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Complying with SSPC-PA2, “Measurement of Dry Coating Thickness with Magnetic Gages”

Complying with SSPC-PA2, “Measurement of Dry Coating Thickness with Magnetic Gages”. William D. Corbett KTA-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

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Complying with SSPC-PA2, “Measurement of Dry Coating Thickness with Magnetic Gages”

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  1. Complying with SSPC-PA2, “Measurement of Dry Coating Thickness with Magnetic Gages” William D. Corbett KTA-Tator, Inc.

  2. 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)

  3. Learning Objectives/Outcomes • Completion of this webinar will enable the participant to: • Describe the purpose of SSPC-PA 2 • Describe the differences between Type 1 and Type 2 gages • Describe the processes associated with calibration, verification of accuracy and adjustment • Explain Base Metal Reading acquisition • Describe the differences between measurement acquisition using manual verses electronic gages • Describe the frequency and tolerance of measurements • Describe the measurement acquisition process for steel beams. laydowns and test panels

  4. 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

  5. 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

  6. Gage Descriptions • Gage type is determined by magnetic properties employed to measure thickness (not the read-out mode) • Type 1 – Pull-off Gages • Type 2 – Electronic Gages • Gages not addressed by SSPC-PA 2 • Measurement of coatings on non-ferrous metal surfaces • Measurement 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)

  7. Gage Types Type 1 – Pull-off Gages Type 2 – Electronic Gages

  8. Gage Types, continued • Type 1 – Pull-off Gages • Permanent magnet contacts coated surface • Force required to detach magnet is measured • Force interpreted as the coating thickness on scale or display • Scale is nonlinear

  9. Gage Types, continued • Type 2 – Electronic Gages • Electronic circuitry converts reference signal to coating thickness

  10. 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

  11. 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

  12. 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 Verification • Select a reference test block below and above the median anticipated coating thickness • E.g., 5 mils (125 µm), select 3 mil (75 µm) and 7 mil (175 µm) reference standards

  13. 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 accuracy • E.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

  14. 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

  15. Correction for Surface Roughness Measuring Base Metal Effect with Type 1 DFT Gage Average BMR: 21 µm (0.8 mil)

  16. 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)

  17. 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)

  18. Correction for Surface Roughness Table A2 Typical Gage Correction Values Using ISO 8503 Profile Grades

  19. 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

  20. 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

  21. Verification of Type 2 Gage Accuracy • Single Point Verification • Same as described for Type 1 gages • Can use reference blocks or shims (per gage manufacturer) • Tw0 Point Verification • Same as described for Type 1 gages • Can use reference blocks or shims (per gage manufacturer)

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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

  27. Measurement Frequency • Terminology: • Gage Reading: A single reading at one location • Spot 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

  28. Measurement Frequency

  29. 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

  30. 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)

  31. Measurement Frequency

  32. Measurement Frequency Example 1 (US Standard) Structure Size: 900 square feet No. of Areas: 3 areas No. of Spots: 3 Areas x 5 Spots/Area = 15 Spots Minimum No. of Gage Readings: 15 Spots x 3 Readings/Spot = 45 Gage Readings

  33. Measurement Frequency Example 2 (US Standard) Structure Size: 55,000 square feet No. of Areas: 3 + 54 = 57 areas No. of Spots: 57 Areas x 5 Spots/Area = 285 Spots Minimum No. of Gage Readings: 285 Spots x 3 Readings/Spot = 855 Gage Readings

  34. Measurement Frequency Example 3 (metric) Structure Size: 5200 Square Meters No. of Areas: 3 + 51 = 54 areas No. of Spots: 54 Areas x 5 Spots/Area = 270 Spots Minimum No. of Gage Readings: 270 Spots x 3 Readings/Spot = 810 Gage Readings

  35. 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

  36. 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)

  37. 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

  38. Cumulative Thickness Measurements • Nondestructive coating thickness gages: • Most cannot distinguish coating layers • Measure the total cumulative thickness

  39. Cumulative Thickness Measurements EXAMPLE: Specification Primer 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)

  40. 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

  41. 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)

  42. Full Determination Note: Areas 2, 6, 8 and 12 (Toe) may not be measured

  43. 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)

  44. 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)

  45. 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

  46. 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

  47. 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

  48. 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

  49. 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)

  50. Summary • During this webinar, we have: • Described the purpose of SSPC-PA 2 • Described the differences between Type 1 and Type 2 gages • Described the processes associated with calibration, verification of accuracy and adjustment • Explained Base Metal Reading acquisition • Described the differences between measurement acquisition using manual verses electronic gages • Described the frequency and tolerance of measurements • Described the measurement acquisition process for steel beams, laydowns and test panels

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