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An apparatus for gauging the level of a liquid, above which a gas having a dielectric constant within a predetermined dielectric constant range exists, comprises a transmitter for transmitting a microwave signal in a propagation mode in a tube through the gas towards the liquid surface; a receiver f

An apparatus for gauging the level of a liquid, above which a gas having a dielectric constant within a predetermined dielectric constant range exists, comprises a transmitter for transmitting a microwave signal in a propagation mode in a tube through the gas towards the liquid surface; a receiver for receiving the microwave signal reflected against the liquid surface and propagating back through the tube; and a processing device for calculating from the propagation time of the transmitted and reflected microwave signal the level of the liquid. In order to essentially avoid the influence on the calculated level by the dielectric constant of the gas above the liquid, the transmitter is adapted to transmit the microwave signal in a frequency band, at which the group velocity of the microwave signal in the propagation mode in the tube is, within the predetermined dielectric constant range, essentially independent of the dielectric constant.

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1. An apparatus for high accuracy gauging of the level of a liquid in a container, above which level there exists a gas having a dielectric constant within a predetermined dielectric constant range, comprising:a transmitter for transmitting a microwave signal in a first mode of propagation in a tube

1. An apparatus for high accuracy gauging of the level of a liquid in a container, above which level there exists a gas having a dielectric constant within a predetermined dielectric constant range, comprising:a transmitter for transmitting a microwave signal in a first mode of propagation in a tube through said gas towards the surface of said liquid, wherein the walls of said tube is provided with a number of holes so that the liquid in said container can flow laterally in and out of said tube to maintain a unitary level of said liquid inside and outside said tube; a receiver for receiving the microwave signal reflected against the surface of said liquid and propagating back through said tube; and a signal processing device for calculating from the propagation time of the transmitted and reflected microwave signal the level of said liquid in said container, wherein said transmitter is adapted to transmit said microwave signal in a frequency band, at which the group velocity of a microwave signal in said first mode of propagation in said tube is, within said predetermined dielectric constant range, essentially independent of the dielectric constant. 2. The apparatus of claim 1 wherein said transmitter is adapted to transmit said microwave signal in a frequency band, which includes a frequency deviating from an optimum frequency fopt with less than 7%, wherein the optimum frequency is given by where fc0 is the cut-off frequency of said first mode of propagation in said tube, and ε is the center dielectric constant of said dielectric constant range.3. The apparatus of claim 2 wherein said frequency deviates from said optimum frequency fopt with less than 5%.4. The apparatus of claim 2 wherein said frequency deviates from said optimum frequency fopt with less than 3%.5. The apparatus of claim 2 wherein said frequency deviates from said optimum frequency fopt with less than 2%.6. The apparatus of claim 2 wherein said frequency deviates from said optimum frequency fopt with less than 1%.7. The apparatus of claim 2 wherein said frequency is identical with said optimum frequency fopt.8. The apparatus of claim 1 wherein said frequency band has a center frequency, which deviates from said optimum frequency fopt with less than 7%.9. The apparatus of claim 8 wherein said center frequency deviates from said optimum frequency fopt with less than 5%.10. The apparatus of claim 8 wherein said center frequency deviates from said optimum frequency fopt with less than 3%.11. The apparatus of claim 8 wherein said center frequency deviates from said optimum frequency fopt with less than 2%.12. The apparatus of claim 8 wherein said center frequency deviates from said optimum frequency fopt with less than 1%.13. The apparatus of claim 1 wherein said predetermined dielectric constant range is about 1-1.03.14. The apparatus of claim 1 wherein said liquid is comprised of a condensed gas and said gas is comprised of said condensed gas in gaseous phase, which condensed gas being stored in said container at overpressure.15. The apparatus of claim 1 wherein the tube comprises a waveguide.16. The apparatus of claim 1 wherein said tube has a rectangular cross section.17. The apparatus of claim 1 wherein said tube has a circular cross section.18. The apparatus of claim 17 wherein the first mode of propagation, in which said transmitter transmits said microwave signal in said waveguide, is any of H11, H01, and H02.19. The apparatus of claim 18 wherein said tube has a diameter of about 100 mm and said transmitter is adapted to transmit said microwave signal in any of H11 mode at about 2.5 GHz, H01 mode at about 5 GHz, and H02 mode at about 10 GHz.20. The apparatus of claim 18 wherein said tube has a diameter of about 50 mm and said transmitter is adapted to transmit said microwave signal in H01 mode at about 10 GHz.21. The apparatus of claim 1 whereinsaid transmitter transmits said microwave signal in a second mode of propagation in said tube through said gas towards the surface of said liquid; and said receiver receives said microwave signal reflected against the surface of said liquid and propagating back through said tube in said second mode of propagation; and to distinguish portions of said microwave signal received in different ones of said first and second modes of propagation. 22. The apparatus of claim 21 wherein said tube has a circular cross section and said second mode of propagation is any of E01 or H11.23. The apparatus of claim 21 wherein said receiver distinguishes portions of said microwave signal received in different ones of said first and second modes of propagation based on the portions different arrival times at said receiver.24. The apparatus of claim 23 wherein said transmitter transmits said microwave signal in said first and second modes of propagation sequentially.25. The apparatus of claim 21 wherein said transmitter transmits said microwave signal in said first and second modes of propagation spectrally separated.26. The apparatus of claim 21 wherein said signal processing device calculates the dielectric constant of said gas above the level of said liquid based on said received and distinguished portions of said microwave signal received in different ones of said first and second modes of propagation.27. The apparatus of claim 21 wherein said signal processing device calculates a cross section dimension of said tube based on said received and distinguished portions of said microwave signal received in different ones of said first and second modes of propagation.28. The apparatus of claim 27 wherein said tube has a circular cross section and said cross section dimension calculated is the average diameter of said tube along the distance said microwave signal propagates before being reflected against the surface of said liquid.29. The apparatus of claim 21 whereinsaid signal processing device calculates from the propagation time of the transmitted and reflected microwave signal in said second mode of propagation the level of said liquid in said container; and said signal processing device estimates one or more properties of the tube or of the environment in said container based on said calculated levels of said liquid in said container. 30. The apparatus of claim 21 whereinsaid signal processing device calculates attenuations of said distinguished portions of said microwave signal, which are received in different ones of said first and second modes of propagation; and said signal processing device estimates one or more properties of the tube or of the environment in said container based on said calculated attenuations of said distinguished portions of said microwave signal. 31. The apparatus of claim 29 wherein said one or more properties of the tube or of the environment in said container comprises a cross-sectional dimension of said tube, a variation in a cross-sectional dimension along the length of said tube, a concentricity measure of said tube, presence of impurities, particularly solid or liquid hydrocarbons, at the inner walls of said tube, or presence of mist in said gas.32. The apparatus of claim 29 wherein a reflecting reactance is arranged in said tube to give a substantially stronger reflex of the microwave signal in one of the propagation modes than in the other one of the propagation modes.33. The apparatus of claim 1 wherein said microwave signal is a frequency modulated continuous wave signal.34. The apparatus of claim 1 wherein said microwave signal is a pulsed radar signal.35. The apparatus of claim 1 wherein said transmitter is adapted to transmit said microwave signal in a frequency band, which is adjustable.36. A method for high accuracy gauging of the level of a liquid in a container, above which level there exists a gas having a dielectric constant within a predetermined dielectric constant range, and in which container there is arranged a tube provided with a number of lateral holes so that the liquid in said container can flow laterally in and out of said tube to maintain a unitary level of the liquid inside and outside said tube, comprising the steps of:determining a first quantity representative of an inner dimension of said tube; determining based on said first quantity a frequency band, at which the group velocity of a microwave signal in a first mode of propagation in said tube is, within said predetermined dielectric constant range, essentially independent of the dielectric constant; tuning a transmitter to opera:e in said frequency band; transmitting in said frequency band a microwave signal in said first mode of propagation in said tube through said gas towards the surface of said liquid; receiving in said frequency band the microwave signal reflected against the surface of said liquid and propagating back through said tube; determining a second quantity representative of a propagation time of the transmitted and reflected microwave signal; and calculating based on said first and second quantities the level of said liquid in said container. 37. The method of claim 36 wherein said frequency band includes a frequency deviating from an optimum frequency fopt with less than 7%, wherein the optimum frequency is calculated as where fc0 is the cut-off frequency of said first mode of propagation in said tube, and ε is the center dielectric constant of said dielectric constant range.38. The method of claim 37 wherein said frequency deviates from said optimum frequency fopt with less than 7%.39. The method of claim 37 wherein said frequency deviates from said optimum frequency fopt with less than 5%.40. The method of claim 37 wherein said frequency deviates from said optimum frequency fopt with less than 3%.41. The method of claim 37 wherein said frequency deviates from said optimum frequency fopt with less than 2%.42. The method of claim 37 wherein said frequency deviates from said optimum frequency fopt with less than 1%.43. The method of claim 37 wherein said frequency is identical with said optimum frequency fopt.44. The method of claim 36 wherein said frequency band has a center frequency, which deviates from said optimum frequency fopt with less than 7%.45. The method of claim 44 wherein said center frequency deviates from said optimum frequency fopt with less than 5%.46. The method of claim 44 wherein said center frequency deviates from said optimum frequency fopt with less than 3%.47. The method of claim 44 wherein said center frequency deviates from said optimum frequency fopt with less than 2%.48. The method of claim 44 wherein said center frequency deviates from said optimum frequency fopt with less than 1%.49. The method of claim 34 wherein said predetermined dielectric constant range is about 1-1.03.50. The method of claim 36 wherein said liquid is comprised of a condensed gas and said gas is comprised of said condensed gas in gaseous phase, which condensed gas being stored in said container at overpressure.51. The method of claim 36 wherein the first mode of propagation, in which said transmitter transmits said microwave signal in said waveguide, is any of H11, H01, and H02.52. The method of claim 36 whereinsaid microwave signal is transmitted in a second mode of propagation in said waveguide through said gas towards the surface of said liquid; said microwave signal reflected against the surface of said liquid and propagating back through said waveguide in said second mode of propagation is received; and portions of said microwave signal received in different ones of said first and second modes of propagation are distinguished. 53. An apparatus for gauging the level of a liquid in a container, above which level a gas exists, comprising:a transmitter for transmitting a microwave signal in a waveguide through said gas towards the surface of said liquid; a receiver for receiving the microwave signal reflected against the surface of said liquid and propagating back through said waveguide; and a signal processing device for calculating from the propagation time of the transmitted and reflected microwave signal the level of said liquid in said container, wherein said transmitter transmits said microwave signal in at least two different modes of propagation and within a frequency band, which admits propagation of said microwave signal in said at least two different modes in said waveguide; and said receiver receives said microwave signal in said at least two different modes of propagation and to distinguish portions of said microwave signal received in different ones of said at least two different modes of propagation. 54. The apparatus of claim 53 whereinsaid signal processing device calculates from the propagation time of the transmitted and reflected microwave signal in each mode of propagation the level of said liquid in said container; and said signal processing device estimates one or more properties of the waveguide or of the environment in said container based on said calculated levels of said liquid in said container, and to use said estimate of said one or more properties to calculate a corrected level of said liquid in said container. 55. The apparatus of claim 53 whereinsaid signal processing device calculates attenuations of said distinguished portions of said microwave signal, which are received in different ones of said at least two different modes of propagation; and said signal processing device estimates one or more properties of the waveguide or of the environment in said container based on said calculated attenuations of said distinguished portions of said microwave signal, and to use said estimate of said one or more properties to calculate a corrected level of said liquid in said container. 56. The apparatus of claim 54 wherein said one or more properties of the waveguide or of the environment in said container comprises a cross-sectional dimension of said waveguide, a variation in a cross-sectional dimension along the length of said waveguide, a concentricity measure of said waveguide, presence of impurities, particularly solid or liquid hydrocarbons, at the inner wails of said waveguide, or presence of mist in said gas.57. The apparatus of claim 53 wherein a reflecting reactance is arranged in said waveguide to give a substantially stronger reflex of the microwave signal in one of said at least two different modes of propagation than in an other one of said at least two different modes of propagation.58. A method for gauging the level of a liquid in a container, above which level a gas exists, comprising the steps of:transmitting a microwave signal in a waveguide through said gas towards the surface of said liquid; receiving the microwave signal reflected against the surface of said liquid and propagating back through said waveguide; and calculating from the propagation time of the transmitted and reflected microwave signal the level of said liquid in said container, wherein: said microwave signal is transmitted in at least two different modes of propagation and within a frequency band, which admits propagation of said microwave signal in said at least two different modes in said waveguide; said microwave signal is received in said at least two different modes of propagation; and portions of said microwave signal received in different ones of said at least two different modes of propagation are distinguished. 59. An apparatus for gauging the level of a liquid in a container, above which level there exists a gas or gas mixture having a dielectric constant within a dielectric constant range, comprising:a transmitter for transmitting a microwave signal in a mode of propagation in a waveguide through said gas towards the surface of said liquid; a receiver for receiving the microwave signal reflected against the surface of said liquid and propagating back through said waveguide; and a signal processing device for calculating from the propagation time of the transmitted and reflected microwave signal the level of said liquid in said container, wherein said transmitter transmits said microwave signal in a frequency band, whose center frequency is essentially equal to, or close to, an optimum frequency fopt wherein the optimum frequency is calculated as where fc0 is the cut-off frequency of said mode of propagation in said waveguide, and ε is the center dielectric constant of said dielectric constant range.60. The apparatus of claim 59 wherein said dielectric constant range is about 1-1.03.61. The apparatus of claim 60 wherein said liquid is comprised of a condensed gas and said gas is comprised of said condensed gas in gaseous phase, which condensed gas being stored in said container at overpressure.62. An apparatus for gauging the level of a liquid in a container, above which level there exists a gas or gas mixture, comprising:a transmitter for transmitting a microwave signal in a mode of propagation in a waveguide through said gas towards the surface of said liquid; a receiver for receiving the microwave signal reflected against the surface of said liquid and propagating back through said waveguide; and a signal processing device for calculating from the propagation time of the transmitted and reflected microwave signal the level of said liquid in said container, wherein said transmitter transmits said microwave signal at a frequency where the group velocity of said microwave signal in said mode of propagation as a function of the dielectric constant of said gas has a local maximum. 63. The apparatus of claim 62 wherein said liquid is comprised of a condensed gas and said gas is comprised of said condensed gas in gaseous phase, which condensed gas being stored in said container at overpressure.64. An apparatus for gauging the level of a liquid in a container, above which level there exists a gas or gas mixture, comprising:a transmitter for transmitting a microwave signal in a waveguide through said gas towards the surface of said liquid; a receiver for receiving the microwave signal reflected against the surface of said liquid and propagating back through said waveguide; and a signal processing device for calculating from the propagation time of the transmitted and reflected microwave signal the level of said liquid in said container, wherein said transmitter transmits said microwave signal at a frequency where the microwave signal can propagate in said gas in said waveguide only in H11 and E01 modes. 65. The apparatus of claim 64 wherein said liquid is comprised of a condensed gas and said gas is comprised of said condensed gas in gaseous phase, which condensed gas being stored in said container at overpressure.66. A method for gauging the level of a liquid in a container, above which level there exists a gas or gas mixture, comprising the steps of:transmitting a microwave signal in a mode of propagation in a waveguide through said gas towards the surface of said liquid; receiving the microwave signal reflected against the surface of said liquid and propagating back through said waveguide; and calculating from the propagation time of the transmitted and reflected microwave signal the level of said liquid in said container, wherein the frequency and mode of propagation of said microwave signal, and the cross sectional dimension of said waveguide are selected with respect to each other to obtain a group velocity of the microwave signal in the waveguide, which is fairly constant over an predetermined range of dielectric constant values. 67. The apparatus of claim 66 wherein said predetermined range of dielectric constant values is about 1-1.03.68. The apparatus of claim 66 wherein said predetermined range of dielectric constant values is a subset of about 1-1.03.69. The apparatus of claim 66 wherein said liquid is comprised of a condensed gas and said gas is comprised of said condensed gas in gaseous phase, which condensed gas being stored in said container at overpressure.70. An apparatus for gauging the level of a liquid in a container, above which level there exists a gas or gas mixture having a dielectric constant within a dielectric constant range, comprising:a transmitter for transmitting a microwave signal in a first mode of propagation in a waveguide through said gas towards the surface of said liquid; a receiver for receiving the microwave signal reflected against the surface of said liquid and propagating back through said waveguide; and a signal processing device for calculating from the propagation time of the transmitted and reflected microwave signal the level of said liquid in said container, wherein said transmitter transmits said microwave signal in a frequency band, which includes a frequency deviating from an optimum frequency fopt with less than 7%, wherein the optimum frequency is calculated as where fc0 is the cut-off frequency of said first mode of propagation in said waveguide, and ε is the center dielectric constant of said dielectric constant range.71. A method for gauging the level of a liquid in a container, above which level there exists a gas or gas mixture having a dielectric constant within a dielectric constant range, comprising the steps of:transmitting a microwave signal in a first mode of propagation in a waveguide through said gas towards the surface of said liquid; receiving the microwave signal reflected against the surface of said liquid and propagating back through said waveguide; and calculating from the propagation time of the transmitted and reflected microwave signal the level of said liquid in said container, wherein: said microwave signal is transmitted in a frequency band, which includes a frequency deviating from an optimum frequency fopt with less than 7%, wherein the optimum frequency is calculated as where fc0 is the cut-off frequency of said first mode of propagation in said waveguide, and ε is the center dielectric constant of said dielectric constant range.