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TN_US-BA-e-2020
9.
Measuring hot surfaces
The velocity of sound through a given material depends on its temperature. With
increasing temperature, the speed of sound decreases. For most applications with a
surface temperature of less than 100°C no further precautions need to be taken. At
temperatures above this, the change in the speed of sound through the material to be
measured begins to have a noticeable effect on the ultrasonic measurement.
At such high temperatures, it is recommended to first calibrate with a material sample
of known material thickness, which corresponds exactly or approximately to the
temperature of the material to be measured. This allows the instrument to calculate the
exact sound velocity through the hot material.
For measurements on hot surfaces it may also be necessary to use a "high
temperature sound generator". These are specially designed for use at high
temperatures, especially since contact with the material surface should be maintained
for a short period of time for stable measurement.
While the sound generator is in direct contact with the hot surface, it heats up. Thermal
expansion and other effects can have a negative impact on the measuring accuracy.
10.
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 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 is 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.
