GAS METER INCORPORATING CALORIFIC MEASUREMENT

Abstract: A gas meter (10) determining the calorific value of the gas passing through the meter is disclosed. The meter includes a reaction chamber (28) that draws gas from the gas flow path and an air intake (30). The calorific value is determined using a resistor (36), coated with a catalyst material, that forms part of a wheatstone bridge circuit. Oxidation of the gas/air mixture on the surface of the resistor (36) causes a temperature and resistance change dependent on the calorific value of the gas that can be detected by the wheatstone bridge.

Description: GAS METER INCORPORATING CALORIFIC MEASUREMENT BACKGROUND OF INVENTION Conventional gas meters measure consumption based on gas volume flow through the meter. The meter reading is recorded on the consumer's account and multiplied by a conversion factor which is calculated to reflect the typical or average calorific value of the gas being supplied. This converts the volume measurement into an energy figure, which is the true commodity which the consumer is purchasing. Natural gas, liquefied petroleum gas (LPG) and other energy gases are typically mixtures of hydrocarbons, having significant variations in calorific value depending on temperature, pressure, source, the ratios of the particular hydrocarbons and the presence of water and other non-combustible components. These variations may make the prior art energy calculations for individual consumers inaccurate. SUMMARY OF INVENTION The present invention aims to provide an alternative arrangement, and is characterised by the incorporation of means for measuring the calorific value of the gas passing through the meter. In a first form, the invention provides a gas meter for a combustible gas having metering means for measuring gas flow and generating an output, means for determining calorific value of the gas including sampling means for taking a sample of the gas, an air intake, means for contacting the gas sample with air, at least one catalyst member including a catalyst material causing oxidation of at least a part of the gas sample, said oxidation causing a change in one or more properties of said catalyst member dependent on the calorific value of the gas, and means for generating an output representative of said calorific value, said 1 gas meter further including means for receiving outputs from the metering means and calorific value determining means and calculating energy usage. A second form of the invention provides a calorific value determining means for connection to a gas meter for combustible gas, including sampling means for taking a sample of the gas, an air intake, means for contacting the gas sample with air, at least one catalyst member including a catalyst material causing oxidation of at least a part of the gas sample, said oxidation causing a change in one or more properties of said catalyst member dependent on the calorific value of the gas, and means for generating an output representative of said calorific value, said calorific value determining means further including means for receiving an output from the gas meter and calculating energy usage. In one preferred form, the calorific value determining means includes a reaction chamber to which the gas sample and air are introduced and are contacted with the catalyst member. The sampling means preferably includes means for periodically or constantly withdrawing a gas sample from gas passing through the meter. The catalyst material serves to catalyse oxidation of the combustible gas in the reaction chamber. Preferred catalyst materials include noble metals such as platinum, optionally doped with other materials, and preferably formed by thin or thick film deposition with a thickness of from 5000A to lOμm. Properties of the catalyst member which vary responsive to calorific value and are detected preferably include temperature and/or electrical resistance. BRIEF DESCRIPTION OF THE DRAWINGS Further preferred embodiments are now described with reference to the accompanying drawings, in which: Fig. 1 is a schematic of a preferred gas meter arrangement; Fig. 2 shows the reaction chamber; and Fig. 3 is a schematic showing a preferred calorific value sensing circuit. DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 shows a gas meter/regulator unit 10 having a gas inlet 12 for connection to a high, variable pressure gas supply, typically at 40-600 kPa, and an outlet 13 for connection to the gas plumbing of the premises for which the meter/regulator unit is installed. Within the unit 10, the gas flow path is divided into a high, variable pressure region between the inlet 10 and a regulator 14, and a low pressure region downstream of the regulator. The regulator acts to reduce the high gas supply pressure to a lower, substantially constant pressure at which the gas is supplied to the premises, typically in the range of 0.5-3.5 kPa. The regulator 14 may be mechanically operated, such as a conventional spring-biased valve, but preferably is electronically controlled by the processor/controller 16. Located upstream of the regulator 14 in the high pressure region of the gas path is an electronic metering apparatus 18, such as the type consisting of acoustic transducers situated at upstream and downstream ends of a gas flow measurement tube. The transducers are controlled by the processor 16 to transmit and receive acoustic (e.g. ultrasonic) signals through the tube to determine the gas flow velocity through the tube and send outputs to the processor 16. The gas flow velocity is calculated from variations in the time taken for the signal to pass along the tube. Further details of this preferred acoustic metering apparatus are described in Australian Patent No. 682498, the contents of which are incorporated herein by reference. Pressure sensors (not shown) may measure the gas pressure in the high pressure region and generate an output to the processor 16. The pressure sensors may be situated either side of the metering means if it is anticipated that there will be significant pressure drop across the metering means. The unit 10 further includes a gas calorific value detector 24, which periodically (for example hourly) or continuously withdraws a sample of the gas from the gas flow path to determine 3 its calorific value. Preferably, the gas is withdrawn downstream of the regulator so that the gas pressure is substantially constant. Details of the calorific value measurement will be described below. The calorific value detector generates an output signal to the processor 16. The processor 16 receives the outputs from the calorific value detector, the metering apparatus 18 and, optionally, from other sensors such as a gas temperature sensor (not shown) and from this information calculates the total energy value of the gas passing through the unit and into the premises. A cumulative energy reading is communicated to a display 20 on the unit. The processor 16 may also be provided with an external communications link 22 allowing remote reading and control of the meter/regulator unit. For example, if an electronically controlled regulator is used, the unit may have facility for the gas supply authority to send a signal causing the processor 16 to close the regulator valve 14, shutting off the gas supply to the premises. With reference to Fig. 2, the calorific value detector 24 includes a gas sampling passage 26 communicating with the gas flow path through the meter and leading to a reaction chamber 28 having an air intake 30 and a gas outlet baffle 32 vented to atmosphere. If required the gas outlet 32 may be vented so as to return the gas in the reaction chamber 28 to the gas flow path. Within the reaction chamber is a substrate 34 bearing catalyst material which oxidises the sampled gas, producing a change in properties which can be measured and correlated to the gas calorific value by the processor. Details of a preferred measuring arrangement are discussed below with reference to Fig. 3. Suitable catalyst materials include stannous oxide and noble metals such as platinum, optionally containing dopants such as palladium, which oxidise the gas and produce a measurable change in electrical resistance and or temperature of the catalyst member which varies dependent on the heat energy released by that oxidation reaction. In one form, the catalyst member may be a ceramic (e.g. alumina) bead to which is applied a thin, high electrical resistance film of catalyst material about 5000- 4 15000A thick, by known deposition techniques such as vapour deposition, sputtering or spray pyrolysis. Alternatively, a thick film may be applied to the bead by dip coating. If a flat substrate is to be used a resistance film of about 5 to lOμm can be applied in paste form by screen printing and baking at 625°C on a flat substrate. With reference to Fig. 3, a modified wheatstone bridge circuit comprises a catalyst thin film resistor 36, a non-catalyst resistor 37, and a pair of known- value discrete resistors 38. A voltage is applied to opposite ends of the bridge circuit and the voltage across the bridge is measured. The wheatstone bridge circuit is preferably powered only when the gas/air mixture is present in the reaction chamber. Only the catalyst resistor 36 need be within the reaction chamber 28. However, the reference resistor 37 is preferably also disposed in the reaction chamber 28 in order to maintain the operating environments of the resistors 36 and 37 as close as possible. With the resistors 36 and 37 at operating temperature (approximately 400°C) and no oxidation occurring at the catalyst resistor 36, the wheatstone bridge circuit is balanced with zero voltage across the bridge. When the catalyst resistor 36 is contacted with the gas/air sample in the reaction chamber 28 an oxidation reaction occurs. This reaction raises the surface temperature of the catalyst resistor 36 to about 500° depending on the calorific value of the gas. The change in temperature of the resistor creates a corresponding change in the resistance value which results in a measurable change in voltage across the wheatstone bridge. The measured voltage is communicated to the processor 16 which calibrates the voltage with known gas calorific value measurements. In a further embodiment, the thin film resistor in the modified wheatstone bridge of Fig. 3 may be substituted by thick film spirals, about 10 microns thick, of platinum (i.e. the catalyst resistors) and ruthenium oxide (i.e. non-catalyst). The resistance change in the platinum resistor is sensed by the measured voltage and compared to the calibration algorithm stored in the processor in like manner to the Fig. 3 arrangement described above. While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims: CLAIMS 1. A gas meter for a combustible gas having metering means for measuring gas flow and generating an output, means for determining calorific value of the gas including sampling means for taking a sample of the gas, an air intake, means for contacting the gas sample with air, at least one catalyst member including a catalyst material causing oxidation of at least a part of the gas sample, said oxidation causing a change in one or more properties of said catalyst member dependent on the calorific value of the gas, and means for generating an output representative of said calorific value, said gas meter further including means for receiving outputs from the metering means and calorific value determining means and calculating energy usage. 2. A gas meter according to claim 1 wherein said one or more catalyst member properties includes the electrical resistance of the catalyst member. 3. A gas meter according to claim 2 wherein the calorific value determining means further includes means for measuring the electrical resistance of the catalyst member, said output of said calorific value determining means being representative of said resistance. 4. A gas meter according to claim 3 wherein the means for measuring electrical resistance includes a wheatstone bridge circuit, said catalyst member forming a resistor of said circuit, the voltage across said bridge circuit being representative of the resistance of said catalyst member. 5. A gas meter according to claim 2 wherein said catalyst member includes a film of catalyst material substantially 5000-15000A thick deposited on a ceramic substrate. 6. A gas meter according to claim 5 wherein said ceramic is alumina. 7. A gas meter according to claim 1 wherein the catalyst material is one of platinum or doped platinum. 7 8. A gas meter according to claim 7 wherein the catalyst material is platinum doped with palladium. 9. A gas meter according to claim 1 wherein said means for determining calorific value of the gas includes a reaction chamber in communication with a gas flow path through the meter and means for withdrawing gas into said reaction chamber from said gas flow path. 10. A gas meter according to claim 9 wherein the gas is drawn into the reaction chamber from a substantially constant low pressure region of the gas flow path. 11. A calorific value determining means for connection to a gas meter for combustible gas, including sampling means for taking a sample of the gas, an air intake, means for contacting the gas sample with air, at least one catalyst member including a catalyst material causing oxidation of at least a part of the gas sample, said oxidation causing a change in one or more properties of said catalyst member dependent on the calorific value of the gas, and means for generating an output representative of said calorific value, said calorific value determining means further including means for receiving an output from the gas meter and calculating energy usage. 12. A gas meter according to claim 11 wherein said one or more catalyst member properties includes the electrical resistance of the catalyst member. 13. A gas meter according to claim 12 wherein the calorific value determining means further includes means for measuring the electrical resistance of the catalyst member, said output of said calorific value determining means being representative of said resistance. 14. A gas meter according to claim 13 wherein the means for measuring electrical resistance includes a wheatstone bridge circuit, said catalyst member forming a resistor of said circuit, the voltage across said bridge circuit being representative of the resistance of said catalyst member 8 15. A gas meter according to claim 11 wherein said means for determining calorific value of the gas includes a reaction chamber in communication with a gas flow path through the meter and means for withdrawing gas into said reaction chamber from said gas flow path. 16. A gas meter according to claim 15 wherein the gas is drawn into the reaction chamber from a substantially constant low pressure region of the gas flow path.

Inventors: TORPY KEITH MARIO, TRIMM DAVID LAWRENCE, TORPY, KEITH, MARIO, TRIMM, DAVID, LAWRENCE
Applicants: EMAIL LTD, TORPY KEITH MARIO, TRIMM DAVID LAWRENCE, EMAIL LIMITED, TORPY, KEITH, MARIO, TRIMM, DAVID, LAWRENCE
Application year: 1999
Main classification: G01F1
IPC codes: G01N27, G01N33