It depends on what type you have. There are magnetic types(some are mechanical and some are electronic) and ultrasonic types.

Here is what we use....the 345 model
http://www.elcometer.com/international%20index%20pages/international/Coatings/index.htm

John Wright

edit: My link didn't work correctly.....You'll have to look in the product catalogue under DFT gages.

Of the magnetic types of paint thickness gauges there are 2 basic types. The first is a magnetic pull-off mechanism. How it works- a permanent magnet is placed against a painted ferrous metal surface. Then spring tension is gradually increased until the magnet is pulled off. At that point a scale is read to indicate paint thickness. The magnet requires more force to pull off from thin paint than is required for thicker paint.

The other type is a constant pressure ) or "fixed probe" gauge. Very basically, there is a coil in the probe that is controlled by electronic circuitry. The current required to balance an internal spring corresponds to the distance the probe is from the ferrous metal surface. Then a digital read out is provided.
A variation of this is used for non-ferrous (non-magnetic) metals. Those are called eddy current gauges. In those, an eddy current in induced into the metal which also causes a magnetic field that is used to correlate to paint thickness. Again, a digital readout is provided.

As far as ultrasonic gauges, I think you may already know the principle involved, but in case you don't- sound energy is sent into the paint. When the sound gets to the substrate (material that is painted) it is reflected back to the probe. Internal circuitry measures the time required for the pulse to travel through the paint and back, and that correlates to a thickness.

I'm thinking that you are asking a lot of good questions, and everyone is trying to be helpfull in answering them. Have you looked into some books on the subjects of interest? There is a lot of information available that will give you more thorough answers than we can manage here.

Chet

I think there are at least half a dozen types out there. As I prefer the magnetic type - I'll try to put into words it's principle of operation. Basically, magnetic pull-off gages use a magnet, a calibrated spring, and a scale. The atraction between the magnet and the magnetic steel pulls the two together. As the coating thickness separating the two increases, it becomes easier to pull the magnet away. The coating thickness is determined by measuring the force required for this "pull-off" force.

The slight disadvantage with using the magnetic type are that it is quite sensitive to surface roughness and curvature of the surface, among other things.