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Posted on Út zář 09, 2008 04:47
Post Subject: ULTRASONIC THICKNESS MEASUREMENT
HI,
I HAVE A QUERY REGRADING ULTRASONIC THICKNESS MEASUREMENTS OF THIN SHEETS/ SKIN OF THICKNESS RANGE 0.004'' TO 0.030''. I'M USING PANAMETRICS 25DL PLUS AND 35DL WITH WIDE RANGE OF PROBES LIKE M202 M2, V260, M112, M116 .MY APPLICATION IS GENERALLY MEASURING REMAINING THICKNESS AFTER CORROSION BLENDOUT OF ALUMINIUM AIRCRAFT SKIN. I'M FACING EXTREME DIFFICULTY IN MEASURING THICKNESSES BELOW 0.020'' PROBABLY DUE TO MULTIPLE REFLECTIONS AND NEAR ZONE EFFECTS WHICH USUALLY GIVES A ERROR IN READING . FOR EXAMPLE A NOMINAL THICKNESS OF 0.016'' WOULD READ AS 0.032''. ANYBODY ELSE WHO HAS COME ACROSS THE SAME SITUATION AND OVERCOMED IT? IS THERE ANY SPECIFIC PROBE WHICH CAN BE USED FOR THIS APPLICATION. SUGGESSTIONS WOULD BE APPRECIATED.
REGARDS,
ANIKET.
Posted on Út zář 09, 2008 05:07
Post Subject: RE: ULTRASONIC THICKNESS MEASUREMENT
Hello, Aniket --
Sorry to hear that you have having problems. First, I am afraid that a thickness of 0.004" in aluminum is out of range for both the 25DL PLUS and 35DL gages. The minimum aluminum thickness that you would be able to measure, using a Mode 3 setup with an M208 delay line transducer, would be approximately 0.006". With other transducers and setups it will be higher. The gage operating manuals contain range information for each transducer listed with respect to steel. For aluminum, minimums will be about 10% higher because of its faster sound velocity.
The doubling error that you report can be caused by several things:
1. Attempted measurement of material below the minimun that can be resolved with a given transducer.
2. Non-optimum instrument setup.
3. Worn or low sensitivity transducers.
4. Coatings on the inside surface that dampen the multiple echoes in Mode 3 measurements.
For corrosion blending applications, most customers use either a V260 Sonpoen transducer with the DEFM3-V260 setup or an M208 with the DEFM3-M208 setup. Are you using one of those setups, or an optimized custom setup? Transducers other than the V260 and M208 would not normally be recommended for blending applications. Be sure that the delay line is in good condition and that there is couplant between the delay line and the body of the transducer.
You might also try adjusting yourt setup by increasing the initial gain value by a few decibels, and be sure that the Mode 3 Blank setting is no greater than 50-60 nanoseconds.
I am attaching a typical waveform showing a thin aluminum measurement with an M208 transducer and a Model 35 gage, with clean echoes. If you still have problems, can you capture a waveform from the incorrect readings and post that along with your instrument setup? I can then comment further.
Tom Nelligan
Senior Applications Engineer
Olympus NDT/Panametrics-NDT products