UT Calibration Blocks and Reference Standards Explained

Reviewed by Anoop Rayavarapu, ASNT NDT Level III (UT, RT, MT, PT, VT). Updated 18 October 2026.

An ultrasonic result is only as good as what the instrument was set up against. Two kinds of block do that job. A calibration block sets up the instrument and probe: screen range, probe index point and refracted angle. A reference block (often called a reference standard) sets sensitivity: it carries reflectors of known size and depth, such as side-drilled holes, flat-bottom holes or notches, against which indications on the part are compared. A procedure names both, and the examination record identifies the actual blocks used.

Common calibration and reference blocks

Customer specifications and construction codes add their own blocks, so the procedure, not habit, decides which block is used on a job.

Why the block has to match the part

A reference block works only if sound behaves in it the way it behaves in the part. Material, product form, heat treatment and surface finish all change attenuation and coupling, so codes such as ASME Section V Article 4 set requirements for the block material relative to the material examined. Where the part's surface or attenuation differs from the block, a transfer correction measures the difference and adjusts the gain, so that a reference-level echo means the same thing on the block and on the part.

Distance amplitude correction

The same reflector returns a smaller echo the further it is from the probe, because of beam spread and attenuation. A distance amplitude correction (DAC) curve, built by plotting the peak echoes from reference reflectors at several depths, gives the evaluation level at every depth. Time-corrected gain (TCG) does the same job electronically by raising gain with time so that the reference reflectors all appear at the same screen height.

Checks during the examination

Calibration is checked before scanning, at the intervals the procedure sets, whenever the probe, cable, instrument or operator changes, and at the end of the examination. If a check shows that sensitivity or range has drifted beyond the procedure's tolerance, everything examined since the last good check is re-examined. The record of those checks is what lets an auditor accept the results years later.

Traceability and records

Each block carries an identifier and documentation of its material and reflector dimensions, so that the reflector sizes behind every recorded indication can be traced. Lost or unidentified blocks, worn reflectors and blocks of the wrong material are among the findings that most often undermine an otherwise sound ultrasonic examination.

Questions about UT reference standards

What is the difference between a calibration block and a reference block?

A calibration block sets up the instrument and probe, for example screen range, index point and refracted angle. A reference block sets sensitivity with reflectors of known size and depth. Many procedures use one of each, and some blocks serve both purposes.

Does the reference block have to be the same material as the part?

It has to be acoustically comparable. Codes such as ASME Section V Article 4 set material and product-form requirements for the block; where the part's surface or attenuation still differs, a transfer correction adjusts the gain so the reference level means the same on both.

When should calibration be rechecked during an examination?

At the intervals the procedure sets, after any change of probe, cable, instrument or operator, and at the end of the work. A failed check means re-examining everything inspected since the last acceptable check.

For angle-beam weld work, see shear wave angle beam inspection, the calibration block glossary entry and ASTM E164 weld UT. To practise calibration on reference reflectors, use the ultrasonic testing simulator.