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A microwave level gauge for measuring a level of a process material in a tank includes a ceramic seal and a microwave conductor. The ceramic seal is disposed adjacent to an opening in the tank and adapted to isolate circuitry from the process material. The microwave conductor is electrically coupled

A microwave level gauge for measuring a level of a process material in a tank includes a ceramic seal and a microwave conductor. The ceramic seal is disposed adjacent to an opening in the tank and adapted to isolate circuitry from the process material. The microwave conductor is electrically coupled to the circuitry and extends through the hermetic seal and into the process material in the tank. The ceramic seal and an isolating adapter cooperate to isolate the microwave conductor from the process fluid and from external forces.

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What is claimed is: 1. A microwave level gauge for measuring a level of a process fluid in a tank with a tank atmosphere, the microwave level gauge comprising: a hollow adapter body extending from a lower portion that is open to the tank atmosphere to a transmitter mount outside the tank; a connect

What is claimed is: 1. A microwave level gauge for measuring a level of a process fluid in a tank with a tank atmosphere, the microwave level gauge comprising: a hollow adapter body extending from a lower portion that is open to the tank atmosphere to a transmitter mount outside the tank; a connector rod extending from the lower portion and connectable to an antenna in the tank; conductors coupled between the connector rod and the transmitter mount such that microwaves are couplable from the antenna to the transmitter mount; and a primary seal in the lower portion that includes a lower support band brazed to an outer surface of a ceramic seal body, and an upper support band brazed to an outer surface of the ceramic seal body and sealingly joined to the adapter body such that the primary seal contacts the tank atmosphere and seals the conductors from the tank atmosphere. 2. The microwave level gauge of claim 1 wherein the conductors comprise: a threaded rod fastened to the connector rod and extending up through a central passageway in the ceramic seal body to an upper threaded end; and a compression washer mounted to the upper threaded end and providing axial compression to the ceramic seal body. 3. The microwave level gauge of claim 2 further comprising a secondary seal between the cavity and the transmitter mount, the secondary seal including a venting seal between the cavity and an atmosphere outside the tank, the venting seal venting the cavity to the atmosphere under fault conditions. 4. The microwave level gauge of claim 3 wherein the conductors comprise: a spring loaded pin contacting the threaded rod and extending through the secondary seal. 5. The microwave level gauge of claim 1 wherein braze joints between the upper and lower support bands and the ceramic seal body have a shrink fit that provides radial compression to the ceramic seal body. 6. The microwave level gauge of claim 1 wherein the adapter body comprises: a lower adapter body having a threaded upper end and an upper weld surface spaced apart from the threaded upper end; an upper adapter body having a threaded lower end and a lower weld surface spaced apart from the threaded lower end; and wherein the microwave level gauge further comprises: a metal coupler threaded into the threaded upper and lower ends such that the upper and lower adapter bodies are mechanically joined; and a weld between the upper and lower weld surfaces such that the upper and lower weld bodies are sealed to one another. 7. The microwave level gauge of claim 6 wherein the lower adapter body includes a transition from a larger internal diameter to a smaller internal diameter so that the transition partially reflects microwaves such that the transition produces a test pulse. 8. The microwave level gauge of claim 1 further comprising a threaded coupler nut that is threaded on the transmitter mount such that a transmitter can be attached to the transmitter mount in an arbitrary rotational position. 9. The microwave level gauge of claim 1 further comprising: a protective sheath around the antenna, the protective sheath being attached to the lower portion. 10. The microwave level gauge of claim 1 further comprising: a flange attached to the lower portion and having a flange surface that is sealable to a tank opening. 11. A microwave level gauge for measuring a level of a process material in a tank with a tank atmosphere, the microwave level gauge comprising: a housing coupled to an opening in the tank; a ceramic seal disposed within the housing and adjacent to the opening in the tank to isolate the housing from the tank atmosphere, the ceramic seal contacting the tank atmosphere; a microwave conductor extending from the housing and through the ceramic seal; and an isolating adapter extending over the microwave conductor and sealed to the ceramic seal, the isolating adapter and the ceramic seal sealing the microwave conductor from the tank atmosphere. 12. The microwave level gauge of claim 11 wherein the isolating adapter is sealed to the ceramic seal by brazing. 13. The microwave level gauge of claim 12 wherein the brazed seal extends an entire circumference of a coupling joint between the ceramic seal and the isolating adapter. 14. The microwave level gauge of claim 11 and further comprising: circuitry disposed outside of the tank and adapted to detect a reflected microwave signal. 15. The microwave level gauge of claim 11 and further comprising: a secondary seal disposed within the housing, the microwave conductor extending through the secondary seal, the secondary seal being generally impedance matched to the ceramic seal. 16. The microwave level gauge of claim 11 wherein the secondary seal is formed from glass. 17. The microwave level gauge of claim 12 wherein the brazed seal comprises a braze band that translates external forces away from the microwave conductor. 18. The microwave level gauge of claim 11 and further comprising: a microwave antenna coupled to the isolating adapter and extending into the process material in the tank. 19. A microwave level gauge assembly for measuring a level of a process material in a tank with a tank atmosphere, the microwave level gauge assembly comprising: a hermetic seal in contact with the tank atmosphere and fixed over a tank opening to isolate transmitter circuitry from the tank atmosphere, the hermetic seal comprising: a ceramic seal body having an opening sized to receive a microwave conductor; an isolating element disposed over the opening and coupled to the ceramic seal body at a coupling joint; and a brazed seal disposed over the coupling joint for fixedly sealing the isolating element to the ceramic seal body; and a microwave conductor extending through the opening and electrically coupled to the transmitter circuitry on one end and to the isolating element on an opposing end, the microwave conductor being sealed from the tank atmosphere by the hermetic seal. 20. The microwave level gauge assembly of claim 19 wherein the brazed seal is formed of a gold/nickel alloy. 21. The microwave level gauge assembly of claim 19 wherein the isolating element comprises: an isolating body coupled to the ceramic seal body via an upper brazed seal; and an antenna adapter coupled to the isolating body by a lower brazed seal. 22. The microwave level gauge assembly of claim 19 wherein the brazed seal extends around an entire circumference of the coupling joint. 23. The microwave level gauge assembly of claim 19 further comprising: a housing coupled to the tank adjacent to the tank opening, the housing defining a chamber that is isolated from the process material by the hermetic seal. 24. The microwave level gauge assembly of claim 23 wherein the housing comprises: a housing adapter body coupled to the tank; a transmitter housing releasably coupled to the housing adapter by a coupler nut. 25. The microwave level gauge assembly of claim 24 further comprising: a secondary seal disposed between the adapter body and the transmitter housing. 26. The microwave level gauge assembly of claim 25 wherein the secondary seal and the hermetic seal are impedance matched. 27. The microwave level gauge assembly of claim 24 further comprising: a gasket disposed between the adapter body and the transmitter housing, the gasket for sealing a joint between the adapter body and the transmitter housing. 28. The microwave level gauge assembly of claim 24 further comprising: an o-ring seal disposed between the adapter body and the transmitter housing. 29. The microwave level gauge of claim 19 wherein the brazed seal directs external forces along a wetted surface of the isolating adapter and the seal body and away from the microwave conductor. 30. A microwave level gauge for measuring a level of a process fluid in a tank having a tank atmosphere, the microwave level gauge comprising: a ceramic seal disposed in a tank opening and contacting the tank atmosphere, and adapted to seal the tank opening; and a microwave conductor extending through the ceramic seal and into the tank, the microwave conductor being sealed to the ceramic seal at a brazed joint such that the ceramic seal seals the microwave conductor from the tank atmosphere. 31. The microwave level gauge of claim 30 wherein the microwave conductor comprises: a thin-walled tube formed from a corrosion resistive material; and a conductive alloy disposed within the thin-walled tube and formed from a material having a low coefficient of expansion with respect to temperature. 32. The microwave level gauge of claim 31 wherein the conductive alloy comprises Kovar짰 or Alloy 52. 33. The microwave level gauge of claim 30 further comprising: a coaxial tube coupled to the tank and disposed around the microwave conductor in the tank to protect the microwave conductor from external forces. 34. The microwave level gauge of claim 33 and further comprising: a support structure coupled to an inside surface of the coaxial tube and adapted to center the microwave conductor within the coaxial tube. 35. A microwave level gauge for measuring a level of a process fluid in a tank having a tank atmosphere, the microwave level gauge comprising: a microwave conductor electrically coupled to process circuitry outside of the tank and extending through a tank opening disposed in a portion of the tank; and a seal disposed in the tank opening and contacting the tank atmosphere, and extending over the microwave conductor to isolate the microwave conductor from the tank atmosphere. 36. The microwave level gauge of claim 35 wherein the seal comprises: a ceramic seal body disposed in the tank opening, the ceramic seal body defining a conductor opening sized to receive the microwave conductor; and an isolating adapter disposed over the conductor opening and sealed to the ceramic seal body, the isolating adapter electrically coupled to the microwave conductor. 37. The microwave level gauge of claim 36 wherein the isolating adapter comprises: an adapter body sealed to the ceramic seal body; and an antenna adapter sealed to the adapter body, the antenna adapter coupled to the microwave conductor. 38. The microwave level gauge of claim 36 further comprising: a corrosion resistant band disposed around a portion of the isolating adapter adjacent to a joint between the isolating adapter and the ceramic seal, the band being brazed to seal the isolating adapter to the ceramic seal. 39. The microwave level gauge of claim 35 wherein the ceramic seal isolates the microwave conductor from external forces.