Pencil-Type Magnetic Pull-Off
16.2.2 Pencil-Type Magnetic Pull-Off
DFT Gauge
The Pencil Pull-off Gauge, Type 1, is another type of magnetic pull-off gauge. ( Figure 16.7 ) The instrument is a hollow tube, similar in size to a large pencil, with an internal magnet and spring.
Dry Film Thickness Measurement Instrumentation 16-5
turer regarding the use and limitations of this instrument.
Again, repeatability of results depends sig- nificantly on the operator and technique. Because pencil gauges are less accurate than others, that drawback is also a factor in the repeatability of results.
Question any unusually high or low read- ings. If the gauge proves to be inaccurate, replace it.
Figure 16.7 Pencil-Type Magnetic Pull-Off DFT Gauge
Advantages of pencil gauges include:
• No battery or power source required,
16.2.2.1 Proper Use
therefore intrinsically safe
Most pencil-type pull-off gauges have large
• Can use on small parts (bolts), hot parts
magnets and are designed to work in only
(some models), and in hard-to-reach areas
one position (vertical or horizontal) to avoid
• Fits in a shirt pocket for quick spot check-
the effects of gravity. Some versions have
ing
scales designed to compensate for the effects of gravity regardless of their orientation.
16.3 Type 2 DFT Instruments
Electronic gauges are called Type 2 gauges Hold the pencil gauge perpendicular to the in both SSPC PA 2 and ASTM D7091 ( Fig- surface, and place the magnet in contact ure 16.8 ). They use a measuring probe and with the surface. As the gauge body pulls the magnetic induction, hall-effect, and/or away from the coated surface, the magnet eddy current measurement principles in con- remains magnetically attached to the sur- junction with electronic microprocessors to face. Keep a close watch on the indicator. produce a coating thickness measurement. It Note the reading when the spring tension is essential to place the probe directly (in a overcomes the force of attraction and pulls perpendicular position) on the coated sur- the probe from the surface. face to obtain a measurement.
16.2.2.2 Calibration, Verification, and Adjustment
16.3.1 Proper Use
See the relevant sections in the previous dis- Although electronic DFT gauges come from cussion of dial-type instruments.
different manufacturers, the basic procedure for taking measurements is the same. Place
the probe against the coated surface and hold Pencil gauges generally measure up to
16.2.2.3 Operating Parameters
it there while the measurement is taken. Lift around 25 mils (600 µm) with a tolerance of
the probe to relocate and take another mea- between +10% to ±15%, therefore they are
surement. Some gauges have fixed inte- used less frequently than other types of mag-
grated probes, some have separate probes, netic instruments. Check with the manufac-
and yet others have interchangeable integral
16-6 Dry Film Thickness Measurement Instrumentation
and separate probes. The procedure remains stored separately in distinct batch files for the same. The gauge’s screen displays the
future use. Users can transfer the data to coating thickness.
computers and, increasingly, to other digital devices.
Some advanced features are presented later in this section. As always, refer to the manu- facturer’s model-specific operating instruc- tions for detailed instruction and for features new to the user.
There are numerous standards that govern DFT measurement ( Figure 16.9 ). The most commonly used standards are:
• SSPC-PA 2 • ASTM D 7091 • ISO 19840 • IMO Resolution MSC.216 (82), Annex 3
Check the specification for applicable stan- dards, then consult the manufacturers’ data sheet to ensure the gauge complies with the standard. Some gauges have features to automatically determine if the DFT over a large area conforms to certain standards such as SSPC-PA2 or IMO MSC.215 (82) and MSC.216(82) PSPC (performance stan- dard for protective coatings in ballast tanks).