How to calibrate the Elcometer 456 Scan Probe
Calibration of all coating thickness gauges is critical to achieve the highest levels of measurement accuracy. Substrate shape, roughness, composition and even thickness can affect the accuracy of the coating thickness gauge.
The Elcometer 456 Coating Thickness Gauge with Scan Probe allows the user to set the gauge to the exact substrate conditions by using calibration shims, also known as foils. Calibration foils are precisely measured using laboratory equipment and labelled with the thickness values in microns and mils, allowing precise adjustment of the gauge.
When calibrating other Elcometer 456 Coating Thickness Gauge probes it is recommended to calibrate the gauge using foils close to the expected coating thickness.
When calibrating the Scan Probe it is recommended to calibrate over the entire range. This is required because of the influence of the probe cap on the total thickness being measured, as well as ensuring that the probe is set up for the full range of coating thicknesses that may be encountered in a scan.
When calibrating the Elcometer 456 Coating Thickness Gauge Scan Probe it is essential to take the thickness of the protective wear cap into account. As the probe is used over time, the probe cap thickness will decrease. Regular calibration will compensate for this loss and maintain maximum accuracy of the coating thickness gauge. The Scan Probe uses the Elcometer 456 Coating Thickness Gauge’s patented offset feature, ensuring that any cap wear during use is incorporated within the calibration process. The coating thickness gauge even informs the user when to replace the cap.
There are two main methods of calibrating the Elcometer 456 Scan Probe: Smooth for calibrating the gauge on smooth surfaces and Rough or 2 Point calibration for rough surfaces. Both methods require the calibration to start with the protective cap removed from the Scan Probe.
To calibrate the Scan Probe Scale 1, a calibration foil of 1500 µm (60mils) nominal value and an uncoated base or zero test plate are required. Alternatively, two calibration foils can be with nominal values of 1000µm (40 mils) and 500µm (20mil) and an uncoated base.
In the smooth surface calibration for a Scale 1 Probe, the foils with nominal values of 1000µm (40 mils) and 500µm (20mil) should be stacked at angles ensuring the labels do not touch, and placed on an uncoated sample of the substrate material. Following the steps in the easy to follow wizard, take 5 or more measurements on the stacked foils, and if necessary adjust the gauge reading to match the exact thickness of the foils. Then take 5 measurements on the uncoated base material. The gauge is then set to zero. The final step is to fit the wear cap and take 3 more readings on the uncoated base, compensating for the thickness of the cap.
To calibrate the Scan Probe Scale 2, follow the same procedure but use a calibration foil with a nominal value of 4mm (160mils) and an uncoated base or zero test plate.
Rough Calibration or 2 Point Calibration
Calibrating for rough, abrasively blasted surfaces is conducted in a similar way. The stacked 1000µm (40mil) and 500µm (20mil) foils are placed on the uncoated material, 5 measurements are taken, and the gauge is adjusted to match the combined thickness. For the second step the 50µm foil is placed on the surface, 5 measurements are taken, the gauge is adjusted if necessary and then set. As in the smooth calibration, the wear cap is then fitted and 3 measurements taken on the uncoated base material.
Using a 50µm calibration foil instead of the uncoated zero test plate increases the accuracy of the gauge over blasted profiles in accordance with ISO 19480 Annex A. The foil sits over the peaks of the profile and keeps the probe at a consistent level. This means that the gauge will always measure the dry film thickness from the top of the peaks of the profile, ensuring that the DFT measurement accurately indicates whether or not these peaks are adequately covered.