A guided wave radar level gauge for determining the filling level of product in a tank, said level gauge being adapted for mounting on a nozzle of the tank, said level gauge comprising: transceiver circuitry configured to generate and transmit an electromagnetic transmit signal (S T) and receive an electromagnetic return signal (S R) from the tank, the electromagnetic return signal caused by a reflection from a surface of said product, a transmission line probe electrically connected to said transceiver circuitry, said probe in use extending from the level gauge to a bottom of the tank, and being adapted to allow the microwave transmit signal to propagate along the probe toward said product and return the electromagnetic return signal, processing circuitry configured to determine the distance between a reference position and the surface of the product based on a relationship between the transmit signal and the return signal, an attachment collar with an opening configured to be aligned with the nozzle, and a coaxial probe connection comprising a central probe connector suspended in a dielectric sleeve mechanically secured in the attachment collar, characterized in that the attachment collar is to be secured by means of an annular clamping device, and that said central probe connector has an exterior end extending outside the dielectric sleeve beyond said collar opening, wherein said probe is attached to said exterior end of the probe connector, and that said level gauge further comprises: an annular sealing gasket fitted in said collar opening, said annular sealing gasket having an outer periphery configured to be sandwiched between the attachment collar and a flange of said nozzle, and a central opening through which said central probe connector extends, and a distancing sleeve arranged around the exterior end of the probe connector, said distancing sleeve having a radially protruding collar portion between the dielectric sleeve and the sealing gasket, such that, when the probe is attached to the probe connector, an inner periphery of the sealing gasket will be sandwiched between the radially protruding collar portion and an end of the probe, wherein said processing circuitry is configured to detect an impedance change caused by tank content or condensate which has passed through the central opening of the sealing gasket and collected in an annular space immediately surrounding said radially protruding collar portion. transceiver circuitry configured to generate and transmit an electromagnetic transmit signal (S T) and receive an electromagnetic return signal (S R) from the tank, the electromagnetic return signal caused by a reflection from a surface of said product, a transmission line probe electrically connected to said transceiver circuitry, said probe in use extending from the level gauge to a bottom of the tank, and being adapted to allow the microwave transmit signal to propagate along the probe toward said product and return the electromagnetic return signal, processing circuitry configured to determine the distance between a reference position and the surface of the product based on a relationship between the transmit signal and the return signal, an attachment collar with an opening configured to be aligned with the nozzle, and a coaxial probe connection comprising a central probe connector suspended in a dielectric sleeve mechanically secured in the attachment collar, characterized in that the attachment collar is to be secured by means of an annular clamping device, and that said central probe connector has an exterior end extending outside the dielectric sleeve beyond said collar opening, wherein said probe is attached to said exterior end of the probe connector, and that said level gauge further comprises: an annular sealing gasket fitted in said collar opening, said annular sealing gasket having an outer periphery configured to be sandwiched between the attachment collar and a flange of said nozzle, and a central opening through which said central probe connector extends, and a distancing sleeve arranged around the exterior end of the probe connector, said distancing sleeve having a radially protruding collar portion between the dielectric sleeve and the sealing gasket, such that, when the probe is attached to the probe connector, an inner periphery of the sealing gasket will be sandwiched between the radially protruding collar portion and an end of the probe, wherein said processing circuitry is configured to detect an impedance change caused by tank content or condensate which has passed through the central opening of the sealing gasket and collected in an annular space immediately surrounding said radially protruding collar portion. an annular sealing gasket fitted in said collar opening, said annular sealing gasket having an outer periphery configured to be sandwiched between the attachment collar and a flange of said nozzle, and a central opening through which said central probe connector extends, and a distancing sleeve arranged around the exterior end of the probe connector, said distancing sleeve having a radially protruding collar portion between the dielectric sleeve and the sealing gasket, such that, when the probe is attached to the probe connector, an inner periphery of the sealing gasket will be sandwiched between the radially protruding collar portion and an end of the probe, wherein said processing circuitry is configured to detect an impedance change caused by tank content or condensate which has passed through the central opening of the sealing gasket and collected in an annular space immediately surrounding said radially protruding collar portion.