Relating to interface detection

1. A level switch configured to initiate a switching function in the event of a change of medium in contact with said switch, said switch including:

a sensor having a connection end and a distal end, and being configured to sense a change of medium in contact with the distal end of said sensor; and

drive and processing electronics connected to the connection end of the sensor and configured to provide drive signals to the connection end of said sensor, to receive and process received signals resulting from reflection of the drive signals at the distal end of said sensor, and to initiate a switching function dependent on an amplitude and/or phase difference in frequency of said received signals;

wherein said sensor comprises an open ended microwave waveguide formed by an outer electrode and an inner electrode held within said outer electrode with an electrical insulator there-between, said sensor having a length L; and

wherein said sensor is configured to create an impedance mismatch at or adjacent to said connection end.

2. A level switch as claimed in claim 1 wherein said sensor has a main waveguide section of impedance Z and wherein said impedance mismatch comprises a reflection section configured to have an impedance of greater than 1.5 Z.

3. A level switch as claimed in claim 2 wherein said reflection section is configured to have an impedance of substantially 4 Z.

4. A level switch as claimed in claim 1 wherein said reflection section comprises a cavity which is evacuated, or filled with air or a low permittivity foamed material.

5. A level switch as claimed in claim 1 wherein said sensor is configured to operate within a selected frequency band having a substantially central frequency f 0 with a wavelength λ and wherein said impedance mismatch has a dimension in the direction of length L of a multiple of substantially ¼ λ.

6. A level switch as claimed in claim 5 wherein said main waveguide section has a dimension in the direction of L which is a multiple of ½ λ.

7. A level switch as claimed in claim 5 wherein said sensor further includes a pressure resisting section extending between said inner and outer electrodes, said pressure resisting section having a dimension in the direction of L which is a multiple of ½ λ.

8. A level switch as claimed in claim 7 wherein said pressure resisting section is positioned within the length of said main waveguide section.

9. A level switch as claimed in claim 8 wherein the distance between an edge of said pressure resistant section and an end of said main waveguide section is a multiple of ½ λ.

10. A level switch as claimed in claim 1 further including an expansion section formed at said distal end in which said inner electrode is expanded in diameter and said outer electrode is reduced in thickness to accommodate the expansion.

11. A level switch as claimed in claim 10 wherein the dimension of said expansion section in the direction of L is a multiple of ½ λ.

12. A level switch as claimed in claim 11 wherein at said distal end said inner electrode projects beyond said outer electrode in the direction of L by an amount of the order of 1/10 λ.

13. A level switch as claimed in claim 1 wherein an insulating coating is provided over said distal end to prevent short-circuiting between said inner and outer conductors.

14. A level switch as claimed in claim 1 wherein said outer electrode and said inner electrode are substantially co-axial.

15. A level switch as claimed in claim 1 wherein said processing electronics is configured to reduce the frequencies of said received signals before processing into a determination of a switching point.

16. A level switch configured to initiate a switching function in the event of a change of medium in contact with said switch, said switch including:

A sensor having a connection end and a distal end, and being configured to sense a change of medium in contact with the distal end of said sensor; and

drive and processing electronics connected to the connection end of the sensor and configured and configured to provide drive signals to said sensor, to receive and process received signals from said sensor, and to initiate a switching function dependent on an amplitude and/or phase difference in frequency of said received signals,

wherein said sensor comprises an open ended microwave waveguide formed by an outer electrode and an inner electrode held within said outer electrode with an electrical insulator there-between, said sensor having a length L, and

wherein said sensor is configured to operate within a frequency band having a frequency f 0 with wavelength λ located substantially centrally within said band, said sensor including at least one impedance mismatch having a dimension in the direction of length L related to λ.

17. A level switch configured to initiate a switching function in the event of a change of medium at a level of said switch, said switch including:

a sensor having a connection end and a distal end, and being configured operable to sense a change of medium at a level of the distal end of said sensor; and

drive and processing electronics connected to the connection end of the sensor and configured to provide drive signals to the connection end of said sensor, to receive and process received signals resulting from reflection of the drive signals at the distal end of said sensor, and to initiate a switching function dependent on an amplitude and/or phase difference in frequency of said received signals,

wherein said sensor comprises an open ended microwave waveguide formed by an outer electrode and an inner electrode held within said outer electrode with an electrical insulator there-between, the distal end of said sensor being configured for mounting in substantial contact with a wall of a non-conductive vessel containing said medium or in contact with a non-conductive window provided in a wall of a vessel containing said medium, said sensor having a length L,

wherein said sensor is configured to create an impedance mismatch at or adjacent to said connection end.

18. A level switch configured to initiate a switching function in the event of a change of medium at a level of said switch, said switch including:

a sensor having a connection end and a distal end, and being configured to sense a change of medium at a level of the distal end of said sensor; and

drive and processing electronics connected to the connection end of the sensor and configured to provide drive signals to the connection end of said sensor, to receive and process received signals resulting from reflection of the drive signals at the distal end of said sensor, and to initiate a switching function dependent on an amplitude and/or phase difference in frequency of said received signals,

wherein said sensor comprises an open ended microwave waveguide formed by an outer electrode and an inner electrode held within said outer electrode with an electrical insulator there-between, the distal end of said sensor being configured for mounting in substantial contact with a wall of a non-conductive vessel containing said medium or in conduct with a non-conductive window provided in a wall of a vessel containing said medium, said sensor having a length L, and

wherein said sensor is configured to operate within a frequency band having a frequency f 0 with wavelength λ located substantially centrally within said band, said sensor including at least one impedance mismatch having a dimension in the direction of length L related to λ.