7. Calibration
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MiniTest 2500 - MiniTest 4500
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7.15 Shot-blasted and rough surfaces
7.15.1 General
To remove rust in order to ensure a good adhesion of the paint, surfaces are commonly blasted in
pre-treatment. As a result, the base material gets rough. Roughness influences the measuring
results, i.e. readings will be higher than the actual thickness.
The following section describes some steps how to remove the influence of roughness in coating
thickness measurement.
For calibration and for determining the average, it is generally recommended to take at least a set of
10 readings.
If you proceed on thickness measurement according to the steps below, the average thickness over
the peaks will be displayed. Note that the statistics program is of great benefit in this procedure.
7.15.2 Method A (roughness Rz > 20µm)
Carry out a two point-calibration according to section 7.3 use a smooth (non blasted) and clean
calibration sample with the same geometry and the same substrate as the later measuring sample.
Now take approx. 10 readings on the uncoated, shot-blasted sample to produce the mean value
𝑥̅
0
.
After this take approx. 10 further readings on the coated, shot blasted test sample to produce the
mean value
𝑥̅
m
.
The difference between the two mean values is the mean coating thickness
𝑥̅
eff
over the peaks.
The greater standard deviation
σ of the two values
𝑥̅
m
and
𝑥̅
0
should also be taken into
consideration:
𝑥̅
eff
= (
𝑥̅
m
−
𝑥̅
0
)
±
σ
7.15.3 Method B (roughness Rz < 20µm)
-
Carry out a zero calibration of 10 readings on a shot-blasted, uncoated sample. Then carry
out a foil calibration on the uncoated substrate. The foil set should consist of a number of
individual foils of max. 50 microns thickness each and should roughly correspond to the
estimated coating thickness.
-
The coating thickness can be read directly from display and should be averaged from 5...10
single measurements. The statistics function is useful in this context.
7.15.3 Method C Calibration with two calibration foils of different thickness
-
This method also gives reliable results. Simply follow the two-point calibration method using
two foils as described in section without zero setting.
-
For a maximum approach to the respective nature of surface, the foil value can be reached
by using several foils -
50μm each.The mean coating thickness should be calculated from
5...10 readings. The statistics programme is very useful in this context.
Note:
For coatings thicker than 300 μm, the influence of roughness generally is of no importance and it
will not be necessary to apply above calibration methods.