ISO 16809:2012

INTERNATIONAL ISO
STANDARD 16809
First edition
2012-11-15


Non-destructive testing — Ultrasonic
thickness measurement
Essais non destructifs — Mesurage de l'épaisseur par ultrasons




Reference number
ISO 16809:2012(E)
©
ISO 2012

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ISO 16809:2012(E)

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©  ISO 2012
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Published in Switzerland
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ISO 16809:2012(E)
Contents Page
Foreword . iv
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
6.3  Selection of probe . 6
6.4  Selection of instrument . 6
6.5  Materials different from the reference . 7
6.6  Special measuring conditions . 7
7  Instrument setting . 7
7.1  General . 7
7.2  Methods . 8
7.3  Check of settings . 9
8  Influence on accuracy . 10
8.1  Operational conditions . 10
8.2  Equipment . 13
8.3  Evaluation of accuracy . 14
9  Influence of materials . 14
9.1  General . 14
9.2  Inhomogeneity . 14
9.3  Anisotropy . 14
9.4  Attenuation . 14
9.5  Surface conditions . 15
10  Test report . 16
10.1  General . 16
10.2  General information . 16
10.3  Inspection data . 17
Annex A (informative) Corrosion in vessels and piping . 18
Annex B (informative) Instrument settings . 22
Annex C (informative) Parameters influencing accuracy . 25
Annex D (informative) Measuring technique selection . 30
Bibliography . 32

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ISO 16809:2012(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 16809 was prepared by the European Committee for Standardization (CEN) as EN 14127:2011 and was
adopted by Technical Committee ISO/TC 135, Non-destructive testing, Subcommittee SC 3, Ultrasonic testing.

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INTERNATIONAL STANDARD ISO 16809:2012(E)

Non-destructive testing — Ultrasonic thickness measurement
1 Scope
This International Standard 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 referenced documents are indispensable for the application 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 inspection — Vocabulary
ISO 16811, Non-destructive testing  Ultrasonic testing  Sensitivity and range setting
EN 1330-4, Non-destructive testing  Terminology  Part 4: Terms used in ultrasonic testing
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5577 and EN 1330-4 apply.
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:
a) 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).
b) Mode 2: Measure the transit time from the end of a delay line to the first back-wall echo (single echo
delay line mode).
c) Mode 3: Measure the transit time between back-wall echoes (multiple echoes).
d) 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).
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ISO 16809:2012(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
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ISO 16809:2012(E)
5 General requirements
5.1 Instruments
Thickness measurement can be achieved by using the following types of instruments:
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 detection of discontinuities with A-scan presentation of signals. This
type of instrument may also include numerical display of thickness values.
See 6.4.
5.2 Probes
The following types of probes are 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 has to 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 couplant to be used 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 examination.
If the surface is coated, the coating shall have good adhesion to the material. Otherwise it shall be removed.
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ISO 16809:2012(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 International Standard shall have
a basic knowledge of the physics of ultrasonics, 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
measured.
It is assumed that ultrasonic thickness testing is performed by qualified and capable personnel. In order to
[1]
prove this qualification, it is recommended that personnel be certified in accordance with ISO 9712 or
equivalent.
[3]
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 is required 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 two times the probe's diameter is preferred. Poor
contact results in loss of energy, distortion of signal 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 it is necessary for thickness measurements 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 free of 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.
With a knowledge of the material, geometry and thickness to be measured as well as the accuracy required,
the most suitable measuring equipment and mode can be selected. Annex D gives further guidance.
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ISO 16809:2012(E)
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.
c) On curved objects, the diameter of the probe contact area shall be significantly smaller than the diameter
of the test object.
d) The accuracy of the thickness measurement depends on how accurate the time-of-flight can be
measured, depending on the mode of time-measuring (zero crossing, flank-to-flank, peak-to-peak),
depending on the mode chosen (with multiple echoes, mode 3, the accuracy is higher than with modes 1
and 2), depending on the frequencies which can be used (higher frequencies provide higher accuracy
than lower frequencies because of the more accurate time measurement).
e) 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 is 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 method 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 have 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 have to 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 5.1 c) with low-frequency probes shall be used.
6.2.3 In-service measurement of residual wall thickness
During in-service inspection, measurements shall be taken on materials that are subject to corrosion or
erosion. The surfaces may 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 inspection programs are available to achieve this (data logging).
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ISO 16809:2012(E)
With in-service inspection, the environmental conditions are very important. Equipment may be needed which
can withstand high temperatures and harsh environments or has special electrical shielding.
The flow charts in Figures D.3 and D.4 give guidance on in-service thickness measurements.
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 have 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 should cover the expected thickness range.
When measuring small thicknesses, a delay path should be used. The measurement should be done with the
interface echo (delay path / test object) and the first back-wall echo from the test object (mode 2) or the
measurement made 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 will 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. plastics delay
on metals, there will be a phase shift of the interface echo. This requires correction to get accurate results.
Some thickness instruments do this correction automatically.
For small thicknesses, it is also possible to use a dual element probe with a small focal distance.
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 shall 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 at those temperatures.
6.4 Selection of instrument
Selection is done as follows:
 instruments of type 5.1 c) can be used for modes 1 to 4, see Clause 4, and can satisfy the conditions
given in 6.2.2 and 6.2.3;
 instruments of type 5.1 b) can be used for modes 1, 2 and 3 only, see Clause 4, and can satisfy the
conditions given in 6.2.2.1 and 6.2.3;
 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.
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ISO 16809:2012(E)
6.5 Materials different from the reference
See Table B.1.
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 item under test.
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 may 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 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 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
All instrument setting shall be carried out with the same equipment as that to 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 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.
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ISO 16809:2012(E)
vt
d (1)
n
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
7.2.1 General
The method for setting of 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
instrument.
Tables B.1 and B.2 give guidance on the selection of methods for setting instruments.
Differences exist between calibrating a digital thickness instruments [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 can be achieved in either of two ways:
 adjust the displayed reading such that it agrees with the measured known dimensions of the series of
reference blocks;
 adjust or set the material velocity on the instrument to agree with the known velocity of the test object.
7.2.3 A-scan Instruments
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 to the
known thickness of the reference block. Then adjust the nth 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 to the position of the
nth back-wall echo divided by n and n is normally in the range 2 to 10. See Figure 2.
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ISO 16809:2012(E)
Mode 4 can only be used with an A-scan instrument. The instrument shall be set up to operate in through
transmission mode according to the manufacturer’s manual. A transmission pulse indication should 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 Check 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;
h) at other intervals as directed by specific procedural instructions.
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ISO 16809:2012(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 may 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 unce
...