A level gauge for detecting process variables related to a distance to a surface of a content in a tank, comprising a first and second functionally independent circuitry arrangements comprising transceiver circuitry and processing circuitry. The gauge further comprises a transmission line probe connected to the circuitry arrangements, the transmission line probe extending into the content in the tank and being adapted allow propagation of first and second transmission modes, and a feeding arrangement connected to couple electromagnetic signals into the probe in first and second propagation modes.