DeFelsko PosiTector 200 v.1.2 - Manual (Page 16)

Page 16
Measurement of dense (hard) coatings.
A significant echo occurs at the probe/coating interface. The relative strength of
this echo compared to the coating/substrate echo requires that Gate A be
increased.
Measurement of total thickness in multi-layer coating applications.
The echoes from multiple coating/coating/substrate interfaces causes some con-
fusion. The user may need to adjust Gates to ignore reflections from coating/coat-
ing interfaces
Measurement of a coating on a non-homogeneous substrate.
When a coating is applied to a non-homogeneous substrate multiple echoes from
within the substrate can cause the gage to appear to read erratic. In most
instances reducing Gate B will help. Example: Coatings over some types of fiber-
glass or composite.
Measurement of a coating on a rough or porous substrate.
The roughness of the substrate will scatter the echo from the coating/substrate
interface. In some cases multiple closely spaced echoes will be overlapped caus-
ing the gage to appear to read erratic. In other cases the scattering from the sub-
strate absorbs the echo and the gage will not be able to distinguish the coat-
ing/substrate echo. In the case of a porous substrate the coating might be
absorbed into the substrate in an uneven fashion producing the same effect as a
rough substrate. Examples: Coatings over very rough concrete; Coatings over
very soft woods.
The above examples depict general factors that need to be considered when
using ultrasound to measure coating thickness with the PosiTector 200. These
include the relative difference in density of the probe and the coating, the condi-
tion of the coating surface, the relative difference in density of the coating and
substrate and the condition of the coating/substrate interface. There are situations
where combinations of the above conditions result in very challenging measure-
ment applications. Careful use of the Gate settings helps ensure successful
measurement of the application.