DeFelsko PosiTector 200 v.1.2 - Manual (Page 15)

Page 15
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 appli-
cation.
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 den-
sity. 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 instrument
to measure the thickness of the coating. Since most applications are not homo-
geneous the gage will "hear" many echoes when placed on a coating/substrate.
The instrument "hears" ALL reflections within the measurement limits of the gage
and assumes the largest “echo” is the coating/substrate echo. Adjustable meas-
urement Gates have been provided for the user to force the instrument to ignore
reflections from unwanted boundaries within the sample. Several examples below
help to illustrate the use of Gates 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/sub-
strate interface (#1), an adjustment (increase) of Gate A may be required for the
gage to display the weaker (#1) echo.
Application Notes Application Notes
#2
#1 Coating
Substrate
Probe
Couplant