7.3 Special Considerations.
7.3.1 Conductive Coatings.
If the displayed voltage drops sharply when the probe is
placed on the test surface, or if the alarm sounds continuously,
then the coating may be conductive. The usual reasons for this
are detailed below.
Existence of metallic, carbon or other conducting
particles in the coating.
During normal use, the particles in this type of coating are not
linked. However, when the coating is subjected to high
voltages the material between the particles can break down.
This results in the coating becoming conductive and the
detector indicating a flaw.
To overcome this, reduce the voltage sufficiently to avoid
break down of the coating. In certain cases, the coating will
still conduct at voltages which are too low to locate a flaw.
Surface moisture or contamination.
This can cause the instrument to indicate non-existent flaws.
The surface should either be dried using a suitable cloth or
cleaned with a non-conducting, non-destructive cleaner or
solvent.
Note: Ensure that any cleaner or solvent containers are
removed from the test area before repeating the test.
Moisture penetration or absorption.
Moisture can enter materials if the surface is eroded or
scratched and then immersed in water. In this case, allow
adequate time for the coating to dry prior to testing.
Rubber linings.
These may be slightly conductive due to their carbon content.
As with other conductive coatings, reduce the alarm
sensitivity so that the detector indicates a known flaw, but
does not sound when the probe is placed on sound coating.
It may also be necessary to increase the test voltage to
compensate for the current flow through the coating.
15
To Next Page
Loading...
