reason a flag is set in th~ case is that if the ratio of P2 to p, e~cceeds a certain limit, sonic flow will no longer be present in ~the sonic nozzle 52, and the rnass flow calculations will no longer be oorrect. In that case, the microprocessor 140 will automatically calculate the mass flow rate for subsonic nozzle conditions, as was described in detail above. Pinally, as shown in Figure 12, the process 1216 is repea~ed until one of the conditions in step 1240 is satisfied, in which case the process eIecutes step 1244, which closes the valve, updates the cycle time, and directs the program flow to e~cecute ~e End Sequence process 218 shown in Figure 8 and described above.
This completes ~e detailed description of the preferred embodiments of dle natural gas dispensing syst~m 10 accordiDg to the present invention. While some of the obvious and numerous modifications and equivalents have been described herein, still other modifications and cha~ges will readily occur to those having ordinary slcill in the art. For e~ample, none of ~e sealing devices required by this invention have been shown and described herein, as it is well-known to provide various types of seals, such as ~O" ring type seals, to prevent the CNG from lealtiDg, and persons having ordinary slcill in this art could readily provide such seals after becoming familiar with the de~ls of the present invention.
Fur1~her, while this invention has been shown and described to dispense compressed na~ral gas, other fluids could be used just as easily with a system according to the present invention with little or no modification. For instance, the dispensing system shown and described herein could also be used to dispense hydrogen or propane gas. Moreover, more than two sonic nozzles could be connected to the supply plenum to provide an increased number of dispensing hoses from a single dispenser body or plenum. Finally, numerous enhancements of the operating program are possible by reprogramming the microptocessor WO 93/nO264 PCI`/US92/05538 2 ~ 4 5 ~ 24 to malce the appropriate enhancements, as would be obvious to those persons having ordinary skill in the art.
I~ne foregoing is coDsidered as illus~ative only of the principles of ~is invention.
Furdler, since numerous modifica~tions and changes will readily occur to those skilled in ~e S art, it is not desir~d to limit the invention to the e~act construction and operation shown and described, and accordingly, all suitable modifications and equivalents~may be considered as ~lling within ~e scope of d~e invention as defined by ~he claims which follow.

Claims (33)

Claims The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Temperature compensated fluid dispensing apparatus for dispensing fluid from a fluid source to a fluid receiver at an ambient temperature, wherein the fluid in the fluid source has a stagnation temperature and wherein the fluid receiver has a receiver pressure rating at a predetermined pressure and temperature, comprising:
valve means interconnecting the fluid source and the fluid receiver for selectively closing off the interconnection between the source and the receiver; and valve control means connected to said valve means for actuating said valve means to close off the interconnection between the fluid source and the fluid receiver when the fluid receiver has been filled to a pressure equivalent to the receiverpressure rating corrected for the ambient temperature.

2. The temperature compensated fluid dispensing apparatus of claim 1, wherein said valve control means includes:
means for sensing the ambient temperature and for generating an ambient temperature signal related thereto;
means for determining an initial stagnation pressure of the fluid in the fluid receiver and for generating an initial receiver stagnation pressure signal related thereto;
means for dispensing an initial mass of fluid into said fluid receiver;
means for determining an intermediate stagnation pressure of the fluid in the fluid receiver and for generating an intermediate receiver stagnation pressure signal related thereto;
calculation means responsive to said ambient temperature signal for determining a cut-off receiver stagnation pressure based on said ambient temperature and based on the receiver pressure rating, for determining a fluid receiver volume based on said initial receiver stagnation pressure and said intermediate receiver stagnation pressure, and for determining an additional mass of fluid to increase said receiver stagnation pressure to about the cut-off receiver stagnation pressure; and valve actuation means connected to said valve means and to said calculation means for causing said valve means to close off the interconnection between the fluid source and the fluid receiver when said additional mass has been dispensed into said fluid receiver.

3. The temperature compensated fluid dispensing apparatus of claim 2, further comprising a sonic nozzle connected between the fluid source and said valve means.

4. The temperature compensated fluid dispensing apparatus of claim 3, wherein both said means for determining an initial stagnation pressure and said means for determining an intermediate pressure comprise pressure sensing means for sensing the pressure of the fluid in said fluid receiver and for generating a pressure signal related thereto, and wherein said pressure sensing means is located between said sonic nozzle means and said valve means.

5. The temperature compensated fluid dispensing apparatus of claim 4, wherein said pressure sensing means is located between said valve means and the receiver.

6. The temperature compensated fluid dispensing apparatus of claim 3, including means for sensing the stagnation temperature of the fluid in the fluid source and for generating a source stagnation temperature signal related thereto, and wherein said means for sensing the stagnation pressure of the fluid in the fluid receiver is located between said sonic nozzle and said valve means.

7. The temperature compensated fluid dispensing apparatus of claim 6, further comprising flow rate calculation means responsive to said source stagnation temperature signal and to said source stagnation pressure signal for determining a mass flow rate of fluid flowing through said sonic nozzle when the fluid flow through said sonic nozzle is choked.

8. The temperature compensated fluid dispensing apparatus of claim 7, wherein said flow rate calculation means is also responsive to said receiver stagnation pressure signal for determining a subsonic mass flow rate of fluid flowing through said sonic nozzle when said receiver stagnation pressure is too high to allow the sonic nozzle to choice.

9. Temperature compensated fluid dispensing apparatus for independently dispensing fluid from a fluid source into first and second fluid receivers at an ambient temperature, wherein the fluid in the fluid source has a stagnation temperature and wherein each fluid receiver has a receiver pressure rating at a predetermined stagnation pressure and temperature, comprising:
a supply plenum connected to the fluid source;
first valve means interconnecting the fluid source and the first fluid receiver for selectively closing off the interconnection between the source and the first fluid receiver;

second valve means interconnecting the fluid source and the second fluid receiver for selectively closing off the interconnection between the source and the second fluid receiver; and two channel valve control means connected to said first valve means and said second valve means for independently actuating said first and second valve means to dose off the interconnection between the fluid source and the corresponding fluid receiver when the corresponding fluid receiver has been filled to a pressure equivalent to the receiver pressure rating corrected for the ambient temperature.

10. A fluid dispensing system for controlling an amount of fluid flowing from a fluid source to a fluid receiver, wherein the fluid in the fluid source has a stagnation temperature and wherein the fluid in the fluid receiver has a stagnation pressure, comprising:
valve means interconnecting the source and the receiver for selectively closing off the interconnection between the source and the receiver;
means for sensing the ambient temperature for generating an ambient temperature signal related thereto;
means responsive to said ambient temperature signal for determining a cut-off receiver stagnation pressure based on said ambient temperature and in accordancewith predetermined receiver pressure parameters;
means for determining an additional mass of fluid to be added to said fluid receiver to increase the pressure in said receiver to about said cut-off receiver stagnation pressure; and valve control means connected to said valve means and responsive to said means for determining an additional mass of fluid for actuating said valve means to close off the interconnection between the source and the receiver when said additional mass of fluid has been added to said fluid receiver.

11. A fluid metering and control system for measuring and controlling an amount of fluid flowing from a fluid source to a fluid receiver, wherein the fluid in the fluid source has a stagnation temperature and a stagnation pressure and wherein the fluid in the fluid receiver has a stagnation pressure, comprising:
a sonic nozzle interconnecting the source and the receiver;
valve means positioned between said sonic nozzle and the receiver for selectively closing off the interconnection between the source and the receiver;means for sensing the ambient temperature and for generating an ambient temperature signal related thereto;

means for sensing the stagnation pressure of the fluid in the source and for generating a source stagnation pressure signal related thereto;
means for sensing the stagnation temperature of the fluid in the source and for generating a source stagnation temperature signal related thereto;
means responsive to said ambient pressure signal and responsive to said source stagnation temperature signal for determining a mass flow rate of fluid flowing through said sonic nozzle when the fluid flow through said sonic nozzle is choked;
means responsive to said ambient temperature signal for determining a cut-off receiver stagnation pressure based on said ambient temperature and in accordancewith predetermined receiver pressure parameters;
means for determining an additional mass of fluid to be added to said fluid receiver to increase the pressure in said receiver to about said cut-off receiver stagnation pressure; and valve control means connected to said valve means and responsive to said means for determining an additional mass of fluid to be added to said fluid receiver for actuating said valve means to close off the interconnection between the source and the receiver when said additional mass has been added to said fluid receiver.

12. The fluid metering and control system of claim 11, including means for sensing a downstream stagnation pressure downstream of said sonic nozzle and for generating a downstream stagnation pressure signal related thereto.

13. The fluid metering and control system of claim 12, wherein said means responsive to said source stagnation pressure signal and said source stagnation temperature signal is also responsive to said downstream stagnation pressure signal for determining a subsonic mass flow rate of fluid flowing through said sonic nozzle when said downstream stagnation pressure is too high to allow the sonic nozzle to become choked.

14. The fluid metering and control system of claim 13, including means for combining and integrating said mass flow rate flowing through said sonic nozzle when said sonic nozzle is choked with the subsonic mass flow rate of fluid flowing through said sonic nozzle when said sonic nozzle is not choked during the period when said valve means is not closed to produce a total mass of fluid that passed through said sonic nozzle into the receiver.

15. A method of dispensing compressed gas from a pressurized storage tank to a receiver under less pressure than the storage tank, comprising the steps of:
connecting said storage tank and said receiver with a pressure tight dispensing hose;

sensing an ambient temperature before initiating the dispensing cycle;
calculating a cut-off pressure for the receiver based on the ambient temperature tank and based on a predetermined based pressure for the receiver;
sensing an initial pressure of said receiver;
adding a predetermined mass of gas to the receiver;
sensing an intermediate pressure of said receiver;
determining a volume of the receiver and an additional mass of fluid to increase said receiver stagnation pressure to about the cut-off pressure;
initiating the flow of gas through said dispensing hose from said supply tank to said receiver; and terminating the flow of gas when the additional mass has been added to said receiver.

16. The method of claim 15, including the step of:
sensing the stagnation pressure and stagnation temperature of the gas within the storage tank and;
calculating the amount of gas dispensed from said storage tank into said receiver based on the stagnation temperature and pressure of the gas in the storage tank and flowing through a sonic nozzle.

17. The method of claim 15, wherein the step of determining a volume of the receiver and an additional mass of fluid includes the step of reducing the additional mass of the fluid by a predetermined amount if said receiver is less than about 1/4 full of gas on a pressure basis.

18. The temperature compensated fluid dispensing apparatus of claim 1, wherein said valve control means includes;
means for sensing the ambient temperature and for generating an ambient temperature signal related thereto;
means for determining a stagnation pressure of the fluid in the fluid receiver and for generating a receiver stagnation pressure signal related thereto;
calculation means responsive to said ambient temperature signal for determining a cut-off receiver stagnation pressure based on said ambient temperature and based on the receiver pressure rating; and valve actuation means connected to said valve means and to said calculation means and responsive to said receiver stagnation pressure signal for causing said valve means to close off the interconnection between the fluid source and the fluid receiver when said receiver stagnation pressure is about at said cut-off receiver stagnation pressure.

19. The temperature compensated fluid dispensing apparatus of claim 18, further comprising a sonic nozzle connected between the fluid source and said valve means.

20. The temperature compensated fluid dispensing apparatus of claim 19, wherein said means for determining a stagnation pressure includes:
pressure sensing means for sensing the pressure of the fluid and for generating a pressure signal related thereto, and wherein said pressure sensing means is located between said sonic nozzle means and said valve means; and pressure drop correcting means responsive to said pressure signal for correcting the pressure of the fluid to account for frictional effects, so that the pressure of the fluid is substantially equal to the stagnation pressure of she fluid in the fluid receiver.

21. The temperature compensated fluid dispensing apparatus of claim 20, wherein said pressure sensing means is located between said valve means and the receiver.

22. The temperature compensated fluid dispensing apparatus of claim 19, including means for sensing the stagnation temperature of the fluid in the fluid source and for generating a source stagnation temperature signal related thereto, and wherein said means for sensing the stagnation pressure of the fluid in the fluid receiver is located between said sonic nozzle and said valve means.

23. The temperature compensated fluid dispensing apparatus of claim 22, further comprising flow rate calculation means responsive to said source stagnation temperature signal and to said source stagnation pressure signal for determining a mass flow rate of fluid flowing through said sonic nozzle when the fluid flow through said sonic nozzle is choked.

24. The temperature compensated fluid dispensing apparatus of claim 23, wherein said flow rate calculation means is also responsive to said receiver stagnation pressure signal for determining a subsonic mass flow rate of fluid flowing through said sonic nozzle when said receiver stagnation pressure is too high to allow the sonic nozzle to choke.

25. Temperature compensated fluid dispensing apparatus for independently dispensing fluid from a fluid source into first and second fluid receivers at an ambient temperature, wherein the fluid in the fluid source has a stagnation temperature and wherein each fluid receiver has a receiver pressure rating at a predetermined stagnation pressure and temperature, comprising:
a supply plenum connected to the fluid source;

first valve means interconnecting the fluid source and the first fluid receiver for selectively closing off the interconnection between the source and the first fluid receiver;
second valve means interconnecting the fluid source and the second fluid receiver for selectively closing off the interconnection between the source and the second fluid receiver; and two channel valve control means connected to said first valve means and said second valve means for independently actuating said first and second valve means to close off the interconnection between the fluid source and the corresponding fluid receiver when the corresponding fluid receiver has been filled to a pressure equivalent to the receiver pressure rating corrected for the ambient temperature.

26. A fluid dispensing system for controlling an amount of fluid flowing from a fluid source to a fluid receiver, wherein the fluid in the fluid source has a stagnation temperature and wherein the fluid in the fluid receiver has a stagnation pressure, comprising:
valve means interconnecting the source and the receiver for selectively closing off the interconnection between the source and the receiver;
means for sensing the ambient temperature for generating an ambient temperature signal related thereto;
means for determining the stagnation pressure of the fluid in the receiver and for generating a receiver stagnation pressure signal related thereto;
means responsive to said ambient temperature signal for determining a cut-off receiver stagnation pressure based on said ambient temperature and in accordancewith predetermined receiver pressure parameters; and valve control means connected to said valve means and responsive to said receiver stagnation pressure signal for actuating said valve means to dose off the interconnection between the source and the receiver when said receiver stagnation pressure equals said cut-off receiver stagnation pressure.

27. A fluid metering and control system for measuring and controlling an amount of fluid flowing from a fluid source to a fluid receiver, wherein the fluid in the fluid source has a stagnation temperature and a stagnation pressure and wherein the fluid in the fluid receiver has a stagnation pressure, comprising:
a sonic nozzle interconnecting the source and the receiver;
valve means positioned between said sonic nozzle and the receiver for selectively closing off the interconnecting between the source and the receiver;

means for sensing the ambient temperature and for generating an ambient temperature signal related thereto;
means for sensing the stagnation pressure of the fluid in the source and for generating a source stagnation pressure signal related thereto;
means for sensing the stagnation temperature of the fluid in the source and for generating a source stagnation temperature signal related thereto;
means responsive to said ambient pressure signal and responsive to said source stagnation temperature signal for determining a mass flow rate of fluid flowing through said sonic nozzle when the fluid flow through said sonic nozzle is choked;
means for determining the stagnation pressure of the fluid in the receiver and for generating a receiver stagnation pressure signal related thereto;
means responsive to said ambient temperature signal for determining a cut-off receiver stagnation pressure based on said ambient temperature and in accordancewith predetermined receiver pressure parameters; and valve control means connected to said valve means and responsive to said receiver stagnation pressure signal for actuating said valve means to close off the interconnection between the source and the receiver when said receiver stagnation pressure equals said cut-off receiver stagnation pressure.

28. The fluid metering and control system of claim 27, wherein said means responsive to said source stagnation pressure signal and said source stagnation temperature signal is also responsive to said receiver stagnation pressure signal for determining a subsonic mass flow rate of fluid flowing through said sonic nozzle when said receiver stagnation pressure is too high to allow the sonic nozzle to become choked.

29. The fluid metering and control system of claim 28, including means for combining and integrating said mass flow rate flowing through said sonic nozzle when said sonic nozzle is choked with the subsonic mass flow rate of fluid flowing through said sonic nozzle when said sonic nozzle is not choked during the period when said valve means is not closed to produce a total mass of fluid that passed through said sonic nozzle into the receiver.

30. A method of dispensing compressed gas from a pressurized storage tank to a receiver under less pressure than the storage tank, comprising the steps of:
connecting said storage tank and said receiver with a pressure tight dispensing hose;
sensing the ambient temperature before initiating the dispensing cycle;

calculating a cut-off pressure for the receiver based on the ambient temperature tank and based on a predetermined rated pressure for the receiver;
initiating the flow of gas through said dispensing hose from said supply tank to said receiver;
determining the stagnation pressure of said receiver; and terminating the flow of gas when the stagnation pressure of said receiver substantially equals said cut-off pressure.

31. The method of claim 30, including the step of:
sensing the stagnation pressure and stagnation temperature of the gas within the storage tank and;
calculating the amount of gas dispensed from said storage tank into said receiver based on the stagnation temperature and pressure of the gas in the storage tank and flowing through a sonic nozzle.

32. The method of claim 31, including the step of calibrating a pressure transducer used to sense the receiver pressure against a pressure transducer used to sense the storage tank pressure before the gas flow is initiated.

33. The method of claim 30, wherein the step of determining the stagnation pressure of said receiver includes the steps of:
sensing an intermediate pressure of the gas in said dispensing hose; and correcting the intermediate pressure of the gas in said dispensing hose for a pressure drop in said dispensing hose, to determine the stagnation pressure of said receiver.