A reflector reflects electromagnetic waves, which waves are emitted into a conducting structure from at least one antenna situated at a distance (d′) from the reflector. The distance (d′) is calculated based on measured emitted and reflected waves to and from the reflector, respectively. The reflector includes a reflector disc, an elongated body having a first longitudinal end and a second longitudinal end, the first longitudinal end being fastened to the reflector disc and the second longitudinal end being fastenable to an accumulator piston. Said elongated body further comprises at least two telescopically interconnected reflector tubes rendering reversible adjustment of the elongated body's longitudinal length around a predetermined equilibrium length possible. A piston accumulator applying the reflector, and a method for measuring the distance (d′) are also provided.