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A method and apparatus for measuring the flow rate of a single phase fluid, particularly a gas, through a conduit. A pulse of tracer fluid is discharged or injected into a first fluid flowing through a conduit, the concentration of the tracer fluid is measured as a function of time at a downstream f

A method and apparatus for measuring the flow rate of a single phase fluid, particularly a gas, through a conduit. A pulse of tracer fluid is discharged or injected into a first fluid flowing through a conduit, the concentration of the tracer fluid is measured as a function of time at a downstream from where it is injected, and the flow rate of the first fluid is determined based on the concentration measurements. The concentration of tracer fluid is preferably sampled substantially continuously from when the tracer gas first passes the measuring point until substantially all of the tracer fluid has passed the measuring point and the sampled concentrations integrated. The concentration of tracer fluid is preferably determined by measuring the thermal conductivity of the mixture of the first fluid and tracer fluid which is dependent upon the concentration of the tracer fluid.

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The invention claimed is: 1. A method of measuring a flow rate of a first single phase fluid flowing through a conduit, the method comprising: discharging a pulse of tracer fluid into a first fluid flowing through a conduit; determining a concentration of the tracer fluid at a suitable point downst

The invention claimed is: 1. A method of measuring a flow rate of a first single phase fluid flowing through a conduit, the method comprising: discharging a pulse of tracer fluid into a first fluid flowing through a conduit; determining a concentration of the tracer fluid at a suitable point downstream from the discharging; and determining a flow rate of the first fluid through the conduit dependent upon the molar quantity of the tracer fluid discharged and the measured concentration of the tracer fluid at the suitable point downstream, wherein the concentration of the tracer fluid is determined by measuring a change in thermal conductivity of a mixture of first fluid and tracer fluid compared with a thermal conductivity of the first fluid alone, which is a function of the concentration of the tracer fluid. 2. A method according to claim 1, wherein the concentration of the tracer fluid is determined from the measured thermal conductivity using a processing means using suitable algorithms. 3. A method according to claim 1, wherein the concentration of the tracer fluid is determined from the measured thermal conductivity using a look-up table. 4. A method according to claim 1, wherein the concentration of the tracer fluid is measured a plurality of times as the tracer fluid passes the measuring point. 5. A method according to claim 4, wherein the concentration of the tracer fluid is measured or sampled from when the tracer gas first passes the measuring point until substantially all of the tracer fluid has passed the measuring point. 6. A method according to claim 4, wherein the measured or sampled concentrations of tracer fluid are summed. 7. A method according to claim 4, wherein the measured or sampled concentrations of tracer fluid are integrated with respect to time. 8. A method according to claim 1, wherein the concentration of the tracer fluid is sampled substantially continuously as it passes the measuring point. 9. A method according to claim 8, wherein the concentration of the tracer fluid is measured or sampled from when the tracer gas first passes the measuring point until substantially all of the tracer fluid has passed the measuring point. 10. A method according to claim 8, wherein the measured or sampled concentrations of tracer fluid are summed. 11. A method according to any of claim 8, wherein the measured or sampled concentrations of tracer fluid are integrated with respect to time. 12. A method according to claim 1, wherein the measured or sampled concentrations are adjusted to take account of background tracer fluid concentrations. 13. A method to according to claim 1, wherein the flow rate of the first fluid through the conduit is determined dependent upon the molar quantity of tracer fluid injected into the first fluid. 14. A method to according to claim 1, wherein the flow rate of the first fluid through the conduit is determined dependent upon the concentration of tracer fluid injected into the first fluid. 15. A method to according to claim 1, wherein the tracer fluid includes helium. 16. A method to according to claim 1, wherein the first fluid is a gas. 17. A method according to claim 15, wherein the first fluid is natural gas. 18. A method of measuring the flow rate of a first fluid flowing through a conduit according to claim 1, wherein the flow rate is determined independent of information about the conduit geometry. 19. An apparatus for measuring a flow rate of a first single phase fluid flowing through a conduit, the apparatus comprising a device for discharging a pulse of tracer fluid into a first fluid flowing in a conduit; determining means for determining a concentration of the tracer fluid at a point downstream from where the discharging device is arranged to discharge the tracer fluid; and control means for determining a flow rate of a first fluid through a conduit dependent upon the known or determinable molar quantity of the tracer fluid discharged and the concentration of the tracer fluid determined by the determining means, wherein the determining means includes a measuring means for measuring a change in thermal conductivity of the mixture of first fluid and tracer fluid compared with a thermal conductivity of the first fluid alone which is dependent upon the concentration of the tracer gas and the determining means includes conversion means for converting the measured change in thermal conductivity into a value corresponding to the tracer fluid concentration. 20. An apparatus according to claim 19, wherein the conversion means converts the measured change in thermal conductivity into a corresponding tracer fluid concentration using suitable algorithms. 21. An apparatus according to claim 20, wherein the measuring means includes a thermal conductivity detector for detecting the thermal conductivity of the passing fluid which is a function of its concentration of tracer fluid. 22. An apparatus according to claim 19, wherein the conversion means converts the measured change in thermal conductivity into a corresponding tracer fluid concentration using a look-up table. 23. Ah apparatus according to claim 19, wherein the tracer fluid is injected into the first fluid. 24. An apparatus according to claim 19, wherein the control means is arranged to receive a plurality of concentration measurements taken as the tracer fluid passes the measuring means. 25. An apparatus according to claim 24, wherein the control means is arranged to receive substantially continuous concentration samples as the tracer fluid passes the measuring means. 26. An apparatus according to claim 25, wherein the control means is arranged to receive concentration measurements or samples from at least when the tracer fluid first passes the measuring point until substantially all of the tracer fluid has passed the measuring point. 27. An apparatus according to claim 25, wherein the control means sums the measured or sampled concentrations of tracer fluid. 28. An apparatus according to claim 25, wherein the control means integrates the measured or sampled concentrations of tracer fluid with respect to time. 29. An apparatus according to claim 25, wherein the control means is arranged to take account of background tracer fluid concentrations in its determination of the flow rate of a first fluid. 30. An apparatus according to claim 24, wherein the control means is arranged to receive concentration measurements or samples from at least when the tracer fluid first passes the measuring point until substantially all of the tracer fluid has passed the measuring point. 31. An apparatus according to claim 24, wherein the control means sums the measured or sampled concentrations of tracer fluid. 32. An apparatus according to claim 24, wherein the control means integrates the measured or sampled concentrations of tracer fluid with respect to time. 33. An apparatus according to claim 24, wherein the control means is arranged to take account of background tracer fluid concentrations in its determination of the flow rate of a first fluid. 34. An apparatus according to claim 24, wherein the control means is arranged to take account of the molar quantity of tracer fluid injected into the first fluid by the discharging device in its determination of the flow rate of a first fluid. 35. An apparatus according to claim 24, wherein the control means is arranged to take account of the concentration of the tracer fluid discharged into the first fluid by the discharging device in its determination of the flow rate of a first fluid. 36. An apparatus according to claim 19, wherein the control means is arranged to take account of the molar quantity of tracer fluid injected into the first fluid by the discharging device in its determination of the flow rate of a first fluid. 37. An apparatus according to claim 19, wherein the control means is arranged to take account of the concentration of the tracer fluid discharged into the first fluid by the discharging device in its determination of the flow rate of a first fluid. 38. An apparatus according to claim 19, wherein the measuring means includes a thermal conductivity detector for detecting the thermal conductivity of the passing fluid which is a function of its concentration of tracer fluid. 39. An apparatus according to claim 19, wherein the flow rate of a fluid through a conduit is determined independent of information about the geometry of the conduit through which the first fluid flows. 40. An apparatus for measuring a flow rate of a first single phase fluid flowing through a conduit, comprising a discharge device configured to discharge a pulse of tracer fluid into a first fluid flowing in a conduit; a concentration determination unit configured to determine a concentration of the tracer fluid at a point downstream from where the discharge device is arranged to discharge the tracer fluid; and a control unit configured to determine a flow rate of a first fluid through the conduit dependent upon the known or determinable molar quantity of the tracer fluid discharged and the concentration of the tracer fluid determined by the concentration determination unit, wherein the concentration determination unit includes a thermal conductivity measurement device configured to measure a change in thermal conductivity of a mixture of the first fluid and tracer fluid compared with a thermal conductivity of the first fluid alone which is dependent upon the concentration of the tracer gas, and a converter configured to convert the measured change in thermal conductivity into a value corresponding to the tracer fluid concentration.