EN ISO 2178:2016

SLOVENSKI STANDARD
01-julij-2016
1DGRPHãþD
SIST EN ISO 2178:1999
Nemagnetne prevleke na magnetnih osnovah - Merjenje debeline prevleke -
Magnetna metoda (ISO 2178:2016)
Non-magnetic coatings on magnetic substrates - Measurement of coating thickness -
Magnetic method (ISO 2178:2016)
Nichtmagnetische Überzüge auf magnetischen Grundmetallen - Messen der
Schichtdicke - Magnetverfahren (ISO 2178:2016)
Revêtement métalliques non magnétiques sur métal de base magnétique - Mesurage de
l'epaisseur du revêtement - Méthode maguétique (ISO 2178:2016)
Ta slovenski standard je istoveten z: EN ISO 2178:2016
ICS:
17.040.20 Lastnosti površin Properties of surfaces
25.220.40 Kovinske prevleke Metallic coatings
25.220.50 Emajlne prevleke Enamels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 2178
EUROPEAN STANDARD
NORME EUROPÉENNE
April 2016
EUROPÄISCHE NORM
ICS 25.220.40; 25.220.50 Supersedes EN ISO 2178:1995
English Version
Non-magnetic coatings on magnetic substrates -
Measurement of coating thickness - Magnetic method (ISO
2178:2016)
Revêtements métalliques non magnétiques sur métal Nichtmagnetische Überzüge auf magnetischen
de base magnétique - Mesurage de l'epaisseur du Grundmetallen - Messen der Schichtdicke -
revêtement - Méthode magnétique (ISO 2178:2016) Magnetverfahren (ISO 2178:2016)
This European Standard was approved by CEN on 27 February 2016.

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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, 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: Avenue Marnix 17, B-1000 Brussels
© 2016 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 2178:2016 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 2178:2016) 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” 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 October 2016, and conflicting national standards shall
be withdrawn at the latest by October 2016.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent
rights.
This document supersedes EN ISO 2178:1995.
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, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 2178:2016 has been approved by CEN as EN ISO 2178:2016 without any modification
INTERNATIONAL ISO
STANDARD 2178
Third edition
2016-03-15
Non-magnetic coatings on magnetic
substrates — Measurement of coating
thickness — Magnetic method
Revêtement métalliques non magnétiques sur métal de base
magnétique — Mesurage de l’epaisseur du revêtement — Méthode
maguétique
Reference number
ISO 2178:2016(E)
©
ISO 2016
ISO 2178:2016(E)
© ISO 2016, Published in Switzerland
All rights reserved. Unless otherwise specified, 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
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2016 – All rights reserved

ISO 2178:2016(E)
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle of measurement . 2
4.1 Basic principle of all magnetic measurement methods . 2
4.2 Magnetic pull-off method . 2
4.3 Magnetic inductive principle . 3
4.4 Magnetic flux gauge . 5
5 Factors affecting measurement accuracy . 6
5.1 Basic influence of the coating thickness . 6
5.2 Magnetic properties of the base metal . 6
5.3 Electrical properties of the coating materials . 7
5.4 Geometry: base metal thickness . 7
5.5 Edge effect . 7
5.6 Geometry: surface curvature . 7
5.7 Surface roughness . 8
5.8 Cleanliness: lift-off effect . 8
5.9 Probe pressure . 8
5.10 Probe tilt . 8
5.11 Temperature effects . 9
5.12 External electromagnetic fields . 9
6 Calibration and adjustment of the instrument . 9
6.1 General . 9
6.2 Thickness reference standards . 9
6.3 Methods of adjustment .10
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) .15
10 Test report .15
Annex A (informative) Basic principle of all measurement methods .17
Annex B (informative) Basic performance requirements for coating thickness gauges which
are based on the magnetic method described in this International Standard .19
Annex C (informative) Examples of experimental estimation of factors affecting the
measurement .21
Annex D (informative) Example of uncertainty estimation (see Clause 8) .26
Annex E (informative) Basics of the determination of the uncertainty of a measurement of
the used measurement method corresponding to ISO/IEC Guide 98-3 .29
ISO 2178:2016(E)
Annex F (informative) Table of the student factor .31
Annex G (informative) Details on precision .32
Bibliography .37
iv © ISO 2016 – All rights reserved

ISO 2178:2016(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 on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 107, Metallic and other inorganic coatings.
This third edition cancels and replaces the second edition (ISO 2178:1982), which has been technically
revised.
INTERNATIONAL STANDARD ISO 2178:2016(E)
Non-magnetic coatings on magnetic substrates —
Measurement of coating thickness — Magnetic method
1 Scope
This International Standard specifies a method for non-destructive measurements of the thickness of
non-magnetizable coatings on magnetizable base metals.
The measurements are tactile and non-destructive on typical coatings. The probe or an instrument with
integrated probe is placed directly on the coating to be measured. The coating thickness is displayed on
the instrument.
In this International Standard the term “coating” is used for material such as, for example, paints and
varnishes, electroplated coatings, enamel coatings, plastic coatings, powder coatings, claddings.
NOTE This method can also be applied to the measurement of magnetizable coatings on non-magnetizable
base metals or other materials (see ISO 2361).
2 Normative references
The following documents, in whole or in part, are normatively referenced in 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)
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.
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 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”.
ISO 2178:2016(E)
[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 to obtain a measurement result from 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
4.1 Basic principle of all magnetic measurement methods
The magnetic flux density close to a magnetic field source (permanent magnet or electromagnet)
depends on the distance to a magnetizable base metal. This phenomenon is used to determine the
thickness of a non-magnetic coating applied to the base metal.
NOTE 1 Annex A describes the physical background of this effect in more detail.
All the methods covered by this International Standard evaluate the magnetic flux density to determine
the thickness of the coating. The strength of the magnetic flux density is converted into corresponding
electrical currents, electrical voltages or mechanical forces depending on the method used. The values
are either pre-processed by digital means or are directly displayed on a usefully scaled gauge.
NOTE 2 The methods described in 4.3 and 4.4 can also be combined in one and the same probe with another
method, e.g. with the eddy current method according to ISO 2360 or ISO 21968.
Annex B describes the basic performance requirements for coating thickness gauges based on the
magnetic method described in this International Standard.
4.2 Magnetic pull-off method
The magnetic flux density of a permanent magnet and thus the attraction force between a permanent
magnet and a magnetizable base metal decreases with increasing distance. In this way, the attraction
force is a direct measure for the coating thickness of interest.
Instruments working with the magnetic pull-off method consist of at least three units:
— a permanent magnet;
— a pull-off device with continuously increasing pull-off force;
— a display or scale for the coating thickness, which is calculated from the pull-off force.
The pull-off force can be generated by different types of springs or an electromagnetic device.
Some instruments are able to compensate the influence of gravity and allow measurements in all
positions.
All other instruments may only be used in the position specified by the manufacturer.
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ISO 2178:2016(E)
The location of measurement shall be clean and free from liquid or pasty coatings. The permanent
magnet shall be free from particles.
Electrostatic charging can cause additional forces on the permanent magnet or the measuring system
and is therefore to be avoided or shall be discharged before the measurement.
Figure 1 shows a magnetic pull-off gauge.
Key
1 base metal
2 coating
3 magnet
4 scale
5 spring
Figure 1 — Magnetic pull-off gauge
4.3 Magnetic inductive principle
The electrical inductivity of a coil changes when an iron core is inserted into the coil or when an iron
object, e.g. a plate, approaches the coil. Therefore, the electrical inductivity can be used as a measure of
the distance between the coil and a ferromagnetic substrate or as a measure of the coating thickness, if
the coil is placed onto a coated magnetizable base metal.
There are many different electronic methods to evaluate changes of the electrical inductivity or
the reaction of a coil system to a ferromagnetic substrate. Magnetic induction probes for thickness
measurements of coatings on magnetizable materials can consist of one or more coils. Most often two
coils are used (see Figure 2): the first (primary coil) to generate a low frequency alternating magnetic
field and the second (secondary coil) to measure the resulting induced voltage U. If the probe is placed
on a coated magnetizable material (µ > 1) the magnetic flux density (see Annex A) and the induced
r
voltage of the secondary coil vary as a function of the coating thickness. The function between the
induced voltage and the coating thickness is nonlinear and depends on the permeability µ of the base
r
metal. It is usually determined by a calibration. Calibration curves that assign a coating thickness to the
induced voltages can be stored in the gauge.
Different designs and geometries of these kind of probes are used. Very often both coils are
employed together with a highly magnetizable core in order to increase the sensitivity of the probes
and to concentrate the field. In this way, both the coating area, which contributes to the thickness
measurement, and the influence of the geometry of the coated component are reduced (see 5.5 and 5.6).
On the contrary, a two pole probe (see Figure 3) has a wide and open field distribution. The two-pole
probe has area integrating properties, while a one-pole probe measures locally.
ISO 2178:2016(E)
Usually the frequency of the generated field is below the kilohertz range, which avoids eddy current
generation if the coatings are conductive. Therefore, both conductive and nonconductive coatings can
be measured by means of this principle.
Key
1 iron core of the probe I exciting current
~
2 low frequency alternating magnetic field t coating thickness
3 steel/iron substrate U = f(t) measurement signal
4 coating
Figure 2 — Schematic of the magnetic induction principle
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ISO 2178:2016(E)
Key
1 iron core of the probe 4 coating
2 coil system 5 base metal
3 probe tip
Figure 3 — Schematic of a two pole probe
4.4 Magnetic flux gauge
The magnetic flux density close to a magnet depends on the magnetic properties of the substances in
the magnetic field. The magnetic flux density decreases if the fraction of non-magnetizable substances
increases relative to magnetizable substances. This fact is used in magnetic flux gauges (see Figure 4).
The coating (4) is non-magnetizable; the base metal (3) is magnetizable. A magnet (1) creates a
magnetic field. Its field lines pass through both the coating and the base metal. A magnetic flux detector
(5) placed close to the magnet outputs electrical signals, which depends on the coating thickness.
NOTE 1 Magnetic flux detectors are Hall-sensors or magneto resistive sensors.
NOTE 2 The magnet can be a permanent magnet or an electromagnet.
ISO 2178:2016(E)
Key
1 permanent magnet U output voltage
2 static magnetic field a measurement signal
3 base metal
4 coating
5 Hall element as magnetic flux detector
Figure 4 — Flux gauge using a Hall probe
The electric signals of the flux detector are further processed by electronic means. The function
between flux detector output and the coating thickness is nonlinear and depends on the permeability
µ of the base metal. It is usually determined by calibration. Calibration curves that assign a coating
r
thickness to the electric detector output can be stored in the gauge.
5 Factors affecting measurement accuracy
5.1 Basic influence of the coating thickness
The sensitivity of a probe, i.e. the measurement effect, decreases with increasing thickness within the
measurement range of the probe. In the lower measurement range this measurement uncertainty (in
absolute terms) is constant, independent of the coating thickness. The absolute value of this uncertainty
depends on the properties of the probe system and the used sample materials, e.g. the homogeneity of
the base metal permeability, the base metal roughness and the sample surface roughness. In the upper
measurement range of the probe the uncertainty becomes relative to the thickness and is approximately
a constant fraction of that thickness.
5.2 Magnetic properties of the base metal
The permeability of the base metal causes the measurement effect of this method.
The relationship between coating thickness and the measured value depends strongly on the
permeability of the base metal. Consequently, calibration procedures and measurements shall be
made on the same material. Different materials with different permeabilities can cause more or fewer
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ISO 2178:2016(E)
thickness errors as well as local fluctuations of the permeability or variations between different
samples.
Residual magnetism of the base material can also affect the measurements considerably, especially
when static magnetic fields are used (see 4.2 for magnetic pull-off force or 4.4 for magnetic flux gauge).
The base metal can be magnetized by repeated measurements on the same location if a measurement
method with a static magnetic field is used (see 4.2 for magnetic pull-off force or 4.4 for magnetic flux
gauge). This may lead to errors in the thickness readings.
NOTE Examples of the initial permeability of typical steel used is in the range of 100 to 300.
5.3 Electrical properties of the coating materials
Coating thickness measurements can be affected if the probe is operated with an alternating magnetic
field due to eddy currents (see 4.3 for magnetic inductive principle or 4.4 for magnetic flux gauge).
These induced eddy currents can counteract the measurement effect of the magnetic method. The
induced eddy current density increases with increasing conductivity and frequency.
NOTE Usually instruments using measurement methods 4.3 or 4.4 work within a frequency range below
1 kHz. Therefore, induced eddy currents affecting measurement results are only effective for thick coatings
(thickness above 1 mm) with a high conductivity, e.g. copper.
5.4 Geometry: base metal thickness
If the base metal thickness is too small, the interaction of the magnetic field with the base metal is
reduced. This influence can only be disregarded 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 a too low base
metal thickness. 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 (field strength, geometry)
and the magnetic properties of 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 C.2.
5.5 Edge effect
The expansion of the magnetic field is obstructed by geometric limitations of the base metal (e.g. edges,
drills and other). Therefore, measurements made too near to an edge or corner cannot be valid unless
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 A simple experiment to estimate the edge effect is described in C.3.
5.6 Geometry: surface curvature
The propagation of the magnetic field is affected by the base metal surface curvature. 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 base metal 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.
ISO 2178:2016(E)
Measurements performed on parts with too small radius of curvature can result in unreliable results,
even after calibrations. The resulting uncertainty should be considered to determine whether such a
measurement is acceptable or not.
NOTE A simple experiment to estimate the effect of surface curvature is described in C.4.
5.7 Surface roughness
Measurements are influenced by the surface topography of the base material and of the coating. 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 an uncoated base
metal with a roughness equivalent to the coated sample base metal.
If necessary, the definition of the used average coating thickness should be stated between supplier
and client.
NOTE ISO 19840 describes special measurement procedures in cases of application paint and varnishes on
steel with rough surfaces.
5.8 Cleanliness: lift-off effect
If the probe is not placed directly down on to the coating, the gap between probe and coating (lift-off)
will act as an additional coating thickness and will therefore affect the measurement. Lift-off can be
produced unintentionally due to the presence of small particles between probe and coating. The probe
tip shall frequently be checked for cleanliness.
5.9 Probe pressure
The pressure that the probe exerts on the test specimen can affect the instrument reading and shall
always be the same during adjustment and measurements.
The influence of the probe pressure is more pronounced in cases of soft coatings because the probe tip
can be indented into the coating. Therefore, the probe pressure should be as small as possible. Most
commercially available instruments are equipped with spring loaded probes, which ensure a constant
pressure during the placement. A suitable auxiliary device should be used in case the probe is not
spring loaded.
NOTE 1 The contact pressure and the probe tip indentation depth can be reduced by reducing the applied
force or by using a probe with a larger diameter of the probe tip.
NOTE 2 An indentation of the probe tip into soft coatings can be reduced by placing a protective foil with
known thickness onto the coated surface. In this case, the coating thickness is the measured thickness minus the
foil thickness.
5.10 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 causes measurement errors.
The risk of inadvertent tilt can be minimized by probe design or by the use of a probe holding jig.
NOTE Most commercially available instruments are equipped with spring loaded probes, which ensure a
perpendicular placement on the sample surface.
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ISO 2178:2016(E)
5.11 Temperature effects
As temperature changes affect the characteristics of the probe it should be used under approximately
the same temperature conditions as under calibration.
NOTE 1 The influence of temperature variations can be reduced by a temperature compensation of the probe.
The manufacturer’s specification has to be taken into account.
NOTE 2 Temperature differences between probe, electronics of the instrument, environment and sample can
cause strong thickness errors. One example is the thickness measurement of hot coatings.
5.12 External electromagnetic fields
The measurement results can be influenced by strong electromagnetic interfering fields. In cases
showing unexpected results or a strong variation of results, which cannot be explained by other
factors, this reason 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 usage every instrument shall be calibrated or adjusted according to the instructions of the
manufacturer by means of suitable thickness reference standards and base metal. Material, geometry
and surface properties of the base metal used for calibration or adjustment should comply with 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.
During calibration or adjustment the instruments, the standards and the base metal should have the
same temperature as the test specimens to minimize temperature induced differences.
In order to avoid the influence of instrument drifts, periodic control measurements with reference
standards or control samples are recommended. If required, the instrument has to 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 metals or foils,
which are placed onto uncoated base metals.
Foils and coatings shall be non-magnetizable. 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, e.g. U (95 %), i.e. there is a 95 %
probability that the documented thickness value is within the reported uncertainty interval.
Prior to use, foils and coatings are to be checked visually for damage or mechanical wear as this would
cause a wrong adjustment and therefore systematic deviation of all measurement values.
The use of foils as reference standards, compared to selected coated base metals, will enable the foils to
be placed directly on to the base metal, thus matching the shape and geometry exactly.
However, by placing the probe on foils elastic or plastic deformation may occur, which can affect the
measuring result. Moreover, any gap between the pole of the probe, foil and base metal has to be
avoided. Especially for concave specimens, or if the foil is wrinkled or bended, the usually low pressure
of the spring loaded guiding sleeve of the probe may not be sufficient to ensure there is no gap .
ISO 2178:2016(E)
A possible elastic or even plastic deformation of a reference foil used depends on the applied load force
of the probe and the probe tip diameter (see 5.9). Consequently, the calibration of such reference foils
should be carried out with comparable values of the applied force and tip diameter to avoid indentation
differences during the probe calibration. In this way, respective indentation errors are already taken
into account in the foil thickness value, i.e. this value can be smaller than the unaffected geometric
thickness. Both values, the applied force and the tip diameter of the foil calibration should be known
from the reference foil manufacturer in order to estimate possible thickness errors.
NOTE In most cases the foil material is plastics but other materials, e.g. copper alloys, can be used as well.
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. Depending on the instrument types,
the instructions of the manufacturer and the functional range of the instrument under use, adjustments
can be carried out on the following items:
a) a piece of uncoated base metal;
b) a piece of uncoated base metal and a piece of coated base metal with defined coating thickness;
c) a piece of uncoated base metal and several pieces of coated base metal with defined but different
coating thickness;
d) several pieces of coated base metal with defined but different coating thickness.
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 and positioning them with closer coverage over
the expected thickness interval of the coating to be measured.
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 need to be
considered in detail, see Clause 5 and Annex C.
NOTE 1 The process that is used to adapt the probe to the given base metal by placing the probe onto the
uncoated base metal 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 International Standard.
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 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, e.g. 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 and shall consider the
factors affecting measurement accuracy discussed in Clause 5.
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ISO 2178:2016(E)
Before using the instrument and after making changes affecting the measurement accuracy (see
Clause 5) the adjustment of the instrument shall be checked.
To ensure that the instrument measures exactly 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 (e.g. temperature) whose effects are not known
(see Annex D).
As not all changes of measurement conditions and their influences on the measurement accuracy can
be immediately recognized (e.g. drift, wear of the probe) the instrument should be calibrated at regular
time intervals
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