GENERAL: This standard describes the procedures to easure the thickness of a dry film of a nonmagnetic coating applied on a magnetic substrate using commercially available magnetic gages. These procedures are intended to supplement manufacturers’ instructions for the manual operation of the gages and are not intended to replace them.
The procedures for calibration and measurement are described for two types of gages: pull-off gages (Type 1) and constant pressure probe gages (Type 2).
1.3 The standard defines a procedure to determine if the film thickness over an extended area conforms to the minimum and the maximum levels specified.
Gage Reading: A single reading at one point.
Spot Measurement: The average of at least three gage readings made within a 1.5 inch (4 cm) diameter circle.
Gage Types: The gage type is determined by the specific magnetic properties employed in measuring the thickness and is not determined by the mode of data readout, i.e. digital or analog. This standard does not cover gages that measure the effect of eddy currents produced in the substrate.
Type 1—Pull-Off Gages: In pull-off gages, a permanent magnet is brought into direct contact with the coated surface and a calibrated scale measures the force necessary to pull the magnet from the surface. Less force is required to remove the magnet from a thick coating. The scale is nonlinear.
• Type 1A - A magnet is attached to one end of a pivoting balance arm. This assembly is connected to a calibrated helical spring. By rotating a dial, the spring increases the force on the magnet and pulls it from the surface. The Type 1A gages are commonly called “banana” gages.
• Type 1B - A magnet is mounted directly or indirectly to a coil spring. The spring acts perpendicularly to the surface to pull off the magnet. The Type 1B gages are commonly called “pencil” gages.
Type 2—Constant Pressure Probe Gages: A constant pressure probe gage uses a probe which exerts a constant pressure on the coated surface during the entire measuring operation. Electronic circuitry is used to convert a reference signal into coating thickness. (See 8.1.)
USE OF STANDARD: This document contains the following:
• Calibration, verification and measurement procedures (Section 4).
• Required number of measurements for conformance to a thickness specification (Section 5).
• Notes on gage principles and various factors affecting thickness measurement (Section 8).
• A numerical example of thickness measurement over an extended area (Appendix 1).
• A numerical example of verification of the calibration of Type 2 gages using plastic shims (Appendix 2).
