ACFM probes are designed to keep the effect of probe lift-off on sensor readings to a minimum. However, by using different orientations of input current and sensor coils, probes can be made that are much more sensitive to lift-off.

Such probes can be supported by standard ACFM instruments, by using different software.

How does it work?

Probe size directly affects the range of lift-off that can be measured. Small probes are sensitive to small changes in lift-off, but the maximum measurable lift-off is also small – that is the signal output reaches the fixed in-air value at small lift-offs. Conversely, large probes are not very sensitive to small changes in lift-off, but the effect is measurable up to much larger lift-offs.

The probes work on both ferritic and non-ferritic metals, although the change is in the opposite sense. Prior to measuring lift-off, the probe response is calibrated at two fixed distances from a plate of the same material as under the coating. Software then produces a calibration curve for calculating lift-off from sensor output.

The fact that a large probe is needed to measure large lift-offs means that it is not practical to make a probe suitable for use at a wide range of lift-offs. Probes tend to be designed to give the best response at a particular target lift-off. Lift-off probes have been produced for several different applications, but the two most common have been the measurements of marine fouling thickness, and pipe ovality under coatings.