introduction
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ISO 21968-2019 “Measurement of coating thickness of non-magnetic metallic coatings on metallic and non-metallic substrates by phase-sensitive eddy current method”

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This document was prepared by Technical Committee ISO/107, Metal and other Inorganic Coatings.

The second edition cancelled and replaced the technically revised first edition (ISO 21968:2005). The main changes from the previous edition are as follows:

ISO 21968-2019 “Measurement of coating thickness of non-magnetic metallic coatings on metallic and non-metallic substrates by phase-sensitive eddy current method”

This document has been adapted to the current requirements of ISO/IEC Guide 98-3 (also known as “GUM: 1995”);

– Added hints, practical examples, and simple estimates of measurement uncertainties for important factors;

– Added repeatability and reproducibility values for typical applications of the method;

– The attachment has been expanded for further application and experimental estimation of factors affecting accuracy.

Any feedback or questions regarding this document should be directed to the user’s national standards body. A complete list of these institutions can be found at iso.org/members.html.

ISO 21968-2019 “Measurement of coating thickness of non-magnetic metallic coatings on metallic and non-metallic substrates by phase-sensitive eddy current method”

1 Range
This document specifies a method for non-destructive measurement of the thickness of non-magnetic metal coatings on metal and non-metallic base materials using a phase-sensitive eddy current instrument, such as:

a) Zinc, cadmium, copper, tin or chromium on steel;

b) Copper or silver on composites.

Compared to the amplitude-sensitive eddy current method specified in ISO 2360, the phase-sensitive method can be applied to smaller surface areas and stronger surface curvature without thickness errors and is less affected by the magnetic properties of the underlying material. However, the phase-sensitive method is more affected by the electrical properties of the coating material.

In this document, the term “coating” is used for materials such as paints and varnishes, electroplating coatings, enamel coatings, plastic coatings, overcoats and powder coatings.

This method is especially suitable for the measurement of metal coating thickness. These coatings can be non-magnetic metal coatings on non-conductive, conductive, or magnetic substrates, or magnetic coatings on non-conductive or conductive substrates.

Metal coating measurements for metal-based materials are valid only if the product of the conductivity and permeability (σ, μ) of one of the materials is at least twice the product of the conductivity and permeability of the other material. The relative permeability of non-ferromagnetic materials is 1.

ISO 21968-2019 “Measurement of coating thickness of non-magnetic metallic coatings on metallic and non-metallic substrates by phase-sensitive eddy current method”

2 Normative references
The following files are referenced in text in a manner that constitutes part or all of the requirements of this document. For dated references, only the cited version applies. For undated references, a new version of the reference (including any revisions) applies.

ISO 2064, Metallic and other Inorganic coatings – Definitions and practices relating to thickness measurement

ISO 4618, paints and varnishes – Terms and definitions

ISO/IEC Guide 98-3, Uncertainty in measurement – Part 3: Guidance for the expression of Uncertainty in Measurement (GUM: 1995)

3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 2064, ISO 4618 and the following articles apply.

ISO 21968-2019 “Measurement of coating thickness of non-magnetic metallic coatings on metallic and non-metallic substrates by phase-sensitive eddy current method”

3.1 Adjustment of measurement system

A set of operations performed on a measurement system so that it provides a prescribed indication corresponding to a given value of the quantity to be measured

Note 1: The type of adjustment of a measuring system may include zero adjustment, offset adjustment, and range adjustment (sometimes called gain adjustment) of the measuring system.

Note 2: Adjustment of a measurement system should not be confused with calibration (3.2), which is a prerequisite for adjustment.

Note 3: After adjusting the measuring system, the measuring system should usually be recalibrated.

Note 4: Colloquially, the term “calibration” is often incorrectly used instead of the term “adjustment”. Similarly, the terms “verification” and “check” are often used in place of the correct terms “calibration”.

[Source: ISO/IEC Guide 99:2007, 3.11 (also known as “VIM”), modified – article note 4 added.

3.2 Calibration

Under specified conditions, the first step establishes a relationship between a quantitative value with the measurement uncertainty provided by the measurement standard and the corresponding indication with the relevant measurement uncertainty, and the second step uses this information to establish a relationship in order to obtain the measurement result from the indication

ISO 21968-2019 “Measurement of coating thickness of non-magnetic metallic coatings on metallic and non-metallic substrates by phase-sensitive eddy current method”

Note 1: Calibration can be expressed in terms of sentences, calibration functions, calibration diagrams, calibration curves, or calibration tables. In some cases, it may include addition or multiplication correction for indications with associated measurement uncertainty.

Note 2: Calibration should not be confused with adjustment of the measurement system (3.1), often incorrectly referred to as “self-calibration”, nor should it be confused with calibration verification.

Note 3: Generally, the first step in the above definition is itself considered calibration.

[Source: ISO/IEC Guide 99:2007, 2.39 (also known as “VIM”)]

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