TESTING & MEASUREMENT
The Eddy current test method , ( amplitude sensitive ) measures according to ISO 2360 and ASTM 7091 standards . A coil wrapped around the ferrite core is induced with an excitation current . It generates a high-frequency magnetic field that sends loops of electrical current into a base material , in planes perpendicular to the magnetic field . The same coil measures the resulting magnetic field .
In both methods , the measurement coil sends a signal to the instrument ( DFT gauge ), which converts it into numerical units we can understand , such as microns or mils .
In case a coating has the same chemical composition as a substrate , there ’ s no way for a measurement probe to recognise the difference . Hence , it ’ s essential to understand what material a coating consists of and whether it ’ s measurable on a given substrate using a DFT gauge .
The table below demonstrates typical applications for both magnetic induction and eddy current methods . We also extensively talk about magnetic induction and eddy current measurement principles in the “ How to Measure Coating Thickness ” article .
Substrate ( Base Material )
Magnetic Induction
Ferromagnetic ( e . g ., Steel or Iron )
- Non-ferromagnetic coatings
- Electroplated coatings made of chromium ,
The main field of applications zinc , copper or aluminium
- Paint , enamel , varnish or plastic coatings
Table 1 : Measurement principles and their main applications
Eddy Current Electrically conductive nonferrous metal ( e . g ., Aluminium )
- Electrically nonconductive coatings - Paint , enamel , varnish or plastic coatings
- Anodic coatings
Helmut Fischer GmbH offers a wide range of DFT gauges with built-in or interchangeable probes that use magnetic induction and eddy current methods . Learn more about the instruments we suggest following the link below : https :// fischerinstrumentation . co . uk / privacy-policy / product-portfolio / dry-film-thickness-gauge-fmp /
Coating roughness
If a coating is rough , then there will be a higher variation in the measured coating thickness . It happens since a probe tip may rest on peaks or in troughs . Probes that aren ’ t influenced by rough coatings solve this challenge Figure 2 .
FD13H probe . Probes for measurements on virtually all metals . The probes work with two test methods and are therefore able to measure coating thicknesses on non-ferrous metals as well as on ferrous metals . Because of the large pole tip , the probes are also well suited for measurements on rough surfaces .
FTD3.3 probe . This probe utilises the eddy current method . It is suitable for measurements of rough paint , lacquer , plastics , and anodised coatings on Non-ferromagnetic substrates .
V7FKB4 probe . Measures non-ferrous and nonmetallic coatings on steel or iron substrates . Higher repeatability precision than single tip probes when measuring rough surfaces . FKB10 probe . Measures non-ferrous and nonmetallic coatings on steel or iron substrates . Uniquely suited for thick layers ( up to 8mm ). Provide a much higher measurement precision on rough surfaces than single tip probes . Table 1 : Probes for measurements on rough surfaces
The location of the measurement area
The location of the measurement area also plays a significant role in selecting the right DFT gauge for the job . When it comes to testing coating thickness on flat surfaces , most of the time , it ’ s possible t o get away with a standard instrument setup . However , in case a measurement is taken too close to a sample ’ s edge , then readings get affected due to the magnetic field reaching beyond a specimen ( Figure 3 ).
Figure 3 . Graphical representation of an “ edging ” effect
In case a sample is small and a probe inevitably always lands near an edge , you can obtain more accurate results by using an appropriate “ thin tip ” probe . It is also suitable for measurements in intricate locations . Further below you can find two types of “ thin tip ” probes offered by Helmut Fischer GmbH for measurements on ferrous and non-ferrous substrates :
FGA06H-MC probe . Measures non-ferrous and non-metallic coatings on steel or iron substrates . Mechanical design is especially suited for precise probe positioning ( e . g ., near edges ) FTA2.4-MC probe . Measures electrically nonconducting coatings on non-ferromagnetic metal substrates . Mechanical design is especially suited for precise probe positioning ( e . g ., near edges ) Table 2 : Thin tip probes
Figure 2 . A schematic diagram demonstrating how peaks and troughs affect coating thickness readings [ A ] and the effect of wide and dual-tip probes that take consistent readings on rough surfaces [ B ].
Wide and dual-tip probes have a larger measurement area and always sit on peaks . This way , the total thickness of a coating is consistently measured . A list of probes offered by Helmut Fischer GmbH , specifically designed to measure on rough surfaces consistently , can be found in table 1 opposite :
The following probes work with the FMP family of instruments .
The FGA06H-MC probe works with FMP10 deltascope , FMP20 dualscope , FMP30 deltascope , FMP40 dualscope , and FMP100 dualscope . Meanwhile , FTA2.4-MC operates with FMP10 isoscope , FMP20 dualscope , FMP30 isoscope , FMP40 dualscope , and FMP100 dualscope . Follow the link below for more details about the FMP range of instruments and how to access probes ’ technical information .
https :// fischerinstrumentation . co . uk / privacy-policy / product-portfolio / dry-film-thickness-gauge-fmp /
Finally , it ’ s essential to consider that coating thickness measurements can also take place inside bores and holes . In this case , it ’ s often impossible to obtain a reading without a specialist probe .
CONTINUED ON PAGE 36 twitter : @ surfaceworldmag DECEMBER 2020
35