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 gage selected for use should be one in which the measured film thickness falls within the middle 80
percent of each scale. The gage is available in the ranges given in table 4-1.
(2) Nordson WFT Gage. The Nordson WFT gage has several legs, with each of the inner legs
somewhat shorter than the outer legs. The two end legs define the plane of application and penetrate
through the wet film to the substrate. Consequently, some of the legs will dip into the coating while others
will not touch it. The leg, which is just wetted by the coating, indicates the WFT.
c. DFT Measurements. Minimum DFT for the coatings should be determined with one of the DFT gages
(or equal) described below.
(1) General Electric Type B Thickness Gage. The General Electric Type B thickness gage is a
rugged, reasonably portable instrument operating from a 115-volt, 60-Hz power supply. The gage
operates on a magnetic principle and can be used to measure coatings only on a magnetic surface. The
instrument is fast and easy to use, and can be used on slightly curved surfaces as well as on flat
surfaces. Due to the fact that different surfaces will give different readings, it is necessary to standardize
the scale with a foil of known thickness on a bare area of the same thickness and type of surface as the
one on which the measurement is being made. To prevent variances in readings, measurements shall not
be made close to edges and corners. The accuracy of the readings varies with the thickness of foil used
to standardize the instrument. For best results, the standardizing foil shall be close to the actual thickness
of the paint film being measured.
(2) Elcometer Thickness Gage. The Elcometer thickness gage has different ranges available and is a
light, portable instrument that fits into a coat pocket. This gage is operated by means of the variation in
magnetic force between the metal surface and a self-contained permanent magnet. No outside power
source is necessary. Measurements can be made on slightly curved surfaces. To operate the Elcometer
thickness gage, it is necessary to first standardize the dial over a bare area of the same type and
thickness of metal as that on which the coating is to be measured. It is important for accuracy to hold the
meter in the same plane when taking the measurements, as the gage is then set to zero. The results
obtained will be satisfactory for most purposes for which field measurements are made, providing the
necessary precautions for use are taken.
(3) General Electric Permanent Magnet Thickness Gage. The General Electric permanent magnet
thickness gage is a portable, self-contained gage with a dual scale. The low scale ranges from 0 to 7 mils
and the high scale ranges from 1 to 60 mils. The gage is also provided with a go-no-go feature, which can
be set for a minimum and maximum paint thickness.
(4) Mikrotest Thickness Gage. In the Mikrotest thickness portable gage, a small magnet is attached to
one end of a balance, which is at equilibrium; the magnet is connected through a spiral spring with a
micrometer screw. To measure DFT, the spiral spring of the gage is stretched by turning the micrometer
screw until the magnet is pulled away from the coated surface. The coating thickness can then be read
from the calibrated disk or obtained from a calibration curve.
(5) Dermitron Electromagnetic Thickness Gage. The dermitron electromagnetic thickness gage and
other eddy-current type thickness gages are electronic instruments that measure variations in impedance
of an eddy-current-inducing coil by paint coating thickness variations. Gages of this type can be used only
if the electrical conductivity of a coating differs significantly from that of the substrate. Typical
measurement applications involve an organic coating over aluminum or corrosion-resistant steel (CRES).
(6) Tooke Coating Inspection Gage. The Tooke coating inspection gage is an optical thickness gage.
It is used to measure the thickness of different colored individual paint coats or can be used when
nondestructive gages cannot be used. A cut is scribed through the film and measurements are made by
means of a calibrated microscope eyepiece. After the measurements, the scribed marks shall be
repainted.
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