Introduction of Ultrasonic Calibration Block

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In ultrasonic testing applications, calibration and reference standards blocks are used every day. Calibration is the act of evaluating the precision and accuracy of the equipment used.

Most ultrasonic equipment can be used in various applications, so the operator needs to calibrate the equipment to suit the need at hand. This will involve calibrating the equipment settings, the test set up, and the transducer to ensure optimum accuracy and precision.

On the other hand, reference standards validate that the equipment provides similar results consistently and that different structures produce identical results. It also helps to determine the size of the flaw.

These calibration and reference standards come in varying shapes and sizes. The standard you use depends on the application as well as the form and shape of the object being tested. For this dynamic to work, the reference standards block must be made of the same material as the test subject, and the artificially made flaw must closely resemble the actual defect you are testing. Unfortunately, most ultrasonic calibration blocks use drilled holes and notches that are larger and do not represent real flaws at all.

In this section, we look at a few common calibration and reference standards used in ultrasonic testing.

The IIW Type Block

IIW stands for International Institute of Welding, and the ‘type’ comes in because while it was patterned after the ‘true’ IIW block, this calibration block does not follow IIW requirements in IIS/IIW-23-59. IIW Type Calibration Block can be obtained commercially in different materials, and its dimensions are in English units. Manufacturers of IIW Type blocks can include additional calibration features such as circular groves, notches, and scales to meet the need of a specific application.

There are three types of IIW Type blocks, two full-sized and one mini version;

  1. IIW Type US-1 block is deemed the basic ‘true’ IIW block
  2. IIW Type US-2 block was developed for the US Air Force and
  3. IIW Type Mini is half the size of the full-size blocks and weighs about a quarter as much.

IIW type calibration blocks prepare equipment for both angle beam and regular incident inspections. They can be used to set metal distance, sensitivity settings, determine sound exit point and evaluate depth resolution of beam inspection set up.

The Miniature Angle-Beam or ROMPAS Calibration Block

This calibration block was designed for the US Air Force, and it's much smaller and lighter than IIW blocks. Besides doing everything IIW blocks do, it can also make out the beam angle and exit point of the transducer. You can also use it to calibrate distance and sensitivity level for standard and angle beam set up. The AWS Shear Wave Distance/Sensitivity Calibration (DSC) and AWS Shear Wave Distance Calibration (DC) blocks work the same way but conform to different requirements.

AWS Resolution Calibration (RC) Block

The RC block determines the resolution of angle beam transducers as required by the American Welding Society and AASHTO. It has engraved index markers for 45, 60, and 70-degree angle beams.

30 FBH Resolution Reference Block

The 30 FBH resolution block evaluates the near-surface resolution and depth sensitivity of the standard beam setup. A Miniature Resolution Block does the same thing but with a higher resolution.

Distance/Sensitivity (DS) Block

The DS test block is a standard to evaluate the horizontal linearity and dB accuracy as required by AWS and AASHTO.

Distance/Area-Amplitude Blocks

Distance/Area-Amplitude correction blocks are purchased as a ten-block set that can be aluminium, steel, or titanium. Each block has one flat-bottomed plugged hole in different sizes.

Area-Amplitude Blocks

Very similar to Distance/Area-Amplitude correction blocks, this set comes with eight blocks with a constant 3-inch metal path distance. The hole sizes at the bottom of the blocks, however, still vary in size. These blocks are used to measure the relationship between flaw size and signal breadth for different holes.

Distance-Amplitude #3, #5, #8 FBH Blocks

Again, similar to Distance/Area-Amplitude correction blocks, except it’s a 19 block set with flat-bottom holes of the same size and different metal path distance. You can measure the relationship between the metal path distance and signal amplitude by comparing signals from same-size flaws using different depths.

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