Guided wave radar probe with leak detection

1. A probe defining a transmission line for use with a measurement instrument including a pulse circuit connected to the probe for generating pulses on the transmission line and receiving reflected pulses on the transmission line, the probe comprising:

a conductive outer sleeve for mounting to a process vessel;

a center conductor coaxial with the outer sleeve for conducting the pulses;

a primary seal element between the outer sleeve and the center conductor spaced a select distance from a near end of the outer sleeve;

a secondary seal element between the outer sleeve and the center conductor spaced proximate the near end of the outer sleeve, to define a generally tubular space between the primary seal element and the secondary seal element;

a dielectric insert filling a portion of the tubular space proximate the secondary seal element; and

a leak detection tube in the tubular space between the dielectric insert and the primary seal element, the leak detection tube having an inner diameter greater than an outer diameter of the center conductor to define a void, wherein the dielectric insert and the leak detection tube provide a substantially continuous uniform impedance under dry conditions.

2. The probe of claim 1 wherein the probe has an electrical connector at the near end of the outer sleeve.

3. The probe of claim 2 wherein the probe has substantially uniform impedance under dry conditions from the electrical connector to a distal end of the probe, and the substantially uniform impedance is different from impedance of a connector cable to provide a fiducial reference marker at the connector.

4. The probe of claim 1 wherein the primary seal comprises a glass seal.

5. The probe of claim 4 further comprising a metal body supporting the glass seal in the probe.

6. The probe of claim 5 wherein the metal body includes a shoulder supporting the glass seal.

7. The probe of claim 5 wherein the metal body comprises a counterbore receiving the leak detection tube.

8. A probe defining a transmission line for use with a measurement instrument including a pulse circuit connected to the probe for generating pulses on the transmission line and receiving reflected pulses on the transmission line, the probe comprising:

a conductive outer sleeve for mounting to a process vessel;

a center conductor coaxial with the outer sleeve for conducting the pulses, the center conductor being supported in the outer sleeve by a primary seal element spaced a select distance from a near end of the outer sleeve and a secondary seal element proximate the near end of the outer sleeve;

a dielectric insert in the outer sleeve proximate the secondary seal element; and

a leak detection tube in the outer sleeve between the dielectric insert and the primary seal element, the leak detection tube having an inner diameter greater than an outer diameter of the center conductor to define a void, wherein the dielectric insert and the leak detection tube provide a substantially continuous uniform impedance under dry conditions.

9. The probe of claim 8 wherein the probe has an electrical connector at the near end of the outer sleeve.

10. The probe of claim 9 wherein the probe has substantially uniform impedance under dry conditions from the electrical connector to a distal end of the probe, and the substantially uniform impedance is different from impedance of a connector cable to provide a fiducial reference marker at the connector.

11. The probe of claim 8 wherein the primary seal comprises a glass seal.

12. The probe of claim 11 further comprising a metal body supporting the glass seal in the probe.

13. The probe of claim 12 wherein the metal body includes a shoulder supporting the glass seal.

14. The probe of claim 12 wherein the metal body comprises a counterbore receiving the leak detection tube.

15. A method of detecting leaks in a probe defining a transmission line for use with a measurement instrument including a pulse circuit connected to the probe for generating pulses on the transmission line and receiving reflected pulses on the transmission line, comprising:

providing a probe with a conductive outer sleeve and a center conductor coaxial with the outer sleeve for conducting the pulses, the center conductor being supported in the outer sleeve by a primary seal element spaced a select distance from a near end of the outer sleeve and a secondary seal element proximate the near end of the outer sleeve, and a leak detection tube in the outer sleeve between the primary seal element and the secondary seal element, the leak detection tube having an inner diameter greater than an outer diameter of the center conductor to define a void, wherein the probe has a substantially continuous uniform impedance under dry conditions and impedance proximate the leak detection tube changes responsive to failure of the primary seal; and

measuring changes in impedance to detect a leak in the probe.

16. The method of claim 15 wherein the probe has substantially uniform impedance under dry conditions from a near to a distal end of the probe, and the substantially uniform impedance is different from impedance of a connector cable to provide a fiducial reference marker at the connector.

17. The method of claim 15 wherein the primary seal comprises a glass seal.

18. The method of claim 17 further comprising a metal body supporting the glass seal in the probe.

19. The method of claim 18 wherein the metal body includes a shoulder supporting the glass seal.

20. The method of claim 18 wherein the metal body comprises a counterbore receiving the leak detection tube.