Abstract: Microwaves are used to detect the level of a liquid in a vessel. An external chamber, constructed of a material essentially invisible to microwaves, is connected to the vessel and is open to the interior of the vessel above and below the level to be detected. Microwaves are directed through the external chamber and the unabsorbed energy of the microwaves passing through the external chamber is measured and converted to a signal representative of the liquid level in the vessel. Known electrical circuitry is used to generate the microwaves, to measure the unabsorbed microwave energy, and to convert the measured energy to a standard instrument signal representative of the liquid level in the vessel.