MIL-HDBK-1110
attachments for strip recorders for repetitive work or alarms to
produce sounds if minimum thicknesses are not met. For the paint
inspector, these more sophisticated attachments are normally
unnecessary.
10.3.12
Destructive (Nonmagnetic) Dry Film Thickness Gage.
There are several models of Tooke gage described in ASTM D 4138,
Measurement of Dry Film Thickness of Protective Coating Systems
by Destructive Means that measure paint dry film thickness on any
surface by microscopic observations of precision-cut angular
grooves in the film. The gage is not recommended with very soft
or brittle films which distort or crumble, respectively, when
cut.
A dark, thick line is first drawn on the painted
surface for later reference under the magnifier. A groove is
then firmly cut perpendicular across the line with a tungsten
carbide cutter tip as it forms a tripod with two support legs.
The width of the cut is determined visually using the illuminated
magnifier portion of the instrument. Tips with three different
cutting angles are available for use with films of thickness up
to 50 mils. Visual observations are multiplied by 1, 2, or 10,
depending upon the cutting angle of the tip, to determine the
actual film thickness. Thicknesses of individual coats of a
multi-coat system can be determined, if they are differently
colored.
10.3.13
Holiday Detector. Instruments for detecting pinholes
and other flaws in coatings on metal surfaces are used mostly on
waterfront and fuel storage and distribution facilities but
should be used on freshly coated critical metal structures.
Holiday detectors are available in two types: low and high
voltage, as described in NACE RP0188.
10.3.13.1 Low Voltage Holiday Detectors. Low voltage (30 to 90
volts) detectors are used on coatings up to 20 mils in thickness.
These portable devices have a power source (a battery), an
exploring electrode (a dampened cellulose sponge), an alarm, and
a lead wire with connections to join the instrument to bare metal
on the coated structure. A wetting agent that evaporates upon
drying should be used to wet the sponge for coatings greater than
10 mils in thickness. The wetted sponge is slowly moved across
the coated surface so that the response time is not exceeded.
When a holiday is touched, an electric circuit is completed
through the coated metal and connected wire back to the
instrument to sound the alarm. Holidays should be marked after
detection for repair and subsequent retesting.
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