Scale thickness measuring method

A scale thickness measuring method of measuring a thickness of scale attached to an outer peripheral surface of a heat transfer pipe extending in a direction of an axis and having a cylindrical shape centered on the axis by using an eddy-current probe, which is provided with an excitation coil generating an eddy current, and a pair of detection coils which are provided integrally with the excitation coil and detecting axial and circumferential components of a magnetic field formed by the eddy current, the scale thickness measuring method comprising:

• obtaining a reference point in a region of the heat transfer pipe to which the scale is not attached by calculating a difference between the axial and circumferential components of the magnetic field detected by the pair of detection coils;
• moving the eddy-current probe inside the heat transfer pipe in the direction of the axis while inserting the eddy-current probe into the heat transfer pipe;
• calculating the difference between the axial and circumferential components of the magnetic field detected by the pair of detection coils;
• obtaining a drift amount by comparing the difference with the reference point;
• and
• calculating the thickness of the scale from a calibration curve obtained in advance on the basis of the drift amount.