FISCHER FISCHERSCOPE X-RAY XDLM 231, Operator'S Manual

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Back-up cycle Mass per unit area and geometric coating thickness
FISCHERSCOPE
®
X-RAY 101
14 Mass per unit area and geometric
coating thickness
The natural unit of measurement for X-ray fluorescence is mass per
unit area.
Definition "Mass per unit area": Mass per area
The number of atoms or the mass per unit area can be determined
from the fluorescence spectrum. However, this area-related mass
AM (e.g., in g per m² or mg per cm
2
) is not the typically preferred
unit of measurement in coating technology - rather, the coating
thickness Th (e.g., in
μ m) is normally used for the designation of the
coating process or for the specification of the coated product and is
usually requested in the supplier’s document.
Thus, the mass per unit area needs to be converted into the coating
thickness.
The correlation between mass per unit area AM and the geometric
coating thickness Th is
Th = AM /

(1)
where

designates an effective density (in g/cm³) that represents
the mass of the coating element per unit volume.
In combination with a thickness measurement per cross cut, i.e., at
a known coating thickness Th, the density

of the coating can be
computed from it using equation (1). Other coating thickness
measurement methods for determining the geometric coating
thickness may be used as well.
It is then possible to adjust the two methods through proper
selection of the density such that they provide the same
measurement result for the geometric coating thickness
However, a prerequisite is that the density does not fluctuate
significantly during the coating process. This would lead to
deviating coating thickness readings when using the X-ray
fluorescence method.
Physical background
An XRF coating thickness test instrument evaluates the intensities
of the radiation of the coating and substrate materials. The intensity