A gauging instrument sealingly mountable on a nozzle of a tank, for determining a process variable relating to a product in said tank, said gauging instrument comprising: an attachment collar (12; 112) mateable with said nozzle so as to face the product in the tank; a dielectric filling member (20; 120) arranged in said attachment collar and having a radially extending collar portion (28; 128; 128') which abuts an outer rim of said attachment collar (12; 112), said dielectric filling member being substantially impermeable to the product and any further tank contents; one or several fastening elements configured to secure the attachment collar to, and in mating relationship with, said nozzle, such that, when said attachment collar is secured to the nozzle, the radially extending collar portion is sandwiched between the outer rim of the attachment collar and the nozzle, wherein the gauging instrument is a radar level gauge arranged to determine a filling level of the product in the tank, said radar level gauge comprising: a transceiver configured to generate and transmit electromagnetic signals; an antenna arranged to direct an electromagnetic transmit signal transmitted by said transceiver towards said surface of the product and to return back to said transceiver an electromagnetic surface reflection signal resulting from reflection of said electromagnetic transmit signal at said surface; and processing circuitry connected to said transceiver for determining said filling level based on said electromagnetic surface reflection signal; wherein said attachment collar (12, 112) is formed by said antenna, and wherein said dielectric filling member is permeable to said electromagnetic signals, characterised in that the gauging instrument comprises an annular spring element (30; 130) arranged between the outer rim of the attachment collar (12; 112) and the radially extending collar portion (28; 128; 128'), such that, when said attachment collar is secured to the nozzle, the annular spring element (30; 130) exerts a contact pressure on the collar portion against the nozzle to thereby ensure a process seal. an attachment collar (12; 112) mateable with said nozzle so as to face the product in the tank; a dielectric filling member (20; 120) arranged in said attachment collar and having a radially extending collar portion (28; 128; 128') which abuts an outer rim of said attachment collar (12; 112), said dielectric filling member being substantially impermeable to the product and any further tank contents; one or several fastening elements configured to secure the attachment collar to, and in mating relationship with, said nozzle, such that, when said attachment collar is secured to the nozzle, the radially extending collar portion is sandwiched between the outer rim of the attachment collar and the nozzle, wherein the gauging instrument is a radar level gauge arranged to determine a filling level of the product in the tank, said radar level gauge comprising: a transceiver configured to generate and transmit electromagnetic signals; an antenna arranged to direct an electromagnetic transmit signal transmitted by said transceiver towards said surface of the product and to return back to said transceiver an electromagnetic surface reflection signal resulting from reflection of said electromagnetic transmit signal at said surface; and processing circuitry connected to said transceiver for determining said filling level based on said electromagnetic surface reflection signal; wherein said attachment collar (12, 112) is formed by said antenna, and wherein said dielectric filling member is permeable to said electromagnetic signals, characterised in that the gauging instrument comprises an annular spring element (30; 130) arranged between the outer rim of the attachment collar (12; 112) and the radially extending collar portion (28; 128; 128'), such that, when said attachment collar is secured to the nozzle, the annular spring element (30; 130) exerts a contact pressure on the collar portion against the nozzle to thereby ensure a process seal. a transceiver configured to generate and transmit electromagnetic signals; an antenna arranged to direct an electromagnetic transmit signal transmitted by said transceiver towards said surface of the product and to return back to said transceiver an electromagnetic surface reflection signal resulting from reflection of said electromagnetic transmit signal at said surface; and processing circuitry connected to said transceiver for determining said filling level based on said electromagnetic surface reflection signal; wherein said attachment collar (12, 112) is formed by said antenna, and wherein said dielectric filling member is permeable to said electromagnetic signals, characterised in that the gauging instrument comprises an annular spring element (30; 130) arranged between the outer rim of the attachment collar (12; 112) and the radially extending collar portion (28; 128; 128'), such that, when said attachment collar is secured to the nozzle, the annular spring element (30; 130) exerts a contact pressure on the collar portion against the nozzle to thereby ensure a process seal.