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.

Level gauges are generally used in the process control industry to measure the level of material contained in a tank. As used herein, the term “tank” refers to a container, receptacle, vessel or other device for holding gases, liquids or solids. Radar level gauges are often used to measure process fluids or process solid levels in tanks, where the process materials range from benign materials to severely corrosive or abrasive compounds.

One type of level gauge for measuring a level of material in a tank is called a microwave level gauge. Microwaves are high frequency, short wavelength, electromagnetic waves. Microwaves, because of their short wavelengths, tend to travel in straight lines. This property gives rise to their application in radar, where objects are detected by the reflection of microwaves.

Generally, microwave or radar level gauges transmit an electromagnetic pulse into the materials contained within the tank, and measure the level of the process materials using the reflected pulse. As used herein, the term “microwave” refers to a high-frequency electromagnetic wave. The term “microwave pulse” refers to a microwave signal of short duration that is transmitted on a microwave antenna or conductor. The terms “microwave antenna” and “microwave conductor”, as used herein, refer to a conductive structure specifically designed to couple high-frequency electromagnetic energy (or to radiate electromagnetic energy). Typically, the microwave antenna or microwave conductor can both transmit and receive electromagnetic energy.

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.