Background of SSPC PA 2; Overview and Purpose of SSPC-PA 2 (); Purpose of ASTM D ; Definitions; Gage Descriptions; Calibration & Verification. ASTM D focuses on gage use. • SSPC-PA 2 () focuses on acceptability of acquired measurements. • Both address ferrous and non-ferrous metal. This standard is issued under the fixed designation D ; the number immediately following the 1 This practice is under the jurisdiction of ASTM Committee D01 on Paint and . manufacturer’s specifications, n—a statement or set.
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Click Here to Read the Updated Article. Coating thickness shall be measured in accordance aastm SSPC: 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.
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. 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.
The standard has been updated on multiple occasions. Untilthe most recent technical changes were published in Maywith a minor editorial revision in to one of the appendices regarding measurements on test panels.
The revisions took five years to complete. It, too, was published in 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. Magnetic pull-off gages are categorized as Type 1 Fig.
These gages were designed in the s. 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.
Measuring Dry Film Coating Thickness According to SSPC-PA 2KTA University
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 Asfm. 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. To help assure the reliability of the coating thickness measurements, ASTM D 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 asttm. 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. Verification qstm 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 d791 using certified shims. D70091 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. A one-year calibration asmt is a typical starting point suggested by gage manufacturers.
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 If a large number of measurements asmt being obtained, d0791 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 v7091 rechecked. The accuracy of Type 1 magnetic pull-off gages is verified by placing the gage probe onto a certified coated thickness standard Figs. 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. These readings represent the effect of the substrate conditions on the coating thickness measurement device.
Measuring Dry Film Coating Thickness According to SSPC-PA 2
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.
For example, if a gage reading was 5.
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 d701.
The correction curve may or may not cover the full range of the gage, but should cover the intended range of use. This requirement makes Type 1 gages very difficult to use. While some gage operators may simply subtract a adtm amount for example, 0. The accuracy of Type 2 electronic gages can be verified by placing the gage aetm onto a certified coated thickness standard described for Type 1 gages or certified shims Figs.
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. The final step in e7091 process is to adjust the gage on the surface to which the coating will be applied.
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 primeran 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.
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. 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.
An area ast, defined as approximately square feet. Within each area, five randomly spaced spots are selected. Each spot atsm of a ast. 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 ast spot measurement; the average of five spot measurements is asrm area measurement.
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: For areas containing less than square feet of coated surface, every square-foot area must be measured for coating thickness. For areas of coating to 1, square feet, three random areas are selected and measured.
For areas of coating exceeding 1, square feet, three random areas are selected from the first 1, square feet, r7091 with one additional area for each additional 1, square feet. A total of 15 areas must be measured three in axtm first 1, square feet and one additional area in each of the 12 remaining 1,square-foot areas or portions thereof.
This culminates in a total of 75 spot measurements 15 x 5 and a minimum of gage readings 15 x 5 x 3. Asttm spot measurement variances result in area measurements that do not meet the specification, then additional spot x7091 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. S7091 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. 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.
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. If no Restriction Level is specified, then the default d791 Level 3, which is based on the 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. Another change in the version of the standard is the procedure for identifying nonconforming areas Fig. In the edition, if spot or area measurements were out of conformance, each 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 revision. 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. 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 aatm 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.