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Re: Is PVDF for achiving high resolution the best choice? (for thin plastic layers)

Posted by: Robert A. Dayt , E-mail: rockyd@netcom.com, on September 04, 1996 at 17:52:15:

In Reply to: Is PVDF for achiving high resolution the best choice? (for thin plastic layers) posted by : Rolf Diederichs on September 04, 1996 at 17:04:40:

Rolf -

Actually PVDF is the best material for achieving resolution but laser generated sound is often better resolution. Problems with lasers are:

- difficult to focus (needed I think for your application?).
- poor signal to noise of receive.
- very expensive.

You don't say in your post whether you are using a
focused 15 MHz probe but I would assume you are. One
possible way to improve performance in such a difficult
material and geometry is to use a cylindrical focus
probe whose radius and position is adjusted to match
the outer tubing. This makes the geometry more plane
wave like and may improve signals a little. Because you
have at least two materials with different properties it
may not improve as much as you need.

You might be able to get more bandwidth with a PVDF (you
don't mention transducer type) but to improve the
performance dramatically you need to increase the
frequency to 20 or 25 MHz. Do you know if these
frequencies will penetrate? Can you send samples to try
things on. Transducer makers like samples!

You mentioned detecting the positive or negative spike.
Are you doing this in analog electronics or digital? One
good way to do wall thickness measurements is to take
the Hilbert transform of the signal which produces a
kind of video signal but without the phase distortion
conventional RF detectors produce. This method may be to
slow for production applications (100s of milliseconds
typically).

Multiple layer pipe and hose is getting very popular so
I expect we will be called on to NDE more of these types
of materials in the future

: In R. A. Day's article it is pointed out:

: "The bandwidth of PVDF is actually the best of any material so far discovered and
: routinely produces transducers with 0.5 to 1 cycle
: transmitted and received pulses".

: Is it right that there is no doubt about that PVDF should be used for
: gaining the best resolution. Especially for excellent near to surface resolution
: or thin wall thickness measurement it should be applied.

: Just in a case we would need maximal amplitude as well, we
: should consider also other transducer materials.

: Finding a solution for a very thin wall thickness measurement is always a challenge.
: Here is an example I assume that a PVDF transducer should be used.
: What would be the best transducer design (Dia, Freq, Focus) for the application of
:
: Coextrusion Layer Thickness Measurement
: for polyamid gasoline pipes.
: Especially for an In-Line measurement by immersion technique,
: however to measure also the thickness of some layers.
: Product dimensions:
: Dia: about 8mm
: Total wall: about 1mm
: 1-5 Layers: each in a range of 0.1mm - 0.3mm

: I am locking forward your suggestion.

: Rolf Diederichs



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