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Calibration Block: The Ultimate Guide
Table of Contents
Have you ever asked yourself why calibration blocks are so crucial in ultrasonic testing?
The ability of ultrasonic flaw detectors to accurately identify flaws within a component is strongly tied to the consistency in periodic calibration that the flaw detector has undergone. Consequently, the benefits of carrying out regular calibration are numerous.
However, since the nature of the defect suspected and the geometry of the test samples determine the type of flaw detector required, it follows that the kind of calibration and reference standards to employ will be closely related to the geometry of the test sample as well as the specific testing requirements.
This guide gives you all the information you need to know about ut calibration blocks. Without further ado, let’s dive in.
I. What is a Calibration Block for Ultrasonic Testing?
When it comes to applications of ultrasonic testing, reference standards, and calibration are of utmost importance as they serve as a measuring yardstick to ascertain the level of accuracy of flaw detecting devices. Calibration evaluates the precision of the quality assessment tool used for inspection and identifying flaws within a given component. It’s no surprise that cal blocks are used this way since the level of precision of your flaw detecting tool determines the level of result accuracy you can expect per inspection.
Calibration is part of an inspection technician’s daily, weekly, monthly, and yearly routine, as failing to carry out periodic calibration can accumulate errors in the instrument for every inspection undertaken.
The calibration of ultrasonic flaw detectors often depends on the nature of inspection specified for the flaw detector since most UT equipment is versatile and can be used for varied applications. Naturally, calibrating UT equipment will involve setting the transducer, equipment setting, and test setup to guarantee optimum precision and accuracy of test results.
Also, reference standards are equally essential, seeing that they validate the result of the equipment as consistent with established parameters of accuracy. Nonetheless, both calibration and reference standards come in various shapes and sizes to match the shape and size of the test sample and the specific testing application required.
Still, both test material and the reference standard blocks must be made of the same materials. The created sample flaw must be closely similar to the fundamental flaw intended for testing.
II. How Many Types of Ultrasonic Calibration Blocks are there in Ultrasonic Testing?
Like most calibration tools, ut calibration blocks help ascertain the accuracy of flaw detectors and ensure their margin of error is within tolerable limits. However, calibration test blocks can include the following based on their specific uses, material, shape, and size of the tested object.
- IIW type blocks
The International Institute of Welding or IIW type blocks are made using various materials, and its dimensions use imperial units of measurement. However, they can be used for both regular incident inspection and angle beam calibration. Additionally, they often come in three variants, including IIW US-1, IIW US-2, and IIW mini-type blocks.
- AWS resolution calibration blocks
The resolution calibration or RC blocks is the cal block for determining the resolution of angle beam probes following the American Welding Society (AWS) and American Association of State Highway Transport Officials (AASHTO). Essentially, the AWS RC block comes with index markers at 45-, 60-, and 70-degree angle beam engraved in it.
- Distance/Sensitivity DS blocks
DS block conforms with the AWS and AASHTO standards for evaluating the horizontal linearity and the accuracy of the decibels as specified by these standard bodies.
- ASTM distance/area-amplitude blocks
This calibration block often comes as a set of ten correctional blocks made of either steel, aluminum, or titanium drilled with a plug hole at the flat bottom side. These holes are of varying diameters for the ten sets of blocks. Other similar blocks include ASTM distance-amplitude blocks and ASTM area-amplitude blocks.
➤ Related Article: Types of Ultrasonic Calibration Block
➤ Related Article: Top 7 Ultrasonic Calibration Block Manufacturers
III. What is Ultrasonic Calibration Block V1?
V1 block is used to calibrate ultrasonic flaw detectors when conducting a quality inspection of welded joints with ultrasonic testing. Ordinarily, they have an error accuracy of ± 0.5 mm when determining the index points of ultrasonic X-values and ultrasonic oscillations. However, they are also effective when choosing non-testing zones and resolution capacity for straight beams and the horizontal sweep linearity of UT flaw detectors.
On the other hand, its specification includes 4.8kg weight, fine-grained low carbon steel material, and a low attenuation coefficient. Additionally, its longitudinal and shear wave velocities are 5920 ±30m/s and 2670 ±100 m/s, respectively, for material a frequency of 5.0 MHz and temperature of 20±5 degrees Celsius.
IV. What is Ultrasonic Calibration Block V2?
Ultrasonic calibration block V2 is used in calibrating flaw detection devices, especially for welding inspection and quality assessment. Naturally, its specification comes with longitudinal wave velocities of 5890 and 5950 m/s for the block material frequency of 5MHz and a temperature ranging between 15 and 25 degrees Celsius. Also, it has a shear wave velocity of 3240 and 3270 m/s for the block material frequency of 5MHz and a temperature ranging between 15 and 25 degrees Celsius.
Essentially, a calibration block V2 calibrates flaw detectors with sound velocity while operating with straight beam or angle beam probes. Also, it enables the determination of the probe angle of sound waves in steel.
V. What is Distance Amplitude Correction (DAC)?
When you examine the acoustic signals received by a transducer, you quickly realize that the distance of the sound signals from the transducers naturally determines the amplitude of the signal received by the transducers even though the sounds are coming from the same reflective surface. Therefore, to graphically represent these differences as reference level sensitivity on an A-scan display, a Distance Amplitude Correlation (DAC) methodology comes in handy.
Similar discontinuities reflect signals in specific ways that make DAC the most suitable method for evaluating them. However, correlating signal attenuation as a function of depth must first be settled. Nonetheless, DAC always gives tolerance for amplitude loss with time changes represented on the A-scan graphically. Still, specific instruments may enable this sort of representation electronically with almost equivalent results.
While transducer size and frequencies still determine the beam spread and near field length. However, materials change with velocity and attenuation, and a DAC curve still needs to be created for each situation. With a DAC, you can graphically represent both shear and longitudinal operational modes likewise for both immersion and contact inspection methods. The point here is the versatility of using a DAC over other forms of signal representation during an inspection.
VI. How to Develop a Distance Amplitude Correction (DAC) Curve?
Defining the Distance Amplitude Correlation (DAC) is but the first step. Developing the DAC curve is another crucial step in the calibration process, and it involves the display of all A-scan echoes devoid of their non-electronically adjusted height.
Nonetheless, inspectors reference standards and introduce drilled holes on the side (SDH) and drilled holes at the flat bottom sides (FBH). These holes or notches, in some cases, help to identify reflector positions within the test material, usually at different depths in the material.
Additionally, inspectors ensure that the reflector shape and size remain constant throughout the development over the sound path distance irrespective of the reflector type used in creating the DAC.
The procedure starts with calibrating an appropriate sweep distance for material using an applicable reference standard. Secondly, establish side-drilled holes at a location that is a fraction of the thickness of the material. For instance, establish holes at 1/4T and 3/4 T positions along with the depth of the material where T is the thickness of the material.
Thirdly, with an appropriate marker, on the screen, mark the 80% full screen-size peak of the echo produced by positioning the transducers at the first hole position. Note down the gain setting at this 80% screen size marking. Repeat the third step for the second hole without adjusting the gain control, and finally connect the dots of your markings with a single line.
➤ Related Article: How to Choose the Right Ultrasonic Transducer?
VII. What is Transfer Correction?
Generally, echo responses are the tool used for transfer corrections, and they revolve around obtaining similar echoes for both the component and the calibration blocks. Essentially, this method compensates for the attenuation and transfer loss differences both for shear wave compression probes and attenuation compression probes.
A peculiar case exists for degree probes where inspectors use back wall echoes to determine the attenuation correction and establish transfers simultaneously. On the other hand, to get a back wall echo for the shear wave probe, you’ll need to utilize two identical probes using a pitch-catch method. However, a convenient parallel section for both components is required to employ either of the two methods successfully.
VIII. What is Horizontal Linearity Calibration in Ultrasonic Testing?
Horizontal linearity calibration is a time-based test used to calibrate a probe using a sample material that has a parallel face with the probe connected to it. The procedure often involves the collection of several repeated readings of echo coming from the parallel surface of the sample material while monitoring the readings on the digital display screen showing the graphical representations of these signals.
To ascertain that the equipment is calibrated appropriately, the spacing of the signals on the display screen must be spaced equally. For instance, with a 10mm thick sample material, the idea is to get echoes coming from the back wall at equal intervals of 10, 20, 30, etc. These intervals should be displayed on the horizontal axis of the screen.
IX. What is Vertical Linearity Calibration in Ultrasonic Testing?
The vertical linearity calibration is centered around getting the amplification spaced out equally using the gain control of the digital display device. The procedure often involves a gradual increment in the gain using the gain control and ensuring that the gain increase for the 6dB and the 12 dB raises the signal amplitude by double and quadruple, respectively.
However, it also requires ensuring that the first echo is 100% of the full-screen height and subsequently a gradual reduction in gain of 1dB. It is expected that the amplitude will reduce according to specific pre-calculated values to ascertain the adequacy of the equipment.
In closing, calibration blocks are vital in ensuring your flaw detector gives you accurate results and can identify the specific defect that it was calibrated for. However, the consistency of the periodic calibration for your detectors is also critical since neglecting the calibration of your ultrasonic equipment can lead to the accumulation of errors by the equipment over time which can cause unreliable inspection results.
Consequently, the correct type of reference standard and calibration equipment need to be chosen for each type of testing equipment.
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