Sound travels more slowly in hot steel. The elastic moduli soften as temperature rises while the density barely changes, and the compression velocity falls by roughly 1 % for every 55 °C (100 °F) above the temperature at which the instrument was calibrated. A gauge calibrated cold and then applied to a hot line is still converting time to distance with the cold velocity, so every reading comes out high — the wall looks thicker than it is, which is the dangerous direction for a corrosion survey.

The correction is a simple ratio. The true thickness is the displayed thickness scaled by the ratio of hot velocity to calibration velocity, t = t_disp × (1 − k ΔT). At 350 °C the factor is about 0.94, so a 12.70 mm reading is really 11.94 mm — three-quarters of a millimetre, which on a line with a 0.15 mm/yr corrosion rate is five years of remaining life.

Temperature affects more than velocity. The couplant must survive the surface, the probe itself has a duty-cycle limit and its delay line changes velocity as it heats, and the steel has physically expanded — around 0.4 % at 350 °C — so the hot wall is genuinely thicker than the same wall cold. This calculator corrects the velocity error only; where readings are being trended against a cold minimum thickness, the thermal expansion should be considered separately.

The cleanest way to avoid the whole problem is to calibrate on a block at the operating temperature, or on the component itself against a known thickness. Where that is impossible, apply the correction and record on the report that it was applied, at what metal temperature, and with what coefficient.

Compression wave

Worked example

Material coefficient	carbon_steel
Custom coefficient	0.00018182 1/°C
Displayed thickness	12.7 mm
Metal temperature	350 °C
Calibration temperature	20 °C
Velocity used at calibration	5900 m/s
Temperature rise above calibration	330 °C
Correction factor	0.94
Corrected thickness	11.94 mm
Error in the uncorrected reading	-6 %
Velocity at metal temperature	5546 m/s

350 °C metal against a 20 °C calibration is a 330 °C rise. At 1 % per 55 °C that is 330/55 = 6.0 % slower sound, so the correction factor is 0.940 and the true wall is 12.70 × 0.940 = 11.94 mm — the uncorrected reading over-states the wall by 6 %. The compression velocity at temperature is 5900 × 0.940 = 5546 m/s.

Use at your own risk — verify before you act

These calculators support, and never replace, the judgement of qualified NDT and engineering personnel. Results are provided as is, without warranty of any kind, express or implied, and must be independently verified against the governing code edition named in your contract before being used in any inspection, acceptance, rejection, radiation-safety or fitness-for-service decision. By using them you accept full responsibility for how the results are applied; NDT Inspect, its owners and contributors accept no liability for any loss, damage, injury or death arising from their use or from reliance on them. If a result matters to safety, check it by hand and have it reviewed by a competent person.