Background In many situations where a fluid is desired to be delivered either in a commercial or domestic environment, there is a requirement that the fluid delivered be provided at a pressure regulated within practical and/or system determined limits. Often, it is also a requirement that the delivery of the fluid be monitored so that the volume of fluid delivered can be accounted for and billed accordingly. Such arrangements are common in relation to the supply of hydrocarbons such as natural gas, liquefied petroleum gas (LPG), petroleum, oils, methane, water, and many other fluids.

Over the past ten to twenty years, there has been greater emphasis upon accurately monitoring the delivery of such fluids so that reliable and accurate determination of customer billing can be achieved. This has necessitated various advances in fluid flow metering technologies, one example of which is the now relatively widespread interest in ultrasonic flow metering, particularly for gas delivery systems and the like.

Ultrasonic flow meters are characterised by relatively high levels of accuracy compared to their forerunners, but also are subjected to a number of practical constraints, one of those being that such flow meters are typically associated with relatively significant pressure drops across the measuring arrangement. Whilst pressure drops can be minimised using large ultrasonic flow measuring arrangements, such arrangements become cumbersome and do not lend themselves to ready substitution for existing gas meters, particularly in domestic situations where it may be desired by a gas supplier to refit meters in many thousands of residences and the like. As a consequence, practical ultrasonic flow meters intended for use in domestic situations are sized to substitute for existing mechanical flow metering arrangements. Such provides significant size restrictions upon the ultrasonic flow meter and hence designers must seek a balance between meter accuracy and pressure drop minimisation.

Summary of the Invention It is an object of the present invention to substantially overcome, or at least ameliorate, the above mentioned difficulties through a flow metering arrangement which addresses the issues of pressure drop across the meter.

In accordance with one aspect of the present invention there is disclosed a fluid supply apparatus including a fluid inlet, a fluid outlet, a fluid pressure regulator having a pressure chamber arranged following said inlet, a fluid flow meter arranged between said pressure regulator and said outlet, and pressure transfer means configured to transfer a fluid pressure at said outlet to said pressure chamber to thereby permit said pressure regulator to adjust a flow of said fluid to maintain said fluid pressure substantially at a predetermined value.

Preferably, the flow meter is an ultrasonic flow meter having an elongate measuring tube of reduced cross-sectional area compared to at least said input.

Advantageously, said pressure transfer means comprises a transfer tube for fluidically connecting said pressure chamber with said outlet.

Typically, the pressure chamber comprises a dead volume substantially isolated from said inlet and an intermediate portion between said pressure regulator and said flow meter.

Brief Description of the Drawings A number of aspects of the prior art and of the preferred embodiment will be described with reference to the drawings in which: Fig. 1 is a schematic cross-sectional representation of a prior art gas pressure regulator; and Fig. 2 is a schematic cross-sectional representation of a combined gas flow meter and pressure regulator of the preferred embodiment.

Detailed Description Fig. 1 illustrates a gas pressure regulator 1 which includes a relatively high pressure gas inlet 2 and a relatively low pressure gas outlet 3. Arranged between the inlet 2 and outlet 3 is a housing 4 divided into two portions by means of a diaphragm 5, one of those portions forming a pressure chamber 6 in fluid communication with the outlet 3.

The diaphragm 5 is supported upon a rigid plate 7 which is biased for resilient movement by a spring 8 mounted within the housing 4. A force exerted by the spring 8 is arranged to be adjustable by means of an adjusting nut 9.

Extending from the plate 7 and through the pressure chamber 6 is a piston 10 upon the periphery of which is formed a valve 11 configured to open and close an orifice 12.

As will be appreciated by those skilled in the art, as the pressure on the outlet 3 drops, so too will the pressure within the pressure chamber 6 thus permitting the spring 8 to expand causing the piston 10 to extend in such a manner so that the orifice 12 opens.

The opening of the orifice 12 allows greater quantities of high pressure gas from the inlet 2 to pass through the orifice 12 into the pressure chamber and the outlet 3. This additional gas acts against the diaphragm 5 and plate 7 to counteract and balance the force applied by the spring. 8. In this fashion, the pressure regulator 1 acts to provide a relatively constant pressure at the outlet 3 for a range of flow rates of gas passing between the inlet 2 and the outlet 3.

Fig. 2 illustrates a gas meter-regulator system 20 which incorporates a pressure regulator portion 21 including components corresponding to those shown in Fig. 1 and whose reference numbers have been elevated by the value 20 and to which a similar description applies. In the arrangement of Fig. 2, the system 20 also includes a ultrasonic gas flow meter portion 35 arranged immediately preceding the outlet 23. The meter portion 35 incorporates two ultrasonic transducers 36 arranged to transmit ultrasonic signals therebetween via a fluid measuring tube 37 which, as illustrated, is of a reduced cross-sectional size compared to the outlet 23 and an intermediate channel 40 formed between the pressure regulator portion 21 and the flow meter portion 35. Typically, the flow meter portion 35 would have associated therewith an electronics arrangement configured for performing flow and volume calculations using signals passed between the transducers 36. However, such detail of the meter portion 35 is not relevant to the present invention and are omitted from the representation of Fig. 2 for that reason.

Because of the presence of the ultrasonic transducer 36 and of the reduced cross- section of the measuring portion 37, a significant pressure drop can occur across the flow meter portion 35. If such an arrangement were placed upon the outlet 3 of the arrangement of Fig. 1, a pressure drop may occur that may be sufficient to prevent such an arrangement being practically used in some gas reticulation systems.

According to the present invention, the arrangement shown in Fig. 2 addresses this problem through arranging the flow meter portion 35 within a gas pressure feedback loop which, in Fig. 2, is embodied by a transfer tube 38 that interconnects the outlet 23 with the pressure chamber 26. In this fashion, the outlet pressure present at the outlet 23

is continually transferred and referred back to the pressure chamber 26 to act upon the diaphragm 25 and pressure plate 27. The pressure plate 27 and piston 30 act to move the valve 31 to open and close the orifice 32 in a corresponding manner to the arrangement shown in Fig. 1. As before, the adjusting nut 29 can be adjusted to ensure that the outlet pressure of the system 20 can be set at a desired value, for example corresponding to that which would be used in the arrangement of Fig. 1.

As seen in Fig. 2, the intermediate channel 40, which is in fluid communication with the inlet 22 via the orifice 32, is isolated from the pressure chamber 26 by means of a isolator membrane 39 positioned within supports 41 arranged on the piston 30 and adjacent walls of the pressure chamber 26. The gas within the intermediate channel 40 is thereby prevented from entering the pressure chamber 26 and thus erroneously influencing the pressure being applied against the diaphragm 25 and plate 27.

Further, whereas in Fig. 1 the pressure chamber 6 forms a path for substantive flow between the inlet 2 and outlet 3, in Fig. 2 the pressure chamber 26 forms a substantial volume of the gas.

The meter-regulator system 20 of Fig. 2, provides that the flow meter portion 35 can be utilised without impacting in any substantial way upon the desired outlet pressure.

The pressure regulator portion 21 acts to continually open and close the orifice 32 via the operation of the valve 31 in response to changes of pressure sensed via the transfer tube 38.

The arrangement of Fig. 2 has a number of advantages. Notably, the arrangement of Fig. 2 allows the use of ultrasonic gas flow measuring arrangements which are smaller than arrangements either existing or currently proposed. These smaller sizes permit higher levels of accuracy in fluid flow measuring, yet in prior art arrangements cause greater pressure drops across the metering arrangement. Through using the pressure feedback system, those pressure drops are compensated for by providing a substantially constant outlet pressure than can be predetermined and/or preset.

Because the arrangement 20 of Fig. 2 can accommodate higher pressure drops across the fluid flow meter portion, the cross-sectional area of the measuring tube 37 (which is traditionally cylindrical), can be reduced. Reduction in size of the measuring tube, and in particular its cross-sectional area, is known in the art to substantially reduce the propagation and incidence of high order acoustic modes which are known to provide errors in flow metering technology, particularly where the ringaround (or singaround) method of ultrasonic propagation is used. The arrangement of Fig. 2 provides for shorter

metering tubes and metering tubes of smaller cross-sectional area which provides for higher accuracy in metering technology whilst providing for reliable outlet pressure and thus supply to a consumer.

The foregoing describes only one embodiment of the present invention and modifications, can be made thereto without departing from the scope of the present invention. For example, whilst the gas meter 35 described is seen as a direct path ultrasonic gas flow meter, other types of flow meters may also be used. For example, other measuring paths such as inclined paths as well as N-shaped and W-shaped ultrasonic transmission paths may be used.