Measurement method for detecting damage to a turbine blade and turbine

A measurement method for the early detection of damage to a blade of an impeller of a turbine, the method comprising

• during operation of the turbine,
• in a rotational direction of the blade along a circumference which surrounds the impeller, at a plurality of points, in each case a plurality of magnetic fields are generated next to one another substantially in an oscillation direction of the blade, which magnetic fields are influenced by a tip of a blade leaf of the blade during transit, wherein, in the turbine, measuring areas are arranged at a plurality of points along the circumference in the direction of rotation, wherein the measuring areas are formed with a plurality of exciter coils and with a plurality of rows of probes arranged next to one another,
• acquiring, as a result of the influence at the plurality of points, positional values of the tip,
• forming a positional profile of the blade leaf from the positional values,
• determining a frequency from the positional profile,
• comparing the frequency with specific frequency values and
• in the event of a sudden and/or sharp change in the frequency, detecting an alarm event.