INSPECTION , TESTING & MEASUREMENT
Substrate thickness
Thin substrate thickness can also have a dramatic influence on readings . Measurement distortion happens when the magnetic field
Figure 5 . Measurement distortion due to a thin metal substrate
reaches beyond the substrate material thickness ( Figure 5 ).
Figure 4 . Measurement deviations due to geometry
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 non-metallic 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 .
https :// fischerinstrumentation . co . uk / privacy-policy / product-portfolio / dry-film-thickness-gauge-fmp /# mot
When a substrate is too thin , standard probes may give misleading readings even after a corrective calibration . In these instances , Helmut Fischer GmbH offers a list of high-frequency probes . Their distinguishing feature is that eddy currents are induced in the skin of relatively thin substrates , to allow reliable coating thickness measurements without influence from the variation of substrate thickness .
Final Instrument Choice
The market is saturated with a broad range of coating thickness gauges , so it may initially seem too overwhelming to choose an instrument from the wide range of options available , especially when it doesn ’ t accurately explain what the right choice is .
In these instances , it may become extremely tempting to go for the cheapest option that the market offers . However , as attractive as it may be , the final decision should always be guided by the coating thickness measurement applications .
At Helmut Fischer GmbH , purchasing a coating thickness gauge isn ’ t just a matter of Googling online and pressing the “ buy now ” button , of course . Providing the customer has the right knowledge to make an informed decision ; it can become that simple .
The link further below leads to gauges , that are offered by Helmut Fischer GmbH – DFT gauges with a built-in probe for straightforward applications and more advanced instruments that can accommodate numerous specialist probes for more challenging measurement tasks .
https :// fischerinstrumentation . co . uk / privacy-policy / product-portfolio / dry-film-thickness-gauge-fmp /
https :// fischerinstrumentation . co . uk / how-to-choose-a-correct-coatingthickness-gauge /
Telephone : 01590 684100 Visit : www . fischerinstrumentation . co . uk twitter : @ surfaceworldmag SEPTEMBER 2020
57