Elcometer

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Modern coating thickness gauges, of which the Elcometer 456 belongs to, are a technical marvel. More sophisticated models have features such as the ability to save measurement data and send them to a smartphone for easy analysis. To top it all off, these gauges are also easy to use compared to the older, mechanical, analog models, with an LCD display that is both accurate and easy to read. Just like all tools however, thickness gauges are only as effective as the capability of its handler.

Even with all the technology the gauge brings to the table, there are still some things you have to consider when using modern thickness gauges. For example, when you’re dealing with an uneven surface, you can’t use the same measurement method as you would when dealing with a flat surface. Generally, modern gauges have a setting that specifically deals with these common difficulties but the user will first have to know what exactly these special cases are.

It’s all in the probe[edit | edit source]

Ultimately, no matter what type of coating thickness gauge you’re dealing with; electromagnetic, eddy current, ultrasonic, etc, the heart of all of these gauges is the probe. The probe is a highly sophisticated tool that essentially performs the main measurement function, it’s the one that sends out the ‘signal’, for a lack of a better word, that is used for measurement purposes and one that receives the reflected ‘signal’ and reads that data that is then converted into the actual thickness of the coating.

In thickness gauges, the probe is the part of the gauge that directly makes contact with the surface and the accuracy of the measurement performed heavily rely on how you deal with this contact. Thickness measurement is rarely, if ever, straightforward. Most coated materials have curves, bends and edges and are simply uneven. The challenge is how you could get a reliably consistent reading across all of these uneven surfaces with a single gauge and simple calibration.

Planes, trains and automobiles[edit | edit source]

Transportation is one of the most common subjects in coating. Vehicles such as cars, airplanes and especially ships are always coated in some form in order to ensure their surface durability. The hull of the ship and the underline that is in constant contact with the water have to contend with corrosion given how are they exposed to saltwater during operation. Oil and gas pipelines, due to their sensitivity, also require sufficient protection from the environment and that protection comes in the form of coating.

The common theme with all of the objects I’ve mentioned above is that none of them are flat. You could argue that airplanes and ships have a somewhat uniform shape but cars and pipes definitely aren’t. It’s this variety of shapes that coating inspector regularly have to deal with that makes thickness measurement somewhat challenging. As a measurement of coating quality and durability, thickness is an integral parameter and knowing just what to watch out for when it comes to thickness measurement should help you avoid getting inaccurate readings.

Take into account curvature compensation[edit | edit source]

The primary thing you have to look out for is curves, even ones that have a relatively high radius. Thickness measurement is very precise, commonly being measured in microns where 1 micron is equal to 1/1000 millimeter. Even a slight deviation can have a measurable impact in the coating quality. The effect of curvature varies between gauge types but they all share a common theme in that the effect grows progressively worse as the radius of the curvature decreases.

Each thickness gauge normally has a maximum tolerance when it comes to curvature, any more than that and the gauge would be unable to perform an accurate reading. As long as the curvature you’re dealing with is under that tolerance though, there’s a setting within each gauge to enable a curvature compensation method. Make sure you read the manual the gauge came with to figure out how to compensate for curvature.

Compensating for the edge effect[edit | edit source]

The edge effect is when you’re dealing with an abrupt change in the shape of the surface as you would normally see in a corner or on the edge, hence why it’s referred to as the edge effect. Measurements performed near or at the edge tend to not be accurate unless the gauge is specifically set up to compensate for this. Just like curvature, modern thickness gauge usually has a setting that enables them to compensate for edge effect so check the manual or the settings on your thickness gauge.

Surface roughness and the issue of base metal reading (BMR)[edit | edit source]

The uneven surface of the uncoated substrate also has an impact on the accuracy of your measurements, which is detailed in the BMR parameter. In layman’s terms, the BMR is the effect of surface roughness on thickness measurement that is caused by the manufacturing process. Relatively speaking, surface roughness tend to have a smaller impact to the accuracy of your measurement compared to curvature and edge effect and unlike the other two, there’s no setting in thickness gauges that you could use to account for this.

To compensate for surface roughness, the typical solution is to perform more than one reading on a given spot with the end measurement result being an average of all of those readings. For a more comprehensive solution, there is also the option of using a double-tip measurement probe but keep in mind that this would result in a higher measurement cost. Some surfaces are more rough than others so before you begin the coating process, it might be useful to create a roughness profile of the uncoated substrate first to see the level of roughness you’re dealing and decide how to proceed based on your findings.

Elcometer