MIL-HDBK-1110
after each measurement. An alternative circularly notched gage
("hot cake") is rolled perpendicularly through the wet film and
the clearance of the deepest face wetted is noted.
10.3.9.2 Cylindrical Gage. A cylindrical wet film thickness
gage is described in ASTM D 1212. These gages are also rolled
through the paint rather than being pressed into it. They have
an eccentric center wheel with constantly changing clearance
supported by two outer wheels. The position on the exterior
scale corresponding to the point that the wet paint first touches
the eccentric wheel indicates the wet film thickness.
10.3.10
Dry Film Thickness Gages for Coatings on Aluminum,
Copper, and Stainless Steel. Gages are available to determine
the dry film thickness of organic coatings on aluminum, copper,
and stainless steel. Alternating current from the instrument
probe coil induces eddy currents in the metal that in turn induce
magnetic fields that modify the electrical characteristics of the
coil. ASTM D 1400 fully describes the instrument and its
operating procedure.
10.3.11
Magnetic Dry Film Thickness Gages for Coatings on
Steel. There are many different types of gages available for
nondestructively determining the film thickness of cured organic
coatings on metal surfaces. Most rely on the ferromagnetic
properties of steel. Their use is described in detail in ASTM D
1186 and SSPC PA 2. They are available in different thickness
ranges to provide the best accuracy with different coating
thicknesses. Each has a probe or tip that is placed directly on
the coating during measurement.
a) Magnetic thickness gages should be calibrated
before use. It is also a good practice to check the calibration
during and after use. Gage suppliers provide a set of standard
thickness nonmagnetic (plastic or nonferrous metal) shims to
cover their working ranges. The shim for instrument calibration
should be selected to match the expected coating thickness. It
is placed on a bare steel surface and the gage probe placed on it
for calibration. If the instrument scale does not agree with the
shim, it should be properly adjusted. If adjustment is
difficult, the reading for bare steel can be added or subtracted
from field readings to determine actual thicknesses.
b) The steel surface used for calibration should be a
masked-off area of the steel being painted or an unpainted
reference panel of similar steel, if possible. Pull-off gages
are best calibrated using small chrome-plated steel panels of
precise thickness (Standard Reference Material No. 1358,
Certified Coating Thickness Calibration Standard) available from
the National Institute of Standards and Technology (formerly the
159
Previous Page
