Leveller calibration device

A calibrating bar having a neutral plane for calibrating a multi-roll leveller for metal strips, the calibrating bar having a thickness from 1.0 mm to 2.0 mm and a yield strength from 1000 MPa to 2000 MPa, the calibrating bar comprising:

• a first groove on a first face, a first optical fibre being embedded in the first groove via an adhesive;
• a second groove on a second face opposite to the first face, a second optical fibre being embedded in the second groove via a further adhesive;
• the first optical fibre and the second optical fibre comprising a fibre Bragg grating, being parallel;
• the first optical fibre and the second optical fibre being located at the same distance from the neutral plane;
• and
• the first embedded optical fibre and the second embedded optical fibre being connectable to an optical coupler so as to have a sufficient length to extend over all rolls of the multi-roll leveller.