DeFelsko PosiTector 200 v.3.1 Full Manual User Manual

Page 33

Application Notes

The PosiTector 200 probe emits a high frequency sound pulse
that travels into the coating via a coupling gel and reflects from
ANY surface that is different in density. Coating thickness
readings are obtained by measuring the time taken for the
ultrasonic signal to propagate from the probe to the
coating/substrate interface and back. The travel time is divided by
two and multiplied by the velocity of sound in the coating to obtain
the thickness of the coating. The strength of the reflected signal
from the coating/substrate interface determines the ability of the
Gage to measure the thickness of the coating. Since most
applications are not homogeneous the Gage will "hear" many
echoes when placed on a coating/substrate. The Gage "hears"
ALL reflections within the measurement limits of the Gage and
assumes the largest “echo” is the coating/substrate echo (single
coating applications only). Adjustable measurement Ranges have
been provided for the user to force the Gage to ignore echoes
from unwanted boundaries within the sample. Several examples
below help to illustrate the use of the Set Range (pg. 11) feature
for specific applications.

Measurement of coatings with rough surfaces.
When rough coatings are measured, the Gage typically identifies
the thickness from the top of the coating peaks down to the
substrate (#1). Couplant fills the voids between the probe and the
coating (#2) creating an additional interface. If echoes from the
couplant/coating interface (#2) are stronger than the
coating/substrate interface (#1), an adjustment (increase) of
Lo Value may be required for the Gage to display the weaker (#1)
echo.

The PosiTector 200 uses an ultrasonic principle to measure
coating thickness of most coatings on most substrates. An
ultrasonic signal is a very high frequency sound wave. Like the
echoes you hear when you shout in a large hall or canyon the
PosiTector 200 listens for echoes from acoustic boundaries
within your application.