Abstract: A telescopic shock absorber, having: a housing; a cavity located within the housing and containing a liquid and a gas; and a sensor for measuring the level of the liquid in the cavity. The sensor has: a first waveguide having a first end and a second end; and a communications interface operable to transfer electrical signals between the first waveguide to the exterior of the housing, wherein the first waveguide is arranged such that when the shock absorber is in normal use the first end is surrounded by the gas and the second end is immersed in the liquid.