Microwave sealing for radar level gauges

1. A radar level gauge system for measuring a distance to a surface of a content in a container relatively to a measuring position, which is located above the surface and geometrically defined in relation to said container, said radar level gauge comprising:

a power supply interface for receiving electrical power to said radar level gauge,

a communication interface for presenting externally of said radar level gauge information based on said measured distance;

a transmitter for generating and transmitting an electromagnetic transmitter signal;

means for directing said transmitter signal towards said surface and for receiving a reception pulse reflected back from said surface;

a receiver for receiving said reception pulse;

processing circuitry coupled to the transmitter and the receiver for determining said distance and to said communication interface for providing said information;

a fastening structure adapted to secure said means for directing in said measuring position, said fastening structure arranged to provide a spatial orientation of said means for directing, such that the directing of said transmitter signal is generally vertical, wherein said fastening structure has a first sealing surface adapted to mechanically couple with a second sealing surface of a feed-through structure located in an upper boundary of said container;

a sealing unit adapted to provide a pressure tight seal joint between said fastening structure and said feed-through when subjected to pressure between said first and second sealing surfaces; and

means for blocking electromagnetic energy, adapted to suppress net transfer of electromagnetic energy through said sealing unit from the inside to the outside of said container;

wherein the sealing unit comprises a sealing list or gasket made of a pressure sealing material and a microwave attenuating material, whereby the means for blocking electromagnetic energy is integrated in the sealing unit; and

wherein said sealing unit constitutes the single pressure sealing means provided between said fastening structure and said feed-through structure.

2. The radar level gauge system of claim 1 , wherein the means for blocking electromagnetic energy further comprises at least one quarterwave choke.

3. The radar level gauge system of claim 2 , wherein the quarterwave choke is arranged as a recess in at least one of the first sealing surface of the fastening structure and the second sealing surface of the feed-through structure.

4. The radar level gauge system of claim 2 , wherein the quarterwave choke is provided by a protrusion in at least one of the first sealing surface of the fastening structure and the second sealing surface of the feed-through structure.

5. The radar level gauge system of claim 2 , wherein the quarterwave choke is arranged as an open compartment arranged on top of at least one of the first sealing surface of the fastening structure and the second sealing surface of the feed-through structure.

6. The radar level gauge system of claim 2 , wherein the quarterwave choke is at least partly filled with dielectric material.

7. The radar level gauge system of claim 1 , wherein the microwave attenuating material has a resistivity which is less than 1 ohm-meter.

8. The radar level gauge system of claim 1 , wherein the means for blocking electromagnetic energy provides a microwave attenuation of at least 10 dB.

9. The radar level gauge system of claim 8 , wherein the means for blocking electromagnetic energy provides a microwave attenuation of at least 20 dB.

10. A radar level gauge system for determining a filling level of a filling material in a container comprising:

a transmitter for transmitting measuring signals towards the surface of the filling material;

a receiver for receiving echo signals from said surface;

a fastening structure adapted to secure said transmitter and receiver in a measuring position on a feed-through structure located in an upper boundary of said container;

a sealing unit arranged between said fastening structure and said feed-through, and adapted to provide a pressure seal between the inside and the outside of the container; and

means for blocking electromagnetic energy, adapted to block net transfer of electromagnetic energy between the inside and the outside of the container through said sealing unit;

wherein the sealing unit comprises a sealing list or gasket made of a pressure sealing material and a microwave attenuating material, whereby the means for blocking electromagnetic energy is integrated in the sealing unit; and

wherein said sealing unit constitutes the single pressure sealing means provided between said fastening structure and said feed-through.