N = D² / (4λ), λ = c / f
Worked example 10 mm, 5 MHz probe on steel N = 21.19 mm reliable far field from 3N = 63.56 mm
NDT calculators · UT
Ultrasonic Testing calculators.
Beam geometry, angle-beam trigonometry, calibration, thickness and remaining life. Each of the 36 shows the governing formula, a worked example and the standard it comes from.
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Ultrasonic testing 36
Beam geometry, angle-beam trigonometry, calibration, thickness and remaining life.
UT Angle beam and weld scanning
Angle beam sound path, surface distance and depth
Solves the first-leg angle-beam triangle — sound path, surface distance and depth — from any one of the three, and reduces the surface distance to the front of the probe.
ISO 16811 · ASME V Free
UT Beam and sound field
Near field length, circular probe
Near-field (Fresnel zone) length N = D²f/4c for a circular single-crystal probe, with the wavelength it is built on.
General engineering Free
UT Thickness and remaining life
Minimum required thickness — ASME VIII UG-27 and B31.3
Pressure design thickness for cylindrical shells and straight pipe to ASME VIII Division 1 UG-27 or ASME B31.3, with the corresponding maximum allowable working pressure at the measured wall.
ASME VIII · ASME B31.3
UT Angle beam and weld scanning
Critical angles
First and second critical angles for a wedge on a test material, plus the smallest refracted shear angle that can be produced.
ISO 7963 · ISO 2400
UT Angle beam and weld scanning
Snell's law refraction angles
Refracted longitudinal and shear angles in the test material for a given incident angle in the wedge, using sin θ₁/c₁ = sin θ₂/c₂.
ISO 5577
UT Beam and sound field
Ultrasonic wavelength
Wavelength, half-wavelength and pulse period from material velocity and probe frequency, for compression or shear waves.
ISO 7963 · ISO 2400
UT Calibration and set-up
Attenuation coefficient from backwall echoes
Material attenuation in dB/mm from the amplitude difference between two backwall echoes, with the far-field beam-spread correction.
ISO 16811 · ASTM E664
UT Calibration and set-up
Decibel amplitude ratio
Convert between two screen heights and a dB difference, and find the resulting screen height after a stated gain change.
ISO 16811
UT Thickness and remaining life
High temperature thickness correction
Corrects a wall thickness reading taken on hot equipment for the fall in compression-wave velocity with temperature — about 1 % per 55 °C in steel.
API 574 · ASTM E797
UT Calibration and set-up
Transfer correction
Gain correction between the reference block and the component, with a sound-path spread term and the resulting total scanning gain.
ISO 16811 · EN 583-2
UT Angle beam and weld scanning
Skip distance and leg sound path
Half-skip and full-skip surface distances and the shear-wave sound path per leg, giving the scanning band needed to sweep a weld through the full wall.
ISO 17640 · AWS D1.1
UT Calibration and set-up
Velocity and probe zero from a calibration block
Derives the true compression-wave velocity and the probe zero offset from timed echoes on one or two known thicknesses of the material being inspected.
ASTM E797 · ASME V
UT Materials and wave physics
Acoustic impedance
Specific acoustic impedance Z = ρ·c in MRayl, for compression or shear waves, from material density and velocity.
General engineering
UT Beam and sound field
Beam divergence angle
Half-angle and full-angle beam spread from sin θ = k·λ/D, with k selectable for the −6 dB, −20 dB or first-null beam edge.
Engineering reference
UT Thickness and remaining life
Corrosion rate and remaining life
Long-term and short-term corrosion rates from thickness history, and the remaining life to the minimum required thickness, following API 510, 570 and 653 practice.
API 510 · API 570
UT Angle beam and weld scanning
Second, third and fourth leg depth
Folds an angle-beam sound path back through the wall to give the true depth and leg number for reflectors beyond half skip.
ISO 17640 · AWS D1.1
UT Thickness and remaining life
Ultrasonic thickness from time of flight
Converts a measured pulse-echo time of flight into remaining wall thickness using the compression-wave velocity, with the probe zero (delay) subtracted.
ASTM E797 · ASME V
UT Calibration and set-up
Immersion water path
Minimum water path to keep interface multiples clear of the part, the steel path a water path is equivalent to, and the water path needed to focus at a chosen depth.
ASTM E1001
UT Materials and wave physics
Interface reflection and transmission coefficients
Pressure and energy reflection/transmission coefficients at a plane interface at normal incidence, from the density and velocity of both media.
General engineering
UT Beam and sound field
Focal spot size
−6 dB focal spot diameter of a focused probe, BD = 1.02·λ·F/D, with the near-field check that decides whether focusing is possible at all.
Engineering reference
UT Calibration and set-up
Maximum pulse repetition frequency
Highest PRF that will not produce ghost (wrap-around) echoes for a given maximum sound path, and the unambiguous range at a chosen PRF.
ISO 16810
UT Calibration and set-up
DAC curve level and indication assessment
Builds the distance amplitude correction curve from the reference reflector law plus material attenuation, then reports how many decibels an indication sits above or below DAC.
ASME V · ISO 16811
UT Beam and sound field
Axial and lateral resolution
Smallest separation two reflectors can have and still be seen as two — along the beam (from pulse length) and across it (from beam width).
General engineering
UT Angle beam and weld scanning
Curved surface correction for angle beam on pipe
Exact sound path, central angle and arc surface distance for a shear beam scanned circumferentially from the outside of a pipe, plus the largest angle that still reaches the bore.
ISO 17640 · ASME V
UT Beam and sound field
Near field length, rectangular probe
Near-field length for a rectangular or square element, N = k·L²/4λ, with k interpolated from the short/long aspect ratio.
Engineering reference
UT Angle beam and weld scanning
Wedge delay and shear velocity from an IIW block
Derives the angle-beam wedge delay and the true shear velocity from timed radius echoes on an IIW V1 or V2 calibration block, and predicts where the next radius echo should fall.
ISO 2400 · ISO 7963
UT Calibration and set-up
DGS / AVG equivalent reflector size
Sizes an indication as an equivalent flat disc reflector from a back-wall reference, using the far-field DGS relation together with normalised distance and near-field length.
ISO 16811 · ISO 11666
UT Beam and sound field
Focal zone length
Depth range over which a focused beam stays within 6 dB of its peak: FZ = N·S_F²·2/(1 + 0.5·S_F), with the start and end depths.
Engineering reference
UT Thickness and remaining life
Velocity correction for temperature
Sound velocity at an elevated temperature, the thickness over-read that results from ignoring it, and the corrected wall reading.
ASTM E797
UT Thickness and remaining life
Next inspection interval from remaining life
Sets the next thickness inspection interval as the smaller of the code fraction of remaining life and the code maximum, including the short-remaining-life rule in API 510 and 570.
API 510 · API 570
UT Beam and sound field
Beam diameter at range
Width of an unfocused beam at a given sound path from the divergence half-angle, for scan-plan coverage and index spacing.
Engineering reference
UT Thickness and remaining life
Thickness grid statistics and projection
Minimum, mean, range and standard deviation for up to eight readings at a condition monitoring location, with the wall projected forward at a given corrosion rate.
API 570 · API 510
UT Materials and wave physics
Rayleigh (surface) wave velocity
Surface wave velocity from the shear velocity and Poisson's ratio, with the Rayleigh wavelength and its effective penetration depth.
Engineering reference
UT Materials and wave physics
Elastic constants from ultrasonic velocity
Young's modulus, shear modulus, bulk modulus and Poisson's ratio from measured compression and shear velocities and density.
ASTM E494
UT Calibration and set-up
Reference reflector equivalence: FBH, SDH and notch
Compares flat-bottomed holes, side-drilled holes and notches by their far-field echo relative to a plane back wall, and converts each to an equivalent disc diameter.
ASTM E127 · ASME V
UT Materials and wave physics
Lamb wave A0 and S0 velocities in steel plate
Phase and group velocity of the fundamental antisymmetric and symmetric Lamb modes in steel plate against frequency-thickness product, with wavelength and the wedge angle needed to excite the mode.
ISO 20601 · ASTM E2775
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