Complying with SSPC-PA2,
“Measurement of Dry Coating Thickness with
Magnetic Gages”
William D. CorbettKTA-Tator, Inc.
Complying with SSPC-PA 2
• Webinar Content Overview and Purpose of SSPC-PA 2 Definitions Proper Gage Use Acquisition of Measurements Frequency of Measurements Tolerance of Measurements Measuring Coating Thickness on Steel Beams
(girders) Measuring Coating Thickness on Test Panels Potential Changes to SSPC-PA 2 (2011/2012)
Learning Objectives/Outcomes
• Completion of this webinar will enable the participant to:
Describe the purpose of SSPC-PA 2Describe the purpose of SSPC-PA 2 Describe the differences between Type 1 and Type 2 Describe the differences between Type 1 and Type 2
gagesgages Describe the processes associated with calibration, Describe the processes associated with calibration,
verification of accuracy and adjustmentverification of accuracy and adjustment Explain Base Metal Reading acquisitionExplain Base Metal Reading acquisition Describe the differences between measurement Describe the differences between measurement
acquisition using manual verses electronic gagesacquisition using manual verses electronic gages Describe the frequency and tolerance of measurementsDescribe the frequency and tolerance of measurements Describe the measurement acquisition process for steel Describe the measurement acquisition process for steel
beams. laydowns and test panelsbeams. laydowns and test panels
Overview and Purpose of SSPC-PA 2
• Describes the procedures to measure the thickness of dry, nonmagnetic coatings applied to magnetic substrates
• Measurements are acquired using commercially available gages (two “types”)
• Procedures for gage adjustment & measurement acquisition are described
• Procedure for determining conformance to specified thickness range over extended areas is described
Definitions• Gage Reading• Spot Measurement• Calibration• Verification of Accuracy• Gage Adjustment• Coating Thickness Standard (Test Block)• Shim (Foil)• Dry Film Thickness Reference Standard• Accuracy• Structure
Gage Descriptions• Gage type is determined by magnetic
properties employed to measure thickness (not the read-out mode)Type 1 – Pull-off GagesType 2 – Electronic Gages
• Gages not addressed by SSPC-PA 2Measurement of coatings on non-ferrous metal surfacesMeasurement of coatings on non-ferrous surfaces
To qualify for use, gages must have an accuracy of +/- 5% or better (0.1 mil or better when < 1 mil DFT)
Gage Types
Type 1 – Pull-off Gages Type 2 – Electronic Gages
Gage Types, continued
• Type 1 – Pull-off GagesPermanent magnet
contacts coated surface
Force required to detach magnet is measured
Force interpreted as the coating thickness on scale or display
Scale is nonlinear
Gage Types, continued
• Type 2 – Electronic GagesElectronic circuitry
converts reference signal to coating thickness
Gage Calibration
• Performed by the gage manufacturer or qualified laboratory
• Certificate of calibration traceable to a National Metrology Institute required
• No standard calibration interval (established based on experience & work environment)
• One year interval is common
Verification of Type 1 Gage Accuracy
Performed using reference standards (traceable test blocks) Beginning and end of each
work shift If gage is dropped or
readings are suspect
Record: Serial no. of gage & standard Stated & measured thickness
Use of shims (foils) not permitted
Verification of Type 1 Gage Accuracy• Single Point Verification
Select one reference test block representing the mid-range of the anticipated coating thickness
E.g., 4-6 mils (100-150 µm), select 5 mil (125 µm) reference standard
• Tw0 Point VerificationSelect a reference test block below and above
the median anticipated coating thicknessE.g., 5 mils (125 µm), select 3 mil (75 µm) and
7 mil (175 µm) reference standards
Verification of Type 1 Gage Accuracy
• Most Type 1 gages cannot be “adjusted”• Adjustments to the helical spring may void
the gage warranty• Combined tolerance of gage and reference
standard determines gage accuracyE.g., if gage accuracy is 5% and reference
standard accuracy is 3%, combined tolerance is ~6%, calculated as: √ 52 + 32
On a 10 mil reference standard, the gage reading can range from 9.4-10.6 mils
Correction for Surface Roughness
• Base Metal Reading (BMR)
• Effect of surface roughness on coating thickness gage
• NOT surface profile• Measure the prepared,
uncoated substrate; calculate average BMR
• Deduct BMR from measured coating thickness
BMR
Correction for Surface Roughness
AreaArea BMRBMR
11 30 µm (1.2 mils)30 µm (1.2 mils)
22 25 µm (1.0 mils)25 µm (1.0 mils)
33 18 µm (0.7 mil)18 µm (0.7 mil)
44 13 µm (0.5 mil)13 µm (0.5 mil)
55 20 µm (0.8 mil)20 µm (0.8 mil)
66 8 µm (0.3 mil)8 µm (0.3 mil)
77 25 µm (1.0 mil)25 µm (1.0 mil)
88 28 µm (1.1 mils)28 µm (1.1 mils)
99 23 µm (0.9 mil)23 µm (0.9 mil)
1010 13 µm (0.5 mil)13 µm (0.5 mil)
1111 18 µm (0.7 mil)18 µm (0.7 mil)
1212 25 µm (1.0 mil)25 µm (1.0 mil)
Measuring Base Metal Effect with Type 1 DFT Gage
Average BMR: 21 µm (0.8 mil)µm (0.8 mil)
BMR Correction for Multiple Coat Systems
Measured Primer Thickness: 102 µm (4.0 mils)BMR: 13 µm (0.5 mils)Actual Primer Thickness: 89 µm s (3.5 mils)
Measured Primer + Finish Thickness: 178 µm (7.0 mils)
BMR: 13 µm (0.5 mils)Actual Total System Thickness: 165 µm (6.5
mils)
Correction for Surface Roughness
What if access to blast cleaned steel is not available (already coated)?
Appendix A2.3 addresses smooth surface adjustment Verify gage accuracy on a smooth surface
(per gage manufacturer instructions) Deduct “assumed” approximate
correction value from each gage reading (see Table A2)
Correction for Surface Roughness
Table A2Typical Gage Correction Values Using
ISO 8503 Profile GradesISO 8503
Profile GradeCorrection Value (µm)
Correction value (mils)
Fine 10 0.4
Medium 25 1.0
Coarse 40 1.6
Adjustment of Type 2 Gages
• Follow the gage manufacturers step-by-step procedures for gage adjustment
• Instructions vary by gage manufacturer
• Adjustment is typically performed using plastic shims (foils) of known thickness
Verification of Type 2 Gage Accuracy
Verify accuracy per manufacturer instructions
Typically performed using reference standards or shims Beginning and end of each
work shift If gage is dropped or readings
are suspect
Record: Serial no. of gage & standard Stated & measured thickness
Verification of Type 2 Gage Accuracy• Single Point Verification
Same as described for Type 1 gagesCan use reference blocks or shims (per
gage manufacturer)
• Tw0 Point VerificationSame as described for Type 1 gagesCan use reference blocks or shims (per
gage manufacturer)
Verification of Type 2 Gage Accuracy
• If smooth reference standards are used (A), user must correct* for surface roughness
• If shims (foils) are used (over the prepared steel; B), no correction is needed
*Via Base Metal Reading (BMR)
A B
Adjustment of Type 2 Gages
• Aligning a gage’s thickness readings to those of a known thickness value to improve gage accuracy on a specific surface or within a measuring range
• Corrects for: – Substrate properties– Coating– Ambient conditions and
surface temperature
Acquiring Coating Thickness Measurements –
Type 1 Gages• Rotate the thumbwheel
forward to set the magnet (or depress the power button on the digital model)
• Retract the thumbwheel until the magnet breaks contact (digital version breaks contact automatically)
• Read coating thickness from the gage dial or display
Acquiring Coating Thickness Measurements –
Type 2 Gages• Power-up the gage• Position the probe on
the coated surface until a measurement is displayed
• Most models have integral and remote probes
Type 2 Gage Data Management• Many Type 2 gages
have “PA 2” Data Management Systems
• Programmed to collect correct no. of values, perform averaging and indicate conformance
• Features and programs vary by gage manufacturer
Measurement Frequency
• Terminology:Gage Reading: A single reading at one
locationSpot Measurement: The average of at least 3
gage readings made within a 1.5” (4 cm) diameter circle
Area Measurement: The average of 5 spot measurements made within a 100 square foot (10 square meter) area
Measurement Frequency
Measurement Frequency
If the structure is less than 300 square feet, (~28 square meters) each 100 square feet (~10 square meters) is measured
If the structure is between 300 and 1000 square feet (~28 and 100 square meters), arbitrarily select 3 random 100 square foot (~10 square meter) test areas and measure
Measurement Frequency
For structures exceeding 1000 square feet (~100 square meters), arbitrarily select 3 random 100 square feet (~10 square meter) testing areas for the first 1000 square feet (~100 square meters), and 1 random 100 square foot(~10 square meter) testing area for each additional 1000 square feet (100 square meters)
Measurement Frequency
Structure Size: 900 square feet
No. of Areas: 3 areasNo. of Spots: 3 Areas x 5 Spots/Area =
15 SpotsMinimum No. of Gage Readings: 15 Spots x 3
Readings/Spot = 45 Gage Readings
Measurement Frequency Example 1 (US Standard)
Structure Size: 55,000 square feet
No. of Areas: 3 + 54 = 57 areasNo. of Spots: 57 Areas x 5 Spots/Area =
285 SpotsMinimum No. of Gage Readings: 285 Spots x 3
Readings/Spot = 855 Gage Readings
Measurement Frequency Example 2 (US Standard)
Structure Size: 5200 Square Meters
No. of Areas: 3 + 51 = 54 areasNo. of Spots: 54 Areas x 5 Spots/Area =
270 SpotsMinimum No. of Gage Readings: 270 Spots x 3
Readings/Spot = 810 Gage Readings
Measurement Frequency Example 3 (metric)
Measurement Tolerance
Individual gage readings obtained and averaged to generate a spot measurement are unrestricted (unusually low or high readings that can’t be repeated are discarded)
Spot measurements (the average of the gage readings) must be within 80% of the minimum thickness and 120% of the maximum thickness
Area measurements must be within specified range
Measurement Tolerance
EXAMPLE: Target DFT: 4-6 mils (102-152 microns) Individual gage readings unrestricted Spot measurements must be between 3.2 mils
and 7.2 mils (82 microns and 182 microns) Area measurement must be between 4 and 6 mils
(102 and 152 microns) If spot or area measurements are out of
tolerance, measure each 100 sq. ft (~ 10 sq. meter) area coated during that work shift to isolate the nonconforming area (independent of structure size)
Specifying Coating Thickness
Specifications should indicate the range of coating thickness (e.g., 5-7 mils or 127-178 microns), not as a single value (e.g., 5 mils or 127 microns)
Nearly impossible for an applicator to achieve a single thickness value
The Quality Control and Quality Assurance inspectors should not have to assume a range
Cumulative Thickness Measurements
Nondestructive coating thickness gages:
Most cannot distinguish coating layers
Measure the total cumulative thickness
Cumulative Thickness Measurements
EXAMPLE:
SpecificationPrimer thickness: 3-5 mils (76-127 microns)Intermediate Coat Thickness: 4-6 mils (102-152 microns)Finish Coat Thickness: 2-3 mils (51-76 microns)
Target After:Primer Application: 3-5 mils (76-127 microns)Intermediate Application: 7-11 mils (178-279 microns)Finish Coat Application: 9-14 mils (229-356
microns)
Appendix 3: Measuring Coating Thickness on Steel
Beams (Girders) Full Determination Sample Determination
Beams < 20 ft (6 m) Beams 20 ft - 60 ft (6 m-18 m)
Spot measurement tolerance (80% of minimum and 120% of maximum) applies
The average of all spot measurements (per area) must conform to specified range
Measurement locations on stiffeners arbitrarily selected
Stiffener
Appendix 3: Measuring Coating Thickness on Steel
Beams (Girders) Full Determination
Divide beam into 5 equal sections along the length
Web > 36”: Obtain one spot measurement in 14 areas, per section (total of 70 spot measurements)
Web < 36”: Obtain one spot measurement in 12 areas, per section (total of 60 spot measurements)
Full Determination
Note: Areas 2, 6, 8 and 12 (Toe) may not be measured
Appendix 3: Measuring Coating Thickness on Steel
Beams (Girders) Sample Determination
Beam length < 20 ft: Obtain 2 spot measurements randomly distributed in all 12 areas (total of 24 spot measurements)
Beam length 20-60 ft: Obtain 3 spot measurements randomly distributed in all 12 areas (total of 36 spot measurements)
Note: If toe areas are not included, measure in 8 areas (16 or 24 spot measurements)
Appendix 4: Measuring Coating Thickness on
Laydowns Laydown: Group of steel
members laid down to be painted in one shift by one applicator
Full DFT Determination Beams (girders) Miscellaneous parts
Sample DFT Determination Beams < 20 ft (6 m) Beams 20 ft - 60 ft (6 m-
18 m)
Appendix 4: Measuring Coating Thickness on
Laydowns Full DFT Determination
Beams: Same procedure described earlier
Miscellaneous parts: 1 spot measurement per “surface” (minimum of 5 spots)
Spot measurement tolerance (80% of minimum and 120% of maximum) applies
The average of all spot measurements (per area) must conform to specified range
Appendix 4: Measuring Coating Thickness on
Laydowns Sample DFT Determination
Beams: Same procedure described earlier
Miscellaneous parts: 3 spot measurements per part
Spot measurement tolerance (80% of minimum and 120% of maximum) applies
The average of all spot measurements (per area) must conform to specified range
Appendix 5: Measuring Coating Thickness on Test
Panels Minimum panel size:
3” x 6” (7.5 x 15 cm) Maximum panel size:
12” x 12” (30 x 30 cm) Use Type 2 gage Two gage readings from
top, middle and bottom third
At least 0.5” from edge and 1” from other readings
80% min.120% max. applies to gage readings
Appendix 6: Measuring Thickness of Thin Coatings on Abrasive Blast
Cleaned Test Panels• “Thin” is considered 1
mil (25.4 µm) or less• Obtain 10 gage
readings from each of three “zones”
• Calculate the mean and standard deviation in each zone
• The mean of all three zones is the coating thickness
10 gage readings
10 gage readings
10 gage readings
Potential Changes to SSPC PA 2
• Most recent revision is May 2004 (editorial changes to Appendix 6 made in 2009)
• Document revisions and updating in progress• Changes may include:
Re-title, “Procedure for Determining Conformance to Dry Coating Thickness Requirements”
Use in concert with ASTM D 7091* Measurement of coatings on ferrous and non-ferrous metal
surfaces Information on calibration and verification of accuracy
removed (already in ASTM D 7091) *Primarily focus on frequency and tolerance of
measurements (instead of gage use)
Summary
• During this webinar, we have:
Described the purpose of SSPC-PA 2Described the purpose of SSPC-PA 2 Described the differences between Type 1 and Type 2 Described the differences between Type 1 and Type 2
gagesgages Described the processes associated with calibration, Described the processes associated with calibration,
verification of accuracy and adjustmentverification of accuracy and adjustment Explained Base Metal Reading acquisitionExplained Base Metal Reading acquisition Described the differences between measurement Described the differences between measurement
acquisition using manual verses electronic gagesacquisition using manual verses electronic gages Described the frequency and tolerance of measurementsDescribed the frequency and tolerance of measurements Described the measurement acquisition process for steel Described the measurement acquisition process for steel
beams, laydowns and test panelsbeams, laydowns and test panels
Complying with SSPC-PA2, “Measurement of Dry Coating Thickness
with Magnetic Gages”
THE ENDTHE END