William D. Corbett
PCS, KTA-Tator, Inc.
Bill Corbett is the Vice President and Professional Services Group Manager for KTATator, Inc., where he has been employed for 33 years. He chairs SSPC committees C.3.2 on Dry Film Thickness and C.6 (Education). He is an SSPC-approved instructor for four SSPC courses, and he holds SSPC certifications as a Protective Coatings Specialist, Protective Coatings Inspector (Level 3), and Bridge Coatings Inspector (Level 2). He is also a NACE Level 3-certified Coatings Inspector. He was the co-recipient of the SSPC 1992 Outstanding Publication Award, co-recipient of the 2001 JPCL Editors’ Award, recipient of SSPC’s 2006 Coatings Education Award, and recipient of SSPC’s 2011 John D. Keane Award of Merit. Bill Corbett是KTATator公司的副总裁和专业服务集团经理，他在那儿已经工作33年了。他担任SSPC委员会C.3.2干膜厚度和C.6（教育）的主席。他是SSPC四门课程的讲师，并持有SSPC防护涂层专家、防护涂层检验员（3级）和桥梁涂层检验员（2级）证书，他还是NACE3级认证的涂层检验员。他是SSPC1992年杰出出版物奖，2001年JPCL编辑奖，2006年SSPC涂层教育奖和2011年SSPC John D. Keane荣誉奖的获得者。 Coating thickness shall be measured in accordance with SSPC: The Society for Protective Coatings Paint Application Standard No. 2 (SSPC-PA 2) is a simple enough statement, yet this common specification requirement is often misinterpreted or regarded as a document that simply states how to measure the dry film thickness (DFT) of coatings, something we already profess to know how to do. Yet the requirements of SSPC-PA 2 regarding gage calibration, verification of gage accuracy and adjustment procedures, the number of measurements to obtain, and the tolerance of the measurements are complex and should be fully understood by the specification writer before invoking PA 2 in a contract.
涂层厚度应按照SSPC的要求进行测量：防护涂层协会油漆施工标准No.2（SSPC-PA2）是非常简单的条文，但是这么普通的规范要求经常被曲解或被认为是简单的描述成如何测量涂层干膜厚度的文件，有时候我们声称知道如何去做。但是SSPC-PA2关于表的校正、表精度的确认和校正程序、测量点的数量、复杂结构测量误差的要求，在合同引用PA2之前，规范的起草者应完全理解这些内容。
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On more than one occasion, I have heard the question, “When did SSPC-PA 2 and dry film thickness measurement become so complicated?” In fact, when you take a close look, measuring DFT isn’t that complex. We have allowed it to become more technologically complex while making the data easier to analyze. We can gather hundreds of gage readings in a relatively short time; batch the measurements; print the data or upload it to a computer for graphing; report the highest, the lowest, the mean, and standard deviation of the collected data; incorporate digital images of the structure or coated area; and even program the gage to produce an audible signal if a spot measurement is outside of the tolerance range. I am no doubt leaving out other bells and whistles, but my point is that while we are able to do a lot with the readings obtained, measuring DFT involves four or five basic steps.
不止一次，我听说过这个问题，“什么时候SSPC-PA2和干膜厚度测量变得如此复杂？”事实上，当你仔细看，测量DFT并不复杂。我们已经使用更复杂的技术来使数据分析更加容易。我们可以在相对较短的时间内收集数以百计的读数；分批处理这些读数；打印这些读数或上传到电脑上绘制图形；记录收集这些读数的最高点、最低点、平均值和标准偏差；结合结构或涂装区域的数字图像；甚至在仪表中设置了超出单个测量点误差范围的报警信号。毫无疑问的，我不去考虑这些花俏的功能，但我的观点是：虽然我们可以对所获得的读数做很多分析，但是测量DFT只涉及到四或五个步骤。 Step 1: Instrument Calibration
步骤1：仪表标定 Step 2: Verification of Gage Accuracy on Certified Coated Standards or Certified Shims
步骤2：在经校验的涂覆标准或垫片上校验表的准确性 Step 3: Base Metal Reading Acquisition or Gage Adjustment (using certified or measured shims)
步骤3：母材读数获取或表的调整（使用经校验的或测量垫片） Step 4: Measurement of Coating Thickness
步骤4：涂层厚度测量 Step 5: Correction for Base Metal Reading (if acquired).
步骤5：母材读数修正（如果需要的话）After a brief review of the history of SSPC-PA 2, this article will describe each of the five steps, based on the 2012 edition of SSPC-PA 2. Special attention will be given in the article to how PA 2 addresses the required number of coating thickness measurements; the acceptability of gage readings, spot measurements, and area measurements; nonconforming thickness; measuring DFT on coated edges; and measuring DFT on pipe exteriors.
在简短的回顾SSPC-PA2历史之后，本文将按照2012版SSPC-PA2标准来叙述这五个步骤。本文将针对PA2描述的涂层厚度测量点数；表的读数合格率、测量点和测量区域；不合格厚度；在涂层边缘测量DFT；和在管道外表面测量DFT给予特别关注。 （未完待续）Some History SSPC-PA 2 was originally published as a temporary standard 40 years ago in 1973 (73T) as “Measurement of Dry Coating Thickness with Magnetic Gages.” The standard referenced gages like the one shown in
Fig. 1, which are now all but obsolete. The standard has been updated on multiple occasions. Until 2012, the most recent technical changes were published in May 2004, with a minor editorial revision in 2009 to one of the appendices (regarding measurements on test panels). The SSPC Committee on Dry Film Thickness Measurement began revising and updating the 2004 version in 2007. The revisions took five years to complete. The latest edition of the standard (“Procedure for Determining Conformance to Dry Coating Thickness Requirements”) is dated May 2012 and was made available to the industry in July 2012.
In nearly the same timeframe, the 2005 version of ASTM D7091, “Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to NonFerrous Metals” was being revised and updated. It, too, was published in 2012. The most current version of the ASTM standard focuses on proper gage use, while SSPC-PA 2 focuses primarily on the frequency of measurements and the acceptability of the acquired measurements. References to the frequency of measurements were removed from the ASTM standard. The two documents are designed to be used together. It is important to note that both documents address the measurement of the DFT of coatings on ferrous and non-ferrous metal substrates. Before 2012, SSPC-PA 2 addressed measurement of coatings only on steel, a ferrous metal. (The sidebar on
p. 32 in this article summarizes the key changes made to PA 2 in 2012.)
Gage Types SSPC-PA 2 addresses two types of DFT gages, both of which are supplied by a variety of manufacturers. Magnetic pull-off gages are categorized as Type 1 (
Fig. 2).
These gages were designed in the 1950s. While their use has declined, they are still readily available and used by some. For these 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 converted to coating thickness, which is displayed on a scale on the gage. The operating principle is simple. Less force is required to remove the magnet from a thick coating, while more force is required to remove the magnet from a thinner one. The scale is not linear, as will be discussed below.Electronic gages are categorized as Type 2 (
Fig. 3). These gages use electronic circuitry to convert a reference signal into coating thickness and are more popular than Type 1 gages. They are typically regarded to be faster, more accurate, and easier to use.
Gage Calibration, Accuracy Verification, and Adjustment To help assure the reliability of the coating thickness measurements, ASTM D 7091 describes three operational steps that must be performed before taking the measurements. These steps are (1) gage calibration, (2) verification of gage accuracy and (3) gage adjustment. The steps are incorporated by reference in SSPCPA 2 and are completed before obtaining coating thickness measurements to determine conformance to a specified coating thickness range. The steps to verify the accuracy of the gage are based on the principle that
you check the gage by measuring a known thickness before you use the same gage to measure an unknown thickness. Verification of gage accuracy is typically performed using certified coated thickness standards (for Type 1 or Type 2 gages) or certified shims (Type 2 gages). Adjustment of Type 2 gages to compensate for substrate characteristics (described later) is typically performed using certified shims. Measured shims (individually labeled with a stated thickness value) commonly supplied with Type 2 gages can also be used for gage adjustment.Dry film thickness gages are calibrated by the equipment manufacturer, its authorized agent, or an accredited calibration laboratory (under controlled conditions). A test certificate or other documentation showing traceability to a national metrology institution is required. While there is no standard time interval for re-calibration, an interval can be established based on experience, the work environment, and/or the internal equipment calibration procedures of the company using the gage. A one-year calibration interval is a typical starting point suggested by gage manufacturers.
Verifying Gage Accuracy To guard against measuring with an inaccurate gage, SSPC-PA 2 requires that gage accuracy be verified (at a minimum) at the beginning and end of each work shift according to the procedures described in ASTM D 7091. If a large number of measurements are being obtained, the user may opt to verify gage accuracy during measurement acquisition (for example, hourly). If the gage is dropped or suspected of giving erroneous readings during the work shift, its accuracy should be rechecked.
Verifying the Accuracy of Type 1 Gages The accuracy of Type 1 (magnetic pull-off) gages is verified by placing the gage probe onto a certified coated thickness standard (Figs.
4 and
5). A one-point or two-point accuracy verification procedure can be performed; typically, the two-point verification provides greater accuracy. If a one-point verification procedure is adopted, the coated standard should be selected based on the intended range of use. For example, if the intended use is between 4 and 6 mils, then a five-mil coated standard is appropriate. Using the same example, if a two-point verification procedure is adopted, then a two-mil and an eight-mil set of coated standards (slightly below and above the intended range of use) is appropriate.
The final step in the process is to obtain a set of base metal readings (BMRs) to compensate for substrate characteristics including (but not limited to) substrate metallurgy, geometry, thickness/thinness, and roughness (
Fig. 6). These readings represent the effect of the substrate conditions on the coating thickness measurement device. SSPC-PA 2 states that a minimum of 10 (arbitrarily spaced) locations should be measured (one reading per location) and then averaged. This average BMR is then deducted from subsequent coating thickness measurements to remove any effect of the base metal surface and its conditions.
Because Type 1 gages cannot be adjusted, some gage operators believed that a “correction value” could be applied to the coating thickness readings to compensate for the inaccuracy of the gage. For example, if a gage reading was 5.7 mils on a five-mil coated standard, a 0.7-mil “correction value” could be deducted (by the gage operator) from subsequent coating thickness measurements. However, because Type 1 gages are non-linear, one cannot assume a linear (mil-for-mil) correction value across the full range of the gage. While the gage may be out of tolerance by 0.7 mils at 5 mils, it may be out of tolerance by more or less than 0.7 mils at a different thickness. Accordingly, SSPC-PA 2 states that the practice of using a linear correction value is not appropriate.However, Note 6 in the standard states,
“A correction curve can be prepared by plotting the actual gage readings against the stated values on the (coated) test blocks (standards). Subsequent coating thickness measurements can be “corrected” by plotting the measurements along the correction curve. The correction curve may or may not cover the full range of the gage, but should cover the intended range of use. The Base Metal Readings (BMR) described in 6.1 may also need to be plotted on the correction curve.” This requirement makes Type 1 gages very difficult to use. While some gage operators may simply subtract a fixed amount (for example, 0.5 mils) from any reading, such a practice is not in compliance with SSPC-PA 2.
Verifying the Accuracy of Type 2 Gages The accuracy of Type 2 (electronic) gages can be verified by placing the gage probe onto a certified coated thickness standard (described for Type 1 gages) or certified shims (Figs.
7 and
8). The certified shim should be placed onto a smooth, uncoated metal surface to remove any effect of the surface roughness during this process. A one-point or two-point accuracy verification procedure can be performed (as described earlier for Type 1 gages).
Adjusting Type 2 Gages The final step in the process is to adjust the gage on the surface to which the coating will be applied. Adjustment is accomplished by placing a certified or measured shim (or shims) onto the prepared, uncoated metal surface and adjusting the gage (when feasible) to compensate for substrate characteristics including (but not limited to) substrate metallurgy, geometry, thickness/thinness, and roughness (
Fig. 9). The gage reading is adjusted to match the thickness of the shim, which effectively removes any influence from the underlying surface.
This step sounds reasonably straightforward but poses several hidden challenges. First, once the surface is coated (for example, with a primer), an uncoated surface may no longer be available for subsequent gage adjustments, so the user may want to have a similar uncoated surface prepared and reserved for future gage adjustments on a given project. Naturally, this surface must be representative of the metallurgy, geometry, thickness/thinness, and roughness of the actual surface, which can be a challenging requirement.Second, some Type 2 gages cannot be adjusted. In such cases, the user will need to obtain BMRs from the prepared, uncoated substrate (described earlier for Type 1 gages). While many Type 2 (electronic) gages have a “zero-set” function, the gages should never be adjusted to zero unless the surface is smooth.
Required Number of Coating Thickness Measurements The section of SSPC-PA 2, “Required Number of Measurements for Conformance to a Thickness Specification,” causes many users confusion, which can result in either under- or over-inspection. Arguably the most critical section in the document, Section 8, describes how many areas to check, the size of the areas, the number of measurements to obtain in each area, and the steps to take if spot or area measurements do not conform to the specification.SSPC-PA 2 contains three definitions that are critical to understanding this next area of discussion.
Gage Reading: A single instrument reading.
Spot Measurement: The average of at least three gage readings made within a 4-cm (≈1.5-inch) diameter circle. Acquisition of more than three gage readings within a spot is permitted. Any unusually high or low gage readings that are not repeated consistently are discarded. The average of the acceptable gage readings is the spot measurement.
Area Measurement: The average of five spot measurements obtained over each 10 m2 (≈100 ft2) of coated surface, or increment (portion) thereof.
An area is defined as approximately 100 square feet. Within each area, five randomly spaced spots are selected. Each spot consists of a 1.5-inch diameter circle. A minimum of three gage readings is obtained in each spot, culminating in a minimum of 15 gage readings within an area. Unusually high or low gage readings that cannot be repeated consistently are discarded. The average of the three acceptable gage readings is the spot measurement; the average of five spot measurements is the area measurement.
Figure 10, from Appendix 1 in SSPC-PA 2, depicts an approximate 100-square-foot area containing gage readings and spot measurements.
The number of areas that must be measured for coating thickness varies, depending on the size of the coated area. There are three categories of coated area: less than 300 square feet; 300 to 1,000 square feet; and greater than 1,000 square feet. For areas containing less than 300 square feet of coated surface, every 100-square-foot area must be measured for coating thickness. For areas of coating 300 to 1,000 square feet, three random areas are selected and measured. For areas of coating exceeding 1,000 square feet, three random areas are selected from the first 1,000 square feet, along with one additional area for each additional 1,000 square feet.Because areas of coating often exceed 1,000 square feet, our example will be based on this third tier (>1,000 square feet). Let’s assume that the total coated area (perhaps the area coated during a work shift, although SSPC-PA 2 does not equate coated area with work shift) is 12,500 square feet. A total of 15 areas must be measured (three in the first 1,000 square feet and one additional area in each of the 12 remaining 1,000-square-foot areas or portions thereof). This culminates in a total of 75 spot measurements (15 x 5) and a minimum of 225 gage readings (15 x 5 x 3). If spot measurement variances result in area measurements that do not meet the specification, then additional spot measurements are acquired (radiating outward in eight directions from the nonconforming area) to determine the magnitude of the non-conforming thickness. This process is described later in this article.
Acceptability of Gage Readings, Spot Measurements, and Area Measurements While individual gage readings that are unusually high or low (and cannot be repeated consistently) can be discarded, there are limitations on the thickness values representing the spot measurements (the average of three gage readings). A minimum thickness and a maximum thickness are normally specified for each layer of coating. However, if a single thickness value is specified and the coating manufacturer does not provide a recommended range of thickness, then the minimum thickness and maximum thickness for each coating layer are established by SSPC-PA 2 at ±20% of the stated value. For example, if the specification requires 3 mils’ DFT and the coating manufacturer does not provide any additional information regarding a recommended thickness range, then, by default, the specified range is established as 2.4–3.6 mils. Because the coating may not perform at the lower thickness, it is important for the specifier to indicate an acceptable range for each coating layer. To assist the specifier, the 2012 edition of SSPC-PA 2 incorporates a Restriction Level Table (
Fig. 11). The Table enables the specifier to select from five different restriction levels related to spot and area measurements.
Level 1 is the most restrictive and does not allow for any deviation of spot or area measurements from the specified minimum and maximum thickness, while Level 5 is the least restrictive. Depending on the coating type and the prevailing service environment, the specifier can select the DFT restriction level for a given project. The specifier may also invoke a maximum thickness threshold for Level 5 Spot or Area Measurements for a generic product type and/or service environment that will not tolerate an unlimited thickness. If no Restriction Level is specified, then the default is Level 3, which is based on the 2004 version of SSPC-PA 2 (what many users of the standard have become accustomed to).For the purpose of final acceptance of the total DFT, the cumulative thickness of all coating layers in each area must be no less than the cumulative minimum specified thickness and no greater than the cumulative maximum specified thickness.
For example, assume that the specification requires a four- to six-mil application of primer. The actual minimum and maximum spot and area thickness requirements are shown in
Fig. 12 for each of the five restriction levels.
Determining the Magnitude of Nonconforming Thickness Another change in the 2012 version of the standard is the procedure for identifying nonconforming areas (
Fig. 13). In the 2004 edition, if spot or area measurements were out of conformance, each 100-square-foot area coated during the work shift had to be measured, and nonconforming areas had to be demarcated. On a larger structure with multiple applicators, the measurement and documentation process could be extensive, so the approach was changed in the 2012 revision. If a nonconforming area is identified, spot measurements are made at five-foot intervals in eight equally spaced directions radiating outward from the nonconforming area, as shown in
Fig. 13.
If there is no place to measure in a given direction, then no measurement in that direction is necessary. Spot measurements are obtained in each direction (up to the maximum surface area coated during the work shift) until two consecutive conforming spot measurements are acquired in that direction, or until no additional measurements can be made. Acceptable spot measurements are defined by the minimum and maximum values in the contract documents. No allowance is made for variant spot measurements (for example, ±20%), which is consistent with the practice followed when determining the area DFT.On complex structures or in other cases where making spot measurements at five-foot intervals is not practical, spot measurements are taken on repeating structural units or elements of structural units. This method is used when the largest dimension of the unit is less than 10 feet. Spot measurements are obtained on repeating structural units or elements of structural units until two consecutive units in each direction are conforming or until there are no more units to test.
Non-compliant areas are demarcated using removable chalk (or another specified marking material) and documented. All of the area within five feet of any non-compliant spot measurement is considered non-compliant. For a given measurement direction or unit measurement, any compliant area or unit preceding a non-compliant area or unit is designated as suspect, and, as such, is subject to re-inspection after corrective measures are taken.
Appendices to the Standard There are eight appendices in the 2012 version of SSPC-PA 2. Two of the eight appendices were added in 2012 (the remaining were in the 2004 edition) and are highlighted below. The appendices to SSPC-PA 2 are not mandatory but may be invoked by contract documents.
Appendix 6: Method for Measuring the Dry Film Thickness of Coatings on Edges For decades, the industry was cautioned about taking coating thickness measurements within one inch of an edge, let alone on an edge. However, several Type 2 (electronic) gage manufacturers offer a variety of probe configurations, some of which are less affected by proximity to edges and are designed to better measure the thickness of coatings on edges (
Fig. 14). Obviously, the gage operator should consult the gage manufacturer’s instructions before measuring coating thickness on edges.
Before measuring coating thickness on edges, the user should verify the gage and probe for accuracy by placing a thin, flexible shim (certified or measured) onto the prepared, uncoated edge. Adjustments to the gage may or may not be required. This procedure also verifies that the probe configuration will accommodate the edge configuration before acquiring coating thickness data.Once verification of accuracy and adjustments are made, a minimum of three gage readings are taken within 1.5 linear inches of coated edge. The average of the gage readings is considered a spot measurement. The number of spot measurements along the edge will vary, depending on the total length of the coated edge.
Appendix 7: Method for Measuring Dry Film Thickness on Coated Steel Pipe Exterior Appendix 7 was added to accommodate pipe coaters that need to determine coating thickness conformance on non-flat (or non-plate) areas, including smaller pipe sections on a cart or rack and longer pipe spools.Pipe sections loaded onto a cart or rack can be considered a complete unit (
Fig. 15). The total number of spot and area measurements is based on the total square footage of pipe on the cart or rack. The square footage is calculated as shown on
p. 35.
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