Measuring Coated Materials; Material Suitability; Coupling Means - sauter TU-US Instruction Manual

9.4 Measuring coated materials

Coated materials are special because their density (and therefore the speed of sound)
can vary considerably from one piece to another.
Even through a single surface, noticeable differences in the speed of sound can be
detected. The only way to obtain an accurate measurement result is to first perform a
calibration on a material sample of known material thickness. Ideally, this should be
from the same piece as the material to be measured, at least from the same production
series. With the help of the "pre-calibration" the deviations are reduced to a minimum.
An additional important factor when measuring coated materials is that any trapped air
gap causes premature reflection of the ultrasonic beam. This becomes noticeable in a
sudden decrease of the material thickness. While on the one hand this prevents the
exact measurement of the total material thickness, on the other hand the user is
positively alerted to air gaps in the coating.

9.5 Material suitability

Ultrasonic material thickness measurements are based on the fact that a sound is sent
through the material to be measured. Not all materials are suitable for this. Ultrasonic
measurement can be applied practically to a wide range of materials including metals,
plastics and glass. Difficult materials include some cast materials, concrete, wood,
fibreglass and some rubber.

9.6 Coupling means

All ultrasonic applications require a medium to transmit the sound from the probe to
the test material. Typically this is a very viscous medium.
Ultrasound cannot be efficiently transmitted through air.
A variety of coupling means are used. For most applications, propylene glycol is too
suitable. Glycerine is suitable for difficult applications. However, glycerine causes
corrosion of some metals by water absorption.
Other coupling agents for measurements at normal temperatures may contain water,
various oils or greases, gels and silicone fluids. High temperature measurements
require special high temperature coupling agents.
A characteristic of ultrasonic measurement is that the device uses the second rather
than the first echo from the rear surface of the material to be measured when it is in
standard pulse-echo mode. This results in a reading that is twice as large as it should
be.
The responsibility for the appropriate use of the measuring instrument and the
detection of these phenomena lies exclusively with the user himself.
22 TU_US-BA-e-2020