EN ISO 21968:2019
(Main)Non-magnetic metallic coatings on metallic and non-metallic basis materials - Measurement of coating thickness - Phase-sensitive eddy-current method (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)
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
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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.
Contents Page
European foreword . 3
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.
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
ISO 21968:2019(E)
© 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
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Published in Switzerland
ii © ISO 2019 – All rights reserved
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 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.
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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
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 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
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 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
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 21968:2019(E)
the instrument has been specifically adjusted for such measurements. The necessary distance in order
to avoid an impact of the edge effect depends on the probe system (field distribution).
NOTE 1 A simple experiment to estimate the edge effect is described in Annex D.
NOTE 2 Amplitude-sensitive eddy-current instruments as described in ISO 2360 can be substantially more
affected by edge effects.
5.5 Geometry — Surface curvature
The propagation of the magnetic field and consequently the induction of eddy currents are affected by
the surface curvature of the coating and the base material. This influence becomes more pronounced
with decreasing radius of the curvature and decreasing coating thickness. In order to minimize this
influence, an adjustment should be performed on a sample with the same geometry.
The influence of surface curvature depends considerably on the probe geometry and can be reduced
by reducing the sensitive area of the probe. Probes with very small sensitive areas are often called
“microprobes”.
NOTE 1 There are instruments and probes available that are capable of automatically compensating the sample
surface curvature influence if the curvature diameter is known. They can avoid the resulting thickness error.
NOTE 2 A simple experiment to estimate the effect of surface curvature is described in Annex D.
5.6 Surface roughness
Measurements are influenced by the surface topography of the coating and can also be influenced by the
surface topography of a conductive base metal. Rough surfaces can cause both systematic and random
errors. Random errors can be reduced by making multiple measurements, each measurement being
made at a different location, and then calculating the average value of that series of measurements.
In order to reduce the influence of roughness, a calibration should be carried out with reference parts
with a roughness equivalent to the coated sample.
If necessary, the definition of the average coating thickness that is used should be stated between
supplier and client.
NOTE Amplitude-sensitive eddy-current measurement as described in ISO 2360 can be more affected by
base metal roughness.
5.7 Lift-off effect
If the probe is not placed directly onto the coating, the gap between probe and coating (lift-off) will
affect the measurement of the metal coating thickness. The strength of the lift-off effect depends on the
probe design and the resulting field geometry. By using appropriate electronic circuit design and/or a
mathematical algorithm in the instrument, lift-off compensation can be applied for gaps of up to 1 mm.
The strength of the lift-off effect can be small for some probe designs. In this case, an increase of the
lift-off height results mainly in a reduction of the impedance amplitude but only in a small change of the
phase angle as a measure of the thickness (see Figure 3). The remaining influence can be compensated
by an appropriate mathematical algorithm using the measured amplitude and phase angle information.
6 © ISO 2019 – All rights reserved
ISO 21968:2019(E)
Key
sample conductivity σ = 101,088 % IACS
sample conductivity σ = 60,407 % IACS
sample conductivity σ = 43,48 % IACS
sample conductivity σ = 27,688 % IACS
sample conductivity σ = 17,85 % IACS
sample conductivity σ = 9,534 % IACS
sample conductivity σ = 3,551 % IACS
1 increasing lift-off height in µm
2 increasing σ in % IACS
X real part
Y imaginary part
Figure 3 — Lift-off effect
Lift-off compensation shall be verified in accordance with the manufacturer’s instructions by using
electrically nonconductive shims of known thickness, which are inserted between the probe and the
coating.
NOTE 1 A simple experiment to estimate the lift-off effect is described in Annex D.
NOTE 2 Instead of lift-off compensation, the thickness of non-conductive coatings on top of conductive base
metals can be measured by using the amplitude change as measuring effect (see Annex A).
ISO 21968:2019(E)
5.8 Probe pressure
The pressure with which the probe is applied to the test specimen shall be made constant as it can
affect the instrument reading.
NOTE The phase-sensitive eddy-current measurement can be substantially less affected by the pressure
with which the probe is placed onto the sample than the amplitude-sensitive eddy-current method given in
ISO 2360. Contactless measurements are possible.
5.9 Probe tilt
Unless otherwise instructed by the manufacturer, the probe should be applied perpendicularly to the
coating surface as tilting the probe away from the surface normal can cause measurement errors.
The possibility of tilt inadvertently occurring can be minimized by probe design or by using a probe
holding jig.
5.10 Temperature effects
As temperature changes affect the characteristics of the probe, it should be used under approximately
the same temperature conditions as when the instrument was calibrated.
The influence of temperature variations can be reduced by a temperature compensation of the probe.
The manufacturer’s specification shall be taken into account.
Most metals change their electrical conductivity with temperature. Because the measured coating
thickness is influenced by changes in the electrical conductivity of the coating and the base material,
large temperature changes should be avoided (see 5.2).
NOTE Temperature differences between the probe, the electronics of the instrument, the environment and
the sample can cause large thickness errors. One example is the thickness measurement of hot coatings.
5.11 Intermediate coatings
The presence of an intermediate coating can affect the measurement of the coating thickness if the
electrical characteristics of that intermediate coating differ from those of the coating or the base metal.
If a difference does exist, then the measurements will, in addition, be affected by an intermediate
coating thinner than t . If the intermediate coating is thicker than t and non-magnetic, it can be
min min
treated as the base metal (see 5.3).
5.12 External electromagnetic fields
The measurement results can be influenced by strong electromagnetic interfering fields. When
observing unexpected results or a strong variation of results, which cannot be explained by other
factors, this influence should be taken into account. In this situation, a comparison measurement should
be carried out at a location without interfering fields.
6 Calibration and adjustment of the instrument
6.1 General
Before use, every instrument shall be calibrated or adjusted according to the instructions of the
manufacturer by means of suitable thickness reference standards, base material and a reference
standard with sufficient coating thickness as a saturation material. The material, geometry and surface
properties of the base metal used for calibration or adjustment should be similar to those of the test
specimens in order to avoid deviations caused by the factors described in Clause 5. Otherwise, these
influences shall be considered in the estimation of the measurement uncertainty.
8 © ISO 2019 – All rights reserved
ISO 21968:2019(E)
During calibration or adjustment, the instruments, the standards and the base material should have the
same temperature as the test specimens to minimize temperature-induced differences.
In order to avoid the influence of instrument drift, periodic control measurements with reference
standards or control samples are recommended. If required, the instrument shall be re-adjusted.
NOTE Most instruments automatically adjust themselves during a function called “calibration”, carried out
by the operator, whereas the result of the calibration is often not obvious.
6.2 Thickness reference standards
Thickness reference standards for calibration and adjustment are either coated base materials or, if
available, metal foils, which are placed onto uncoated base materials.
Metal foils and coatings shall have the same conductivity as the coatings to be measured. Thickness
values of the reference standards and their associated uncertainties shall be known and unambiguously
documented. The surface area for which these values are valid shall be marked. The thickness values
should be traceable to certified reference standards.
The uncertainties shall be documented with their confidence level, for example U (95 %), i.e. the
probability, that the “true” value is within the reported uncertainty interval around the documented
thickness value, is minimum 95 %.
Prior to use, metal foils and coatings are to be checked visually for damage or mechanical wear as this
would cause an incorrect adjustment and thus systematic deviation of all measurement values.
The use of metal foils as reference standards, compared to selected coated base metals, benefits from
the possibility of placing the foils directly on each base material. The geometry influence and other
factors are then already considered during the adjustment.
6.3 Methods of adjustment
Adjustment of the coating thickness gauges is executed by placing the probes on uncoated and/or one
or more coated pieces of base metal with known coating thickness and a coated piece with a sufficiently
high coating thickness, so that it can be used as a saturation standard. Depending on the instrument
types, the instructions of the manufacturer and on the functional range of the instrument under use,
adjustments can be carried out on the following items:
a) a piece of uncoated base material;
b) a piece of coated base material with a sufficiently high thickness (saturation standard);
c) a piece of uncoated base material and a piece of coated base material with a defined coating
thickness;
d) a piece of uncoated base material and several pieces of coated base material with defined, but
different, coating thicknesses;
e) a piece of uncoated base material, one or several pieces of coated base material with defined, but
different, coating thicknesses and a piece of coated base material with a sufficiently high thickness
(saturation standard);
f) several pieces of coated base material with defined, but different, coating thicknesses;
g) several pieces of coated base material with defined, but different, coating thicknesses and a piece of
coated base material with a sufficiently high thickness (saturation standard).
According to 6.2, the term “coated base material” includes metal foils placed onto uncoated base
material.
ISO 21968:2019(E)
The stated adjustment methods may lead to different accuracies of the measuring results. Thus,
a method should be used that best fits the given application and leads to the desired accuracy. The
measuring uncertainty that can be achieved by the different adjustment methods depends on the
evaluation algorithm of the gauges as well as on the material, geometry and surface condition of the
standards and of the base metals to be measured. If the desired accuracy is not achieved by one method,
a different adjustment method may lead to better results. In general, the measuring uncertainty can
be reduced by increasing the number of adjustment points, which should be properly adapted to the
thickness interval of the coating to be measured.
NOTE 1 The process used to adapt the probe to a given base material by placing the probe onto the uncoated
base material, is often called “zeroing” or “zero point calibration”. However, even this procedure is an “adjustment”
or part of an adjustment process as defined by this document. This type of adjustment is only necessary when the
base material is a conductive material.
NOTE 2 Depending on how many pieces of coated and uncoated base metals are used to adjust the instrument,
the corresponding adjustment method is often called “single-point”, “two-point” or “multiple-point adjustment”.
NOTE 3 The process used to adapt the probe to a given piece of coated base material with a sufficiently high
thickness (saturation standard) is often called “saturation measurement” or “saturation calibration”. However,
even this procedure is an “adjustment” or part of an adjustment process as defined by this document.
The measurement uncertainty resulting from an adjustment of the instrument cannot be generalized
to all subsequent measurements. In each case, all specific and additional influencing factors shall be
considered in detail, see Clause 5 and Annex D.
NOTE 4 Some types of gauges permit resetting the instrument to an original adjustment of the manufacturer.
This adjustment is valid for the manufacturer’s uncoated or coated reference standards only. If these standards
or the same types of standards are used to check the instrument after a period of use, any deterioration of gauge
and probes, for example, wear of the probe by abrasion of the contact pole, can be recognized by observing
deviations of the measuring results.
7 Measurement procedure and evaluation
7.1 General
Every instrument shall be operated according to the manufacturer’s instructions especially considering
the factors affecting measurement accuracy discussed in Clause 5.
Before using the instrument and after changes affecting the measurement accuracy (see Clause 5), the
adjustment of the instrument shall be checked.
To ensure that the instrument measures correctly, it shall be calibrated with valid standards at the
place of inspection each time
a) the instrument is put into operation,
b) the material and geometry of the test specimens are changed, or
c) other conditions of the inspection have changed (for example temperature) where the effects are
not known.
As not all changes of measurement conditions and their influences on the measurement accuracy can
be immediately recognized (such as drift, wear of the probe), the instrument should be calibrated at
regular time intervals while in use.
7.2 Number of measurements and evaluation
The coating thickness should be determined as the arithmetic mean of several single values, which
are measured in a defined area of the coating surface. In addition to the mean, the standard deviation
should be reported (see Annex B). The random part of the measurement uncertainty can be reduced by
10 © ISO 2019 – All rights reserved
ISO 21968:2019(E)
increasing the number of measurements. If not otherwise specified or agreed upon, it is recommended
to measure at least five single values (depending on the application).
NOTE 1 From the standard deviation, a variation coefficient V can be calculated. V corres
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