UT thickness, velocity and gauge error

Thickness from transit time, a material's velocity from a known thickness, how far off a reading is when the gauge is set to the wrong velocity, and the hot-steel correction.

For reference only — verify against your procedure and the governing code.

Find

Velocity the gauge is set to

mm/µs

Actual velocity of the material

mm/µs

Or pick the material

Gauge reading (optional)

Result

Reading error: -6.35% of true thickness
Reads: thin (too low)
True thickness behind the reading: 27.34mm

Example: a gauge left on steel (5.900 mm/µs) reading aluminum (6.300 mm/µs) reads about 6.3 % thin.

Typical longitudinal velocities (ASTM E494 Table X3.1)

Material	mm/µs	m/s
Steel	5.900	5,900
Stainless steel (347)	5.790	5,790
Stainless steel (410)	5.900	5,900
Iron, electrolytic	5.960	5,960
Aluminum	6.300	6,300
Titanium	6.240	6,240
Inconel	5.720	5,720
Nickel	5.630	5,630
Monel	6.020	6,020
Copper	4.700	4,700
Brass	4.370	4,370
Magnesium	5.740	5,740
Tungsten	5.460	5,460
Plastic (acrylic resin)	2.670	2,670

Typical values only; E494 says they vary with composition, processing and test conditions. Shear velocities are on the sound velocity page.

Formula

T = V · t / 2 (t = round-trip transit time)
V = 2T / t; from a gauge reading: V(material) = V(gauge) · T(known) / T(reading)
Reading = T(true) · V(gauge) / V(material); error % = (V(gauge) / V(material) − 1) × 100
Hot steel (E797 §9.5): corrected = reading × (1 − 0.01 × (T(surface) − T(set-up)) / 55 °C)

Units. Computed in m/s, mm and µs (m/s × µs ÷ 2000 = mm). Shown in mm/µs and mm, or in./µs and in.; velocities are also shown in m/s. Temperatures in °C (metric) or °F (imperial).

Assumptions and limits

Pulse-echo, with the instrument's zero offset already removed from the transit time.
The gauge measures time correctly and only the velocity is wrong, so the error is the same percentage at every thickness.
The hot-steel figure is ASTM E797's rule of thumb (about 1 % per 55 °C [100 °F], readings high when hot), an average for many steels and up to about 540 °C [1,000 °F] with suitable equipment. Other materials need their own correction, found by test.
Typical velocities are ASTM E494's Table X3.1 values; E494 says they vary with composition and processing. Standardize on the same material at the same temperature where accuracy matters (E797 §9.2).

Sources

ASTM E797/E797M-15, Standard Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo Contact Method — §4.1 (T = Vt/2), §9.2 (standardize on the same material), §9.5 (high-temperature rule of thumb and its 460 °C worked example).
ASTM E494-20, Standard Practice for Measuring Ultrasonic Velocity in Materials — Appendix X3, Table X3.1 (typical values).
NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B conversion factors.

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