The word calibration has two different definitions in the field of ultrasonic test instrument and systems.
– Electronic calibration: that in done annually.
– Processing Calibration: that we do before we start to test with V1 or V2 calibration block and sensitivity setting by reference blocks which have some artificial flaws.
In manual or automatic systems, processing calibration is the most important and critical phase of the test. If the operator does not pay enough attention or does not use the full sensitivity for to the initial setting of the device or if he does not have the knowledge to it properly, even with the most modern and advanced ultrasonic flaw detectors, we cannot get the best out device and would be problematic for the production of the company. Some of these problems are listed as below:
– The faulty product might be recognized as accepted one and this would be disastrous for the company.
– Because of using a block which is not standard and wrong setting by the operator and high sensitivity of the test, right products might get rejected that would cause unnecessary stops in the production line that would cost a fortune.
After installing and delivering the ultrasonic instruments and systems to the buyers, especially automatic systems, are calibrated with the reference blocks which have artificial flaws according to standard, to be able to detect the flaws and be ready for testing. This calibration is called processing calibration.
This kind of calibration must be done before we start to test and it must be rechecked every four hours. Since doing this calibration is very important and effective in test results, Pejvak Rayan does the first calibration to testify the accurate function of the device and also fully trains the buyers’ personnel to make sure that they learn how to do this calibration.
Because of the importance of calibration blocks, in this phase, we will give some guidance and explanation about calibration blocks.
Choosing and making reference block is the most important and critical part of the test. Because sensitivity and accuracy of the test results are directly related to the reference block and created artificial flaws.
Reference block is similar to the test samples that has some artificial defects with the dimensions and geometry shape according to standard and it is used for the initial setting the device and beginning of the test and it must be done at the beginning and ending of each shift of test. The reference block should have the same material, acoustic properties, dimension and geometry shape as the test products.
Some of the codes and standards has obliged that the calibration blocks must have the same heat treatment as well.
The most important point in making the reference blocks, is creating the artificial defects. Some codes and standard have recommended different types of flaws that we will mention them.
Different artificial flaws recommended by standards are listed as below:
· Throughout holes
· N5 or N10 Notches
· Flat bottom holes
Throughout holes: Is one artificial hole throughout the thickness of the part with specific diameter according to standard. For example, in ERW piping 3/2 mm and SAW 1/6mm hole is a must and it can be in the weld or in the body. This hole must be radial otherwise it would not have a good function and it is used for calibration with angle probs.
N5 or N10 Notches: These are rectangular notches, commonly with 1 mm wide and 50 mm long that could be created in the weld or in the body of reference blocks. The word notch N5 and N10 is related to depth of the notch that could be 5% or 10% of the thickness of the block.
In order to detect surface defects, like cracks, two notches must be created upper and lower surface or inside and outside of the reference block. This kind of defects could be used for detecting surface flaws in different components. These notches must have sharp edges which reflect the sound in the best way. So, for the same reason we recommend to create such notch with EDM. These notches are used to calibrate angle probes for simulating surface cracks.
Flat Bottom Holes: This kind of artificial flaw is used for calibrating normal and T/R probes for testing pipes, casting products and plates for simulating lamination flaws, gas holes, inclusions and etc. The location of this flaw should be in 1/4T, 1/2T and 3/4T of the parts.
This hole should be made in a flat way and as much as possible parallel to the outer side.
Because the production of reference blocks is time-taking and cost money and their frequent use, they need to be kept in safe place with a thorough maintenance. Storing these blocks must be in a way that they do not lose their geometry shape and it also should not have scratches or dent while is being stored especially in the sharp edges of the artificial flaws. After using the block, we must grease the areas that we have flaw to keep them from rust. To depo them, we’d better to cover the flaws with pieces of cloth or something to protect edges from losing their sharpness due to external forces.