MIL-HDBK-1110
National Bureau of Standards), Gaithersburg, MD 20899. These
panels should not be used on magnetic flux gages, because the
mass of steel is insufficient for their proper operation. Shims
from pull-off gages should not be interchanged with those from
magnetic flux gages.
c) About five field measurements should be made for
every 100 square feet of painted surface. Each of these five
measurements should be an average of three separate gage readings
taken within an inch or two of each other. Measurements should
be made at least 1 inch away from edges and corners.
10.3.11.1 Pull-Off Gages. Pull-off gages measure film thickness
by stretching a calibrated spring to determine the force required
to pull an attached permanent magnet from a coated steel surface.
The simplest type of pull-off instrument is the pencil gage with
a coil spring attached to the magnet. It is held in a vertical
position on the coated steel and lifted away slowly until the
magnet pops off the surface. The paint thickness is indicated by
the position of the indicator on the calibrated scale. The
attractive force of the magnet varies inversely with the paint
thickness.
Banana gages (long, narrow instruments) represent
another form of pull-off gage. They are more versatile and
precise than pencil gages. A helical spring is stretched by
manually turning a graduated dial, and a pin pops up when the
magnet is lifted. At least one company sells an automatic gage
with a dial that turns and stops automatically. Cheaper models
have a rubber foot contact for the painted surface. More
expensive models have a more durable tungsten carbide foot for
greater durability and precision. "V" grooves are cut in the
probe housing of these gages and the electrically operated flux
gages described below to permit more accurate measurement of
paint dry film thicknesses on cylindrical surfaces.
10.3.11.2 Flux Gages. Magnetic flux gages measure changes in the
magnetic flux within the probe or the instrument itself. Flux
changes vary inversely with distance between the probe and the
steel. Mechanically operated instruments of this type have a
horseshoe magnet that is placed directly on the coating, and
readings are made from the position of a needle on a calibrated
scale.
Electrically operated magnetic flux instruments have a
separate instrument probe that houses the magnet. Thickness
measurements are presented in a digital read-out. Some of these
gages have a probe attached to the instrument to permit greater
accessibility, especially in laboratory work. They may also have
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