Direct reading thickness measurements of dielectric walls using microwave technics

Hinken, Johann; FI Test- und Messtechnik GmbH (FIT-M); Germany

Hinken, J.; FI Test- und Messtechnik GmbH; Germany
Ziep, C.; FI Test- und Messtechnik GmbH; Germany

ID: ECNDT-0445-2018
Download: PDF
Session: Microwave, Terahertz, and Infrared NDE
Room: H1
Date: 2018-06-13
Time: 15:40 - 16:00

Johann Hinken and Christian Ziep
FI Test- und Messtechnik GmbH, Germany,

The standard method to measure wall thicknesses is based on ultrasonics. In certain cases, however, there may be drawbacks and limitations given by the need of a coupling medium, a temperature dependent sound velocity, a high sound attenuation, and an extra needed data evaluation. If the walls are from dielectric, i.e. electrically insulating material, a microwave based gauge may be advantageous.

Following the basic ideas of the successful development of a microwave based thickness gauge for paint on carbon composites, a method has been developed to measure non-destructively dielectric wall thicknesses. A probe, i.e. microwave antenna, is attached to the outer surface of the wall and at a fixed frequency the reflection coefficient is measured with respect to its real and imaginary part. In the complex plane, for increasing wall thickness this first of all results in a clockwise rotation with 360o increase each half a material wavelength. The resulting ambiguity at more than one rotation, however, does practically not occur because of an increased attenuation of the reflected signal with increasing wall thickness. The result is a spiral instead of a multiply cycled circle.

Wall thickness measurements on TPC tubes from the automobile industry are shown as an example. This includes the generation of a calibration spiral from few calibration points. A frequency within the ISM band at 24 GHz is used. Furthermore the outline of a practical direct reading instrument is shown.