Device for determining the distance (Δx) between at least one rotor blade and a wall of a turbine engine, said wall surrounding the at least one rotor blade, which device comprises the following parts: - a waveguide for guiding electromagnetic waves (31a, 31b, 32a, 32b) having different frequencies and emitting electromagnetic waves having at least one frequency in the direction of the rotor blade through at least one waveguide opening which faces the rotor blade, - at least one means for injecting the electromagnetic waves (31a, 31b) having different frequencies into the waveguide, - at least one means for receiving reflected portions (32a, 32b) of the electromagnetic waves (31a, 31b) to be injected into the waveguide, and an evaluation unit for evaluating the reflected portions (32a, 32b) to be received of the electromagnetic waves (31a, 31b) to be injected, comprising means for comparing the respective phases of the electromagnetic waves (31a, 31b) to be injected with the phases of the corresponding reflected portions (32a, 32b) of the electromagnetic waves (31a, 31b) to be injected, wherein a phase comparison value can be ascertained for each frequency by means of the evaluation unit and the distance (Δx) can be determined from a comparison of the phase comparison values, wherein - the waveguide is provided with a sealing element (70a, 70b, 70c) which - is embodied as transmissible for the electromagnetic waves (31a, 32a) having the at least one frequency, and - has two surfaces (71a, 71b, 71c, 72a, 72b, 72c) disposed opposite each other in the guide direction of the electromagnetic waves (31a, 31b, 32a, 32b), characterised in that the two mutually opposing surfaces (71a, 72a) of the sealing element are embodied as flat and parallel to each other, the sealing element being embodied as reflectable for electromagnetic waves (31b, 32b) of at least one first frequency and transmissible for electromagnetic waves (31a, 32a) of at least one second frequency, and the evaluation unit can furthermore be embodied to determine a temperature-compensated distance (Δx) from a comparison of the phase comparison values of the portions of the electromagnetic waves to be injected that are reflected by the sealing element and the portions of the electromagnetic waves to be injected that are transmitted through the sealing element and reflected at the rotor blades. - a waveguide for guiding electromagnetic waves (31a, 31b, 32a, 32b) having different frequencies and emitting electromagnetic waves having at least one frequency in the direction of the rotor blade through at least one waveguide opening which faces the rotor blade, - at least one means for injecting the electromagnetic waves (31a, 31b) having different frequencies into the waveguide, - at least one means for receiving reflected portions (32a, 32b) of the electromagnetic waves (31a, 31b) to be injected into the waveguide, - the waveguide is provided with a sealing element (70a, 70b, 70c) which - is embodied as transmissible for the electromagnetic waves (31a, 32a) having the at least one frequency, and - has two surfaces (71a, 71b, 71c, 72a, 72b, 72c) disposed opposite each other in the guide direction of the electromagnetic waves (31a, 31b, 32a, 32b), characterised in that the two mutually opposing surfaces (71a, 72a) of the sealing element are embodied as flat and parallel to each other, the sealing element being embodied as reflectable for electromagnetic waves (31b, 32b) of at least one first frequency and transmissible for electromagnetic waves (31a, 32a) of at least one second frequency, and the evaluation unit can furthermore be embodied to determine a temperature-compensated distance (Δx) from a comparison of the phase comparison values of the portions of the electromagnetic waves to be injected that are reflected by the sealing element and the portions of the electromagnetic waves to be injected that are transmitted through the sealing element and reflected at the rotor blades. - is embodied as transmissible for the electromagnetic waves (31a, 32a) having the at least one frequency, and - has two surfaces (71a, 71b, 71c, 72a, 72b, 72c) disposed opposite each other in the guide direction of the electromagnetic waves (31a, 31b, 32a, 32b), characterised in that the two mutually opposing surfaces (71a, 72a) of the sealing element are embodied as flat and parallel to each other, the sealing element being embodied as reflectable for electromagnetic waves (31b, 32b) of at least one first frequency and transmissible for electromagnetic waves (31a, 32a) of at least one second frequency, and the evaluation unit can furthermore be embodied to determine a temperature-compensated distance (Δx) from a comparison of the phase comparison values of the portions of the electromagnetic waves to be injected that are reflected by the sealing element and the portions of the electromagnetic waves to be injected that are transmitted through the sealing element and reflected at the rotor blades.