TESTING & MEASUREMENT
Sample geometry has one of the most significant impacts on the coating thickness measurement readings , often producing a deviation from the real value ( Figure 4 ). In this case , operators are forced to conduct a re-calibration for every shape . It adds complications to the measurement procedure with increased testing times and a substantially higher room for errors .
Figure 4 . Measurement deviations due to geometry
Table 3 demonstrates suitable probes for measurements in hard to read locations .
FAW3.3 probe . Measures electrically nonconducting coatings on non-ferromagnetic metal substrate materials . It is suited for measurements on plane specimens or in pipes boreholes and recesses . It can also be used when surfaces exhibit a damp condition ( acidic contamination of test surface ). FAI3.3-150 probe . Measures electrically nonconducting coatings on non-ferromagnetic metal substrate materials . Suited for measurements in pipes , boreholes , grooves , etc . Smallest permissible inside diameter : 9 mm . Maximum insertion depth : 150 mm .
FGABI1.3-150 probe . Measures non-ferrous and non-metallic coatings on steel or iron substrates . Suited for measurements in bore holes , pipes , or grooves . Smallest permissible inside diameter : 9mm . Maximum insertion depth : 150mm
FGABI1.3-260 probe . Measures non-ferrous and non-metallic coatings on steel or iron substrates . Suited for measurements in boreholes , pipes , or grooves . Smallest permissible inside diameter : 9mm . Maximum insertion depth : 260mm
Table 3 : Probes for coating thickness measurements in hard to reach locations
The list of probes in table 3 works with the FMP family of instruments .
Sample Geometry
Most of the time , the final sample geometry may be more complicated with round and concave surfaces , and as a result , the measuring can be problematic . As an example , in the automotive industry , most of the measurements acquired on car bodies are either on a convex or a concave surface .
To avoid errors generated from sample geometry , it is possible to use curvature compensation probes . In this way , an accurate coating thickness reading is always obtained , regardless of a specimen shape . Helmut Fischer GmbH offers a list of patented probes for measurements on ferrous and non-ferrous substrates you can find in the table below .
FTD3.3 probe . Measures paint , lacquer , plastic , and anodised coatings on non-ferromagnetic metal substrates . Excellent curvature compensation down to approximately 4 mm diameter shapes . Patented design . It is suited for measurements on curved surfaces such as car bodies FGA06H probe . Measures non-ferrous and nonmetallic coatings on steel or iron substrates . Uniquely suited for small test areas and curved surfaces . High wear resistance of the tungsten carbide tip . It is not suited for very rough surfaces .
Table 4 : Curvature compensation probes
A list of probes from table 4 works with the FMP family of instruments .
Coating thickness extremes
Minimum and maximum coating thickness may require two different probes to acquire accurate results . At the same time , if acquired measurements are to be in the nanometer range , one will need to consider alternative methods , such as micro-sectioning or X-ray fluorescence ( XRF ).
The further away a probe is from the substrate , the weaker the resulting magnetic field , and vice-versa . As a result , extremely thin coatings of less than 3 microns have a much more significant variation in their measurement data . Here , the signal is too strong for the probe to recognise the difference between the presence and the absence of a coating .
When measuring thin coatings , it is advisable to use a motorised measurement stand . This instrument accessory ensures consistent readings are achieved and eliminates the possibility of thickness deviations caused by manual operation .
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Substrate thickness
Thin substrate thickness can also have a dramatic influence on readings . Measurement distortion happens when the magnetic field reaches beyond the substrate material thickness ( Figure 5 ).
Figure 5 . Measurement distortion due to a thin metal substrate
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