This standard is issued under the fixed designation D ; the number 1 This practice is under the jurisdiction of ASTM Committee D01 on Paint and. Related. In nearly the same timeframe, the version of ASTM D, “Standard Practice for Nondestructive Measurement of Dry Film Thickness of. ASTM D describes three operational steps that must be performed prior to coating thickness measurement to help assure the reliability of.
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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 is complex and should be fully understood by the specification writer before invoking it in a d70991. We have allowed technology to make it more complex, yet easier. The standard has been updated on multiple occasions, and It too was published inand was updated again 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.
All references to the frequency of measurements were removed from the ASTM standard.
The two documents are designed to be used in conjunction with each another. It is also aastm to note that both documents address the measurement of coating thickness on both ferrous and non-ferrous metal substrates. SSPC-PA 2 addresses two types c7091 dry film thickness gages, asrm of which are supplied by a variety of manufacturers.
Magnetic pull-off gages are categorized as Type 1 Figure 2. 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 quite 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.
Electronic gages are categorized as Type 2 Figure 3. These gages use electronic xstm to convert a reference signal into coating thickness and are more popular that Type 1 gages. They are typically regarded to be faster, more accurate and easier to dd7091. Gage Calibration, Accuracy Verification and Adjustment. D70911 D describes three operational steps that must be performed prior to coating thickness measurement to help assure the reliability of the measurements.
These steps include 1 gage calibration, 2 verification of gage accuracy and 3 gage adjustment. These steps are incorporated by reference in SSPC-PA 2 and are completed before obtaining coating thickness measurements to determine conformance to a specified coating thickness range.
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 aatm 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.
Measuring Dry Film Coating Thickness According to SSPC-PA 2
Dry film thickness gages are calibrated by the equipment manufacturer, their 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 is usually established based upon experience awtm the work environment. A one-year calibration interval is a typical starting point suggested by gage manufacturers.
To guard against measuring with an inaccurate gage, SSPC-PA 2 requires that gage accuracy d091 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 is being obtained, the user may opt to verify gage accuracy during measurement acquisition e. If the gage is dropped or suspected of giving erroneous readings during the work shift, its accuracy should be rechecked.
Measuring Coating Thickness According To SSPC-PA 2 – Update 2015
The accuracy of Type 1 magnetic pull-off gages is verified by placing the gage probe onto a certified coated thickness standard Figures 4 and 5.
Also, uncoated shims of flat plastic sheet with assigned values traceable d70991 a national metrology institution. 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 5-mil coated standard is appropriate. Using the same example, if a two-point verification procedure is adopted, then a 2 -mil and an x7091 set of coated standards slightly below and above the intended range of use is appropriate.
Unless explicitly permitted by the gage manufacturer, shims certified or measured are not permitted for verification of accuracy of Type 1 gages. These readings represent themeasurement device.
SSPC-PA 2 states that a minimum of 10 locations arbitrarily spaced should be measured one reading per locationthen averaged. This average base metal reading 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 reads 5. However, since Type 1 gages are non-linear, one cannot assume a linear correction value across the full range of the gage.
While the gage may be out of tolerance by 0.
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 Figures 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 for Type 1 gages.
The final step in the 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.
Asmt, once the surface is coated e. Secondly, some Type 2 gages cannot be adjusted. In this case, the user will need to obtain base metal readings BMR from the prepared, uncoated substrate described earlier for Type 1 gages. Measurements to Determine Conformance to a Specification.
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 what 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. 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. An area is defined as approximately square feet. Within each area, five randomly spaced spots are selected. A minimum of three gage readings are obtained in each spot, culminating in a minimum of fifteen 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. It depicts an approximate square foot area containing gage readings and spot measurements. The number of areas that must be measured for s7091 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 aastm, 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, plus one additional area for each additional 1, square feet.
A total of 15 areas must be measured 3 in the first square feet and one additional area in each remaining square foot area or portion thereof; in this example This culminates in a total of 75 spot measurements 15 x 5 and a minimum of gage readings 15 x 5 x 3.
If spot measurement variances result in area measurements e7091 do not meet the specification, then additional spot measurements are acquired radiating outward in eight directions from the non-conforming area to determine the magnitude of the non-conforming thickness. This process is described later in this article.
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 and a maximum thickness are normally specified for each layer of coating. For example, if the specification requires 3 mils dry film thickness and the coating manufacturer does not provide any additional information astmm a recommended thickness range, then by default the specified range is established as 2.
Therefore, it is important for the qstm to indicate an acceptable range for each coating layer, as the coating may not perform at the lower thickness. This 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 asmt thickness, while Level 5 is the least restrictive.
Depending on the coating type and the prevailing service environment, the specifier can select the dry film thickness restriction level for a given project. For the purpose of final acceptance of the total dry film thickness, 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 4 to 6 mil application of primer. The actual minimum and maximum spot and area thickness requirements are shown in Figure 12 for each of the five restriction levels. Determining the Magnitude of Non-conforming Thickness. If a non-conforming area is identified, spot measurements are made at 5-foot intervals in eight equally spaced directions radiating outward from the nonconforming area as shown in Figure 13 below.
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. That is, no allowance is made for variant spot measurements e.
On complex structures or in other cases where making spot measurements at 5-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.
Measuring Dry Film Coating Thickness According to SSPC-PA 2KTA University
Non-compliant areas are demarcated using removable d70091 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 asm is designated as suspect, and as such is subject to 7d091 after corrective measures are performed. Section 10, Reporting was added to the standard in This section lists the items to report when documenting dry film thickness data, including: Depending upon the application, the individual gage readings may also be recorded.
Three appendices were added within the past 3 years or so two were added in and one was added in ; the remaining were in the edition.