Budget Paint Thickness Gauges and alternative substrates?

[CCC4me]

I'm in the market for a PTG and have been looking at the Highline II (or other versions of it) to save some money.

It has been said it can't read on plastic/fiberglass/carbon fiber. Why can't we just calibrate it (using the non-FE mode) on top of a bare piece of carbon fiber, fiberglass, or ABS plastic before measuring on those surfaces? Wouldn't that work?

[Audios S6]

The NF modes of measuring utilize what is called the eddy principle. A magnetic charge is induced onto the NF substrate. Aluminum is not magnetic, but it is a good conductor of electricity, so this principle works. This is often how aluminum is sorted at recycling facilities. A magnetic charge is applied and the cans are drawn on to a particular sorting line.

Carbon fiber, fiberglass and plastic are poor conductors and as such the eddy principle will not work on them.

[Audios S6]

Follow-up, I don't know for certain, but my guess is that $1k+ units use ultrasonic methods. With this method, an ultrasonic wave is pulsed at the substrate. At the interface between layers, some of the wave is reflected back. This allows the unit to measure all the layers on nearly any solid substrate.

[CCC4me]

Thanks Audios.

How conductive does the substrate need to be? Carbon fiber is fairly conductive.

[Audios S6]

I couldn't say what the cut-off is for conductivity, it likely varies depending on the meter.

Carbon fiber is conductive, but the encapsulating resin is not. You may be able to get a reading, but it would include the resin.