STEP
Delay Line and Immersion Transducer Setup
Set the zero offset to zero and set velocity to the approximate velocity of the test material.
If the velocity is unknown, you can start with a generic value of approximately 5.900 mm/uS or .2320 in/uS for steel and similar metals, or 2.500 mm/us or .1000 in/uS for most engineering plastics and composites.
Set the range to a value large enough to cover the thickest calibration sample plus the acoustic length of the delay line or water path. If testing metal, the acoustic length of a typical delay line at steel velocity will be approximately twice its mechanical length, and the acoustic length of a water path will be approximately four times its mechanical length.
Identify the delay line or water path echoes. The excitation pulse should be at the left edge of the screen and delay line or water path echoes will follow.
Delay line echoes can be verified by touching the tip with a wet fingertip; echo amplitude will be reduced. Front surface echoes in immersion tests can be verified by blocking the sound beam with a fingertip; the echo will disappear.
When you have identified the first delay line or water path echo, adjust the zero offset to move it to the left edge of the screen, as seen in the second image below.
Couple the transducer to the calibration samples and observe the echoes.
Adjust the instrument’s pulser and receiver settings as required to obtain clean echoes. Settings typically include pulse frequency, energy, damping, receiver filtering, and gain.
When testing thin materials with delay line transducers, it is usually advantageous to work with unrectified RF signals for cleaner peak resolution.
Couple the transducer to a thin sample.
Place a measurement gate on the echo and set echo height to approximately 80%. Initiate a zero calibration as described in the instrument operating manual.
Enter the known thickness of the thin sample.
Couple the transducer to a thick sample that is within its measurement range.
Move the measurement gate to that echo and set echo height to approximately 80%. Initiate a velocity calibration as described in the instrument operating manual.
Enter the known thickness of the thick sample.
Conclude the calibration process as described in the instrument manual.
Recheck the thick and thin samples and verify that thickness measurements are accurate.
STEP 1
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