William D. Corbett KTA-Tator, Inc.

William D. Corbett KTA-Tator, Inc.
Complying with SSPC-PA2, “Measurement of Dry Coating Thickness with Magnetic Gages” William D. Corbett KTA-Tator, Inc. This one-hour webinar is on complying with The Society for Protective Coatings standard for measurement of coating thickness, known as Paint Application Standard No. 2, or SSPC PA2.

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) The webinar will discuss a variety topics relating SSPC’s Paint Application Standard No. 2, including: The purpose of SSPC’s dry film thickness standard A few basic definitions listed in the standard Proper gage use, including calibration, verification of accuracy and gage adjustment Acquisition of coating thickness measurements The number of gage, spot and area measurements to acquire based on the size of the structure How far out of specification the measurements are permitted to be, or the tolerance of the acquired measurements The frequency and location of measurements on steel beams or girders as well as test panels; and A brief discussion on some potential changes to the standard in 2011 or 2012.

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 There are seven learning outcomes or objectives for this webinar. Completing this webinar will enable you to: Describe the basic purpose of SSPC’s standard on measurement of dry coating thickness Describe the differences between measurement acquisition using Type 1 magnetic pull-off and Type 2 electronic gages Describe the procedures and frequency for gage calibration, verifying gage accuracy and gage adjustments Explain base metal reading and describe when and how to obtain it, and it’s significance relating to true coating thickness Describe the differences in procedures when measuring coating thickness using manual gages verses electronic gages; Describe the number of coating thickness measurements to obtain and how far out of tolerance the measurements can be and still conform to the specification; and Describe the frequency and location of measurements for steel beams, laydowns in steel fabrication shops and on 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 SSPC’s paint Application Standard No. 2, or PA 2 is perhaps one of the most commonly specified standards for measurement of coating thickness. The standard describes procedures for measuring the thickness of non-magnetic coatings applied to magnetic substrates; essentially carbon steel. According to the standard, coating thickness measurements are obtained using two types of commercially-available gages produced by a variety of gage manufacturers across the US and overseas. Many of these gages are depicted in this webinar; however there is no intent to show favoritism of one gage over another. SSPC PA2 describes the procedures for gage calibration, verification of gage accuracy using traceable standards, gage adjustment and measurement acquisition. Finally, the standard provides a procedure for determining whether a coating or coating system conforms to the specified thickness range over extended areas of a structure.

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 Several terms are used throughout this webinar. It will be helpful to define these terms upfront. All of these terms and associated definitions are provided in SSPC PA2. Gage Reading is a single reading at a single point Spot measurement is the average of at least three gage readings made within a 1 ½” or 4 cm diameter circle Calibration is the controlled and documented process of measuring traceable calibration standards and verifying that the results are within the stated accuracy of the gage. Calibration is typically performed by the gage manufacturer or a qualified laboratory under controlled conditions using a documented process. The standards used for the calibration process are such that the combined uncertainties of the resultant measurement are less than the stated accuracy of the gage. Verification of Accuracy is an accuracy check performed by the gage user using known reference standards. Gage Adjustment is the act of aligning the gage’s thickness readings to match those of a known sample in order to improve the accuracy of the gage on a specific surface or in a specific measuring range. Most Type 2 gages can be adjusted on a coated part or on a shim where the thickness of the coating or the shim is known. Coating Thickness Standard or Test Block is a smooth ferromagnetic substrate with a nonmagnetic coating of a known thickness which is used to verify the accuracy of a coating thickness gage. Shim or foil is a thin strip of nonmagnetic plastic, metal or other material of known uniform thickness used to verify the accuracy of a dry film thickness gage or used for gage adjustment. A dry film thickness reference standard is a sample of known thickness which is used to verify the accuracy of the gage, such as a coated thickness standard or shim. Accuracy is the consistency between a measured value and the true value of the coating thickness standard. Structure is defined by the standard as a unit comprised of one or more connected steel members comprising a bridge, ship, etc. It is possible for a single steel shape such as a steel beam or pipe to be considered a structure, if it is coated in a shop.

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) 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, for example digital or analog. Type 1 gages are considered magnetic pull-off and Type 2 gages are considered electronic The standard does not cover gages that measure coating thickness on non-ferrous metal surfaces like aluminum or non-ferrous surfaces like concrete. To qualify under this standard, a gage must have an accuracy ± 5% or better. For thicknesses less than 1 mil, the gage must have an accuracy of 0.1 mil or better.

Gage Types Type 1 – Pull-off Gages Type 2 – Electronic Gages
A variety of Type 1 magnetic pull-off gages are shown to the left, while several manufacturers of Type 2 electronic gages are shown on the right. An electronic gage uses electronic circuitry to convert a reference signal into coating thickness. Type 2 gages are typically more accurate that Type 1 gages.

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 For pull-off gages, a permanent magnet is brought into direct contact with the coated surface. The force necessary to pull the magnet from the surface is measured and interpreted as the coating thickness value on a scale or display on the gage. Less force is required to remove the magnet from a thick coating. Type 1 gages have nonlinear scales and any adjusting feature is linear in nature. Any adjustment of these gages will limit the DFT range for which the gage will provide accurate readings, and is not recommended. Furthermore, the application of a “correction factor” is not appropriate, since the value is a non-linear correction.

Gage Types, continued Type 2 – Electronic Gages
Electronic circuitry converts reference signal to coating thickness A Type 2 electronic gage uses electronic circuitry to convert a reference signal into coating thickness. As a general statement, Type 2 gages are typically more accurate that Type 1 gages, and data acquisition is typically faster. Also, most Type 2 gages can store readings and provide statistical analysis of the data, and once the data is generated and stored, it can be uploaded into a computer software program or downloaded to a printer.

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 As we defined earlier, gages must be calibrated by the manufacturer or a qualified lab. A Certificate of Calibration or other documentation showing traceability to a national metrology institute is required. There is no standard time interval for re-calibration, nor is one absolutely required. Calibration intervals are usually established based upon experience and the work environment. A one-year calibration interval is a typical starting point suggested by gage manufacturers.

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 To verify gage accuracy, measure the thickness of a series of reference standards covering the expected range of coating thickness. To prevent acquiring measurements with an inaccurate gage, the gage is checked at least at the beginning and the end of each work shift with one or more of the reference standards. If the gage is dropped or suspected of giving erroneous readings during the work shift, its accuracy must be rechecked. When documenting gage adjustment processes, record the serial number of the gage, the reference standard used, the stated thickness of the reference standard as well as the measured thickness value obtained, and the method used to verify gage accuracy. If the same gage, reference standard, and method of verification are used throughout a job, they only need to be recorded once. The stated value of the standard and the measured value must be recorded each time accuracy is verified. Shims of plastic or of non-magnetic metals which are acceptable for verifying the accuracy of Type 2 electronic gages are not to be used for verifying the accuracy of the Type 1 gages.

Single Point Verification Select one reference test block representing the mid-range of the anticipated coating thickness E.g., 4-6 mils ( 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 When verifying the accuracy of Type 1 gages, the user can employ a single point or two-point process. For single point verification, a single reference test block is selected which is at or close to the thickness to be measured. The thickness range over which this adjustment achieves the required accuracy will vary with gage design. Assuming that the coating thickness to be measured is 4-6 mils, or microns, then a reference standard of approximately 5 mils or 125 microns should be used verify gage accuracy. For two point verification, two reference standards are selected - one above and one below the expected film thickness to be measured. Assuming that the coating thickness to be measured is 5 mils or 125 microns, then reference standards of 3 mils or 75 microns and 7 mils or 175 microns are appropriate for establishing a range of 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: √ On a 10 mil reference standard, the gage reading can range from mils Most Type 1 gages should not be adjusted. Any adjustments made to the helical or wound spring within the first year can void the warranty on the gage. The gage can be verified for accuracy using a reference standard. The combined tolerance of the gage and reference standard is used to verify gage accuracy. For example if the accuracy of the gage is 5% and the accuracy of the reference standard is 3%, the combined tolerance of approximately 6% is calculated by using the square root of the sum of both accuracies, squared. On a 10 mil reference standard, the gage reading can range from mils and still be considered accurate.

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 The specified dry film thickness of each coating layer is measured from the tops of the peaks of the surface profile. This is depicted by the thick yellow bar on the slide. However, most coating thickness gages must reach down into the surface roughness to satisfy the magnetic properties of the gage. This is depicted by the thin yellow line part way down into the profile on the slide. As a result, the effect of the surface profile on the thickness gage must be measured and subtracted from the coating thickness measurement. This in known as a base metal reading or BMR, which is depicted by the gap between the thick yellow bar and thin yellow line on the illustration. Once the Type 1 thickness gage is verified for accuracy, the next step is to measure and record the Base Metal Reading or BMR. This is accomplished by placing the gage magnet on the prepared, uncoated substrate and obtaining a measurement. The BMR will vary widely, ranging from 0.1 mil to over 1 mil. Therefore, the user should take several measurements of the base metal and calculate the average. The average BMR is subtracted from the thickness of each coat, in order to assess the thickness of the coating film above the peaks of the surface profile. The BMR is the effect of surface roughness on a coating thickness gage – it is not surface profile. There is no correlation between surface profile depth and the effect of this roughness on a coating thickness gage). It is the BMR and not the surface profile that is deducted from the coating thickness. Also, the gage is never adjusted to zero on the uncoated surface. BMR

Area BMR 1 30 µm (1.2 mils) 2 25 µm (1.0 mils) 3 18 µm (0.7 mil) 4 13 µm (0.5 mil) 5 20 µm (0.8 mil) 6 8 µm (0.3 mil) 7 25 µm (1.0 mil) 8 28 µm (1.1 mils) 9 23 µm (0.9 mil) 10 11 12 Measuring Base Metal Effect with Type 1 DFT Gage A base metal reading is calculated by randomly measuring a number of locations on the structure, then averaging all of the values. In this example, 12 base metal measurements were acquired ranging from 0.3 to 1.2 mils for an average of 0.8 mil. Average BMR: 21 µm (0.8 mil)

BMR Correction for Multiple Coat Systems
Measured Primer Thickness: µm (4.0 mils) BMR: µm (0.5 mils) Actual Primer Thickness: µm s (3.5 mils) Measured Primer + Finish Thickness: 178 µm (7.0 mils) Actual Total System Thickness: µm (6.5 mils) Coating thickness measurements are corrected for the base metal reading independent of which layer or layers is being measured. For example, if the measured primer thickness is 4 mils or 102 microns and the base metal reading was 0.5 mil or 13 microns, the actual primer thickness above the peaks of the surface profile is 3.5 mils or 89 microns. Continuing with the example depicted on the slide, if the measured primer & finish coat thickness is 7 mils or 178 microns, the base metal reading remains 0.5 mil or 13 microns, so the actual system thickness above the peaks of the surface profile is 6.5 mils or 165 microns.

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) If access to the bare blast cleaned substrate is not available because the coating already covers it, a smooth surface can be used to adjust the gage. The procedure for this is described in Appendix A2.3 of the standard. First, adjust the gage on a smooth surface according to the manufacturer’s instructions, using a reference standard. Measurements of coating thickness taken on the blast cleaned substrate will be higher than the true value by an amount dependant on the surface profile and the gage probe design. Therefore an assumed correction value to compensate for the base metal effect must be subtracted from each gage reading to correct for the effect of the surface roughness. The resulting corrected reading represents the thickness of the coating over the peaks. Table A2 in the appendix to the standard provides some guidance on the assumed amount to be deducted, depending on the coarseness of the surface profile.

Table A2 Typical Gage Correction Values Using ISO 8503 Profile Grades ISO 8503 Profile Grade Correction Value (µm) Correction value (mils) Fine 10 0.4 Medium 25 1.0 Coarse 40 1.6 The table illustrates typical gage correction values that can be applied to coating thickness measurements when the true effect of the base metal cannot be measured. Four fine profiles, for example 1-2 mils or microns, a correction factor of 0.4 mil or 10 microns may be appropriate. For medium profiles, for example mils or microns, a correction factor of 1 mil or 25 microns may be appropriate. For coarse profiles, for example 4-5 mils microns, a correction factor of 1.6 mil or 40 microns may be appropriate.

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 Similar to Type 1 gages, the user can employ a single point or two-point process when verifying the accuracy of Type 2 gages. The step-by-step procedures for verifying gage accuracy vary widely between gage manufacturers. It is beyond the scope of this webinar to describe each; however the user should carefully follow the manufacturers instructions when verifying gage accuracy and when making adjustments to the gage. Most Type 2 gages can be verified for accuracy using reference standards or plastic shims. Use of plastic shims is more economical, as most gage manufacturers supply a set of shims with their gage. If needed, calibrated shims are available from many gage manufacturers. These are sold separately.

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 As we described previously, you should always verify the accuracy of the gage according to the manufacturer’s instructions. To prevent acquiring measurements with an inaccurate gage, the gage is checked at least at the beginning and the end of each work shift with one or more of the shims or reference standards. If the gage is dropped or suspected of giving erroneous readings during the work shift, its accuracy must be rechecked. When documenting gage adjustment processes, record the serial number of the gage, the reference standard of shim used, the stated thickness of the reference standard or shim as well as the measured thickness value obtained, and the method used to verify gage accuracy. If the same gage, reference standard or shim, and method of verification are used throughout a job, they only need to be recorded once. The stated value of the standard of shim and the measured value must be recorded each time accuracy is verified.

Single Point Verification Same as described for Type 1 gages Can use reference blocks or shims (per gage manufacturer) Tw0 Point Verification Similar to Type 1 gages, the user can employ a single point or two-point process when verifying the accuracy of Type 2 gages. For single point verification, a single reference test block or shim is selected which is at or close to the thickness to be measured. For two point verification, two reference standards or shims are selected - one above and one below the expected film thickness to be measured.

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) If the gage manufacturer permits the use of reference standards to verify gage accuracy – as shown in (A) on the slide, the user must then acquire a base metal reading and deduct it from each coating thickness measurement, since the gage was verified for accuracy on a smooth surface but the coating was applied over a roughened surface. Conversely, if shims or foils are used to verify gage accuracy – as shown in (B) on the slide, and the shim is placed over the prepared, uncoated steel, then no correction for base metal is required, since the gage has been adjusted to the shim thickness resting on the peaks of the surface profile. The application of a base metal correction is tied to the type of reference material used to verify gage accuracy and not the type of gage. This is a common misunderstanding. 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 The final step in verifying gage accuracy is to align the Type 2 gage to a known value to improve gage accuracy on the specific type and design of surface or within a specific measurement range. Some refer to this step as gage optimization. In this case, the gage is adjusted to match the shim or foil thickness by placing a shim or foil of known thickness directly onto the prepared, uncoated structure or part under the same conditions of air and surface temperatures that the coating will be measured under. This also compensates for curvature of the component or structure, the alloy of the steel, proximity to edges or other surface conditions.

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 Examine the magnet to make sure there is no coating residue, magnetic filings, or other contaminants on it. Place the gage on coated surface to be measured. If you are using a manual magnetic pull-off gage, use your finger or thumb to rotate the scale wheel forward or counterclockwise until the magnet makes contact with the surface. Slowly and steadily turn the scale wheel backwards or clockwise until the magnet breaks contact with the surface. Stop turning the scale wheel immediately after the magnet breaks contact. Read the coating thickness as the number on the scale that lines up with the hairline on the plastic scale cover. If you are using an automatic magnetic pull-off gage, place the end of the instrument containing the magnet on the coated surface and rotate the dial forward until the magnet makes contact with the surface. Push in and release the counterweight button on the underside of the handle. The dial will automatically retract until the magnet breaks contact with the surface. Once this happens, the gage will automatically stop. Read the coating thickness as the number on the scale that lines up with the hairline on the plastic scale cover. If you are using a digital magnetic pull-off gage, place the end of the instrument containing the magnet on the coated surface. Push and release the black button on the underside of the handle. The digital read-out on the top of the gage will display the reading, and an audible signal will sound.

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 Examine the gage probe to make sure there is no coating residue or other contaminants on it. Power up the gage. Place the gage probe onto the coated surface to be measured. The gage will signal when a measurement is taken and the value is displayed. Lift the gage between measurements. Most gages are available with integral or 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 Many of the Type 2 gages are equipped with data management systems that are programmed to collect the correct number of thickness values and perform averaging of the gage readings and spot and area measurements, along with indications of conformance to the specification. The features of the data management systems vary by manufacturer.

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 SSPC PA 2 contains a procedure for frequency of coating thickness measurements. While we already defined some of the terms relating to frequency, they are review here. A gage reading is a single reading at one location, and a spot measurement is the average of at least 3 gage readings made within a 1.5 inch or 4 cm diameter circle. An area measurement is the average of the 5 spot measurements made within a 100 square foot or 10 square meter area on a structure.

The figure shown on the slide was extracted from SSPC PA 2. It illustrates three gage readings taken in each of 5 spots in an area of approximately 100 square feet or 10 square meters. The average of the three gage reading in each of the 5 spots has been calculated.

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 The number of areas that are measured is based on the size of the structure being coated. For structures not exceeding 300 square feet or 30 m2, each 100 square foot or 10 m2 area is measured. As a result, the maximum number of areas to be measured will be 3. For structures greater than 300 square feet but not exceeding 1000 square feet or 100 in area, three 100 square foot or 10 m2 areas are arbitrarily selected and measured.

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) For structures exceeding 1000 square feet or 100 m2 in area, three 100 square foot or 10 square meter areas are measured in the first 1000 square or 100 m2. For each additional 1000 square feet or portion thereof, one additional 100 square foot or one 10 m2 area is arbitrarily selected by and measured. Other size areas or number of spot measurements may be specified by the owner in the job specifications for the size and shape of the structure to be coated.

The figure on this slide illustrates the frequency of coating thickness measurements on large, flat steel plates. The top third of the figure indicates the measurement frequency based on an area less than or equal to 300 square feet. The middle third of the figure indicates the measurement frequency based on an greater than 300 square feet and less than or equal to 1000 square feet. The bottom third of the figure indicates the measurement frequency based on an area greater than 1000 square feet.

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 = Spots Minimum No. of Gage Readings: 15 Spots x 3 Readings/Spot = Gage Readings In Example 1, the structure is approximately 900 square feet. As a result, three 100 square foot areas are arbitrarily selected and measured. This culminates in the acquisition of a minimum of 45 gage readings acquired from 15 spots.

Measurement Frequency Example 2 (US Standard)
Structure Size: 55,000 square feet No. of Areas: = 57 areas No. of Spots: 57 Areas x 5 Spots/Area = Spots Minimum No. of Gage Readings: Spots x 3 Readings/Spot = Gage Readings In Example 2, the structure is approximately square feet. As a result, three 100 square foot areas are arbitrarily selected and measured in the first 1000 square feet, then one additional 100 square foot area is arbitrarily selected and measured in each additional1000 square foot area, for a total of 57 areas . This culminates in the acquisition of a minimum of 855 gage readings acquired from 285 spots.

Measurement Frequency Example 3 (metric)
Structure Size: Square Meters No. of Areas: = 54 areas No. of Spots: 54 Areas x 5 Spots/Area = Spots Minimum No. of Gage Readings: Spots x 3 Readings/Spot = Gage Readings In Example 3, the structure is approximately 5200 square meters. As a result, three 10 square meter areas are arbitrarily selected and measured in the first 100 square meter area, then one additional 10 square meter area is arbitrarily selected and measured in each additional100 square meter area, for a total of 54 areas. This culminates in the acquisition of a minimum of 810 gage readings acquired from 270 spots.

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 Individual gage readings that are averaged together to create a spot measurement are unrestricted. That is, readings that are unusually high or low within the spot that cannot be repeated are discarded. The average of the individual gage readings is a spot measurement. The spot measurement must be within 80% of the minimum specified thickness and 120% of the maximum specified thickness. Stated differently, spot measurements can be high or low by as much as 20%; while individual gage readings can overrun or under-run by a greater amount. The average of the spot measurements must fall within the specified coating thickness range. That is, the 20% allowance only applies to the spot measurement and not the area measurement.

Measurement Tolerance
EXAMPLE: Target DFT: 4-6 mils ( 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) In this example the specified coating thickness is 4 to 6 mils or 102 to 152 microns. The individual gage reading that are obtained are unrestricted. The average of the gage readings – the spot measurements can range between 3.2 and 7.2 mils or microns, which is 80% of 4 mils and 120% or 6 mils, or 80% of 102 microns and 120% of 152 microns. The average of the five spot measurements – the area measurement must be between 4 and 6 mils, or 102 to 152 microns. If spot or area measurements are outside of the allowable tolerance, then each 100 square foot area or 10 square meter area coated during that work shift must be measured to isolate the nonconforming area, independent of the size of the structure.

Specifying Coating Thickness
Specifications should indicate the range of coating thickness (e.g., 5-7 mils or 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 Properly prepared coating specifications will provide the contractor with a coating thickness range for each coating layer, since it is essentially impossible to apply a coating to achieve a single thickness value. That is, if 6 mils is the target thickness, the specifier should allow a range of 5 to 7 mils or 127 to 178 microns. If a coating thickness range is not specified, then all parties involved will need to determine the range. No one should have to assume a coating thickness range.

Cumulative Thickness Measurements
Nondestructive coating thickness gages: Most cannot distinguish coating layers Measure the total cumulative thickness Most coating thickness gages cannot distinguish coating layers. They are designed to measure the total distance between the gage probe and the substrate. This distance is interpreted by the gage as coating thickness. As a result, the thickness of multiple layer coating systems is measured cumulatively as the coating layers are applied one on top of another.

Cumulative Thickness Measurements
EXAMPLE: Specification Primer thickness: mils ( microns) Intermediate Coat Thickness: mils ( microns) Finish Coat Thickness: mils (51-76 microns) Target After: Primer Application: mils ( microns) Intermediate Application: mils ( microns) Finish Coat Application: mils ( microns) In this example, the specification calls for 3 to 5 mils of primer, 4 to 6 mils of intermediate coat and 2 to 3 mils of finish coat. After the primer is applied, the gage should indicate 3 to 5 mils for each area measured. However, once the intermediate coat is applied over the primer, the gage should indicate the total cumulate thickness of the primer and mid-coat layers, which in this example is 7 to 11 mils. Similarly, once the finish coat is applied the gage should indicate the total cumulate thickness of all three layers, which in this example is 9 to 14 mils. It is not recommended to deduct the thickness of previously measured coating layers, as it is unlikely that measurements are acquired from the exact same locations for each coat.

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 There are currently six appendices to the SSPC PA2 standard. None of the appendices are mandatory unless they are invoked by the contract documents for a project. Appendix 3 of the standard provides a frequency for measuring coating thickness on steel beams or girders. A full determination and sample determination are described. The sample determination is based on two lengths of steel beams – those that are less than 20 feet or 6 meters and those that range from 20 feet up to 60 feet or 18 meters. The tolerance of each of the spot measurements is the same as was previously described, which was 80% of the minimum specified thickness and 120% of the maximum specified thickness. Similarly, the average of all of the spot measurements must conform to the specified thickness range. The measurement locations on stiffeners are arbitrarily selected. Stiffener

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) For a full determination, the total length of the beam is divided into five equal sections. For example, a beam that is 75 feet long is divided into five 15 foot sections. For steel beams fabricated with a web that is greater than or equal to 36” in height, one spot measurement is acquired in each of 14 locations per section, for a total of 70 spot measurements. Each spot measurement is the average of at least three gage readings. For steel beams fabricated with a web that is less than 36” in height, one spot measurement is acquired in each of 12 locations per section, for a total of 60 spot measurements. Again, each spot measurement is the average of at least three gage readings.

Full Determination The figure in the slide illustrates the locations of the spot measurements for a full determination based on two web sizes. Note that for the larger beam, two spot measurements are taken on either side of the web verses just one spot measurement on either side of the smaller beam. The flange edges or toe may not be measured depending on the thickness of the flange. Note: Areas 2, 6, 8 and 12 (Toe) may not be measured

Sample Determination Beam length < 20 ft: Obtain 2 spot measurements randomly distributed in all 12 areas (total of 24 spot measurements) Beam length ft: Obtain 3 spot measurements randomly distributed in all 12 areas (total of 36 spot measurements) When performing a sample determination, the number of spot measurements is based on the beam length. For beam lengths less than 20 feet, two spot measurements randomly distributed in all twelve areas, or a total of 24 spot measurements are acquired. The 12 locations were illustrated in the previous figure. For beam lengths between 20 and 60 feet, three spot measurements randomly distributed in all twelve areas, or a total of 36 spot measurements are acquired. Again, each spot measurement is the average of at least three gage readings, and the edges of the flanges may not be included, reducing the overall number of 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 of SSPC PA2 describes a process for measuring coating thickness on laydowns in a shop. A laydown is considered a group of steel members laid down to be coated in one shift by a single applicator. A full dry film thickness determination or sample dry film thickness determination can be performed on beams.

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 For a full dry film thickness determination on steel beams, the number of spot measurements is dependent on beam length as described previously. For miscellaneous parts, one spot measurement is obtained on each surface of the part. If the part has fewer than five surfaces, multiple spot measurements are acquired on the larger surfaces to bring the total to five. If the total area of the part is over 100 square feet, 5 spot measurements are taken which are randomly distributed over the part for each 100 square feet or portion thereof. No single spot measurement can be less than 80% of the specified minimum dry film thickness and no single spot measurement can be more than 120% of the specified maximum dry film thickness. The average value of the spot measurements on each surface must conform to the specified dry film thickness. If there is only a single spot measurement on a surface, it must conform to the specified dry film thickness.

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 For a sample dry film thickness determination on steel beams, the number of spot measurements is dependent on beam length as described previously. For miscellaneous parts, three spot measurements randomly distributed on the part are obtained. Each spot measurement must conform to the specified dry film thickness.

Appendix 5: Measuring Coating Thickness on Test Panels
Minimum panel size: ” x 6” (7.5 x 15 cm) Maximum panel size: ” 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 5 in SSPC PA2 describes a procedure for measuring coating thickness on test panels. The frequency of measurements is based on a minimum panel size of 3” x 6” and a maximum size of 12” x 12”. Using a properly adjusted Type 2 electronic gage, obtain to gage readings from the top, middle and bottom thirds of each coated panel. Each gage reading should be ½” from any panel edge and at least 1” from any other gage reading. Each gage reading must be within 80% of the minimum specified coating thickness and within 120% of the maximum coating thickness. The average of the six gage readings must fall within the specified thickness range.

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 Using a properly adjusted Type 2 electronic gage, obtain ten gage readings randomly distributed in the top third of the panel. Compute the average and standard deviation of the ten readings. Similarly, take ten readings from the middle third and ten readings from the bottom third of the test panel and compute the average and standard deviation for these areas. If test panels smaller than 4” x 9” are used, the measurements are taken at least one-half inch from any edge. For test panels that are 4” x 9 inches and larger, readings are taken at least one-half inch from any edge and at least one inch from any other gage reading. Any unusually high or low gage reading, can be discarded. The dry film thickness of the coating on the test panel is the average of the three zones. 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) The latest technical revision to SSPC PA2 was in May 2004, although editorial changes were made to Appendix 6 in Since SSPC routinely reviews and updates standards every 5 years or so, the current version is in the process of revisions and updating under Committee C.3.2. While no one currently knows what the final version will look like, here are a few highlights of changes that are being considered: First, the committee is considering re-titling the standard to read, “Procedure for Determining Conformance to Dry Coating Thickness Requirements” Second, the information currently in SSPC PA2 that addresses gage calibration and verification of accuracy may be removed, since ASTM has an existing standard, ASTM D 7091, that already provides this information. That is, SSPC PA2 will be complementary to ASTM D 7091, rather than be duplicative, or conflict with another coating thickness standard. Third, SSPC PA 2 may address coating thickness measurement on both ferrous and non-ferrous metal surfaces. Currently it only addresses magnetic surfaces. Finally, the information currently in ASTM D7091 regarding frequency of measurements is being removed. The ASTM standard will focus primarily on gage use. The primary focus of SSPC PA 2 will be frequency and tolerance of coating thickness measurements, rather than gage use. Again this will enable the ASTM and SSPC standards to be complementary to one another.

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 In summary, we have Described the basic purpose of SSPC’s standard on measurement of dry coating thickness; Described the differences between measurement acquisition using Type 1 magnetic pull-off and Type 2 electronic gages; Described the procedures and frequency for performing gage calibration, verification of gage accuracy and gage adjustments; Explained base metal reading and described when and how to obtain it, and it’s significance relating to true coating thickness; Described the differences in procedures when measuring coating thickness using manual gages verses electronic gages; Described the number of coating thickness measurements to obtain and how far out of tolerance the measurements can be and still conform to the specification; and we Described the frequency and location of measurements for steel beams, laydowns in steel fabrication shops and on test panels. Potential changes to the standard were also described. Whether these changes will be included in the next revision to SSPC PA2 is unknown at this time.

Complying with SSPC-PA2, “Measurement of Dry Coating Thickness with Magnetic Gages”
THE END Thank you for attending his one-hour webinar on complying with SSPC Paint Application Standard No. 2

William D. Corbett KTA-Tator, Inc.

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William D. Corbett KTA-Tator, Inc.

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