The CGM-7 is a modern microprocessor based instrument for measuring crack depth in metals
undergoing materials testing. Building on the success of the CGM-5R this unit improves upon the
CGM-5R specification by offering two channel operation, higher frequency and current outputs, full
computer control and a variety of expansion options.
It utilises the alternating current potential drop method (ACPD) which is an established technique
covered by the ASTM 647 standard. The technique involves passing an alternating current through
the metal under test and measuring the resultant voltage drop that is created across the specimen.
The presence of a growing defect will alter this voltage and by suitable calibration, a measure of
the defect depth can be obtained.