Method for improving the accuracy of ultrasonic thickness measurements by calibrating for surface finish

摘要

The invention provides a method for measuring thickness of sample of material having a surface finish in a time of flight system using an ultrasonic thickness gauge, comprising the steps of: defining the time of flight system as a function of t'=h' DIVIDED Va' +C(f) where t' is the time of flight of an ultrasonic wave though the calibration sample, h' is a mechanically measured thickness of the calibration material, Va' is one half the velocity of sound through the material, and C(f) is a surface finish dependent time offset calibrated for the finish of the near and far side surfaces; passing an actual wave with the velocity of 2Va through the sample; measuring the time of flight (t) of the actual ultrasonic wave though the sample; providing the surface finish dependent time offset calibrated for the finish of each surface of the material; and establishing a corrected thickness measurement (CTM) as a function of the surface finish dependent offset (C(f)) which satisfies the condition CTM=Vax(t-C(f)).