Calibration, if said in simpler words, is a process by which an instrument is configured. This process then provides a result for a sample within an acceptable range. The fundamental basis of an instrumental design is completely eliminating or minimising the factors that become a medium of faulty measurements.
Calibration is one such process that varies from instrument to instrument. The ultrasonic flaw detector calibration procedure usually involves “calibrators”. This involves dealing with different instruments to test samples of one or more known values.
Now, the result obtained from the calibration process is used to derive a relationship between the technique of measurement and the known values. This process, in turn, makes the instrument more aware to produce results that are highly accurate, as that of those obtained otherwise.
So, it can be concluded that after the calibration process, the instrument will provide more accurate results.
Ultrasonic Flaw Detector Calibration Procedures
Sensory zero-point adjustment (Transducer Zero Compensation), material sound velocity calibration (Material Velocity Calibration), and null migration calibration (Zero Offset Calibration) are the three main classes of ultrasonic device calibration. Currently, the popular ” 2 calibrations (Two-Point Calibration) ” uses a mix of ” zero migration calibration ” and ” material sound velocity calibration “.
As we already discussed, different instruments require different ultrasonic flaw detector calibration procedures.
Ultrasonic Flaw Detector Calibration Modes
Nearly all industrial equipment may be calibrated by applying the algorithms Zero Offset (ZO) and Material Velocity (MatVel) over reference signals with a reference block repeatedly. The Auto Calibration feature is also a highly useful technique to establish the time-based calibration.
Ultrasonic velocity is usually thought of as a steady value in relation to the material. This assertion is backed up by a collection of circumstances including an elastic and non-dispersive material in a constant air pressure and temperature. The frequency needs to be high enough to create a wavelength range that is substantially shorter than the size of the standardised piece.
Another time base calibration parameter that the technician must select in order to complete the calibration is the ultrasonic velocity. In most instrument panels, the ultrasonic velocity is referred to as Material Velocity (MatVel).
The Zero Offset
The zero offset function compensates for the transit time inside the probe wedge (when using a delay line contact probe or an angle beam probe), and more broadly, the transit time prior to entering the object surface.
While dealing with contact ultrasonic transducers, zero offsets subtract a specified time duration from the thickness or depth measurement, usually to correct for wedge latency or other preset offsets, such as wear plate thickness or delay line thickness while dealing with delay line or dual element transducers.
Classical Digital Calibration
Two reference signals from two comparable reflectors at varying distances are used in the standard calibration procedure. The first step is to set the Zero Offset and Delay parameters to zero while starting the calibration. Applying ultrasonic velocity charts, it is also advised to insert the material velocity value as near to the actual value as feasible.
The calibration must be excellent if the material velocity directly introduced by the operator matches the real material velocity, as the user may verify by surrounding the peak level with the barrier and comparing the projected sound route to the real distance. It’s conceivable, though, that the sound route displayed differs from the real distance. The explanation is that the material velocity chosen from an ultrasonic table does not correspond to the real material velocity of a calibration test block. As a result, the earlier corrected Zero Offset value is an inexact number due to the instrument calculations.
When the Zero Offset value is entered, the new time value is calculated by using the following equation:
tn = th — toff
tn – time used for the calculation
th – the time measured from the instant where the excitation pulse was emitted
toff – Zero Offset value.
Most recent generations of industrial flaw detection ultrasonic instruments feature an autocalibration mode, and the concept is very simple. It involves the following steps
- The first step in the Auto Calibration mode is to enter the precise numbers d1 and d2 for the angle beam case on the instrument panel. The distance difference can be easily measured by the instrument using the following equation:
dp = d2 – d1
The time difference can be measured by the instrument using the equation:
tp = t1h – t2h
- It is useful to evaluate the measured value of time t1h and t2h depending on whatever point of the vertical position of the indicator is chosen to perform the measurement. The flaw detection UT has the phases – peak and flank for this purpose.
With the gate set at a 10% threshold, a higher calibration is achieved in the flank mode, according to observation. It is suggested that the gate be kept at the same level until the calibration is complete.
- The material velocity is automatically calculated using the equation:
cer = dp/tp
- Now that material velocity is calculated, t1 can be calculated using,
t1 = d1/cer
- Finally, the Zero Offset Value is determined using:
toff = t1h – t1
On completion of the above-mentioned steps, the time base calibration is done via autocalibration mode in ultrasonic flaw detectors.
What is the Need For Ultrasonic Flaw Detector Calibration Procedure?
Now that the ultrasonic flaw detector calibration procedure is clear, there is another thing you should be aware of, i.e., the need for calibrating. An ultrasonic flaw detector includes a procedure where a high-frequency sound wave is pushed through a material.
In case of the presence of any irregularities, an echo is picked up by the ultrasound instrumentation during the calibration process. Obviously, if ultrasound waves are involved, it won’t be a matter of surface only. There are a lot of factors involved, such as:
- Construction material
- Operator skill
So, these were the ultrasonic flaw detector calibration procedure for linear, non-linear, and discrete instruments.
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