SIST EN ISO 16809:2019

SLOVENSKI STANDARD
01-november-2019
Nadomešča:
SIST EN 14127:2011
Neporušitvene preiskave - Ultrazvočno merjenje debeline (ISO 16809:2017)
Non-destructive testing - Ultrasonic thickness measurement (ISO 16809:2017)
Zerstörungsfreie Prüfung - Dickenmessung mit Ultraschall (ISO 16809:2017)
Essais non destructifs - Mesurage de l'épaisseur par ultrasons (ISO 16809:2017)
Ta slovenski standard je istoveten z: EN ISO 16809:2019
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 16809
EUROPEAN STANDARD
NORME EUROPÉENNE
June 2019
EUROPÄISCHE NORM
ICS 19.100 Supersedes EN 14127:2011
English Version
Non-destructive testing - Ultrasonic thickness
measurement (ISO 16809:2017)
Essais non destructifs - Mesurage de l'épaisseur par Zerstörungsfreie Prüfung - Dickenmessung mit
ultrasons (ISO 16809:2017) Ultraschall (ISO 16809:2017)
This European Standard was approved by CEN on 8 April 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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, 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 16809:2019 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
The text of ISO 16809:2017 has been prepared by Technical Committee ISO/TC 135 "Non-destructive
testing” of the International Organization for Standardization (ISO) and has been taken over as
which is held by AFNOR.
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 December 2019, and conflicting national standards
shall be withdrawn at the latest by December 2019.
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 14127:2011.
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, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 16809:2017 has been approved by CEN as EN ISO 16809:2019 without any modification.

INTERNATIONAL ISO
STANDARD 16809
Second edition
2017-11
Non-destructive testing — Ultrasonic
thickness measurement
Essais non destructifs — Mesurage de l'épaisseur par ultrasons
Reference number
ISO 16809:2017(E)
©
ISO 2017
ISO 16809:2017(E)
© ISO 2017, Published in Switzerland
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ii © ISO 2017 – All rights reserved

ISO 16809:2017(E)
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Measurement modes . 1
5 General requirements . 3
5.1 Instruments . 3
5.2 Probes . 3
5.3 Couplant . 3
5.4 Reference blocks . 3
5.5 Test objects . 3
5.6 Qualification of personnel . 4
6 Application of the technique . 4
6.1 Surface conditions and surface preparation . 4
6.2 Technique . 4
6.2.1 General. 4
6.2.2 Measurement during manufacture . 5
6.2.3 In-service measurement of residual wall thickness . 5
6.3 Selection of probe . 6
6.4 Selection of instrument . 6
6.5 Materials different from the reference material . 6
6.6 Special measuring conditions . 7
6.6.1 General. 7
6.6.2 Measurements at temperatures below 0 °C . 7
6.6.3 Measurements at elevated temperatures . 7
6.6.4 Hazardous atmospheres . . 7
7 Instrument setting . 7
7.1 General . 7
7.2 Methods of setting . 8
7.2.1 General. 8
7.2.2 Digital thickness instruments . 8
7.2.3 A-scan instrument . 8
7.3 Checks of settings . 9
8 Influence on accuracy .10
8.1 Operational conditions.10
8.1.1 Surface conditions .10
8.1.2 Surface temperature .10
8.1.3 Metallic coating .11
8.1.4 Non-metallic coating .11
8.1.5 Geometry .12
8.2 Equipment .12
8.2.1 Resolution .12
8.2.2 Range .13
8.3 Evaluation of accuracy .13
8.3.1 General.13
8.3.2 Influencing parameters .14
8.3.3 Method of calculation .14
9 Influence of materials .14
9.1 General .14
9.2 Inhomogeneity .14
9.3 Anisotropy .14
ISO 16809:2017(E)
9.4 Attenuation .14
9.5 Surface conditions .14
9.5.1 General.14
9.5.2 Contact surface .15
9.5.3 Reflecting surface .15
9.5.4 Corrosion and erosion . .15
10 Test report .16
10.1 General .16
10.2 General information .16
10.3 Measurement data .17
Annex A (informative) Corrosion in vessels and piping .18
Annex B (informative) Instrument settings .23
Annex C (informative) Parameters influencing accuracy .26
Annex D (informative) Selection of measuring technique .32
Bibliography .37
iv © ISO 2017 – All rights reserved

ISO 16809:2017(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 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.
This document was prepared by Technical Committee ISO/TC 135, Non-destructive testing,
Subcommittee SC 3, Ultrasonic testing.
This second edition cancels and replaces the first edition (ISO 16809:2012), which has been technically
revised. The main changes compared to the previous edition are as follows:
— editorial improvements have been made;
— the terminology has been adapted to the latest edition of ISO 5577;
— Formulae (5) and (6) have been corrected.
INTERNATIONAL STANDARD ISO 16809:2017(E)
Non-destructive testing — Ultrasonic thickness
measurement
1 Scope
This document specifies the principles for ultrasonic thickness measurement of metallic and non-
metallic materials by direct contact, based on measurement of time of flight of ultrasonic pulses only.
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 5577, Non-destructive testing — Ultrasonic testing — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5577 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
— IEC Electropedia: available at http://www.electropedia.org/
4 Measurement modes
The thickness of a part or structure is determined by accurately measuring the time required for a
short ultrasonic pulse generated by a transducer to travel through the thickness of the material once,
twice or several times.
The material thickness is calculated by multiplying the known sound velocity of the material with the
transit time and dividing by the number of times the pulse transits the material wall.
This principle can be accomplished by applying one of the following modes, see Figure 1.
1) Mode 1: Measure the transit time from an initial excitation pulse to a first returning echo, minus a
zero correction to account for the thickness of the probe's wear plate and the couplant layer (single-
echo mode).
2) Mode 2: Measure the transit time from the end of a delay line to the first back wall echo (single-
echo delay line mode).
3) Mode 3: Measure the transit time between back wall echoes (multiple-echo mode).
4) Mode 4: Measure the transit time for a pulse travelling from the transmitter to a receiver in contact
with the back wall (through-transmission mode).
ISO 16809:2017(E)
Mode 1 Mode 2
Mode 3 Mode 4
Key
A transmit/receive probe D transmission pulse indication
A1 transmit probe E1 to E3 back wall echoes
A2 receive probe F interface echo
A3 dual-element probe G delay path
B test object H received pulse
C sound path travel time
Figure 1 — Measurement modes
2 © ISO 2017 – All rights reserved

ISO 16809:2017(E)
5 General requirements
5.1 Instruments
The following types of instruments shall be used to achieve thickness measurement:
a) dedicated ultrasonic thickness measurement instruments with numerical display showing the
measured value;
b) dedicated ultrasonic thickness measurement instruments with numerical display showing the
measured value and A-scan presentation (waveform display);
c) instruments designed primarily for the detection of discontinuities with A-scan presentation of
signals. This type of instrument can also include numerical display of thickness values.
See 6.4.
5.2 Probes
The following types of probes shall be used; these are generally longitudinal wave probes:
— dual-element probes;
— single-element probes.
See 6.3.
5.3 Couplant
Acoustic contact between probe (probes) and material shall be provided, normally by application of a
fluid or gel.
The couplant shall not have any adverse effect on the test object, the equipment or represent a health
hazard to the operator.
For the use of the couplant in special measuring conditions, see 6.6.
The coupling medium should be chosen to suit the surface conditions and the irregularities of the
surface to ensure adequate coupling.
5.4 Reference blocks
The measuring system shall be calibrated on one or more samples or reference blocks representative of
the object to be measured, i.e. having comparable dimensions, material and structure. The thickness of
the blocks or the steps should cover the range of thickness to be measured. Either the thickness or the
sound velocity of the reference blocks shall be known.
5.5 Test objects
The object to be measured shall allow for ultrasonic wave propagation.
There shall be free access to each individual area to be measured.
The surface of the area to be measured shall be free of all dirt, grease, lint, scale, welding flux and
spatter, oil or other extraneous matter that could interfere with the testing.
If the surface is coated, the coating shall have good adhesion to the material. Otherwise it shall be
removed.
ISO 16809:2017(E)
When measuring through coating its thickness and sound velocity need to be known unless mode 3
is used.
For further details, see Clause 8.
5.6 Qualification of personnel
An operator performing ultrasonic thickness measurement according to this document shall have
a basic knowledge of the physics of ultrasound, and a detailed understanding and training related to
ultrasonic thickness measurements. In addition, the operator shall have knowledge of the product and
material to be tested.
It is assumed that ultrasonic thickness testing is performed by qualified and capable personnel. In order
to prove this qualification, it is recommended that personnel be qualified in accordance with ISO 9712
or equivalent.
NOTE For categories III and IV according to Pressure Equipment Directive 97/23/EC, Annex I, 3.1.3, there is
a requirement for personnel to be approved by a third-party organization recognized by a member state.
6 Application of the technique
6.1 Surface conditions and surface preparation
Using the pulse-echo method means that the ultrasonic pulse needs to pass the contact surface between
test object and the probe at least twice: when entering the object and when leaving it.
Therefore, a clean and even contact area with at least twice the probe's diameter is preferred. Poor
contact will result in loss of energy, distortion of signals and sound path.
To enable sound propagation all loose parts and non-adherent coatings shall be removed by brushing or
grinding.
Attached layers, like colour coating, plating, enamels, may stay on the object, but only a few thickness
meters are able to exclude these layers from being measured.
Very often, thickness measurements need to be done on corroded surfaces, e.g. storage tanks and
pipelines. To increase measuring accuracy the contact surface should be ground within an area at least
two times the probe's diameter. This area should be clean from corrosion products.
Care should be taken not to reduce the thickness below the minimum acceptable value.
6.2 Technique
6.2.1 General
The task of ultrasonic thickness measurements can be separated into two application areas:
— measurement during manufacture;
— in-service measurements of residual wall thickness.
Each area has its own special conditions which require special measuring techniques.
Using a knowledge of the material, geometry and thickness to be measured and the required accuracy,
the most suitable measuring equipment and mode shall be selected as follows (Annex D gives guidance):
a) depending on the thickness and the material, frequencies from 100 kHz with through-transmission
on highly attenuative materials up to 50 MHz on thin metal sheets shall be used;
b) if dual-element probes are used, then compensation for V-path error is required;
4 © ISO 2017 – All rights reserved

ISO 16809:2017(E)
c) on curved objects, the diameter of the probe contact area shall be significantly smaller than the
diameter of the test object.
The accuracy of the thickness measurement depends on how accurate the time of flight can be measured,
on the mode of time-measuring (zero crossing, flank-to-flank, peak-to-peak), on the mode chosen (with
multiple echoes, mode 3, the accuracy is higher than with modes 1 and 2), and on the frequencies which
can be used (higher frequencies provide higher accuracy than lower frequencies because of the more
accurate time measurement).
Ultrasonic thickness measurement is often required over an area of the component to be measured.
Where this is the case, consideration should be given to the spacing between each measurement. Such
spacing should be even and the use of a grid is recommended. The grid size should be selected to give a
balance between the confidence in the results and the work content involved.
Measuring the thickness ultrasonically means measuring the time of flight and then calculating the
thickness assuming a constant sound velocity (see Clause 7). If the velocity is not constant within the
path the ultrasonic pulse has travelled, the accuracy of the measurement will be severely affected.
6.2.2 Measurement during manufacture
6.2.2.1 Modes 1, 2 and 3
Where the pulse-echo mode is used, the flow charts in Figures D.1 and D.2 give guidance on the selection
of the best technique and equipment.
Thickness measurement on clean parallel surfaces may be carried out with simple numerical display
thickness instruments.
On composite materials which generate echoes in addition to the back wall echo, it is recommended
that thickness instruments with A-scan displays [type 5.1 b) or 5.1 c)] be used to select the correct echo
of the thickness measurement.
6.2.2.2 Mode 4
If the material is highly attenuative and large thicknesses need to be measured, no echo technique can
be used, i.e. only through-transmission (mode 4) is applicable.
Two probes on opposite sides of the test object shall then be used. The instrument therefore shall allow
for operation with separate transmitter and receiver (TR mode). In most cases, the frequency shall be
lower than 1 MHz. Special low-frequency instruments from group c) in 5.1 with low-frequency probes
shall be used.
6.2.3 In-service measurement of residual wall thickness
During in-service testing, measurements need to be taken on materials that are subject to corrosion or
erosion. The surfaces can be rough and contain pitting or other defects (see Annex A) which are areas
of low reflectivity.
For these applications, the use of dual-element probes is recommended. The sensitivity shall be set
manually to detect the bad reflecting areas.
Where it is necessary to take a lot of measurements, the readings shall be values with the information
on the location of the measuring point. Special testing programs are available to achieve this (data
logging).
With in-service testing, the environmental conditions are very important. Equipment can be needed
which can withstand high temperatures and harsh environments, or has special electrical shielding.
The flowcharts in Figures D.3 and D.4 give guidance on in-service thickness measurements.
ISO 16809:2017(E)
6.3 Selection of probe
Having chosen a suitable measurement procedure according to 6.2, i.e. a general decision for a probe
type (single- or dual-element) has been made, there are other parameters that need to be considered
when matching the probe to the measuring conditions.
Wide-band probes offer a shorter pulse than narrow-band probes, thus giving a suited flank or peak to
start and stop the time-of-flight measurement, giving a better resolution when measuring thin sheets
or coatings.
Additionally, a wide frequency band always gives a stable echo even when attenuating materials need
to be measured.
Probe size and frequency shall be chosen to cover the measurement range by a narrow sound beam to
get an echo from a well-defined area.
For dual-element probes the focal range shall cover the expected thickness range.
When measuring small thicknesses, a delay path shall be used. The measurement shall be done with
the interface echo (delay path/test object) and the first back wall echo from the test object (mode 2) or
to make the measurement using mode 3. The material of the delay path shall be chosen to generate a
suitable interface echo. Using the same material as the test object does not generate an interface echo.
When the material of the delay path has a lower acoustic impedance than the material to be tested, e.g.
a plastics delay line on metals, there is a phase shift of the interface echo. This shall be corrected to get
accurate results. Some thickness instruments do this correction automatically.
For small thicknesses, a dual-element probe with small focal distance may be used.
When measuring on hot surfaces, the delay path shall act as a thermal barrier.
The material chosen for delay shall withstand the temperatures of the test object. The influence
of the temperature on the acoustical properties of the delay path needs to be known (drift of sound
attenuation and velocity). Data sheets of the probe manufacturers show the range of temperatures a
probe is suitable for and the time it can be used on those temperatures.
6.4 Selection of instrument
Selection of instruments of type 5.1 a), b) or c) shall be made as follows:
a) instruments of type 5.1 c) shall be used for modes 1 to 4 (see Clause 4) and shall satisfy the
conditions given in 6.2.2 and 6.2.3;
b) instruments of type 5.1 b) shall be used for modes 1, 2 and 3 only (see Clause 4) and shall satisfy the
conditions given in 6.2.2.1 and 6.2.3;
c) instruments of type 5.1 a) may be preset by the manufacturer to work only in one of the modes 1, 2
or 3 (see Clause 4).
The instruments shall be selected to satisfy the individual requirements given in 6.2.2.1 or 6.2.3.
See also Annex D.
6.5 Materials different from the reference material
See Tables B.1 and B.2.
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ISO 16809:2017(E)
6.6 Special measuring conditions
6.6.1 General
There shall be strict observation of all legislative procedures governing the safe use of chemicals and
electrical equipment.
Where there is a requirement for high-accuracy measurements, the calibration or reference blocks used
should be at the same temperature as the test object.
6.6.2 Measurements at temperatures below 0 °C
For measurements below 0 °C, the couplant chosen shall retain its acoustic characteristics and have a
freezing point below the test temperature.
Most probes are rated for use between −20 °C and +60 °C. At temperatures below −20 °C, specially
designed probes can be required and contact time should be limited as recommended by the
manufacturer.
6.6.3 Measurements at elevated temperatures
For measurements above 60 °C, a high-temperature probe is required and the couplant shall be designed
for use at the test temperature.
It is also recommended that, when using A-scan equipment, it should have a “freeze” mode to allow the
operator to assess the signal response. The probe contact time shall be limited to the minimum time
necessary to achieve a measurement as recommended by the manufacturer.
6.6.4 Hazardous atmospheres
In the measurement of thickness in hazardous atmospheres, there shall be strict compliance with
prevailing safety regulations and standards.
In explosive atmospheres, the probe, cable and equipment combination shall be classified as intrinsically
safe and relevant safety certification and or documentation shall be checked and completed prior to use.
In corrosive atmospheres, the couplant shall not react adversely with the environment and shall retain
its acoustic properties.
7 Instrument setting
7.1 General
The instrument setting shall be carried out with the same equipment as that which will be used for
the measurements. Instrument setting shall be carried out in accordance with the manufacturer’s
instructions or other valid standards or procedures.
It should be noted that this clause covers only the setting of the instrument (in service). The verification
of the equipment characteristics is not considered but can be performed according to the design
specification.
Ultrasonic instruments do not measure thickness; they measure time of flight. The thickness is
calculated by the application of a factor which is the sound velocity of the material.
dv=×tn/ (1)
ISO 16809:2017(E)
where
d is the thickness;
v is the sound velocity;
t is the measured time;
n is the number of transits through the test object (see Figure 2).
7.2 Methods of setting
7.2.1 General
The method for setting the instrument shall suit the measuring mode and the equipment and probe
in use. The setting shall be carried out under comparable operating conditions as those of the
measurement.
Annex B gives guidance on the selection of methods for setting instruments.
Differences exist between calibrating a digital thickness instrument [types 5.1 a) and b)] and an A-scan
instrument [type 5.1 c)].
7.2.2 Digital thickness instruments
See also 5.1 a) and 5.1 b).
Many digital thickness instruments can be used in measurement modes 1, 2 and 3. The setting of the
instrument may be achieved in either of two ways:
a) by adjusting the displayed reading such that it agrees with the measured known dimensions of the
series of reference blocks;
b) adjusting or setting the material velocity on the instrument to agree with the known velocity of the
test object.
7.2.3 A-scan instrument
See also 5.1, c).
Refer to ISO 16811 for information regarding the time-base setting of an A-scan instrument.
When using mode 1 with an A-scan instrument, the horizontal time base is set such that the
transmission pulse indication and the first back wall echo from the reference block are displayed at
convenient positions on the screen to agree with a screen graticule or the digital display.
When using mode 2 with an A-scan instrument, adjust the transmission pulse indication such that it is
off the screen and the interface echo is at zero on the graticule. Then adjust the first back wall echo to
be at the mark relating to the known thickness of the reference block.
When using mode 3 with an A-scan instrument, adjust the first back-wall echo to be at the mark relating
th
to the known thickness of the reference block. Then adjust the n back wall echo to be at the mark
relating to the n times the known thickness of the reference block. When measuring the test object, the
zero point of the graticule will correspond to the surface of the test object. The object thickness is equal
th
to the position of the n back wall echo divided by n. n is normally in the range 2 to 10. See Figure 2.
Mode 4 can only be used with an A-scan instrument. The instrument shall be set to operate in through-
transmission mode according to the manufacturer’s manual. A transmission pulse indication should
8 © ISO 2017 – All rights reserved

ISO 16809:2017(E)
be available to represent the zero-time pulse. Set this to align with the zero on the graticule, and the
received pulse is set to align with a known thickness on the graticule.
Key
A transmit/receive probe
B test object
C sound path travel time
D transmission pulse indication
E1 to En back wall echoes
Figure 2 — Instrument setting for mode 3
7.3 Checks of settings
Checks of the settings of a thickness measuring system shall be carried out with a reference test piece
a) on completion of all measurement work,
b) at regular intervals during the work session, at least once a day,
c) at regular intervals during the work session,
d) if probes or cables are changed,
e) if material types are changed,
f) if the material or equipment temperature changes significantly,
g) if major operating controls are adjusted or considered altered, and
h) at other intervals as directed by specific procedural instructions.
ISO 16809:2017(E)
8 Influence on accuracy
8.1 Operational conditions
8.1.1 Surface conditions
8.1.1.1 Cleanliness
The cleanliness of the test object affects its thickness measurement. Inadequate surface preparation
can lead to inconsistent results.
Adhering dirt and scale shall be removed by brushing before measurement.
8.1.1.2 Roughness
Roughness interferes with the estimate of thickness (overvaluation) and modifies the coefficients of
reflection and transmission at the interface.
In circumstances where there is significant roughness, the sound path is increased and the contact
surface is reduced. The measurement uncertainty increases with decreasing thickness.
If the surface opposite to the input surface (back wall surface) is rough, the echo can be deformed; this
can result in measurement error.
8.1.1.3 Surface profile
Scanning on an irregular surface with a contact probe necessitates the use of a thick couplant layer.
This can create beam distortion.
When using modes 1, 2 or 4 the transit time through the couplant layer can be included in the reading,
which will result in an additive error. For a ratio of velocities of the couplant and the material of 1/4,
this error can thus be equal to four times the actual couplant thickness.
The coupling medium should be chosen to suit the surface conditions and the irregularities of the
surface to ensure adequate coupling.
8.1.2 Surface temperature
Temperature modifies the sound velocity (in both the material and in any delay path and face of the
probe) and also the overall acoustic attenuation.
So, for all measurements, if maximum accuracy is required, then the temperature variation and effect
upon the following additional items shall be considered:
— references: standards, gauges, test blocks;
— system: instrument, probes, etc.;
— process and methods: couplant, test object.
Sound velocity decreases with increase in temperature in most metals and plastics, whereas it can be
seen to increase in glass and ceramics.
The influence of temperature on the velocity of sound in metals is normally insignificant. The longitudinal
−1 −1
(compressional) wave velocity in most steels decreases only by approximately 0,8 ms °C .
The influence of temperature on plastics is significant. For acrylic, which is normally used for probe
−1 −1
delays, the coefficient is −2,5 ms °C Compensation for this shall be applied.
.
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ISO 16809:2017(E)
8.1.3 Metallic coating
Apparent increase of the material thickness (or even apparent decrease in the case of heat-treated
material) can be seen when cladding (constitution, composition, thickness, cladding process, number
of layers, etc.) is not taken into account. In general, coatings (added layers) increase the travelled sound
path, i.e. also the time of flight of the received echoes (see Figure 3).
The measurement accuracy required shall dictate whether the plating should be considered.
For example, with the instrument calibrated for steel:
−1
— Steel 1 mm at v = 5 920 ms ;
−1
— Zinc 20 µm at v = 4 100 ms ;
— Actual thickness 1 mm + 20 µm = 1,02 mm;
−3 −6
11× 0 m 20×10 m
() ()
−7
+ =×1,738 10
...