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:

1. 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:

2. a ceramic seal body having an opening sized to receive a microwave conductor;

3. an isolating element disposed over the opening and coupled to the ceramic seal body at a coupling joint;

4. a brazed seal disposed over the coupling joint for fixedly sealing the isolating element to the ceramic seal body; and

5. 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.

In a conduction wave guide assembly, the antenna typically extends from a transmitter assembly into the process materials. For example, a microwave pulse travels along the antenna, and is reflected back when the pulse encounters a material with a different dielectric constant. Generally, the pulse is affected by the change in the dielectric constant at the surface of the process material within the tank. Various techniques may be used to analyze the reflected microwave pulse (including, for example, time domain reflectometry).

Since the process materials within the tank tend to be corrosive and are often stored under pressure, there is a process seal positioned between the sensor and the potentially aggressive materials within the tank. Conventionally, an O-ring or a teflon seal was used to isolate the process fluid from the transmitter housing and sensor electronics. In addition to isolating the electronics from the process fluid, such seals were typically constructed of materials selected so as not to cause microwave reflection.

While such seals typically met microwave transmission requirements, the seals were not particularly well-suited for high pressure/temperature applications.