Method and apparatus for determining the thickness of a coating on a

Abstract

A system for non-destructively determining the thickness of a coating on a metal substrate includes using a first probe to locally heat the coating so as to establish a temperature gradient therein. The probe is configured so as to also provide electrical contact to the coating surface. This first probe is formed from a material having high thermal conductivity, and is at least partially plated with a material having high resistance to oxidation. Preferably, the probe plating material is chosen to also have high resistance to mechanical wear. The testing system also includes an electrical power supply for controllably heating the first probe in order to maintain the temperature thereof at a constant predetermined value. A second probe provides an electrical return contact in order to form a circuit for measuring the thermoelectric voltage between the two probes. This thermoelectric voltage may be conveniently measured by an AC amplification circuit. The thermoelectric voltage reading obtained may be processed by several different methods in order to provide the desired indication of the presence of a coating and/or its thickness. In one embodiment, the first probe is configured so as to be rollable along the coating surface, and the testing system further includes an electrical signal generator for determining the speed of movement of the rollable probe. In such an embodiment, the true coating thickness is determined from the measured thermoelectric voltage and the speed of movement of the probe.