ISO 29821:2018

INTERNATIONAL ISO
STANDARD 29821
First edition
2018-01
Condition monitoring and diagnostics
of machines — Ultrasound — General
guidelines, procedures and validation
Surveillance des conditions et diagnostic d'état des machines —
Ultrasons
Reference number
©
ISO 2018
© ISO 2018
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ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle of the airborne and structure-borne method . 2
4.1 General . 2
4.2 Application of airborne and structure-borne ultrasound within condition
monitoring programmes . 2
4.3 Correlation with other technologies . 2
5 Ultrasound equipment . 3
5.1 General . 3
5.2 Kinds of sensors . 5
5.3 Airborne sensor choice . 5
5.4 Structure-borne sensor choice . 6
5.5 Instrument characteristics . 6
5.5.1 General. 6
5.5.2 Frequency response . 6
6 Data collection guidelines . 7
6.1 General . 7
6.2 Comparative ultrasound . 7
6.3 Baseline method — Quantitative ultrasound . 7
7 Training requirements . 9
8 Assessment criteria . 9
8.1 General . 9
8.2 Error sources, accuracy and repeatability .11
9 Interpretation guidelines .12
10 Diagnosing ultrasonic problems .12
10.1 Principles of diagnostics using ultrasound .12
10.2 Generation of ultrasound .13
10.2.1 Surface friction .13
10.2.2 Fluid flow .13
10.2.3 Ionization .13
10.2.4 Impacting.13
11 Sensitivity validation guidelines .13
12 Monitoring interval .13
13 Data interpretations .13
14 Reporting .14
Annex A (informative) Example of a compressed air leak survey .15
Annex B (informative) Typical examples of ultrasound test reports .18
Annex C (informative) Example of a generic sensitivity validation procedure — Ultrasonic
tone generator method .22
Bibliography .24
Foreword
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URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 108, Mechanical vibration, shock and
condition monitoring, Subcommittee SC 5, Condition monitoring and diagnostics of machine systems.
This first edition of ISO 29821 cancels and replaces ISO 29821-1:2011 and ISO 29821-2:2016, which has
been technically revised.
iv © ISO 2018 – All rights reserved

Introduction
This document provides specific guidance on the interpretation of ultrasonic readings and wave
files or frequency and time domain printouts (sometimes called “sound characteristics”) as part of a
programme for condition monitoring and diagnostics of machines. Airborne (AB) and structure-borne
(SB) ultrasound can be used to detect abnormal performance or machine anomalies. The anomalies are
detected as high frequency acoustic events caused by turbulent flow, ionization events, impacts and
friction, which are caused, in turn, by incorrect machinery operation, leaks, improper lubrication, worn
components, and/or electrical discharges.
Airborne and structure-borne ultrasound is based on measuring the high frequency sound that is
generated by either turbulent flow, friction, impacts or by the ionization created from the anomalies.
The inspector therefore requires an understanding of ultrasound and how it propagates through the
atmosphere and through structures as a prerequisite to the creation of an airborne and structure-borne
ultrasound programme. Ultrasonic energy is present with the operation of all machines. It can be in the
form of friction, turbulent flow, impacts and/or ionization as a property of the process, or produced
by the process itself. As a result, ultrasonic emissions are created and these are an ideal parameter
for monitoring the performance of machines, the condition of machines, and for diagnosing machine
anomalies. Ultrasound is an ideal technology to do this monitoring because it provides an efficient way
to quickly and non-invasively determine the location of an anomaly with little setup and in a very short
period of time.
INTERNATIONAL STANDARD ISO 29821:2018(E)
Condition monitoring and diagnostics of machines —
Ultrasound — General guidelines, procedures and
validation
1 Scope
This document
— gives guidelines for establishing severity assessment criteria for anomalies identified by airborne
(AB) and structure-borne (SB) ultrasound,
— specifies methods and requirements for carrying out ultrasonic examination of machines, including
safety recommendations and sources of error, and
— provides information relative to data interpretation, assessment criteria and reporting.
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 13372, Condition monitoring and diagnostics of machines — Vocabulary
ISO 13379-1, Condition monitoring and diagnostics of machines — Data interpretation and diagnostics
techniques — Part 1: General guidelines
ISO 13381-1, Condition monitoring and diagnostics of machines — Prognostics — Part 1: General
guidelines
ISO 17359, Condition monitoring and diagnostics of machines — General guidelines
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13372 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at https://www.iso.org/obp
3.1
airborne and structure-borne ultrasound
AB&SB ultrasound
non-destructive test method used to inspect for airborne and structure-borne ultrasound above 20 kHz
created from or through a medium
3.2
background noise
unwanted noise present in a signal which cannot be attributed to a specific cause
Note 1 to entry: This ultrasonic noise can emanate from the area surrounding the inspection, which can cause
false indications.
3.3
scanning
moving a receiving transducer or an array of transducers around a suspected source of ultrasound to
verify the location
3.4
sonic reflection
airborne ultrasound reflected off a solid surface possibly indicating a false reading
3.5
contact module
waveguide in the form of a rod that is coupled to a receiving transducer that receives ultrasounds by
making physical contact with the subject and test equipment, for structure-borne ultrasounds
4 Principle of the airborne and structure-borne method
4.1 General
Airborne and structure-borne ultrasound is a physical wave that occurs within the test subject
(material or machinery component) or in the atmosphere and is detected externally either close to or
at a distance from the test subject. This technology is based on the detection of high-frequency sounds.
Most ultrasonic instruments employed to monitor equipment detect frequencies above 20 kHz, which is
above the range of human hearing (20 Hz to 20 kHz). The differences in the way low-frequency and high-
frequency sounds travel help to explain why this technology can be effective for condition monitoring.
Low-frequency sounds maintain a high intensity of sound volume and travel further than high-frequency
sounds. High-frequency sounds are more directional. As high-frequency sound waves propagate from
the point of generation, their intensity level decreases rapidly with distance depending on the elasticity
and density of the medium traversed, which helps to identify the origin of a sound source.
Airborne ultrasound is propagated through an atmosphere (air or gas) and detected with an ultrasonic
microphone while structure-borne ultrasound is generated within and propagated through the
structure and is usually detected with a contact module, although other sensors may be used. These
contact modules do not require any coupling agent, as the detection frequencies are low enough that,
unlike traditional pulse-echo ultrasound, small air gaps between the contact probe and the structure
under test do not significantly attenuate the received signal. If permanently mounted sensors are used,
careful mounting techniques should be utilized to avoid signal attenuation or resonances, or both. The
structure can be a machine or any component of a machine or a system.
4.2 Application of airborne and structure-borne ultrasound within condition
monitoring programmes
Ultrasound is not normally used as a primary monitoring technique in typical condition monitoring
programmes. The exceptions to this are when ultrasound is preferred as a non-invasive indicator of
impending failure or performance deterioration or when rapid pressure or vacuum leak localization is
necessary to lessen machine performance degradation. Table 1 shows typical examples of ultrasound
applications to machine condition monitoring.
4.3 Correlation with other technologies
Traditionally, airborne and structure-borne ultrasonic inspection is used in a condition-monitoring
programme to detect characteristics of failure modes that have been previously identified by another
technology. There are instances where airborne or structure-borne ultrasound is the first indicator of
a failure mode, such as in the detection of faulty slow-speed bearings and/or insufficient lubrication
in rolling element bearings. Airborne or structure-borne ultrasound can also be used to identify a
potential safety hazard to an inspector using an alternate technology, for example, in the inspection of
enclosed electrical systems. Airborne and structure-borne ultrasound are used to determine if an arc
flash hazard is present before opening the cabinet for an infrared thermographic inspection.
2 © ISO 2018 – All rights reserved

Acoustic emission is the phenomenon of radiation of acoustic (elastic) waves in solids that occurs when
a material undergoes irreversible changes in its internal structure. Acoustic emission is traditionally
utilized to monitor items that are under stress for the formation and location of cracks. These include
pressure vessels, pipelines. Many of the acoustic emission applications are similar to the structure-
borne ones described in this document. Further information on acoustic emission can be located in
ISO 22096.
Table 1 — Ultrasonic application examples
Pressure or vacuum leak
a a
Machine description Mechanical Electrical
a
detection
Heat exchangers AB — —
Boilers AB — —
Condensers AB — —
Control air systems AB — —
Valves SB — —
Steam traps SB — —
Motors — SB SB
Pumps AB SB SB
Gears/gear boxes — SB —
Fans — SB —
Compressors AB SB SB
Conveyors — SB and AB SB
Switchgear — AB and SB AB and SB
Transformers — SB AB/SB
Insulators — — AB
Junction boxes — — SB
Circuit breaker — — SB
Turbines AB SB —
Generators (utility) AB SB AB/SB
Lubrication — SB —
High-speed bearings — SB and AB —
Low-speed bearings — SB and AB —
a
AB: airborne; SB: structure-borne.
5 Ultrasound equipment
5.1 General
AB&SB ultrasonic instruments are typically hand-held, portable and battery operated for ease of
use in the field. Online, non-portable systems are also utilized mainly for condition monitoring
where an anomaly can occur and shall be addressed at the inception rather than when a route-
based inspection is scheduled. Most online applications target a narrow range of applications where
amplitude is the primary parameter that is monitored and false indications are less likely to occur. It is
recommended that the system consist of an instrument, ultrasonic transducers and headphones. It is
highly recommended that the demodulated signal output be appraised through headphones to enable
discrimination between competing sources. This allows the practitioner to recognize and prevent the
acquisition of poor quality data. The system shall provide for the detection of acoustic energy that
is either airborne or structure-borne in the range above 20 kHz and shall translate (demodulate or
heterodyne) this energy into an audible signal that can be seen on a signal strength indicator and heard
through the headphones. The signal strength is usually displayed in decibels and commonly referred
to as “decibel value”. The demodulated or heterodyned signal is representative of the amplitude and
frequency characteristics of the original ultrasonic signal. The ultrasonic physical pressure wave
or pressure variation which is received and measured by the ultrasonic instrument is demodulated
and converted to a corresponding level having the unit decibel (not standard definition); a sound
pressure level, L , is referenced to the threshold level of the AB&SB ultrasonic instrument, where the
p
mathematical expression is L dB = 20 log r , where r is the amplitude ratio.
p 10 a a
Currently, instrument sensitivity can vary between different manufacturers. Each manufacturer
establishes its own threshold level (0 dB) as there are no standards to uniformly define this threshold
level. There can even be different levels of sensitivity for different instruments produced by a single
manufacturer. If a condition monitoring application requires a comparison or trending of signal
strength readings over time, care should be taken to use instruments that have the same sensitivity so
that comparable data can be obtained. When making comparisons between instrument readings, the
dB readings shall be of the same type.
The main housing contains ultrasonic transducers that receive the ultrasound signal and convert it to
an amplified electrical signal. Next, this signal is fed into the main instrument where it is amplified
again, then demodulated or heterodyned. The demodulation or heterodyne principle is used to convert
the non-audible ultrasonic frequencies down to the audible level suitable for humans to hear and for
interfacing with recording and analysing devices. The same principle is used in AM radio broadcasting
and reception. In the demodulation or heterodyne process, the audio signal is a direct translation of the
original signal and this demodulated signal is used for further analysis (see Figure 1).
The demodulated or heterodyned signal allows the inspector to identify a relevant sound source and
to determine the event or condition producing the ultrasound (e.g. air leaks in the same area as an
electrical discharge can cause confusion to an unskilled inspector). The demodulated signal can also be
used to determine the location of the irrelevant ultrasound that could lead to a false reading.
Therefore, the headphone output signal is not a “divided” signal where the audio frequency is multiplied
by a number and ends up with the ultrasonic frequency. In the demodulation (heterodyne) process, the
incoming ultrasonic signal is mixed with an internal oscillator signal and the difference is amplified
and then sent to the headphone output and the meter circuit. A good analogy would be a piano key
being struck once a second (1 Hz); the resultant sound would contain the resonant frequency of the
string that the piano key is linked to, modulated by the 1 Hz of the key being struck. If the piano string
signal (carrier frequency) were removed, what would be left is the 1 Hz signal (modulation frequency)
of the key being depressed.
The ultrasonic detection modules only detect high-frequency noise caused by friction or turbulent
flow and do not respond to low-frequency acceleration, displacement or audible sounds. In the case
of bearings, ultrasound is created by the motion of the rotating elements. As a bearing deteriorates,
defects form on the rotating surfaces and when a rotating element interacts with the defect, it produces
an acoustic event or fault indication. The actual fault frequencies of the affected bearing modulate
the high-frequency components of the generated ultrasonic noise or signal. The signal after the
demodulation or heterodyning would only leave the original modulation. For example, in a bearing,
if the fault frequency is 48 Hz, the instrument detects the ultrasonic component that is modulated by
the 48 Hz fault frequency. When that signal is demodulated or heterodyned, the audio signal at the
headphones does not contain the ultrasonic signal, but contains the 48 Hz fault frequency signal.
In high-speed bearings, if one were to analyse the demodulated or heterodyned ultrasound signal with
a spectral (FFT) analyser, and compare it to the signal from an accelerometer, the signals would be
qualitatively similar. With low-speed bearings at speeds typically below 10 r/min, standard vibration
accelerometers would have low signal strength due to the lack of enough energy to stimulate the
piezoelectric sensing element with the calibration mass attached. For example, there are ultrasonic
sensors currently used in mining operations to provide a signature from a 16,8 m diameter bearing
operating at a speed less than 1 r/min for input from an ultrasonic detector into a portable FFT analyser
for analysis and archival.
In addition to mechanical condition analysis, signal analysis of the heterodyned signals received from
electrical discharges can help identify the severity of the condition and can also help distinguish the
4 © ISO 2018 – All rights reserved

difference between “loose” or 50 Hz to 60 Hz vibrating components such as a transformer winding and
the actual electrical discharges.
Key
1 transducer pre-amp 10 RMS-to-DC converter
2 variable gain amplifier 11 digital I/O
3 demodulation circuit 12 sensitivity/frequency adjustment knob
4 mixer 13 store button
5 oscillator 14 CPU and digital controls
6 low-pass filter 15 gain control
7 audio amplifier 16 frequency control
8 phone output 17 converter input
9 line output 18 display
Figure 1 — Block diagram example of an ultrasonic detector
5.2 Kinds of sensors
Airborne ultrasound is propagated through an atmosphere (air or gas) and detected with an ultrasonic
microphone, while structure-borne ultrasound is generated within and propagated through a structure
and is usually detected with a contact module, although other sensors may be used. A guide for which
sensor should be chosen can be found in Table 1.
5.3 Airborne sensor choice
An ultrasonic instrument with fixed sensors might have limitations with respect to field of reception
and might not be suitable for all applications. For ultrasonic instruments with interchangeable sensors,
there is normally a choice of two kinds of sensors: wide-angle and parabolic.
For machine condition monitoring, wide-angle airborne sensors are particularly useful for gaining an
overall assessment of the machine condition utilizing the maximum machine area for comparison of
ultrasonic signatures. This allows the comparison of multiple components in a single machine. This
module type is also useful in confined-space areas where the access area can be very small.
Parabolic sensors are useful for remote component locations such as elevated conveyors, equipment,
vessels and outdoor substations, where access is limited and the machine, system, or component of
either, is a great distance away. The narrow field of reception is helpful especially for pinpointing leaks
in overhead piping or in determining which phase in a high-voltage electrical tower has an electrical
discharge.
5.4 Structure-borne sensor choice
Structure-borne sensors are used to non-invasively detect internal abnormal performance or machine
anomalies. There is normally a choice of hand-held contact, magnetically coupled or permanently
installed (threaded) sensors.
The contact sensor (stethoscope) is most commonly used when a machine, system or component needs
to be quickly scanned to determine where an anomaly or fault condition is located. It is also effectively
used to get into tight spaces to gain access to a good monitoring point. For inspection points that are
just out of reach, extension contact rods can be used. For measurement points that are in difficult to
reach or in unsafe areas, permanent remote contact sensors can be used.
Magnetically coupled contact sensors remove the measurement variation associated with hand-held
contact sensors. They are therefore ideal in circumstances where a long sampling time is required or
where there are multiple inspectors taking readings on the same sampling point. An example would
be when monitoring an electrical transformer, as a slight movement of a contact sensor can sound
very similar to a partial discharge inside the transformer, which would cause a false indication of an
anomaly.
5.5 Instrument characteristics
5.5.1 General
When selecting an ultrasonic instrument, the sensitivity, frequency response and ability to record the
heterodyned (demodulated) ultrasonic signal output should be carefully considered with respect to
the intended applications. Some manufacturers recommend that applications require monitoring at
different frequencies for the best results. Other applications require a recording of the heterodyned
(demodulated) sound signature for further analysis and for reporting.
5.5.2 Frequency response
If using an airborne or structure-borne ultrasonic instrument with heterodyned (demodulated)
frequency tuning capability, the inspector should be aware that there are certain monitoring frequencies
that enhance the data that are acquired for specific applications. These monitoring frequencies
are primarily due to the propagation of the ultrasonic wave through specific media, but can also be
influenced by the resonance of the ultrasonic sensor. Examples of typical monitoring frequencies are
shown in Table 2.
Table 2 — Typical monitoring frequencies
Acquisition Application Frequency
method kHz
Airborne Leaks, electrical 40
Bearings, mechanical 30
Structure-borne Valves, steam traps 25
Electrical – sealed leaks – underground 20
6 © ISO 2018 – All rights reserved

6 Data collection guidelines
6.1
...