Non-magnetic metallic coatings on metallic and non-metallic basis materials - Measurement of coating thickness - Phase-sensitive eddy-current method (ISO 21968:2019)

This document specifies a method for using phase-sensitive eddy-current instruments for non-destructive measurements of the thickness of non-magnetic metallic coatings on metallic and non-metallic basis materials such as:
a) zinc, cadmium, copper, tin or chromium on steel;
b) copper or silver on composite materials.
The phase-sensitive method can be applied without thickness errors to smaller surface areas and to stronger surface curvatures than the amplitude-sensitive eddy-current method specified in ISO 2360, and is less affected by the magnetic properties of the basis material. However, the phase-sensitive method is more affected by the electrical properties of the coating materials.
In this document, the term "coating" is used for materials such as, for example, paints and varnishes, electroplated coatings, enamel coatings, plastic coatings, claddings and powder coatings.
This method is particularly applicable to measurements of the thickness of metallic coatings. These coatings can be non-magnetic metallic coatings on non-conductive, conductive or magnetic base materials, but also magnetic coatings on non-conductive or conductive base materials.
The measurement of metallic coatings on metallic basis material works only when the product of conductivity and permeability (σ, μ) of one of the materials is at least a factor of two times the product of conductivity and permeability for the other material. Non-ferromagnetic materials have a relative permeability of one.

Nichtmagnetische metallische Überzüge auf metallischen und nichtmetallischen Grundwerkstoffen - Messung der Schichtdicke - Phasensensitives Wirbelstromverfahren (ISO 21968:2019)

Dieses Dokument legt ein Verfahren unter Anwendung phasensensitiver Wirbelstromgeräte zur zerstörungsfreien Messung der Dicke nichtmagnetischer metallischer Überzüge auf metallischen und nichtmetallischen Grundwerkstoffen fest, wie beispielsweise:
a)   Zink, Cadmium, Kupfer, Zinn oder Chrom auf Stahl;
b)   Kupfer oder Silber auf Verbundwerkstoffen.
Das phasensensitive Verfahren kann ohne Schichtdickenfehler auf kleinere Oberflächen und stärkere Oberflächenkrümmungen angewendet werden als das nach ISO 2360 festgelegte amplitudensensitive Wirbel¬stromverfahren und wird weniger stark durch die magnetischen Eigenschaften des Grundwerkstoffs beeinflusst. Dagegen wird das phasensensitive Verfahren stärker durch die elektrischen Eigenschaften des Schichtwerkstoffs beeinflusst.
In diesem Dokument wird der Begriff „Beschichtung“ bzw. „Überzug“ verwendet für z. B. Lacke und Anstrichstoffe, galvanische Überzüge, Email, Kunststoffschichten, Umhüllungen und Pulverlacke.
Dieses Verfahren ist besonders für Messungen der Dicke metallischer Überzüge geeignet. Diese Überzüge können nichtmagnetische metallische Überzüge auf nichtleitenden, leitenden oder magnetischen Grundwerkstoffen sein, aber auch magnetische Überzüge auf nichtleitenden oder leitenden Grundwerkstoffen.
Die Messung von metallischen Überzügen auf metallischem Grundwerkstoff funktioniert nur, wenn das Produkt der Leitfähigkeit und der Permeabilität (σ, µ) eines der Werkstoffe mindestens das Zweifache des Produktes der Leitfähigkeit und der Permeabilität des anderen Werkstoffs beträgt. Nichtferromagnetische Werkstoffe haben eine relative Permeabilität von 1.

Revêtements métalliques non magnétiques sur des matériaux de base métalliques et non métalliques - Mesurage de l'épaisseur de revêtement - Méthode par courants de Foucault sensible aux variations de phase (ISO 21968:2019)

Le présent document spécifie une méthode utilisant des instruments fonctionnant par courants de Foucault sensibles aux variations de phase pour le mesurage non destructif de l'épaisseur des revêtements métalliques non magnétiques sur des matériaux de base métalliques et non métalliques, tels que:
a) le zinc, le cadmium, le cuivre, l'étain ou le chrome sur de l'acier;
b) le cuivre ou l'argent sur des matériaux composites.
La méthode sensible aux variations de phase peut être appliquée sans erreur d'épaisseur à des surfaces planes plus petites et à des surfaces courbes plus accentuées que la méthode par courants de Foucault sensible aux variations d'amplitude, spécifiée dans l'ISO 2360, et est moins affectée par les propriétés magnétiques du matériau de base. Toutefois, la méthode sensible aux variations de phase est davantage affectée par les propriétés électriques des matériaux de revêtement.
Dans le présent document, le terme «revêtement» est utilisé pour désigner des produits tels que, par exemple, les peintures et vernis, les revêtements électrolytiques, les revêtements en émaux, les revêtements plastiques, les placages et les revêtements en poudre.
Cette méthode s'applique tout notamment au mesurage de l'épaisseur des revêtements métalliques. Ces revêtements peuvent être des revêtements métalliques non magnétiques sur des matériaux de base non conducteurs, conducteurs ou magnétiques, mais aussi des revêtements magnétiques sur des matériaux de base non conducteurs ou conducteurs.
Le mesurage des revêtements métalliques sur des matériaux de base métalliques ne fonctionne que lorsque le produit de la conductivité et de la perméabilité (σ, μ) de l'un des matériaux représente au moins deux fois le produit de la conductivité et de la perméabilité de l'autre matériau. Les matériaux non ferromagnétiques ont une perméabilité relative de un.

Nemagnetne kovinske prevleke na kovinskih in nekovinskih osnovnih materialih - Merjenje debeline nanosa prevleke - Metoda vrtinčnih tokov (ISO 21968:2019)

Ta dokument določa metodo za uporabo fazno občutljivih instrumentov za preiskave z vrtinčnimi tokovi za neporušitvene meritve debeline nemagnetnih kovinskih prevlek na kovinskih in nekovinskih osnovnih materialih, kot so: a) cink, kadmij, baker, kositer ali krom na jeklu; b) baker ali srebro na kompozitnih materialih. Fazno občutljivo metodo je mogoče uporabiti brez napak debeline na manjših površinah in na večjih ukrivljenostih površine kot metodo vrtinčnih tokov, občutljivo za spremembe amplitude, ki je določena v standardu ISO 2360, pri čemer nanjo manj vplivajo magnetne lastnosti osnovnega materiala. Vendar pa na fazno občutljivo metodo bolj vplivajo električne lastnosti premazov. V tem dokumentu se izraz »prevleka« uporablja za materiale, kot so barve in laki, elektrolitske prevleke, emajlirane prevleke, plastične prevleke, obloge in praškaste prevleke. Ta metoda je še posebej uporabna za meritve debeline kovinskih prevlek. Te prevleke so lahko nemagnetne kovinske prevleke na neprevodnih, prevodnih ali magnetnih osnovnih materialih in tudi magnetne prevleke na neprevodnih ali prevodnih osnovnih materialih. Merjenje kovinskih prevlek na kovinskih osnovnih materialih deluje samo, ko je zmnožek prevodnosti in prepustnosti (s, μ) enega od materialov najmanj faktor dvakratnega zmnožka prevodnosti in prepustnosti drugega materiala. Relativna prepustnost neferomagnetnih materialov je enaka vrednosti ena.

General Information

Status
Published
Publication Date
15-Oct-2019
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Due Date
16-Oct-2019
Completion Date
16-Oct-2019

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SLOVENSKI STANDARD
SIST EN ISO 21968:2019
01-december-2019
Nadomešča:
SIST EN ISO 21968:2005
Nemagnetne kovinske prevleke na kovinskih in nekovinskih osnovnih materialih -
Merjenje debeline nanosa prevleke - Metoda vrtinčnih tokov (ISO 21968:2019)
Non-magnetic metallic coatings on metallic and non-metallic basis materials -
Measurement of coating thickness - Phase-sensitive eddy-current method (ISO
21968:2019)
Nichtmagnetische metallische Überzüge auf metallischen und nichtmetallischen

Grundwerkstoffen - Messung der Schichtdicke - Phasensensitives Wirbelstromverfahren

(ISO 21968:2019)
Revêtements métalliques non magnétiques sur des matériaux de base métalliques et

non métalliques - Mesurage de l'épaisseur de revêtement - Méthode par courants de

Foucault sensible aux variations de phase (ISO 21968:2019)
Ta slovenski standard je istoveten z: EN ISO 21968:2019
ICS:
25.220.40 Kovinske prevleke Metallic coatings
SIST EN ISO 21968:2019 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 21968:2019
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SIST EN ISO 21968:2019
EN ISO 21968
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2019
EUROPÄISCHE NORM
ICS 25.220.40 Supersedes EN ISO 21968:2005
English Version
Non-magnetic metallic coatings on metallic and non-
metallic basis materials - Measurement of coating
thickness - Phase-sensitive eddy-current method (ISO
21968:2019)

Revêtements métalliques non magnétiques sur des Nichtmagnetische metallische Überzüge auf

matériaux de base métalliques et non métalliques - metallischen und nichtmetallischen Grundwerkstoffen

Mesurage de l'épaisseur de revêtement - Méthode par - Messung der Schichtdicke - Phasensensitives

courants de Foucault sensible aux variations de phase Wirbelstromverfahren (ISO 21968:2019)

(ISO 21968:2019)
This European Standard was approved by CEN on 15 September 2019.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this

European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references

concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN

member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by

translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management

Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,

Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and

United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels

© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 21968:2019 E

worldwide for CEN national Members.
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SIST EN ISO 21968:2019
EN ISO 21968:2019 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 21968:2019
EN ISO 21968:2019 (E)
European foreword

This document (EN ISO 21968:2019) has been prepared by Technical Committee ISO/TC 107 "Metallic

and other inorganic coatings" in collaboration with Technical Committee CEN/TC 262 “Metallic and

other inorganic coatings, including for corrosion protection and corrosion testing of metals and alloys”

the secretariat of which is held by BSI.

This European Standard shall be given the status of a national standard, either by publication of an

identical text or by endorsement, at the latest by April 2020, and conflicting national standards shall be

withdrawn at the latest by April 2020.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CEN shall not be held responsible for identifying any or all such patent rights.

This document supersedes EN ISO 21968:2005.

According to the CEN-CENELEC Internal Regulations, the national standards organizations of the

following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,

Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,

Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of

North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the

United Kingdom.
Endorsement notice

The text of ISO 21968:2019 has been approved by CEN as EN ISO 21968:2019 without any modification.

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SIST EN ISO 21968:2019
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SIST EN ISO 21968:2019
INTERNATIONAL ISO
STANDARD 21968
Second edition
2019-09
Non-magnetic metallic coatings
on metallic and non-metallic basis
materials — Measurement of coating
thickness — Phase-sensitive eddy-
current method
Revêtements métalliques non magnétiques sur des matériaux de
base métalliques et non métalliques — Mesurage de l'épaisseur
de revêtement — Méthode par courants de Foucault sensible aux
variations de phase
Reference number
ISO 21968:2019(E)
ISO 2019
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SIST EN ISO 21968:2019
ISO 21968:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2019

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
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SIST EN ISO 21968:2019
ISO 21968:2019(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle of measurement .......................................................................................................................................................................... 2

5 Factors affecting measurement uncertainty ........................................................................................................................... 5

5.1 Basic influence of the coating thickness ........................................................................................................................... 5

5.2 Electrical properties of the coating ....................................................................................................................................... 5

5.3 Geometry — Base material thickness ................................................................................................................................. 5

5.4 Geometry — Edge effects ............................................................................................................................................................... 5

5.5 Geometry — Surface curvature ................................................................................................................................................ 6

5.6 Surface roughness ................................................................................................................................................................................ 6

5.7 Lift-off effect .............................................................................................................................................................................................. 6

5.8 Probe pressure ........................................................................................................................................................................................ 8

5.9 Probe tilt ....................................................................................................................................................................................................... 8

5.10 Temperature effects ............................................................................................................................................................................ 8

5.11 Intermediate coatings ....................................................................................................................................................................... 8

5.12 External electromagnetic fields ................................................................................................................................................ 8

6 Calibration and adjustment of the instrument ..................................................................................................................... 8

6.1 General ........................................................................................................................................................................................................... 8

6.2 Thickness reference standards ................................................................................................................................................. 9

6.3 Methods of adjustment .................................................................................................................................................................... 9

7 Measurement procedure and evaluation .................................................................................................................................10

7.1 General ........................................................................................................................................................................................................10

7.2 Number of measurements and evaluation ...................................................................................................................10

8 Uncertainty of the results .........................................................................................................................................................................11

8.1 General remarks .................................................................................................................................................................................11

8.2 Uncertainty of the calibration of the instrument ...................................................................................................11

8.3 Stochastic errors .................................................................................................................................................................................12

8.4 Uncertainties caused by factors summarized in Clause 5 .............................................................................13

8.5 Combined uncertainty, expanded uncertainty and final result ..................................................................13

9 Precision ....................................................................................................................................................................................................................14

9.1 General ........................................................................................................................................................................................................14

9.2 Repeatability (r) ..................................................................................................................................................................................14

9.3 Reproducibility limit (R) ..............................................................................................................................................................16

10 Test report ................................................................................................................................................................................................................17

Annex A (informative) Eddy-current generation in a metallic conductor ..................................................................18

Annex B (informative) Basics of the determination of the uncertainty of a measurement of

the used measurement method corresponding to ISO/IEC Guide 98-3 ...................................................24

Annex C (informative) Basic performance requirements for coating thickness gauges based

on the phase-sensitive eddy-current method described in this document ..........................................26

Annex D (informative) Examples for the experimental estimation of factors affecting the

measurement accuracy ...............................................................................................................................................................................28

Annex E (informative) Table of the student factor ..............................................................................................................................33

Annex F (informative) Example of uncertainty estimation ........................................................................................................34

Annex G (informative) Details on precision ...............................................................................................................................................37

Bibliography .............................................................................................................................................................................................................................39

© ISO 2019 – All rights reserved iii
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SIST EN ISO 21968:2019
ISO 21968:2019(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso

.org/iso/foreword .html.

This document was prepared by Technical Committee ISO/107, Metallic and other inorganic coatings.

This second edition cancels and replaces the first edition (ISO 21968:2005), which has been technically

revised. The main changes compared with the previous edition are as follows:

— this document has been adapted to the current requirements of ISO/IEC Guide 98-3 (also known as

“GUM: 1995”);

— hints, practical examples and simple estimations of the measurement uncertainty for most important

factors have been added;

— repeatability and reproducibility values for typical applications of the method have been added;

— the annex has been expanded with further applications and experimental estimations of factors

affecting the accuracy.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2019 – All rights reserved
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SIST EN ISO 21968:2019
INTERNATIONAL STANDARD ISO 21968:2019(E)
Non-magnetic metallic coatings on metallic and non-
metallic basis materials — Measurement of coating
thickness — Phase-sensitive eddy-current method
1 Scope

This document specifies a method for using phase-sensitive eddy-current instruments for non-

destructive measurements of the thickness of non-magnetic metallic coatings on metallic and non-

metallic basis materials such as:
a) zinc, cadmium, copper, tin or chromium on steel;
b) copper or silver on composite materials.

The phase-sensitive method can be applied without thickness errors to smaller surface areas and to

stronger surface curvatures than the amplitude-sensitive eddy-current method specified in ISO 2360,

and is less affected by the magnetic properties of the basis material. However, the phase-sensitive

method is more affected by the electrical properties of the coating materials.

In this document, the term “coating” is used for materials such as, for example, paints and varnishes,

electroplated coatings, enamel coatings, plastic coatings, claddings and powder coatings.

This method is particularly applicable to measurements of the thickness of metallic coatings. These

coatings can be non-magnetic metallic coatings on non-conductive, conductive or magnetic base

materials, but also magnetic coatings on non-conductive or conductive base materials.

The measurement of metallic coatings on metallic basis material works only when the product of

conductivity and permeability (σ, μ) of one of the materials is at least a factor of two times the product

of conductivity and permeability for the other material. Non-ferromagnetic materials have a relative

permeability of one.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 2064, Metallic and other inorganic coatings — Definitions and conventions concerning the measurement

of thickness
ISO 4618, Paints and varnishes — Terms and definitions

ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in

me a s ur ement (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 apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https: //www .iso .org/obp
© ISO 2019 – All rights reserved 1
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SIST EN ISO 21968:2019
ISO 21968:2019(E)
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
adjustment of a measuring system

set of operations carried out on a measuring system so that it provides prescribed indications

corresponding to given values of a quantity to be measured

Note 1 to entry: Types of adjustment of a measuring system can include zero adjustment of a measuring system,

offset adjustment, and span adjustment (sometimes called gain adjustment).

Note 2 to entry: Adjustment of a measuring system should not be confused with calibration (3.2), which is a

prerequisite for adjustment.

Note 3 to entry: After an adjustment of a measuring system, the measuring system shall usually be recalibrated.

Note 4 to entry: Colloquially the term “calibration” is frequently, but falsely, used instead of the term “adjustment”.

In the same way, the terms “verification” and “checking” are often used instead of the correct term “calibration”.

[SOURCE: ISO/IEC Guide 99:2007, 3.11 (also known as “VIM”), modified — Note 4 to entry has been added.]

3.2
calibration

operation that, under specified conditions, in a first step, establishes a relation between the quantity

values with measurement uncertainties provided by measurement standards and corresponding

indications with associated measurement uncertainties and, in a second step, uses this information to

establish a relation for obtaining a measurement result from an indication

Note 1 to entry: A calibration may be expressed by a statement, calibration function, calibration diagram,

calibration curve, or calibration table. In some cases, it may consist of an additive or multiplicative correction of

the indication with associated measurement uncertainty.

Note 2 to entry: Calibration should not be confused with adjustment of a measuring system (3.1), often mistakenly

called “self-calibration”, nor with verification of calibration.

Note 3 to entry: Often, the first step alone in the above definition is perceived as being calibration.

[SOURCE: ISO/IEC Guide 99:2007, 2.39 (also known as “VIM”)]
4 Principle of measurement

Phase-sensitive eddy-current instruments work on the principle that a high-frequency electromagnetic

field generated by the probe system of the instrument will produce eddy currents in the coating on which

the probe is placed and in the base material beneath the coating in case this base material is conductive

(see Figure 1). These induced currents cause a change of the electromagnetic field surrounding the

probe coil system and therefore result in a change of the amplitude and the phase angle of the probe coil

impedance. The induced eddy-current density is a function of the coating thickness, the conductivity of

the coating material, the used frequency of the probe system and the base metal conductivity. If the

thickness of a coating of constant conductivity is increased for a given frequency, the impedance vector

describes a so-called local function of the thickness in the impedance plane (see Figure 2). Each point of

this local curve connects a phase angle of the impedance vector with the respective coating thickness.

Consequently, this impedance angle (phase shift) can be used as a measure of the thickness of the

coating on the conductor by means of a calibration with reference standards (see also Annex A).

In order to measure a change of the coil impedance phase angle, the test coil is usually part of a coil

system and is coupled with the exciting coil on one ferrite core such as in a transformer (see Figure 1).

The changes of phase angle and amplitude due to the impact of the induced eddy currents can be

measured, for example, using a lock in amplifier. These values are usually pre-processed by digital

means and the resulting thickness is then calculated and displayed.
2 © ISO 2019 – All rights reserved
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SIST EN ISO 21968:2019
ISO 21968:2019(E)

The probe and measuring system/display may be integrated into a single instrument.

NOTE 1 Annex C describes the basic performance requirements of the equipment.
NOTE 2 Factors affecting measurement accuracy are discussed in Clause 5.
Key
1 exciting current 4 measured signal U = f(t(φ))
2 ferrit core of probe 5 base material (conductive)
3 high-frequency alternating magnetic field 6 induced eddy currents
Figure 1 — Phase-sensitive eddy-current method
© ISO 2019 – All rights reserved 3
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SIST EN ISO 21968:2019
ISO 21968:2019(E)
Dimensions in micrometre
Key
1 conductivity local curve for the frequency f
2 thickness local curve of Cu for the frequency f
3 conductivity local curve for the frequency f
4 thickness local curve of Cu for the frequency f
5 coil in air (unaffected)
X real part
Y imaginary part

Figure 2 — Thickness local curves of Cu in the normalized impedance plane for two frequencies

f and f
1 2
For each instrument, there is a maximum measurable thickness of the coating.

Since this thickness range depends on both the applied frequency of the probe system and the electrical

conductivity of the coating, the maximum thickness should be determined experimentally, unless

otherwise specified by the manufacturer.

An explanation of eddy-current generation and the calculation of the maximum measurable coating

thickness, t , is given in Annex A.
max
4 © ISO 2019 – All rights reserved
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SIST EN ISO 21968:2019
ISO 21968:2019(E)

However, in the absence of any other information, the maximum measurable coating thickness, t ,

max
can be estimated using Formula (1):
t ≈⋅08, δ (1)
max 0
where δ is the standard penetration depth of the coating material (see Annex A).
5 Factors affecting measurement uncertainty
5.1 Basic influence of the coating thickness

The sensitivity of a probe, i.e. the measurement effect, depends on the used frequency, the conductivity

of the coating and the base material, and the properties of the probe system. Besides the properties of

the probe system, the resulting uncertainty of the thickness also depends on the sample materials, such

as the homogeneity of the coating and base metal conductivity and roughness.
5.2 Electrical properties of the coating

The conductivity of the coating as well as the base material determine the induced eddy-current density

for a given probe system and frequency. Consequently, the coating and base metal conductivity cause

the measurement effect for this method. The relationship between coating thickness and the measured

value depends strongly on the conductivity of both the coating and base material. Therefore, calibration

procedures and measurements shall be made on the same material. Different materials with different

conductivities as well as local fluctuations of the conductivity or variations between different samples

can cause (more or less) errors in the thickness reading.
5.3 Geometry — Base material thickness

In cases of a conductive base material (base metal), the generation of eddy currents by the coil's

magnetic field in the depth of the base metal is obstructed if the base metal thickness is too small. This

influence can only be neglected above a certain critical minimum base metal thickness.

Therefore, the thickness of the base metal should always be higher than this critical minimum base

metal thickness. An adjustment of the instrument can compensate for errors caused by thin base metal.

However, any variation in thickness of the base metal can cause increased uncertainty and errors.

The critical minimum base metal thickness depends on both the probe system (frequency, geometry)

and the conductivity of the coating and the base metal. Its value should be determined experimentally,

unless otherwise specified by the manufacturer.

NOTE A simple experiment to estimate the critical minimum base metal thickness is described in Annex D.

However, in the absence of any other information, the required minimum base metal thickness, t ,

min
can be estimated from Formula (2):
t =⋅3 δ (2)
min 0
where δ is the standard penetration depth of the base metal (see Annex A).

In cases of a non-conductive and non-magnetic base material, the base material thickness does not

affect the measurement results and consequently it shall not be considered as an influencing factor.

5.4 Geometry — Edge effects

The induction of eddy currents is obstructed by geometric limitations of the coating (such as edges,

drills and others). Therefore, measurements made too near to an edge or corner may not be valid unless

© ISO 2019 – All rights reserved 5
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...

SLOVENSKI STANDARD
oSIST prEN ISO 21968:2018
01-december-2018
1HPDJQHWQHNRYLQVNHSUHYOHNHQDNRYLQVNLKLQQHNRYLQVNLKRVQRYQLKPDWHULDOLK
0HUMHQMHGHEHOLQHQDQRVDSUHYOHNH0HWRGDYUWLQþQLKWRNRY ,62
Non-magnetic metallic coatings on metallic and non-metallic basis materials -
Measurement of coating thickness - Phase-sensitive eddy-current method (ISO/DIS
21968:2018)
Nichtmagnetische metallische Überzüge auf metallischen und nichtmetallischen

Grundwerkstoffen - Messung der Schichtdicke - Phasensensitives Wirbelstromverfahren

(ISO/DIS 21968:2018)
Revêtements métalliques non magnétiques sur des matériaux de base métalliques et

non métalliques - Mesurage de l'épaisseur de revêtement - Méthode par courants de

Foucault sensible aux variations de phase (ISO/DIS 21968:2018)
Ta slovenski standard je istoveten z: prEN ISO 21968
ICS:
25.220.40 Kovinske prevleke Metallic coatings
oSIST prEN ISO 21968:2018 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN ISO 21968:2018
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oSIST prEN ISO 21968:2018
DRAFT INTERNATIONAL STANDARD
ISO/DIS 21968
ISO/TC 107 Secretariat: KATS
Voting begins on: Voting terminates on:
2018-10-08 2018-12-31
Non-magnetic metallic coatings on metallic and non-
metallic basis materials — Measurement of coating
thickness — Phase-sensitive eddy-current method

Revêtements métalliques non magnétiques sur des matériaux de base métalliques et non métalliques —

Mesurage de l'épaisseur de revêtement — Méthode par courants de Foucault sensible aux variations de phase

ICS: 25.220.40
THIS DOCUMENT IS A DRAFT CIRCULATED
This document is circulated as received from the committee secretariat.
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
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WHICH REFERENCE MAY BE MADE IN
Reference number
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ISO/DIS 21968:2018(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION. ISO 2018
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ISO/DIS 21968:2018(E)
ISO/DIS 21968:2018(E)
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oSIST prEN ISO 21968:2018
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COPYRIGHT PROTECTED DOCUMENT

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utilized otherwise in any form or by any means, electronic or mechanical, including

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Permission can be requested from either ISO at the address below or ISO’s member body in the

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© ISO 2018 – All rights reserved
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oSIST prEN ISO 21968:2018
ISO/DIS 21968:2018(E)
Contents Page

Foreword ........................................................................................................................................................................... v

1 Scope ....................................................................................................................................................................1

2 Normative references ....................................................................................................................................1

3 Terms and definitions ....................................................................................................................................2

4 Principle of measurement ............................................................................................................................2

5 Factors affecting measurement uncertainty ..........................................................................................5

5.1 Basic influence of the coating thickness ..................................................................................................5

5.2 Electrical properties of the coating ...........................................................................................................5

5.3 Geometry: base material thickness ...........................................................................................................5

5.4 Geometry: edge effects ...................................................................................................................................6

5.5 Geometry: surface curvature .......................................................................................................................6

5.6 Surface roughness ...........................................................................................................................................6

5.7 Lift-off effect ......................................................................................................................................................6

5.8 Probe pressure .................................................................................................................................................8

5.9 Probe tilt .............................................................................................................................................................8

5.10 Temperature effects .......................................................................................................................................8

5.11 Intermediate coatings ....................................................................................................................................8

5.12 External electromagnetic fields .................................................................................................................8

6 Calibration and adjustment of the instrument .....................................................................................9

6.1 General ................................................................................................................................................................9

6.2 Thickness reference standards ..................................................................................................................9

6.3 Methods of adjustment ..................................................................................................................................9

7 Measurement procedure and evaluation ............................................................................................. 10

7.1 General ............................................................................................................................................................. 10

7.2 Number of measurements and evaluation .......................................................................................... 11

8 Uncertainty of the results .......................................................................................................................... 11

8.1 General remarks ........................................................................................................................................... 11

8.2 Uncertainty of the calibration of the instrument .............................................................................. 12

8.3 Stochastic errors ........................................................................................................................................... 13

8.4 Uncertainties caused by factors summarized in Clause 5 .............................................................. 13

8.5 Combined uncertainty, expanded uncertainty and final result ................................................... 14

9 Precision .......................................................................................................................................................... 14

9.1 General ............................................................................................................................................................. 14

9.2 Repeatability (r) ............................................................................................................................................ 14

9.3 Reproducibility limit (R) ............................................................................................................................ 16

10 Test report ...................................................................................................................................................... 18

(informative) Eddy current generation in a metallic conductor ............................................. 19

A.1 General ............................................................................................................................................................. 19

A.2 Example 1: conductive coating on a non-conductive base material .......................................... 20

A.3 Example 2: conductive coating on a conductive and/or magnetic base metal ....................... 21

A.4 Example 3: non-conductive coating on a conductive and/or magnetic base metal ............. 23

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(informative) Basics of the determination of the uncertainty of a measurement of

the used measurement method corresponding to ISO/IEC Guide 98-3 ................................... 25

B.1 General ............................................................................................................................................................. 25

B.2 Type A ............................................................................................................................................................... 25

B.3 Type B ............................................................................................................................................................... 26

(informative) Basic performance requirements for coating thickness gauges which

are based on the phase-sensitive eddy current method described in this document ........ 27

C.1 Technical specification ............................................................................................................................... 27

C.2 Check/verification of instruments and probes ................................................................................. 27

C.2.1 Prior to the supply, after repair and at regular intervals after use ........................................... 27

C.2.2 Performed on site......................................................................................................................................... 28

(informative) Examples for the experimental estimation of factors affecting the

measurement accuracy .............................................................................................................................. 29

D.1 General ............................................................................................................................................................. 29

D.2 Edge effect ....................................................................................................................................................... 29

D.3 Base metal thickness ................................................................................................................................... 30

D.4 Surface curvature ......................................................................................................................................... 31

D.5 Conductivity and permeability of the base metal ............................................................................. 32

D.6 Lift-off-height ................................................................................................................................................. 33

(informative) Table of the student factor ........................................................................................ 35

(informative) Example of uncertainty estimation (see Clause 8) ........................................... 36

F.1 Sample details ............................................................................................................................................... 36

F.2 Steps .................................................................................................................................................................. 36

F.2.1 the example sample is measured by the following steps. .............................................................. 36

F.2.2 All other possible factors affecting the measurement accuracy are considered to be

negligible in this example (edge effect, base metal thickness, curvature, temperature

drift, etc.) ......................................................................................................................................................... 38

F.2.3 Further conclusions: it is obvious that the resulting uncertainty is limited by the

largest uncertainty component, in this case the uncompensated lift-off effect (see

5.7). Therefore, an increase of the number of repeated measurements would reduce

u , however, the combined uncertainty would not be strongly affected in this way.

sto
However the compensation of the lift-off effect would reduce the combined

uncertainty to ................................................................................................................................................ 38

(informative) Details on precision .................................................................................................... 39

G.1 General notes on the round-robin test ................................................................................................. 39

G.2 Samples ............................................................................................................................................................ 39

G.3 Film thickness gauges ................................................................................................................................. 39

G.4 Calibration ...................................................................................................................................................... 39

G.5 Number of measurements ........................................................................................................................ 39

G.6 Evaluation ....................................................................................................................................................... 39

Bibliography ................................................................................................................................................................. 41

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Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national

standards bodies (ISO member bodies). The work of preparing International Standards is normally

carried out through ISO technical committees. Each member body interested in a subject for which a

technical committee has been established has the right to be represented on that committee.

International organizations, governmental and non-governmental, in liaison with ISO, also take part in

the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all

matters of electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following

URL: www.iso.org/iso/foreword.html.

The committee responsible for this document is ISO/107, Metallic and other inorganic coatings.

This second edition cancels and replaces the firstedition (ISO 21968:2005), which has been technically

revised with changes as follows.

— adaption of this document to the current requirements of ISO/IEC Guide 98 3 (GUM:1995);

— inclusion of hints, practical examples and simple estimations of the measurement uncertainty for

most important factors;

— inclusion of a repeatability and reproducibility values for typical applications of this method;

— expansion of the Annex with further applications, experimental estimations of factors affecting the

accuracy;
— editioriel amendments according to the current ISO directives part 2.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www.iso.org/members.html.
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DRAFT INTERNATIONAL STANDARD ISO/DIS 21968:2018(E)
Non-magnetic metallic coatings on metallic and non-metallic
basis materials — Measurement of coating thickness — Phase-
sensitive eddy-current method
1 Scope

This document specifies a method for using phase sensitive eddy current instruments for non-

destructive measurements of the thickness of non-magnetic metallic coatings on metallic and non-

metallic basis materials such as:
1. zinc, cadmium, copper, tin or chromium on steel;
2. Copper or silver on composite materials.

The phase sensitive method can be applied without thickness errors to smaller surface areas and to

stronger surface curvatures than the amplitude sensitive eddy current method specified in ISO 2360,

and is less affected by the magnetic properties of the basis material. However, the phase sensitive

method is more affected by the electrical properties of the coating materials.

In this document the term “coating” is used for materials such as, for example, paints and varnishes,

electroplated coatings, enamel coatings, plastic coatings, claddings and powder coatings.

This method is particularly applicable to measurements of the thickness of metallic coatings. These

coatings can be non-magnetic metallic coatings on non-conductive, conductive or magnetic base

materials, but also magnetic coatings on non-conductive or conductive base materials.

When measuring metallic coatings on metallic basis material, the product of conductivity and

permeability (σ, ) of one of the materials should be at least a factor of 2 times the product of

conductivity and permeability for the other material. Non-ferromagnetic materials have a relative

permeability of 1.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 2064, Metallic and other inorganic coatings — Definitions and conventions concerning the

measurement of thickness
ISO 4618, Paints and varnishes — Terms and definitions

ISO 5725-1:1994, Accuracy (trueness and precision) of measurement methods and results — Part 1:

General principles and definitions

ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in

measurement (GUM:1995)
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3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 2064 and ISO 4618 and the

following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
3.1
adjustment of a measuring system

set of operations carried out on a measuring system so that it provides prescribed indications

corresponding to given values of a quantity to be measured

Note 1 to entry: Adjustment of a measuring system can include zero adjustment, offset adjustment, and span

adjustment (sometimes called gain adjustment).

Note 2 to entry: Adjustment of a measuring system should not be confused with calibration, which is a

prerequisite for adjustment.

Note 3 to entry: After an adjustment of a measuring system, the measuring system must usually be recalibrated.

Note 4 to entry: Colloquially the term “calibration” is frequently, but falsely, used instead of the term “adjustment”.

In the same way, the terms “verification” and “checking” are often used instead of the correct term “calibration”.

[SOURCE: ISO/IEC Guide 99:2007, 3.11 (also known as “VIM”), modified – Note 4 to entry has been

added.]
3.2
calibration

operation that, under specified conditions, in a first step, establishes a relation between the quantity

values with measurement uncertainties provided by measurement standards and corresponding

indications with associated measurement uncertainties and, in a second step, uses this information to

establish a relation for obtaining a measurement result from an indication

Note 1 to entry: A calibration may be expressed by a statement, calibration function, calibration diagram,

calibration curve, or calibration table. In some cases, it may consist of an additive or multiplicative correction of

the indication with associated measurement uncertainty.

Note 2 to entry: Calibration should not be confused with adjustment of a measuring system, often mistakenly

called “self-calibration”, nor with verification of calibration.

Note 3 to entry: Often, the first step alone in the above definition is perceived as being calibration.

[SOURCE: ISO/IEC Guide 99:2007, 2.39 (also known as “VIM”)]
4 Principle of measurement

Phase-sensitive eddy current instruments work on the principle that a high frequency electromagnetic

field generated by the probe system of the instrument will produce eddy currents in the coating on

which the probe is placed and in the base material beneath the coating in case this base material is

conductive (see Figure 1). These induced currents cause a change of the electromagnetic field

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surrounding the probe coil system and therefore result in a change of the amplitude and the phase

angle of the probe coil impedance. The induced eddy current density is a function of the coating

thickness, the conductivity of the coating material, the used frequency of the probe system and the base

metal conductivity. If the thickness of a coating of constant conductivity is increased for a given

frequency the impedance vector describes a so-called local function of the thickness in the impedance

plane (see Figure 2). Each point of this local curve connects a phase angle of the impedance vector with

the respective coating thickness. Consequently, this impedance angle (phase shift) can be used as a

measure of the thickness of the coating on the conductor by means of a calibration with reference

standards (see also Annex A).

In order to measure a change of the coil impedance phase angle the test coil is usually part of a coil

system and coupled with the exciting coil on one ferrite core like in a transformer (see Figure 1). The

changes of phase angle and amplitude due to the impact of the induced eddy currents can be measured

e.g. using a lock in amplifier. These values are usually pre-processed by digital means and the resulting

thickness is then calculated and displayed.

The probe and measuring system/display may be integrated into a single instrument.

NOTE 1 Annex C describes the basic performance requirements of the equipment.
NOTE 2 Factors affecting measurement accuracy are discussed in Clause 5.
Key
1 exiciting current 4 measured signal U=f(t())
2 ferrit core of probe 5 induced eddy currents
3 high frequency alternating magnetic field 6 base material (conductive)
Figure 1 — Phase-sensitive eddy current method
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Key
thickness local curve of Cu for the frequence f
thickness local curve of Cu for the frequence f
1 conductivity 2 thickness 3 coil air (unaffected)
X real part
Y imaginary part

Figure 2 — thickness local curve of Cu in the normalized impedance plane for two frequencies f

and f
For each instrument there is a maximum measurable thickness of the coating.

Since this thickness range depends on both the applied frequency of the probe system and the electrical

conductivity of the coating, the maximum thickness should be determined experimentally, unless

otherwise specified by the manufacturer.

An explanation of eddy current generation and the calculation of the maximum measurable coating

thickness, t , is given in Annex A.
max
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However, in the absence of any other information, the maximum measurable coating thickness, t ,

max
can be estimated using formula (1).
𝑡 ≈ 0,8 ∙ 𝛿 (1)
max 0
where
δ is the standard penetration depth of the coating material (see Annex A).
5 Factors affecting measurement uncertainty
5.1 Basic influence of the coating thickness

The sensitivity of a probe, i.e. the measurement effect depends on the used frequency, the conductivity

of the coating and the base material, and the properties of the probe system. The resulting uncertainty

of the thickness depends besides properties of the probe system on the sample materials, e.g. the

homogeneity of the coating and base metal conductivity and roughness.
5.2 Electrical properties of the coating

The conductivity of the coating but also of the base material determine the induced eddy current

density for a given probe system and frequency. Consequently, the coating and base metal conductivity

cause the measurement effect for this method. The relationship between coating thickness and the

measured value depends strongly on the conductivity of both the coating and base material.

Consequently, calibration procedures and measurements shall be made on the same material. Different

materials with different conductivities as well as local fluctuations of the conductivity or variations

between different samples can cause (more or less) errors in the thickness reading.

5.3 Geometry: base material thickness
In case of a conductive base material (base metal) the
...

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