A rotor arrangement including: a rotor stage having an annular array of rotor blades each blade having a tip; a casing that surrounds the rotor stage and defines a clearance between the tips and the casing; and an over tip leakage measurement system including a multi-hole total pressure probe mounted to a stationary component downstream of the rotor stage; the probe arranged such that the axis of at least one of the holes is substantially aligned with an expected flow direction from the rotor stage.