Gauge for testing dry film thickness of protective coatings

Paul Boughton

The new ISO19840 standard deals with the verification of nominal dry film thickness values, the adjustment of gauges, the definition of inspection areas, sampling plans, measuring and acceptance/rejection criteria for coatings applied to blast cleaned steel surfaces.
The Elcometer4562 Coating Thickness Gauge is suited for this type of work having many features appropriate to the method in the standard.
The measurement method used in ISO19840 is outlined in ISO2808:1999 Method 10, using magnetic coating thickness gauges. When such gauges are adjusted to measure accurately on smooth steel surfaces, the gauge will read high on coatings applied to a rough surface, ie shot or grit blasted. ISO19840 describes how readings can be corrected for this effect using correction values based on the assessment of the substrate profile described in ISO8503 – 1.
Correction values are given for fine, medium and coarse blast surfaces, eg for a medium blast the correction value is 25µm. This value is subtracted from the gauge reading to give the individual dry film thickness.
Inspection areas are defined as designated areas for which a sampling plan has been established. The whole structure can be a single inspection area but it is often best to divide the structure in to a number of inspection areas, particularly if some parts of the structure have special considerations, such as difficult access, areas that are difficult to coat or have special design features such as stiffeners, brackets, etc.
The sampling plan defines the number of readings for an inspection area by the size of the area. The readings must be taken randomly and, for an area up to 1m2, a minimum of five readings are required. If a single reading fails to meet the nominal thickness value it is allowed to be validated once. For larger areas more readings are required and consequently more readings can be validated.
The acceptance criteria are statistically based and take in to account the uncertainty of measurements of thin films on roughened surfaces.
A typical medium-blast steel surface may have a peak-to-valley profile height of 75µm and some of
the primer coats will have nominal thickness values
of 60µm, so it is clear that a high level of precision
is not practical, particularly so for field measurements.
For the acceptance of an inspection area, the average of the readings should be equal to or greater than the nominal thickness.
Each individual reading should be greater than 80percent of the nominal value but the number of these readings and their value should not pull down the average below the nominal thickness. If these criteria are not met then the inspection area is rejected.
Following on from the success of the original Elcometer 456, launched early in 2001, the new Elcometer 4562 Coating Thickness Gauge takes the performance of the original and combines it with improved functionality – making it even easier to use and more versatile.
The Elcometer 4562 is available with either an integral (internal) or separate probe option, in three model versions (Basic, Standard or Top). It has a number of new features designed to make the gauge even easier to use, including:
* User Definable Opening Screen: The gauge can display unique User information, eg: company logo, serial numbers, etc.
* Simpler Menu Management, in 25 languages: Menu functions can be hidden – enabling the user to see only the key functions which are in daily use.
* Multiple Batch Calibration: Individual batches can have different calibration adjustments.
* Calibration Lock: Prevents accidental changes to calibration settings.
* Enlarged Statistics: View the statistics in large characters.
* Full Compliance to ISO19840: The gauge can be set up to automatically subtract the correction value.
* Improved Data Management Software: A simple to use and more powerful data management software program is available.
The correction value calibration adjustment method, also known as zero offset, allows the appropriate correction value to be entered in to the gauge so all readings are automatically corrected. The gauge displays and stores the corrected values.
The statistical capability of the Elcometer 4562 allows the automatic calculation of the average of the readings in an inspection area and it also provides minimum and maximum reading limits to warn if the 80percent of nominal thickness limit is being breached and, where appropriate, if the maximum thickness is exceeded.
The Top version of the gauge can be used to collect batches of readings with each batch representing an individual inspection area and stored in the memory of the gauge.
The data can then be uploaded to a computer using either Elcometer Data Transfer Software, EDTS+, to put the readings in to a spreadsheet, or using the Elcometer Data Collection Software, EDCS+, the data management software that allows batches of readings to be recorded, charted, printed and included in flexible report-generated documents for the client.
ISO19840 provides a useful standard for specifiers, paint suppliers, paint contractors and paint inspectors, as it provides a structured approach to the essential process of applying protective coatings to meet the service requirements for the structure.
With the correct coating system applied to the specification, the corrosion protection for the structure will be maintained for the expected life of the coating and repair costs will be minimised.
The Elcometer 4562 Coating Thickness Gauge has been developed with features that make its use in accordance with ISO19840 both simple and quick, ensuring that the paint system has been applied as required by the specification and providing proof of that fact.

John Fletcher is Technical Support Manager with Elcometer Instruments Limited, Manchester, England.


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