ASTM B117 01

Prévia do material em texto

FORD LABORATORY TEST METHOD BI 117-01 
 
Date Action Revisions 
 
2001 03 19 Revised Editorial – no technical change A. Cockman 
1989 04 24 
Printed copies are uncontrolled Page 1 of 3 
 
 
Copyright © 2001, Ford Global Technologies, Inc. 
THICKNESS MEASUREMENT OF PAINT FILMS 
 
 
Application 
 
This procedure details recommended practices and equipment for measurement of thickness of paint films 
on metallic substrates. 
 
 
Apparatus Required 
 
Permascope or Leptoscope, or equivalent 
 
Note: If other measuring principles are applied these must be indicated in the test report and if required 
coordinated with Product Engineering. 
 
Measurement Principle 
 
Two (2) types of instruments can be used depending on whether the substrate is magnetic iron or steel, or 
non-magnetic metals. 
 
The instrument consists of a gauge head connected to an indicating unit. A magnetic flux is produced when 
the probe tips are placed in contact with a paint film over a ferritic substrate. The paint film acts as an air 
gap in the magnetic circuit. The magnetic impedance of the gauge head varies as a function of the air gap 
length, i.e. the thickness of the paint film, and thickness is read directly on the scale of the recording 
instrument. 
 
When the substrate is a non-magnetic metallic, an Eddy current system is utilized. A high frequency 
magnetic field is created in the probe's measuring system and Eddy currents are generated when the probe 
is placed onto the non-magnetic substrate. Magnitude of the Eddy current depends on distance between 
metal and probe, shape and electrical conductivity of specimen. 
 
The gauge must be standardized against base materials of the same composition, geometry, heat treatment 
and surface treatment (such as phosphating or galvanizing) for maximum accuracy. 
 
Note: 
 
The following factors may have an influence on the results: 
 
1. Surface roughness and waviness of the surface. 
 
 
 
FORD LABORATORY TEST METHOD BI 117-01 
 
 Page 2 of 3 
 
 
Copyright © 2001, Ford Global Technologies, Inc. 
2. Magnetic conditions of the substrate material. 
 
3. Thickness of the substrate material. 
 
4. Hardness of the coating and pressure applied to gauge head during test. (A constant pressure 
probe should be used where feasible.) 
 
5. Curvature of the surface (measurement should be taken parallel to the curvature axis). 
 
6. Distance from the measuring point to an edge of the material under test. Edge effects should be 
avoided. 
 
7. Deviation of the magnetic probe axis from a perpendicular to the area to be measured. (The probe 
should be normal to the measured surface.) 
 
8. Area of test piece, minimum 30 mm x 30 mm. 
 
9. Some standards from instrument manufacturers are only of nominal film thickness. (see Note on 
Page 3) 
 
Calibration 
 
Thickness standards are furnished with the instrument and are used to calibrate the gauge as follows: 
 
1. Warm up the instrument for 20 minutes before use. 
 
2. Select the proper scale range for the estimated coating thickness. 
 
3. Calibrate the instrument as prescribed in the manufacturer's manual. 
 
4. For test panels and bodies the calibration must be performed on steel body skin panels ground to a 
surface finish of 0.9 - 1.8 micrometers, unless otherwise specified. 
 
Calibrations in cases when either an uncoated specimen is not available or the magnetic properties of the 
coated and uncoated specimens are different: 
 
1. Select proper scale range (first reading). 
 
2. Place gauge head on coating (second readings). 
 
3. Insert suitable thickness standard between coating and gauge head. 
 
4. If the second reading on the instrument is not equal to the original reading plus the thickness of the 
standard, the calibrating control should be adjusted until the instrument indicates the expected 
value of coating thickness plus standard. 
 
 
 
FORD LABORATORY TEST METHOD BI 117-01 
 
 Page 3 of 3 
 
 
Copyright © 2001, Ford Global Technologies, Inc. 
5. Repeat this procedure two (2) or three (3) times to adjust the gauge correctly for the particular base 
metal. 
 
 
Procedure 
 
Paint over Magnetic or Non-Magnetic Metal Substrates 
 
1. Warm up the instrument for 20 minutes before use. 
 
2. Select scale range for estimated thickness. 
 
3. Adjust or zero gauge on an uncoated area of the base metal of the test specimen. This should be 
done on an area having the same contour as the test specimen and may require removal of some 
paint with a suitable solvent or paint remover. 
 
4. Select a standard according to the requirements of the Instrument manufacturer and insert between 
the uncoated specimen and gauge head or probe and adjust indicator to read the thickness of the 
standard. 
 
5. Place probe on painted area of the specimen and read thickness of coating. 
 
Possible errors with Paint on Finished Bodies 
 
Measurements on areas, where due to different type of body construction, increased metal 
thickness exists, also edges, sharp radii, and closeness to trim moulding may not be accurate 
because of interaction with the flux lines of the instrument. 
 
Note: 
 
Standards and instrument correlation checks against certified coating thickness Calibration Standards for 
non-magnetic coatings on steel should be carried out at a minimum of six (6) monthly intervals (Source of 
certified standards, US Department of Commerce, Washington, D.C.). 
 
 
Evaluation 
 
Report type of equipment, scale range, location of measuring points, average, minimum-maximum range, 
and if appropriate Standard Deviation. 
 
 
Chemicals, materials, parts, and equipment referenced in this document must be used and handled 
properly. Each party is responsible for determining proper use and handling in its facilities.