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Ultrasonic Testing by the Numbers
Sound speed, echoes, thickness gauging, corrosion surveys, remaining life, angle-beam weld testing and long-path shaft inspection
How ultrasonic testing works and how to use it with numbers: sound speed, echoes, beams, thickness surveys, remaining life, weld testing, attenuation, sampling and records.
Start readingAbout this book
Tap a steel bar and it rings: the atoms pass a disturbance from one to the next at a speed set by how stiffly they are bound and how heavy they are. Ultrasonic testing uses the same physics at a few million cycles per second. A probe sends a short pulse of sound into a part, the pulse reflects from the far wall or from anything that interrupts the metal, and the instrument times the echo. Time multiplied by sound speed gives a distance: a wall thickness, the depth of a crack, the position of a lack of fusion in a weld. The echo’s height says something about the reflector’s size, and its absence says something about what is not there.
This book shows how to use ultrasonic testing with numbers. It starts with how sound travels in steel, what happens at interfaces and what a probe’s beam looks like. It then works through the jobs a maintenance or quality engineer most often pays for: thickness gauging and its calibration, corrosion surveys and the remaining life and inspection interval they lead to, angle-beam testing of butt welds and the sizing and evaluation of indications, and the long sound paths of shaft inspection, where attenuation and beam spread decide what can be found. It finishes with the procedure, the people, the sampling and the records that turn readings into decisions.
It is for maintenance, reliability, inspection and quality engineers, NDT technicians, fabricators and plant owners. It assumes engineering or trade training and comfort with basic mechanics and statistics; the wave physics is explained as it is used, starting from the idea of atoms connected by springs. Each chapter works the running case with real numbers, includes a plausible mistake and how it was found, links the BIZOMAT tools that do the chapter’s calculations, and ends with an exercise and its worked answer.
The running case
Every chapter follows one plant. This is an illustration. A raw sugar mill in the Mackay district of Queensland crushes cane from June to November and uses the slack season, December to May, for maintenance and inspection. Its inspection contractor’s level 2 ultrasonic technician and the mill’s reliability engineer work on four items:
- Juice line JL-4: DN150 carbon steel pipe, 168.3 mm outside diameter and 7.11 mm nominal wall, carrying heated mixed juice at about 105 °C and a design pressure of 1.0 MPa. Twelve condition monitoring locations are measured every slack season.
- Flash vessel V-12: a carbon steel vessel of 1,200 mm inside diameter with a 10 mm shell, designed for 0.70 MPa, that flashes condensate to low-pressure steam. Its shell is surveyed on a grid, sometimes while still hot.
- The V-12 insert plate: a new 12 mm shell insert plate around a replacement nozzle, joined to the shell by full-penetration butt welds that are tested with angle-beam probes.
- No. 3 mill top roll shaft: a forged steel shaft inspected from its end face with a straight-beam probe for fatigue cracks at the journal fillets, 650 mm and 2,550 mm from the end.
Dimensions, readings, signal amplitudes, rates, prices and results are the mill’s working figures or are calculated from stated assumptions. They do not describe BIZOMAT work or any real mill, inspection company, instrument maker or customer. Material properties are typical values for illustration; real acceptance limits and inspection intervals come from the design code, the standards a contract calls up and the owner’s competent inspection body. Prices are in Australian dollars excluding GST.
How to use the book
Read Chapters 1 to 3 first: they explain sound speed, echoes and beams, which every later chapter uses. Chapters 4 to 6 form a sequence on thickness: gauging and calibration, survey design, and remaining life. Chapters 7 and 8 go together on weld testing. Chapter 9 can be read on its own after Chapter 3, and Chapter 10 brings the procedure, sampling, records and cost together.
Every calculation in the book was checked by running it, and the BIZOMAT tools linked from each chapter were checked against the calculations they perform. Where a tool was built for a different purpose but does one step of a chapter’s calculation, such as the liquid compression tool’s wave speed, the chapter says which output is used and which are not.
Source limits and professional responsibility
This book is educational. Ultrasonic testing of pressure equipment, structural welds and rotating machinery is governed by design codes, product standards, contract specifications and written procedures, and is performed and interpreted by personnel certified for the method, such as under ISO 9712. The book names the standards that apply to its examples by number and scope only; it does not reproduce their requirements, and they must be read in full before use. Fitness-for-service and inspection-interval decisions on real equipment remain with the owner and the competent people responsible for it.
Who this book is for
Maintenance, reliability, inspection and quality engineers, NDT technicians, fabricators and plant owners who specify, perform, audit or act on ultrasonic thickness surveys, weld tests and shaft inspections.
What you will learn
- Explain how sound travels in steel, why its speed follows from stiffness and density, and how echoes form at interfaces and defects.
- Choose a probe and frequency from the wavelength, near field and beam spread, and judge what a beam can and cannot resolve.
- Calibrate a thickness gauge, quantify its uncertainty, design a corrosion survey and turn the readings into a corrosion rate, remaining life and inspection interval.
- Locate and size reflectors in a butt weld with angle-beam probes, Snell's law and a distance–amplitude curve.
- Budget the signal on a long, attenuating path, and set out a procedure, sampling plan and record that make an ultrasonic result a sound decision.
Inside this book
Chapters
- 01How sound travels in steelWhy sound speed in a metal follows from its stiffness and density, longitudinal and shear waves, wavelength, time of flight, and the velocity settings every ultrasonic reading depends on.
- 02Echoes, interfaces and couplantAcoustic impedance, how much sound reflects and crosses at each interface, decibels, why couplant is needed, what a coating does to a reading and why cracks reflect so strongly.
- 03Probes, beams and the near fieldHow a probe makes a pulse and a beam, near field length, beam spread, axial resolution and the dead zone, and how to choose a probe size and frequency for a thickness, weld or shaft job.
- 04Thickness gauging and calibrationTwo-point velocity and zero calibration, the V-path error of dual-element probes, temperature correction for hot walls, a Type 1 gauge study and an uncertainty budget for wall readings.
- 05Corrosion surveys that find the worst wallCondition monitoring locations, spot readings and scanned bands, general and local wall loss, comparing surveys, corrosion coupons, and how to design a survey that finds the thinnest metal.
- 06Remaining life and inspection intervalsFrom survey readings to a decision: minimum required thickness for pipe and vessel shells, corrosion rate, remaining life, its uncertainty, the next inspection date and when to repair or replace.
- 07Angle-beam testing and Snell's lawHow a wedge refracts sound into an angled shear beam, critical angles, beam index and angle checks, skip distance, and how to locate a reflector in a butt weld from its sound path.
- 08Sizing and evaluating weld indicationsReference reflectors, distance–amplitude curves, transfer correction, echo amplitude against DAC, length sizing by the 6 dB drop, characterising indications and applying an acceptance specification.
- 09Attenuation, grain and long sound pathsWhy sound weakens in steel, scattering by grains and its steep rise with frequency, measuring attenuation from back-wall echoes, and a decibel budget for finding a crack 2.55 m into a mill roll shaft.
- 10Procedures, people, sampling and costWhat a written ultrasonic procedure must fix, how inspectors are qualified, what a 10% weld sample really protects against, random selection, trustworthy records and what testing costs.
Apply This in Practice
Use these BIZOMAT tools to try the method from this reading with your own numbers.
Explained in this book
Pressure Vessel Shell and Head Thickness Comparison
Compares required thickness of a shell with hemispherical, ellipsoidal and torispherical heads for the same pressure, showing which part of a vessel has least margin and should be surveyed closest.
Applies this method
Acceptance Sampling Plan (n and c for Producer and Consumer Risk)
Designs the sample size and acceptance number for stated producer and consumer risks and checks a proposed weld sample against them, as done for the 120-weld juice heater bypass.
Applies this method
Acceptance Sampling Probability (OC Curve)
Shows the probability that a 10% weld sample with no rejects allowed accepts a lot at a given defective share, the calculation behind the chapter's warning that small samples protect weakly.
Applies this method
Corrosion Remaining Life and Inspection Interval
Takes two wall readings, Barlow's pressure thickness and a structural minimum to give the corrosion rate, remaining life and next inspection date, as worked for the juice line's worst elbow.
Applies this method
Gauge Repeatability and Bias Check (Type 1 Study: Cg, Cgk, Bias t-Test)
Works the Type 1 study of a thickness gauge on a step block: bias, its t-test, Cg and Cgk from 25 repeated readings, judged against the band of wall change the survey must resolve.
Applies this method
Liquid Adiabatic Compression (Temperature Rise, cp − cv, Adiabatic Bulk Modulus, Wave Speed)
Its wave-speed output, √(adiabatic bulk modulus ÷ density), gives the sound speed in water or oil couplant; the chapter uses it for water at 20 °C (1,486 m/s) to set immersion water paths.
Used in How sound travels in steel
Applies this method
Measurement Uncertainty Budget (Combined, Expanded, Test Ratio)
Combines the step block's certificate, display resolution and repeatability into an expanded uncertainty for one wall reading; its Abbe and expansion terms are set to zero for ultrasonic use.
Applies this method
Pressure Vessel Shell Plan (Required Thickness, Plate Chosen, MAWP, Test Water)
Gives the required shell and 2:1 head thickness and the MAWP of the plate by vessel code formulas, the minimum against which the flash vessel's surveyed wall and remaining life are judged.
Related Tools and Examples
Useful alongside
Corrosion Rate from Mass Loss
Gives the general corrosion rate from a coupon's mass loss, which the chapter compares with ultrasonic rates on straight pipe and uses to judge an inhibitor within weeks rather than years.
Related to Corrosion surveys that find the worst wall
References and source notes
The manuscript and worked examples are original BIZOMAT writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 3 November 2026.
- Y. M. Galperin, lecture notes for course FYS 448, Department of Physics, University of Oslo. Introduction to Modern Solid State Physics (2001). Copyright work, used only as a map of ideas: lattice vibrations and the speed of long elastic waves, longitudinal and transverse modes, phase and group velocity, and the mechanisms that damp sound in solids. No text, equations as set out there, figures or problems are reproduced, and all cases and calculations are BIZOMAT originals.
- J. Krautkrämer and H. Krautkrämer, Springer-Verlag. Ultrasonic Testing of Materials (1990). Fourth edition. Background for wave modes, reflection and transmission at interfaces, sound fields of probes, attenuation and the techniques of thickness, weld and forging testing used throughout.
- International Organization for Standardization. ISO 16809, Non-destructive testing — Ultrasonic thickness determination (2025). Named by number and scope only: principles for determining thickness by the time of flight of ultrasonic pulses (adopted in Australia as AS ISO 16809). Its requirements are not reproduced and must be read in full.
- Standards Australia. AS 2207, Non-destructive testing — Ultrasonic testing of fusion welded joints in carbon and low alloy steel (2007). Named by number and scope only (reconfirmed 2017): manual A-scan testing of full and partial penetration welds in ferritic steel 6 mm and thicker. Check its current status with Standards Australia before use.
- International Organization for Standardization. ISO 17640 and ISO 11666, Non-destructive testing of welds — Ultrasonic testing: techniques, testing levels and assessment; acceptance levels (2018). Named by number and scope only. ISO 11666 sets acceptance levels that apply to testing performed to ISO 17640; a revision of ISO 11666 was in progress in 2026.
- Standards Australia and Standards New Zealand. AS/NZS 3788, Pressure equipment — In-service inspection (2024). Named by number and scope only: minimum requirements for in-service inspection and fitness-for-service assessment of pressure equipment. Inspection intervals for real equipment come from it and from the owner's competent inspection body, not from this book.
- National Institute of Standards and Technology. NIST/SEMATECH e-Handbook of Statistical Methods (2012). Background for the gauge study, uncertainty, regression and sampling methods used in Chapters 4, 5, 6 and 10 and in the BIZOMAT tools linked from them.