PAR = Peak Anodizing Resistance. It's the point you see while anodizing that has the highest resistance. And, since V = I*R, and I is fixed (we're all Constant Current anodizing, right?
) it's also the point where voltage peaks.
Why do we care about PAR? In theory, when peak resistance is reached, the anodizing layer is at it's thickest.
This next part might not be 100% correct, but this is how I visualize it:
Resistance in a wire is defined as: R = (1/A)*(Resistivity*Length) where resistivity is an intensive property of the material used for the wire and A = cross sectional area.
In a similar manner, I believe we can adapt this to anodizing. While anodizing at a constant current, we see the resistance rise up to a certain point, then start falling again. What changes to cause this increase in resistance? The thickness of the anodizing layer. The cross sectional area can be assumed constant, and the resistivity of the anodized layer should be constant, so in theory, the voltage (during constant current anodizing) is also an indicator of layer thickness.
I'm working on some testing right now to verify this hypothesis. Hopefully we'll have some quantification of it soon.