You want to calibrate your ultrasonic flaw detector but don’t know how.
Calibrating your ut flaw detector as a quality assessment professional is one skill that pays off with so many benefits, especially when it has to do with the accuracy of your ultrasonic flaw detector. While you don’t need to calibrate your ultrasonic flaw detector every day, the probability of accumulated errors after consistently using your ultrasonic flaw detector only means that it will be inaccurate by a long margin in a little while. Hence the importance of this process is never lost on most non-destructive testing professionals. The problem? The process is not well-known by most professionals. So, if you have this same challenge, you are in luck, as this article will give you all the necessary information to calibrate your ultrasonic flaw detector.
Without further ado, let’s dive in.
I. What is Ultrasonic Flaw Detector Calibration?
Calibration involves setting your measuring instrument with a standard unchanging reference to ensure the accuracy and precision of your measuring device. For an ultrasonic flaw detector, this means adjusting it using a reference material that meets certain specifications. However, it often occurs in two stages: the manufacturer and the operator calibration. The manufacturer ensures that the flaw detector meets the standard manufacturing specification outlined in the relevant code. Conversely, the user calibration is often inspection specific, and it ensures that the ultrasonic flaw detector meets the specification for the anticipated flaw to be detected.
II. Why Ultrasonic Flaw Detector Calibration is Necessary?
Reasons for calibrating ultrasonic flaw detectors are numerous. However, the most important reason is that it ensures that the ultrasonic flaw detector is accurate and void of errors. Ultrasonic flaw detectors tend to become slightly inaccurate but still within acceptable tolerance when used for an inspection. But over time, the cumulative margin of error increases, and the inaccuracy becomes big and beyond the acceptable tolerance levels. Periodically calibrating your ultrasonic flaw detector ensures that this margin of error is reduced to the barest minimum and without overtly affecting the result of the flaw detector.
III. Ultrasonic Flaw Detector Calibration
Setting up for an ultrasonic flaw detector calibration is often done in three different ways. The details are given below.
- Zero Offset Calibration – This setup considers the time elapsed during the wave travel before entering the test sample and then equates it to the time elapsed as it travels through a layer of the test sample. This time is often designated as t0.
- Material Velocity Calibration – Also known as the ultrasonic velocity setup, this calibration method depends primarily on the material to be inspected and the ambient temperature obtainable during the setup. However, it expects some basic assumptions to be in place before commencing the calibration. These assumptions include an elastic material, a frequency lower than the dimensions obtainable in the test sample but high enough to establish a wavelength, a constant atmospheric temperature and pressure, and a non-diffusive material.
- Auto Calibration – This method requires that specific settings be in place before calibration. These settings include using the ultrasonic velocity tables to adjust the material velocity values to be closer to the real values as much as possible. Also, the delay and zero offset values need to be set to zero. This method considers the speed of two similar signal reflectors sending two separate signals from different distances.
IV. Three Calibration Process Related to Ultrasonic Flaw Detector
For ultrasonic flaw detectors, three processes are often considered calibration processes.
- Velocity/Zero calibration
Converting time to distance measurements using the speed of sound in test materials to program the flaw detector is often referred to as the velocity/zero calibration. Also, the zero offset setting’s echo shape or transducer type requirements are critical. This type of calibration often considers the dimensions measured by the flaw detector, including the distance and thickness of the material, using precisely timed echoes. The ultrasonic flaw detector’s accuracy depends on the careful measures taken during this calibration exercise, and errors might occur in the readings if the calibration is not carefully and correctly done. Thankfully, the calibration process is pretty straightforward and with the possibility of storing different material calibrations in the memory of the device.
- Reference calibration
Using similar materials or test blocks as reference standards to set up a testing operation is often referred to as reference calibration. However, for ultrasonic flaw detectors, the signal’s amplitude received from standard references is usually the benchmark utilized for this type of calibration. User-defined procedures often give the details required for reference calibration used for specific tests.
- Calibration Certification
This process involves documenting an ultrasonic flaw detector’s linearity and measurement accuracy given some specific test conditions. This measurement accuracy is often juxtaposed with the manufacturer’s given tolerances. Additionally, distance and amplitude certifications are given for ultrasonic flaw detectors. Yet, the certification must still be conducted following relevant codes and standards such as EN 12668 or E-317.
V. How to Calibrate Ultrasonic Flaw Detector?
Calibrating an ultrasonic flaw detector requires calibrating specific parameters to detect flaws accurately. These parameter calibrations include calibrating probe delay and X-value, workpiece sound velocity, and incident angle. The following steps are used to calibrate ultrasonic flaw detectors.
- Steps to calibrate Zero point and normal probe’s velocity
- Firstly, set probe in position A with the right coupling.
- Ensure the test range is twice the workpiece’s thickness and greater than 200mm. Also, set the sound velocity to 5920m/s and gain to the correct value.
- Select the speed/delay from the CAL menu, also set the S-ref1 and S-ref2 to greater than 100mm and 200mm respectively.
- Next, move gate A then confirm the echo1 using the plus (+) key. Also repeat the step for echo2.
- And you are done setting the zero point and normal probe’s velocity.
- Steps for calibrating the angle probe for X-value
- Firstly, set probe in position B with the right coupling.
- Adjust gain to a suitable value.
- R100 arc will give the peak echo transmitted on its surface
- Note the reading on the housing of the probe to get the X-VALUE.
- Steps for calibrating the angle probe for delay
- Firstly, set the angle probe in position B with the right coupling.
- Next, set the range and the sound velocity to above 100mm and 3250m/s respectively while setting the gain to a suitable value
- R50 and R100 arc will give the peak echo transmitted on its surface after moving the probe
- Select the speed/delay from the CAL menu, also set the S-ref1 and S-ref2 to greater than 50mm and 100mm respectively.
- Next, move gate A then confirm the echo1 for R50 using the plus (+) key.
- Next, move gate A then confirm the echo2 for R100 using the plus (+) key.
- The speed/delay is instantly set and calibrated with the actual velocity also computed.
- Steps for calibrating the angle probe for angle
- Firstly, set the angle probe in position C with the right coupling.
- Enter the value of the aperture, depth of hole, and calibration type as 50mm, 30mm, and angle on the main menu.
- Also, set the gain to the correct value
- The probe should also be adjusted to the highest echo conforming to a 50mm hole.
- Gate A should also be adjusted to this 50mm hole
- To finish the calibration, use the plus (+) key in the CAL sub-menu.
➤ Related Article: How to Calibrate Ultrasonic Thickness Gauge?
In conclusion, calibrating your ultrasonic flaw detector is not difficult. All you need to do is follow the steps highlighted in this article, and you’ll be fine. However, it would be best if you took careful measures while calibration your ultrasonic flaw detector as this will ensure that the calibration is accurate, and the flaw detector is without errors. Also, ensure you carry out this process frequently, as slight errors in the readings can become cumulatively massive over time. Still, refer to this article from time to time to ensure you acquaint yourself with the processes highlighted here.
So, there you have it.