Ever since the development of the coating thickness gauge in the mid-1940s, dry film thickness measurement has relied on individual measurements being compared with a coating’s specification.
However, with the introduction of digital coating thickness gauges and coating thickness measurements becoming more accurate and repeatable, and easier than before, the task has become much more simplified.
This is according to coatings inspection equipment manufacturer and supplier Elcometer, which says modern gauges, such as the Elcometer 456, have significant processing power built in – allowing users to automatically compare thickness values with a coating’s specification, display trend graphs, and commit the reading and the date and time it was taken to memory.
The gauge can also transfer data wirelessly to a cellphone, recording the global positioning satellite coordinates of precisely where the measurement was taken. Measurement speeds have also increased significantly, almost doubling, from about 40 readings a minute in the 1980s, to in excess of 70 readings a minute when using the new Elcometer 456.
The measurement speed is used by manufacturers to indicate how quickly an accurate reading can be taken and, therefore, how soon any subsequent reading can be made.
“If two inspectors are measuring the dry film thickness of a pipeline, inspector one is on one side of the pipe and inspector two is on the opposite side. Both are tasked to take three spot measurements every 5 m. If inspector one is using a gauge with a measurement rate of 70 readings a minute and inspector two is using a gauge with 40 readings a minute, it will not be too long before inspector one is significantly further ahead of inspector two,” illustrates Elcometer.
Other than the time it takes to move to the next measurement location, the limiting factor for increasing the measurement speed – thereby reducing the time taken to undertake a coating thickness inspection – is the time required to lift the probe off and replace it back on to the coated substrate.
If the inspector can set up the gauge to automatically take a predetermined number of readings, without the need to lift the probe off the surface, then the speed of the measurement time can be increased even further.
Additionally, if the probe could be dragged across the coated surface, without damaging the probe or the coating, the need for prescribing the number of readings to be taken over a defined area can be brought into question.
The Elcometer 456 with Ultra/Scan Probes
When connected to the latest Elcometer 456 coating thickness gauge, the newly developed Elcometer 456 Ultra/Scan probe has a reading rate, in scan mode, in excess of 140 readings a minute. This further enhances the speed and accuracy of field-based dry film coating thickness measurement on ferrous and nonferrous substrates.
Each Ultra/Scan probe has been designed to use a snap-on replaceable end cap, so that the sliding action required to scan a coated surface does not cause any wear to the probe tip – crucial to maintaining the accuracy of the probe over its life.
Using the Elcometer 456’s patented offset feature, the thickness of the cap is excluded from any coating thickness measurement and, as the cap wears during use, the wear effect is also accounted for. The gauge will even display a warning message when the wear cap needs to be replaced.
The Elcometer 456 Ultra/Scan probe can be used in either scan or auto repeat modes.
When selected, the scan mode allows users to slide the Ultra/Scan probe over the entire surface area. As the probe is lifted off the surface, the gauge not only displays the average coating thickness, but also the highest and lowest coating thickness values over the entire scan. Each set of three readings is then stored into memory, together with the date and time of the scan.
The values can also be displayed on the gauge’s display as a run chart. During each scan, the Elcometer 456 displays the live thickness reading together with an analogue bar graph, which graphically indicates the thickness relative to both the nominal thickness and any user-defined thickness limits.
The user can simply drag the probe continuously across a coated surface and, upon removing the probe from the surface, at a glance, view the display to see the average, highest and lowest readings. The user is also able to see whether they are inside or outside any predetermined limits. Therefore, invaluable and immediate information is provided.
The scan mode can also be used with a ‘hold’ function. This feature allows the probe to be momentarily lifted from the surface to clear areas such as welded joints or cut-outs and be placed back in contact with the coating to continue data recoding without interruption of the data set.
Using the pipeline example to demonstrate how useful the scan feature can be, the inspector can simply walk along the pipe – with the probe in contact with the coating – and upon removing the probe from the pipe, immediately assess the high, low and average coating thickness values on the gauge screen.
In auto repeat mode, as the probe slides across the surface, more than two readings are taken every second, with each individual reading stored in the memory of the gauge.
With a reading rate in excess of 140 readings a minute, the auto repeat mode significantly speeds up the inspection of large areas of coatings, which requires the recording and analysis of individual readings.
Working with Standards and Test Methods
International standards and test methods often describe the number of individual gauge readings to be taken in a spot measurement and/or the number of spot measurements required over a defined surface area.
For example, the Society for Protective Coatings’ (SSPC’s) Coating Application Standard No 2, which outlines the procedure for determining conformance to dry coating thickness requirements, state that a minimum of three gauge readings must be taken for each spot measurement and five spot measurements mus be taken over a 10 m2 area.
Using the Elcometer 456’s counted average and fixed batch modes with the Ultra/Scan probe set to auto repeat, the gauge automatically takes three readings, stores the average and is ready for the next set of three readings.
In this way, the user no longer has to lift the probe off the surface in between each gauge reading – reducing the time it takes to measure according to the SSPC, or similar test methods, by up to 40%.