INSPECTION , TESTING & MEASUREMENT
How to Choose a Correct Coating Thickness Gauge
The importance of measuring coating thickness is discussed extensively in the “ How to Measure Coating Thickness ” article . When it comes to selecting a handheld portable coating thickness gauge , an incorrect instrument choice may result in a measurement failure ( e . g ., inability to obtain a value ) or completely wrong results .
Even when a false reading is obtained with an incorrect gauge for the job , there ’ s also a high risk of assuming it ’ s accurate .
The above factors increase the chances of failing to meet a desirable coating thickness specification . The consequences may lead to the added costs of rework or lost contracts due to customer dissatisfaction .
This article explores the factors that influence choosing the most suitable gauge for a specific application , first time , and ensure the measurements are fully representable of the actual coating thickness .
This method is limited to powder coating applications , where a thickness needs to be predicted before the final curing stage . Once the depth is known , it ’ s possible to potentially reduce the powder consumption that leads to leaner coating application processes with financial and ecological advantages .
However , for coatings is their stable ( cured ) state , Dry Film Thickness ( DFT ) gauges are ideal instruments for these applications . They also test layers non-destructively and cover a significantly wide range of applications . In this instance , a measurement probe needs to be applied to a surface for a reading to take place .
This article guides a reader to choose the appropriate DFT gauge for measurements of coatings in their finished “ cured ” state .
Coating and Base Material
DFT Gauges work using either magnetic induction or eddy current measurement methods . The main difference between the techniques is that the first measures the coating thickness on Ferrous ( Fe ) while the latter on Non-Ferrous ( NFe ) substrates . Some gauges incorporate
INFLUENCING FACTORS
When it comes to choosing a suitable coating thickness gauge , it ’ s essential to consider several crucial factors . Each one of them will determine what type of instrument and measurement probe are required .
The physical state of a coating
Initially , for coatings in a liquid state or the so-called ‘ uncured state ’, the traditional Dry Film Thickness ( DFT ) gauges won ’ t work . For such applications , non-contact powder coating thickness gauges are required . They function by placing the probe at a calibrated distance from a measured surface , and readings are obtained using the ultrasonic measurement principle .
Figure 1 . Schematic diagram of probes incorporating magnetic induction ( left ) and eddy current amplitude sensitive ( right ) methods .
both principles in one to cover all possible base materials . The principle of operation can be seen in Figure 1 .
The magnetic induction measurement method works by generating a low-frequency magnetic field that is produced by excitation current I ~. A measurement coil μ-measures the resulting magnetic field .
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 .
54 SEPTEMBER 2020 read online : www . surfaceworld . com