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Measure metal thickness Ultrasonics?

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Full Member level 6
Aug 18, 2012
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Does anyone here have any experience measuring the thickness of plate steel using ultrasonic or other electronic techniques?

The steel plates are in the 1/2 to 2" thickness range.

We made a precision Eddy Current probe for Monel Steel 1mm thick 1cm diameter using 100kHZ, 200KHz centre tapped differential coil in contact with tube inside surface while probing for metallurgical defects, such as a change in wall thickness of <0.1mm or a pinhole of <<0.1mm.

With 0.5 to 2" the loops you make must be > thickness and use a frequency that has a skin depth>= sheet thickness.

In order to measure the anomaly we used a custom precision quadrature PLL to measure real and reactive impedance using a calibration mark to rotate the data whereby with the correct frequency the two different defects will be 90 deg apart. The resolution of our net vector impedance meter was around 10 ppm.

I did this in 1978, but remember all the details as if it were yesterday.

In your case you just want to measure thickness.

ABB I see now has a patent on this. They have discovered that the Eddy Current time response of a current pulse has 3 stages and the last stage is thickness.

So by analyzing the change in slopes by differentiation, one can disable the reset on an integrator at t2 and compute the integral for a duration of X us or until below the noise threshold and capture the distance reading.


Thank you, can you provide some perspective on the coil size and voltage/current levels of the pulse required?

I'm thinking of taking a different approach to this problem.

I did some reading about brake wear indicators, there is a patent that treats the rotor as one plate and the brake pad backing plate as the other plate of a capacitor, think of the brake pad as the dielectric material, as the brake pad wears the two plates come closer together.

My application is very similar in that the thick steel plate will wear and I have been asked to measure the amount of wear, however there will be no rotor wearing against the steel plate just intermittent scuffing that will wear the plate down over time.

Any ideas?

One way could be to use the edge of your iron disc to fill an air gap in the C core of an inductor. As the disc gets thinner, the air gap increases, so making the inductance less.
This will work if you have a measurement resolution and accuracy with thermal shift on the order of 10ppm such as a frequency counter in LC resonant oscillator as L is very NTC and PTC caps are limited in value like P250?

The big IF is temperature stability.

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