DeFelsko PosiTector UTG Notice D'instruction page 6

Ultrasonic Thickness Probes –
Theory of Operation
PosiTector UTG probes transmit an ultrasonic pulse into the
material to be measured. This pulse travels through the material
towards the other side. When it encounters an interface such as air
(back wall) or another material, the pulse is reflected back to the
probe. The time required for the pulse to propagate through the
material is measured by the gage, represented as t1 and t2 below.
Single-echo PosiTector UTG probes (and PosiTector UTG M and
UTG P probes in single-echo mode) determine wall thickness by
measuring t1 (uncoated) or t2 (coated), dividing it by two and then
multiplying by the velocity of sound for that material (steel). See
Figure 1.
t
Probe
2
t
Probe
1
t t
>
2 1
Figure 1
For uncoated materials t 1 relates directly to material thickness.
When a material is coated the propagation time is increased and
is shown above as t 2 .
Coatings such as paint have a slower velocity of sound than that
of metal. Thus the single-echo technique will produce a thickness
result greater than the actual combined coating + metal thickness.
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