ISO 18175:2004

INTERNATIONAL ISO
STANDARD 18175
First edition
2004-03-15


Non-destructive testing — Evaluating
performance characteristics of ultrasonic
pulse-echo testing systems without the
use of electronic measurement
instruments
Essais non destructifs — Évaluation des caractéristiques fonctionnelles
des systèmes de contrôle ultrasonore par réflexion sans utilisation
d'instruments de mesure électroniques





Reference number
ISO 18175:2004(E)
©
ISO 2004

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ISO 18175:2004(E)
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ISO 18175:2004(E)
Contents Page
Foreword. iv
1 Scope. 1
2 Normative references . 2
3 Terms and definitions. 2
4 Principle . 2
5 Significance and use . 2
6 Procedures for obtaining ultrasonic response data . 3
7 Report. 19
8 Precision and bias . 20
9 Keywords . 20
Annex A (informative) Specific design for Figure 6 resolution reference block. 21

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ISO 18175:2004(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 18175 was prepared by Technical Committee ISO/TC 135, Non-destructive testing, Subcommittee SC 3,
Accoustical methods.

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INTERNATIONAL STANDARD ISO 18175:2004(E)

Non-destructive testing — Evaluating performance
characteristics of ultrasonic pulse-echo testing systems
without the use of electronic measurement instruments
WARNING — This International Standard does not purport to address all of the safety concerns, if any,
associated with its use. It is the responsibility of the user of this International Standard to establish
appropriate safety and health practices and determine the applicability of regulatory limitations prior
to use.
1 Scope
This International Standard describes procedures for evaluating the following performance characteristics of
ultrasonic pulse-echo examination instruments:
 horizontal limit and linearity;
 vertical limit and linearity;
 resolution-entry surface and far surface;
 sensitivity and noise;
 accuracy of calibrated gain controls.
Relevant terminology can be found in ASTM Terminology E 1316 and IEEE Standard 100. Evaluation of these
characteristics is intended to be used for comparing instruments or, by periodic repetition, for detecting long-
term changes in the characteristics of a given instrument that may be indicative of impending failure, and
which, if beyond certain limits, will require corrective maintenance.
Ultrasonic examination instruments using pulsed-wave trains and A-scan presentation (rf or video) may also
be evaluated. The procedures are applicable to shop- or field-conditions and additional electronic
measurement instrumentation is not required.
This International Standard establishes no performance limits for examination systems; if such acceptance
criteria are required, these must be specified by the using parties. Where acceptance criteria are implied
herein they are only for the sake of example and are subject to more or less restrictive limits imposed by
customer's and end user's controlling documents. The specific parameters to be evaluated, conditions and
frequency of test, and report data required, must also be determined by the user. This International Standard
may be used for the evaluation of a complete examination system, including transducer, instrument,
interconnections, fixtures and connected alarm and auxiliary devices, primarily in cases where such a system
is used repeatedly without change or substitution. This International Standard is not intended to be used as a
substitute for calibration of a system to inspect any given material.
Required test apparatus includes selected reference blocks and a precision external attenuator (where
specified) in addition to the instrument to be evaluated.
Precautions relating to the applicability of the procedures and interpretation of the results are included.
Alternate procedures, such as examples described in this International Standard, or others, may only be used
with customer approval.
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ISO 18175:2004(E)
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 5577:2000, Non-destructive testing — Ultrasonic Inspection — Vocabulary
ASTM E 114-95, Standard Practice for Ultrasonic Pulse-Echo Straight-Beam Examination by the Contact
Method
ASTM E 127-98, Standard Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Standard
Reference Blocks
ASTM E 214-01, Standard Practice for Immersed Ultrasonic Examination by the Reflection Method Using
Pulsed Longitudinal Waves
ASTM E 428-00, Standard Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic
Inspection
ASTM E 1316-02a, Standard Terminology for Nondestructive Examinations
JIS Z 2352, Method for assessing the overall performance characteristics of ultrasonic pulse echo testing
instrument
IEEE Std 100, IEEE Standard Dictionary of Electrical and Electronic Terms, Wiley-Interscience, New York
3 Terms and definitions
For the purposes of this document the terms and definitions described in ISO 5577, ASTM E 1316 and
IEEE Std 100 apply.
4 Principle
4.1 An examination system to be evaluated comprises an ultrasonic pulse-echo instrument, transducer,
interconnecting cables and couplant. For immersion examination systems suitable fixturing is required.
4.2 When checking an entire system to be used for a given inspection, test conditions are selected that are
consistent with the intended end-use as determined by the user.
4.3 The ultrasonic response from appropriate reference blocks is obtained, and presented in numerical or
graphical form.
5 Significance and use
5.1 This International Standard describes procedures applicable to both shop- and field-conditions. More
comprehensive or precise measurements of the characteristics of complete systems and their components will
generally require laboratory techniques and electronic equipment such as oscilloscopes and signal generators.
Substitution of these methods is not precluded where appropriate; however, their usage is not within the
scope of this International Standard.
5.2 This International Standard does not establish system acceptance limits, nor is it intended as a
comprehensive equipment specification.
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ISO 18175:2004(E)
5.3 While several important characteristics are included, others of possible significance in some
applications are not covered.
5.4 Since the parameters to be evaluated and the applicable test conditions shall be specified, the practice
described in this International Standard shall be prescribed only by those familiar with ultrasonic NDT
technology and the required tests shall be performed either by such a qualified person or under his/her
supervision.
5.5 Implementation may require more detailed procedural instructions in the format of the using facility.
5.6 In the case of evaluation of a complete system, selection of the specific tests to be made shall be made
cautiously. If the related parameters are not critical in the intended application, then their inclusion may be
unjustified, e.g., vertical linearity might be irrelevant for a go/no-go test with a flaw gate alarm, while horizontal
linearity may be required only for accurate flaw depth or thickness measurement from the instrument display.
5.7 No frequency of system evaluation or calibration is recommended or implied. This is the prerogative of
the using parties and is dependent on application, environment and stability of equipment.
5.8 Certain sections are applicable only to instruments having receiver gain controls calibrated in decibels
(dB). While these may sometimes be designated “gain,” “attenuator” or “sensitivity” on various instruments,
the term “gain controls” will be used in this International Standard when referring to those which specifically
control instrument receiver gain but not including reject, electronic distance-amplitude compensation or
automatic gain control.
5.9 These procedures can generally be applied to any combination of instrument and transducer of the
commonly used types and frequencies, and to most straight-beam examinations, either contact or immersed.
Certain sections are also compatible with angle-beam, wheel, delay-line and dual probe techniques. Their use,
however, shall be mutually agreed upon and so identified in the test report.
5.10 The validity of the results obtained will depend on the precision of the instrument display readings. This
is assumed to be ± 0,04 in (± 1 mm), yielding between 1 % and 2 % of full scale (fs) readability for available
instrumentation having suitable screen graticules and display sharpness.
6 Procedures for obtaining ultrasonic response data
6.1 General
6.1.1 A procedure, using this International Standard as a guide, shall be prepared for periodic checking of
each specific type of instrument or system to be used. For each procedure determine, from the requesting
documents, the instrument examination range to be evaluated, select the appropriate probe, fixtures and
reference blocks, and establish the required display conditions. Unless otherwise required, mid-range values
are suggested for most panel controls and “reject” shall be off unless specifically desired to be evaluated. It
may be desired to vary the instrument controls from these initial values. If so, it is important to observe and
report any anomalous effects on the parameters being evaluated when the controls are so varied.
6.1.2 When a procedure requires a change in receiver gain by the use of a calibrated control, it is assumed
that those which increase sensitivity with higher panel readings are designated “gain” and those which
decrease sensitivity with higher readings are designated “attenuation.” Fine (reference) gain controls, when
available, are usually not calibrated in decibels and increase sensitivity with clockwise rotation.
6.1.3 Although the procedures described in this International Standard do not cover the use of electronic
distance-amplitude compensation, its use is not precluded. If it is used to affect any one or combination of
characteristics, measured under this document, then all characteristics should be evaluated with the same
level of compensation as was used on any one, and this level should be referenced in the report. If desired by
the using parties, a dual set of test data may be made both with and without distance-amplitude compensation.
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ISO 18175:2004(E)
6.1.4 If the display screen does not provide a suitable internal graticule, and deflection measurements are
being made, fix the eye relative to the external scale in order to minimize parallax. This practice assumes
reading precision of within 2 % of full scale. If, for any reason, this is not feasible for the instrument under test,
estimate the probable accuracy and include this in the report. Readability can sometimes be improved by the
use of an external scale attached to the display screen face having 50 or 100 divisions for full scale.
6.1.5 When tests are being done by the contact method, position the probe securely and make certain that
couplant changes are not measurably affecting the results. See ASTM Practice E 114.
6.1.6 When using the immersion method, allow adequate time for thermal stabilization, remove bubbles and
particles from the probe and test surfaces and maintain the probe manipulator and reference blocks in stable
positions. See ASTM Practice E 214.
6.2 Horizontal limit and linearity
6.2.1 Significance
Horizontal limit and linearity have significance when determination of depth of a discontinuity is required. A
specified minimum trace length is usually necessary to obtain the horizontal readability desired. Nonlinearity of
sweep trace may affect accuracy of flaw depth or thickness determination made directly from the display
screen.
6.2.2 Apparatus
A reference block is required that will give several (preferably 11) noninterfering multiple back reflections for
the sweep range and other test conditions of interest. Any block having good ultrasonic transmittivity, flat
parallel faces, and a thickness of about one-tenth of the specified sweep range will usually be adequate. The
aluminum blocks shown in Figure 1 will be satisfactory for mid-range frequencies and sweep settings on most
instruments when the beam is directed through the thickness T. For other test frequencies or very large
probes, different block dimensions or other block designs may be required to eliminate interference. The
couplant system used, either contact or immersed, shall provide stable indications during the measuring
procedure. A horizontal scale permitting reading accuracy as specified in 6.1.4 is required.
NOTE An encapsulated transducer-targets assembly may be used for this purpose.
6.2.3 Procedure
Couple the appropriate block to the probe so that the sound beam does not intercept any test holes. Adjust
the instrument gain, sweep-delay and sweep-length controls to display 11 noninterfering back reflections. Set
the amplitude of each back reflection at 50 % fs before measuring its position. Further adjust the sweep
controls (range, centering or delay) to position the leading edge of the third and ninth back reflections at the
20 % and 80 % scale divisions respectively (with each set in turn at 50 % fs). After the third and ninth back
reflections are positioned accurately on the 20 % and 80 % divisions as described, read and record the scale
positions of each other multiple. Alternatively, if sweep-delay is not available, position the second and eighth
back reflections at the 20 % and 80 % scale divisions respectively; read and record the scale positions of the
initial pulse start and of the remaining multiples. An example of an acceptable alternate procedure is given in
JIS Z 2352 wherein the leading edge of the first reference block back-wall signal is set to correspond to zero
on the horizontal scale and the sixth one set to full scale. Departures of the second to fifth back wall signals
from scale graduations at 20 %, 40 %, 60 % and 80 % of full scale are noted and used to express the degree
of nonlinearity of the time base.
NOTE Either more or fewer reflections can be used by suitably modifying the procedure; e.g., six back reflections
may be used if interference echoes are obtained with 11, in which case the second back reflection is positioned at the
20 % scale division and the fifth back reflection at the 80 % scale division. Measurement of the horizontal position of each
multiple echo should be made at the same amplitude on the leading edge of the indication. Any specific value may be
selected if it is used consistently. Typically used values are baseline break, half amplitude or signal peak.
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ISO 18175:2004(E)
6.2.4 Interpretation of data
6.2.4.1 Horizontal limit is given by the maximum available trace length falling within the display screen
graticule lines expressed in linear units (inches or millimetres). Unless otherwise noted, this is also assumed
to represent 100 % fs. Failure to obtain full-scale deflection may indicate an equipment malfunction.
6.2.4.2 Linearity test results shall be presented in tabular form and may also be plotted in the manner
shown in Figure 2. The deviation is given by the displacement (in % full scale) from the straight line through
the set-up points representing ideal linearity. For the test point shown (sixth multiple at 55 % fs) the error is
5 % fs. Maximum nonlinearity is given by the “worst case” test point. Linear range is given by the set of
contiguous points falling entirely within a specified tolerance.

Table of Dimensions
Imperial block Metric block
Dimension on Figure 1 in mm
Dimension Tolerance Dimension Tolerance
A 1,25 0,05 32 1
B 1,00 0,05 25 1
C 0,75 0,05 19 1
D 1,00 0,05 25 1
E 0,75 0,05 19 1
H 3,00 0,05 75 1
T 1,00 0,01 25,0 0,2
W 2,00 0,05 50 1
d and d 0,047 dia. 0,005 1,2 dia. 0,1
1 2
All surfaces:
Flatness 0,001 0,02
Parallelism 0,001 0,02
Finish 63 µin or smoother 1,5 µm or smoother
NOTE Material: 7075T6 aluminium; plug-drilled holes with water-soluble plastic.
Figure 1 — Suggested test blocks for evaluation of horizontal and vertical linearity
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ISO 18175:2004(E)

NOTE An example of a read point for this diagram is 55,6.
Key
X position of signal on sweep trace, % fs
Y back reflection number
1 set point (80,9)
2 ideal linearity line
3 deviation (5 % fs)
4 set point (20,3)
Figure 2 — Example of data plot for determination of horizontal linearity
6.3 Vertical limit and linearity
6.3.1 Significance
Vertical limit and linearity have significance when echo signal amplitudes are to be determined from the
display screen or corresponding output signals, and are to be used for evaluation of discontinuities or
acceptance criteria. A specified minimum trace deflection and linearity limit may be required to achieve the
desired amplitude accuracy. For other situations they may not be important, e.g., go/no-go examinations with
flaw alarms or evaluation by comparison with a reference level using calibrated gain controls. This practice
describes both the two-signal ratio technique (Method A) and the input/output attenuator technique (Method B).
Both methods assume that the test indications used for measurement are free of interferences resulting from
nearby signals such as the initial pulse, interface echo, or adjacent multiples. If linearity is of concern under
such conditions, e.g. for near-surface signals, it may be evaluated by the procedure described in 6.4.3.
Method A (ratio technique) discloses only nonlinearity that occurs in the instrument circuitry between the gain
controls being used to set the amplitudes and the display.
Method B (input/output technique) evaluates the entire receiver/display system at constant gain as established
initially by the panel controls. Because of this and other differences, the two methods may not give identical
results for linearity range. Further, Method A may not disclose certain types of nonlinear response shown by
Method B.
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ISO 18175:2004(E)
6.3.2 Method A
6.3.2.1 Apparatus
A reference block is required that produces two noninterfering signals having an amplitude ratio of 2 to 1.
These are compared over the usable screen height as the instrument gain is changed. The two amplitudes
are referred to as H and H (H > H ). The two signals may occur in either screen order and do not have to
A B A B
be successive if part of a multiple-echo pattern. Unless otherwise specified in the requesting document, any
reference block that will produce such signals at the nominal test settings specified can be used. For many
commonly used probes and test conditions, the reference block shown in Figure 1 is usually satisfactory when
the beam is directed along the H dimension toward the two holes. The method is applicable to either contact
or immersion tests; however, if a choice exists, the latter may be preferable for ease of set-up and coupling
stability.
NOTE An encapsulated transducer-targets assembly may be used for this purpose.
6.3.2.2 Procedure
To obtain test data, position the probe so that two echo signals are obtained having amplitudes in the ratio of
about 2 to 1. Determine that there is sufficient range in the gain controls to vary H (the larger) from 10 % fs to
A
100 % fs. Manipulate the probe and adjust the instrument controls until H and H meet the conditions listed
A B
in Table 1. The preferred values are desired because the data can be more easily presented and evaluated.
However, positioning difficulties or lack of a fine gain or pulse-length control may not permit obtaining the
exact values. When optimum set-up conditions are established, secure the probe in place, observing the
precautions noted in 6.1. Adjust the gain controls in steps so that H is set in increments of 10 % or less from
A
10 % fs to 100 % fs. Read and record the values of H and H within the accuracies prescribed in 6.1.4.
A B
NOTE To better define the response characteristic, particularly near the upper and lower limits, additional readings
may be taken at smaller gain increments.
Table 1 — Vertical linearity range by Method A using two-signal (ratio)
technique/initial values for H and H giving ratios of 1,8 to 2,2
A B
H % fs H % fs
A B
Preferred values
60 30
Acceptable values
65 30 to 36
64 29 to 36
63 29 to 35
62 28 to 34
61 27 to 34
60 27 to 33
59 27 to 33
58 26 to 32
57 26 to 32
56 25 to 31
55 25 to 31
NOTE Preferred setup values permit determination of
vertical linearity range directly from the data plot of Figure 3
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ISO 18175:2004(E)
6.3.2.3 Interpretation of data
Vertical limit is given by the maximum vertical deflection (baseline to peak for video and peak to peak for rf)
within the usable graticule range that can be obtained from a large reflector (e.g., the reference block
surfaces) as the gain is increased. This is reported in linear units (inches or millimetres) and equivalent
graticule divisions are noted. Unless otherwise stated, this is assumed to represent 100 % fs. Failure to obtain
full-scale deflection may indicate an equipment malfunction. Linearity test data may be reported in tabular
form or preferably presented graphically. Unless otherwise specified in the requesting document, vertical
linearity range should be determined graphically using the method shown in Figure 3. If the preferred set-up
condition (H = 60 % fs, H = 30 % fs) is established initially, the test results may be plotted directly on the
A B
scales shown. The limit lines provide a graduated tolerance for H of ± 1 graph division starting at the set-up
B
point (to provide for reading error) to ± 6 divisions at the extremes. Ideal linearity is defined by a straight line
extending from the origin through any set-up point to full scale. The linear range is determined by
interconnecting adjacent data points and noting the first locations above and below the set-up point that
intersects the limit lines. The upper linearity limit is given by the corresponding value for H and the lower by
A
that for H . If the preferred set-up values were not obtained, a new linearity line and corresponding limits shall
B
be constructed following the same approach.
NOTE 1 If the requesting document specifies that the test results be presented in ratio form (i.e., H /H versus H ) the
A B A
necessary values can be calculated from the tabular data and presented in any format specified. To establish linearity
limits the desired tolerances must also be stated.
NOTE 2 If the instrument graticule cannot be read directly in % of full scale, the recorded values of H and H should
A B
be converted to percentages of full scale before plotting. If that is not done, new coordinates with appropriate scale and
limit lines must be constructed.
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ISO 18175:2004(E)

Key
X H % fs
B
Y H % fs
A
1 ideal linearity line
Figure 3 — Data plot for determination of vertical linearity range by Method A (ratio technique)
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ISO 18175:2004(E)

Key
1 pulse echo instrument, through-transmission mode 6 step attenuator
2 transmit jack 7 alternate line for one-probe operation
3 receive jack 8 search unit 1
4 shielded terminator 9 search unit 2
5 coaxial cables 10 delay path
a
In.
b
Out.
c
When in use, disconnect line C. See also 6.3.3.1.
Figure 4 — Recommended system configuration for determination of vertical linearity (Method B)
and gain control calibration
6.3.3 Method B
6.3.3.1 Apparatus
This method requires the use of an auxiliary external-step attenuator meeting the following minimum
specifications which are usually certified by the supplier:
 frequency range: dc to 100 MHz
 impedance: 50 Ω or 75 Ω
 attenuation: 0 dB to 80 dB in 1 dB steps
 accuracy: ± 0,2 dB for any indicated dB step size
The instrument shall be operable in a through-transmission mode with the attenuator inserted between the
source of the received signal and the receiver input jack as shown in Figure 4. Either the single-search-unit or
the alternative two-search-unit configuration may be used. The attenuator shall be connected to the receiver
input with a short length [6 ft (1,8 m) or less] of coaxial cable of no more than 30 pf/ft (9,14 pf/m) capacitance.
The terminator should be a shielded, noninductive resistor preferably mounted in a coaxial connector. See the
Note below regarding termination errors.
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ISO 18175:2004(E)
In the single-search-unit configuration the pulser is shunted by the attenuator input. Therefore, to isolate the
pulser and protect the atten
...