Separation measurement method and separation measurement device

1. A method for measuring a separation between blade tips of rotor blades on a rotor and a casing annularly surrounding the rotor blade tips, and separated radially outward from the rotor blade tips when the rotor is rotating;

the method comprising:

measuring the separation in contactless fashion using at least one separation measurement device which measures in contactless fashion, the separation measurement device including a cylinder and an arresting device;

arresting movement of the cylinder in at least one position by moving a part of the arresting device toward the cylinder so that the part of the arresting device contacts the cylinder; and

releasing the cylinder from an arrested position by moving the part of the arresting device away from the cylinder so that the part of the arresting device does not contact the cylinder.

2. The method as claimed in claim 1 wherein:

the separation measurement device further includes a confocal chromatic separation sensor or an acoustic separation sensor.

3. The method as claimed in claim 1 , further comprising:

inserting the separation measurement device into a through bore which extends radially through the casing and is accessible from outside for carrying out the separation measurement.

4. The method as claimed in claim 3 , wherein:

the separation measurement device and the through bore are formed such that, in a normal state, the casing extends radially further inward than the separation measurement device accommodated in the through bore.

5. The method as claimed in claim 1 , further comprising:

performing the separation measurement at a plurality of circumferential positions around the casing.

6. The method as claimed in claim 5 , wherein the circumferential positions are evenly spaced apart from one another.

7. A separation measurement device for performing a method as claimed in claim 1 , comprising:

a tubular housing; and

a separation sensor configured to measure in contactless fashion and which is accommodated in the housing at a location and in a manner such that the sensor is movable axially along an axis of the sensor between an operating position, wherein a tip of the separation sensor projects outward from the separation measurement device, and a withdrawn position, wherein the tip of the separation sensor is within the separation measurement device;

wherein the arresting device is configured such that the separation sensor can be arrested or held at least in its operating position.

8. The separation measurement device as claimed in claim 7 , wherein the separation sensor is a confocal chromatic separation sensor or an acoustic separation sensor.

9. The separation measurement device as claimed in claim 7 , further comprising:

at least one spring arranged and configured such that its spring force opposes movement of the separation sensor from its withdrawn position into its operating position.

10. The separation measurement device as claimed in claim 7 , wherein:

the cylinder is accommodated in a longitudinally movable fashion in the housing and the separation sensor is arranged in the cylinder.

11. The separation measurement device as claimed in claim 10 ,

wherein the separation sensor has a rod-shape; and

an axial blind bore is provided at the free end of the cylinder in which the rod shaped sensor is arranged.