1. Presented by: David Beamish DeFelsko Corporation
September 2015
Update to SSPC-PA 9 Paint Application Specification
Measurement of Dry Coating Thickness Using Ultrasonic Gages
Presented by: David Beamish DeFelsko Corporation
Thank you Josiah and welcome everyone. I appreciate you joining me for this one-hour webinar. I will will be describing the July 2015 version, the latest version, of SSPC’s standard known as Paint Application Standard No. 9, or simply PA 9.
This standard describes procedures for thickness measurement of coatings on non-metal substrates.
The technical nature of this subject requires that I read portions of this presentation, so please bear with me as I labor through some of those sections.

2. “The New PA 9” Content Background of SSPC-PA 9
Overview and Scope of SSPC-PA 9 (2015)
Purpose of ASTM D
Gage Descriptions
Calibration & Verification of Accuracy
Measurement Procedures
Frequency and Number of Measurements
Conformance to Specified Thickness
Content of the two Appendices
Today’s webinar will cover a variety topics relating to PA9, including:
The background of the standard and reasoning for the change in content
An overview of the standard
The purpose of ASTM’s Test Method for ultrasonic coating thickness measurement and how it ties in with PA9
Gage descriptions, including calibration and verification of accuracy
The measurement acquisition process
The number of gage, spot and area measurements required
How far out of specification the measurements are permitted to be, or the tolerance of the acquired measurements, and finally
The content of the two appendices to the standard will be covered.

3. The full name of SSPC’s PA 9 document is, “Measurement of Dry Coating Thickness Using Ultrasonic Gages”.
This document is related to, and was originally based upon, another popular paint application specification called PA 2, “Procedure for Determining Conformance to Dry Coating Thickness Requirements”.
The 2 documents
-share calibration and verification procedures,
-they require the same numbers of readings per coated surface area,
-and they determine the extent of nonconforming areas the same way.
But they differ primarily in the types of instruments they address for measuring coating thickness.
PA 2 is limited to gages which employ magnetic or eddy current principles to measure coatings over metal substrates. PA 9 is limited to coating thickness gages which employ an ultrasonic principle to measure over a variety of substrates, usually non-metal. Substrates include Concrete, Wood, Wallboard, Plastic, Fiber, and Composites.
While there are many similarities, here are 3 notable differences between PA9 and PA2, all of which I’ll describe in more detail later.
1- PA9 has a larger spot size of 6” vs PA2’s 1 1/2” spot size in which to take at least 3 readings.
2- PA9 deals with the subject of gage adjustment only in a non-mandatory appendix because the ultrasonic measurement principle is less affected by substrate roughness and composition than magnetic gages, hence less emphasis on the need for a gage adjustment. There is, for example, no BMR, or base metal reading.
3- A final major difference is that PA9 builds on the functionality of ASTM D6132, whereas PA2 refers to D7091

4. Overview
The document focuses on the correct method for taking measurements
Includes criteria for determining:
Number and location of measurements
Whether or not the DFT conforms to project specifications
Applies to ultrasonic devices specifically designed to measure coating thicknesses
Procedures described in SSPC-PA 9 are intended to supplement manufacturers’ operating instructions
Like for their magnetic counterparts, there are many standards and test methods which describe various test conditions and provide testing guidelines for how to obtain a single calibrated reading with an ultrasonic coating thickness gage.
PA 9 takes that process a step further by providing criteria for determining the number of readings needed to obtain an accurate DFT, or dry film thickness measurement, and where those measurements should be located.
The traditional use of the term “ultrasonic gage” or “UT gage” applies to a wall thickness gage. But most wall thickness gages are unable to measure coating thicknesses because coatings are usually much thinner than the steel walls those gages were designed to measure.
Like these UT gages, ultrasonic coating thickness gages use a transducer in the probe to send pulses into the coating and then measure the time taken by that pulse to return to the probe tip.
Finally, procedures described in SSPC-PA 9 are intended to supplement the manufacturers’ operating instructions.

5. Background of SSPC-PA 9
There is only 1 previous version which was published 2008.
Current version dated July 20, 2015
Work on the original PA 9 began in 2005 when non-destructive measurement of coatings on concrete became more prevalent and the need for conformance guidelines came into existence. The first draft was based on the procedure described in PA 2 for steel substrates.
PA 9 began life as a procedure in an SSPC training course and eventually was developed as a consensus standard through SSPC committee review. The first version was published in At that time, the standard referred to gages like the ones shown in this slide, which are now obsolete. The second version of PA 9, which is the current version, was published earlier this year, 2015, to reflect important updates to both PA 2 and to ASTM D6132.

6. Background of SSPC-PA 9
SSPC-PA 9 extends the functionality of ASTM D6132
ASTM D describes “how” to use the gage
SSPC-PA 9 (2015) focuses on acceptability of acquired measurements by describing “where” and “how often”
And that’s where we start. ASTM’s D6132 serves as a fundamental starting point for PA 9. It’s title is, “Standard Test Method for Nondestructive Measurement of Dry Film Thickness of Applied Organic Coatings Using an Ultrasonic Coating Thickness Gage”. It describes the use of ultrasonic film thickness gages to measure, accurately and nondestructively, the dry film thickness of organic coatings applied over a substrate of dissimilar material. It essentially focuses on “how” to properly use the measuring instrument while SSPC-PA9 focuses primarily on the acceptability of the measurements.
To develop the latest revisions to both documents (D6132 and PA9), industry professionals serving on both committees worked to harmonize these documents and reduce duplication. For instance, references to frequency of measurements were removed from the ASTM standard and placed into PA9.
So now, these two documents complement one another. They are designed to be used in conjunction with one another. Both address a variety of surfaces such as concrete, wood, wallboard, plastic and composite material.

7. Scope of SSPC-PA 9
Describes procedures to measure the thickness of dry, homogeneous coatings applied on a variety of surfaces
Homogeneous: Refers to a coating of uniform composition throughout that does not contain large, solid particles such as sand.
Ultrasonic gages are not recommended for coatings such as mortars which are heavily filled with aggregate.
Due to the nature of ultrasonic testing, both ASTM and SSPC standards describe the non-destructive testing of homogeneous (or homo-geneous) coatings only, coatings that are uniform in composition throughout and that do not contain large, solid particles such as sand.
The principle of ultrasonic measurement requires that a pulse be transmitted through the coating twice, there and back. Non-homogeneous coatings may prevent the signal from returning with enough strength to be measured. Solid particles such as sand may scatter the signal.

8. Scope of SSPC-PA 9
Describes procedures to measure the thickness of dry, homogeneous coatings applied on a variety of surfaces
Thickness readings can be verified using destructive means –ASTM D4138
The only way to be certain a coating with solid particles can be accurately measured by this method is by testing a sample using other means. A suitable method is described in ASTM D4138, entitled “Standard Test Methods for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive, Cross-Sectioning Means”.

9. Scope of SSPC-PA 9
Measurements are acquired using purpose-built ultrasonic coating thickness gages
Procedures for gage calibration, verification of accuracy and adjustment are described
Procedure for determining conformance to specified thickness range over extended areas is described
Standard is not intended to prescribe a frequency of measurement for a coating failure investigation
Standard contains two non-mandatory appendices
SSPC PA9 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.
It is important to note that this is a quality inspection standard and is not intended to prescribe a frequency of measurement during a coating failure investigation, as the number and location of measurements may vary widely from that prescribed by this standard.
Also, the standard contains 2 appendices that are non-mandatory unless they are invoked by a contract document.

10. Gage Description
Must be capable of measuring coatings over non-metallic substrates
Gages shall be calibrated and verified according to the manufacturer’s instructions and ASTM D6132
Ultrasonic thickness gages measure polymer coating thicknesses from ~0.3 to 300 mils. The upper limit depends on the acoustical properties of the material being measured and the transducer in the probe.
Coating thickness gages that use ultrasonic measurement techniques are becoming increasingly popular. They support or replace destructive methods for measuring the thickness of coatings over a variety of substrates.
Quality professionals are already familiar with various aspects of ultrasonic testing wherein high-frequency sound energy is used to conduct examinations and make measurements.
Wall-thickness measurement is perhaps the most common of ultrasonic tests. Precision ultrasonic wall-thickness gages permit quick thickness measurement of objects without requiring access to both sides. For coating measurement, however, these gages are not ideal. They do not have sufficient sensitivity to measure the thickness of most coatings.
Purpose-built ultrasonic coating thickness gages must have a minimum measuring accuracy of +5% and be made ready to measure according to both the manufacturer’s instructions and to ASTM D6132. Various probes are available, so the correct probe should be selected in order to measure the expected coating thickness range. Most gages can measure polymer coating thicknesses from approximately 0.3 to 300 mils or 8 μm to 7.6 mm. The upper limit depends on the acoustical properties of the material being measured and the transducer in the probe.

11. Ultrasonic vibrations reflect off coating interfaces
Gage Description
How does it work?
Ultrasonic testing works by sending an ultrasonic vibration into a coating using a probe, or transducer, with the assistance of a couplant gel applied to the surface. The vibration travels through the coating until it encounters a material with different mechanical properties—typically the substrate but perhaps a different coating layer. The vibration, partially reflected at this interface, travels back to the transducer. Meanwhile, a portion of the transmitted vibration continues to travel beyond that interface and experiences further reflections at any material interfaces it encounters.
Because a potentially large number of echoes could occur, the gage is designed to select the maximum or “loudest” echo from which to calculate a thickness measurement. The loudest echo usually comes from the coating/substrate interface as depicted by the thicker arrow in this image.
Some instruments have the ability to measure the individual layers in a multi-layer application. The user simply enters the number of layers to measure, in this case two, and the gage measures the two loudest echoes. The gage ignores softer echoes from coating imperfections and substrate layers.
Ultrasonic vibrations reflect off coating interfaces

12. Three Steps Calibration and verification of gage accuracy (section 5)
Measurement procedures (section 6)
Conformance to specified thickness (section 7)
PA 9 has the user follow 3 major steps to ensure they acquire accurate, repeatable measurements and to help come to a conformance conclusion.
Step 1 follows ASTM D6132 to ensure accurate coating thickness measurements are obtained.
Step 2 is to acquire a sufficient number of readings over representative locations to obtain meaningful thickness results.
The final step is to identify non conforming areas.
Let’s begin with step 1.

13. Calibration and Verification
Step 1 of 3
Shall be performed in accordance with ASTM D6132
It is very important that all gages be calibrated and adjusted following the manufacturer’s instructions and be in accordance
with ASTM D6132 which describes the use of ultrasonic film thickness gages to nondestructively measure dry film thickness of organic coatings applied over a substrate of dissimilar material. Measurements may be made on field structures, on commercially manufactured products, or on laboratory test specimens.

14. ASTM D6132
ASTM D6132 describes 3 operational steps to ensure accurate measurement:
Calibration
Verification of Accuracy
Adjustment
These steps must be completed before coating thickness data acquisition.
The ASTM D6132 standard test method describes three operational steps associated with assuring accurate measurement, including gage calibration, verification of accuracy, and adjustment. Each of these steps must be completed before coating thickness measurements are acquired to determine conformance to a coating specification.
Calibration is an operation that is typically performed by the manufacturer or by a qualified lab and usually results in a Certificate of Calibration being issued.
Verification is an accuracy check performed by the user using known reference standards such as coated metal plates or Certified Plastic Shims.
Adjustment is the act of aligning the gage’s thickness readings to match those of a known sample. This temporarily changes the velocity setting inside the gage to match the speed of sound through the particular coating being measured.
Let’s go into a little more detail on these important points.

15. Gage Calibration
“Calibration of coating thickness gages is performed by the equipment manufacturer, their authorized agent, or by an accredited calibration laboratory in a controlled environment using a documented process.”
Certificate of Calibration traceable to a National Metrology Institute can be issued
No standard time interval for re-calibration, (established based on experience & work environment) but 1-year is typical
Coating thickness gages must be calibrated by the manufacturer, their authorized agent, or by an accredited calibration laboratory in a controlled environment using a documented process.
Upon completion, a Certificate of Calibration or other documentation showing traceability to a national metrology institute can be issued.
Recalibration, also referred to as recertification, is the process of performing a calibration on a used instrument. Recalibrations are periodically required throughout the life cycle of an instrument since probe surfaces are subject to wear that may affect the linearity of measurements.
There is no standard time interval for re-calibration, nor is one absolutely required. Calibration intervals are usually established by the user based upon experience and the work environment. A one-year calibration interval is a typical starting point suggested by most gage manufacturers.

16. Gage Verification
An accuracy check performed by the user on known reference standards comprising of at least one thickness value close to the expected coating thickness.
The process is intended to guard against measuring with an inaccurate gage.
Verification is an accuracy check performed by the user on known reference standards prior to gage use for the purpose of determining the ability of the coating thickness gage to produce reliable values. The process is intended to verify that the gage is still functioning as expected.
Verifications are typically performed to guard against measuring with an inaccurate gage
at the start or end of a shift,
before taking critical measurements,
when an instrument has been dropped or damaged, or
whenever erroneous readings are suspected.

17. Gage Verification
Shall be performed, at a minimum, at the beginning and end of each work shift
If the gage is dropped or suspected of giving erroneous readings during the work shift, its accuracy shall be rechecked
PA 9 requires verification be performed, at a minimum, at the beginning and end of each work shift. The user is also advised to verify gage accuracy hourly when a large number of measurements are being obtained. If the gage is dropped or suspected of giving erroneous readings during the work shift, its accuracy must be rechecked.
If deemed appropriate by the contracting parties, initial agreement can be reached on the details and frequency of verifying gage accuracy.
If readings do not agree with the reference standard, all measurements made since the last accuracy check are suspect. The gage should be removed from service and returned to the manufacturer for repair and/or recalibration.
The use of check standards like those shown here are not intended as a substitute for regular calibration and confirmation of the instrument, but their use may prevent the use of an instrument which, within the interval between two formal confirmations, ceases to conform to specification.

18. Gage Verification Record the: Serial number of the gage
The reference standard used
The state thickness of the reference standard as well as the measured thickness value obtained
The method used to verify gage accuracy
For the purposes of documentation, the user is required to 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, the information need be recorded only once. The stated value of the standard and the measured value shall be recorded each time accuracy is verified.
So that was Calibration and Verification.
PA9 relegates the final gage preparation step, adjustment, to non-mandatory Appendix 2. This is because, unlike magnetic gages, it is not considered to be a critical step in the ultrasonic measurement process. The primary purpose of an adjustment is to improve measurement accuracy on such things as soft coating materials. An adjustment usually takes the form of changing the displayed measurement to match the known thickness of a reference sample.

19. Three Steps Calibration and verification of gage accuracy (section 5)
Measurement procedures (section 6)
Conformance to specified thickness (section 7)
So now that we are certain the gage is ready to measure accurately, we move onto the next major PA 9 step, and that is to acquire a sufficient number of readings over representative locations to obtain meaningful thickness results. This is where we leave ASTM D6132 and focus solely on SSPC-PA 9, which describes the frequency of coating thickness measurements.

20. Measurement Procedure
Step 2 of 3
Gage Reading: A single instrument reading.
Spot Measurement: The average of at least 3 gage readings made within a 6” diameter circle.
Any unusually high or low gage readings that are not repeated consistently are discarded.
Area Measurement: The average of 5 separate spot measurements over each 100 ft2 of coated surface.
Repeated gage readings, even at points close together, often differ due to small surface irregularities of the coating and the substrate. Therefore, a minimum of three gage readings shall be made for each spot measurement of the coating. For each new gage reading, move the probe to a new location within the 6-inch, or 15 cm diameter circle defining the spot.
Each gage reading shall be at least 2 inches or 5 cm from the other gage readings. Discard any outliers. Outliers are single, unusually high or low gage readings that are not repeated consistently.
The average of the acceptable gage readings is the spot measurement.
Unless otherwise specified in the procurement documents or project specification, an area measurement is the average of 5 spot measurements over each 100 square feet or 10 square meters of coated surface
So we know how many measurements to take in an area, an area defined as 100 sq. feet. But how do we determine how many areas to measure on a project. Well here, PA9 gives us some guidance. And this has not really changed significantly from the last version of PA9 except for some of the wording.

21. Area Measurement
Average of 5 separate spot measurements over each 100 ft2 of coated surface
For areas not exceeding 300 ft2, each 100 ft2 is measured
For areas ft2 , three 100 ft2 areas are selected and measured
For areas greater than 1,000 ft2, the first 1,000 ft2 has three 100 ft2 area measurements selected. For each additional ft2, one area measurement is selected
The number of areas that are measured is based on the size of the coated area.
For areas of coating not exceeding 300 square feet (or 30 square meters), you measure every 100 square foot (or 10 square meter) area of coating. As a result, the maximum number of areas to be measured will be 3.
For areas of coating that are between 300 and 1,000 square feet (or 100 square meters), three random 100 square foot areas are arbitrarily selected by the inspector and measured. Again, the maximum number of areas to be measured will be 3.
The third category is for areas of coating greater than 1,000 square feet. For these, which are very common especially on large projects, we check 3 areas in the first 1,000 square feet as stated in number 2. Then we check one more area for each additional 1,000 square feet of coated area, or increment thereof. Those areas are arbitrarily selected.

22. Area Measurement for 100 ft2
Here is an example of how a 100 square foot area, 10’ x 10’, could be measured using PA 9. This example is described in Appendix 1.
In this area we have selected five spots, each 6” in diameter. Inside each spot circle we have taken at least 3 gage readings which are averaged to obtain a spot measurement. So we have 5 spot circles, each with its own calculated spot measurement. For example, spot 3 measurement is 21 mils, which is the average of the 3 gage readings inside that spot.
The average of all 5 spot measurements is taken to be the thickness for this area. So we average 25, 30, 21, 30 and 23. In this case the area measurement is calculated to be 26 mils.

23. Measurement Frequency
The figure on this slide illustrates the frequency of coating thickness measurements on large, flat coated surfaces. This figure is not in the standard.
The top third of the figure indicates the measurement frequency based on a coated area less than or equal to 300 square feet. You check each 100 sq.ft area.
The middle third of the figure indicates the measurement frequency based on a coated area greater than 300 square feet and less than or equal to 1,000 square feet. Three random areas are measured.
The bottom third of the figure indicates the measurement frequency based on a coated area that is greater than 1,000 square feet, in fact it measures 3500 sq.feet. So we have three 100 sq.ft areas in the first 1000 sq.feet in the bottom portion. Then we have 3 additional 1000 sq. foot areas that we check one area in. That’s a total of six 100sq.foot areas in 3500 sq. feet. Three areas in the first 1,000 and 1 more area in each addition 1,000 sq.feet or portion thereof.

24. Measurement Frequency Example
Size of Coated Area: 900 square feet
No. of Areas: 3
No. of Spots: 15 (3 Areas x 5 Spots/Area)
Minimum No. of Gage Readings:
45 (15 Spots x 3 Readings/Spot)
Here is an example. A coated area is approximately 900 square feet. So we are between 300 and 1000 sq. 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.

25. Measurement Frequency Example
Size of Coated Area: 55,000 square feet
No. of Areas: 57 (3 + 54)
No. of Spots: 285 (57 Areas x 5 Spots/Area)
Minimum No. of Gage Readings:
855 (285 Spots x 3 Readings/Spot)
In this second example, the coated area is approximately 55,000 square feet. As a result, three 100 square foot areas are arbitrarily selected and measured in the first 1,000 square feet, then one additional 100 square foot area is arbitrarily selected and measured in each additional 1,000 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.

26. Nonconforming Area
If the coating thickness for a 100 ft2 area is not in compliance:
Determine the spot DFT at 5 ft. intervals in 8 equally spaced directions radiating outward from the nonconforming 100 ft2 area up to the limit of area coated during the work shift
Acquire spot measurements in each direction until 2 consecutive conforming spot measurements are acquired in that direction or until no additional measurements are possible
One of the most significant changes to PA 9 was the procedure used to determine the magnitude of a non-conforming area. In 2008, when an area did not conform, the standard required that each 100 square foot area coated during the shift be measured to determine how bad the problem was. If 8,000 square feet were coated during a shift, then 80 areas, comprised of 400 spot measurements and 1200 gage readings would have to be acquired, analyzed, and the defective areas demarcated for potential rework, which could severely impede production and the project schedule.
The PA 2 committee recognized this problem and put forth a considerable effort to create a better approach that would still properly address deficient or excessive film builds. Their resultant procedure was adopted by the PA 9 committee and added to the current 2015 version of the document.
When a defective area is discovered, additional spot measurements at 5 foot or 1.5 meter intervals are made in 8 directions radiating outward from the nonconforming area, up to the limit of the area coated during the work shift. Each spot must conform to the specified thickness, not the allowable spot tolerance. When 2 consecutive spots conform, measuring can stop in that direction. Any area within 5 feet of a nonconforming measurement is considered suspect and must be re-inspected after corrective measures have occurred.
For repeating structural units or parts, one spot measurement is made on each unit and the process repeated until the spot readings on 2 consecutive units conform to the specification.

27. Initial Nonconforming 100 ft2 Area
Dashed line indicates boundary of area painted during work shift OK OK
STOP
STOP OK
STOP OK NO
5 ft
5 ft
5 ft
Suspect area preceding a nonconforming spot must be re-measured after corrections are made OK OK
5 ft
5 ft
5 ft
Dashed line indicates boundary of area painted during work shift
Initial Nonconforming 100 ft2 Area
Dashed line indicates boundary of area painted during work shift OK
STOP
5 ft
STOP OK NO
5 ft
5 ft
(Limit of area coated during work shift) OK
STOP
This slide was developed by SSPC to illustrate the procedure used to determine the magnitude of nonconforming thickness. The initial nonconforming area is shown as a blue square, which triggered a more thorough inspection of the surrounding coated area. Additional spot measurements were obtained in eight directions radiating outward from the defective area. Each arrow represents approximately 5 linear feet. Once two consecutive spots conformed to the specification, measurement acquisition stopped as shown, for example, in the 12 o’clock position. Note that only one spot measurement was acquired at, for example, the 6 o’clock position since the boundary was established by the area coated during the work shift.
5 ft
5 ft
5 ft OK
(Limit of area coated during work shift)
5 ft OK
(Limit of area coated during work shift)
STOP
STOP
Dashed line indicates boundary of area painted during work shift
MegaRust 2008

28. Three Steps Calibration and verification of gage accuracy (section 5)
Measurement procedures (section 6)
Conformance to specified thickness (section 7)
The third and final major step of PA 9 deals with conformance.

29. Conformance to Specified Coating Thickness
Step 3 of 3
Specifications normally indicate a min/max range of coating thickness for each layer (e.g., 5-7 mils)
When only a single thickness value is specified by manufacturer:
range is established at +20% of the stated thickness value.
E.g., if the stated thickness value is 7 mils, then the minimum and maximum thickness shall be 5.6 to 8.4 mils.
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 or 150 microns is the target thickness, the specifier should allow a range of, say, 5 to 7 mils, or 125 to 175 microns.
If a coating thickness range is not specified and no range is indicated by the coating manufacturer, then PA 9 establishes the range as plus-minus 20% of the stated value.
For example, if the specifier and manufacturer require 7 mils or 175 microns thickness, the acceptable range for the coated area becomes 5.6 mils to 8.4 mils, or 140 to 210 microns.

30. Table 1 Coating Thickness Restriction Levels
Gage Reading
Spot
Reading
Area Measurement
Level 1
Minimum
Unrestricted
As specified
Maximum
Level 2
120% of maximum
Level 3
80% of minimum
Level 4
150% of maximum
Level 5
Both the original 2008 and the current 2015 versions of PA 9 contain a table of coating thickness restriction levels for gage, spot and area measurements. Five levels are included.
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.
Level 5 is the least restrictive, allowing unrestricted maximum thickness of the gage, spot and area measurements.
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 example, a maintenance coating project may invoke Level 4, which allows the coating to be 50% higher than the maximum specified thickness, provided the area measurement falls within specification.
If no restriction level is specified, then Level 3 is the default.
Note that Level 3 allows the spot measurements to be lighter or heavier than specified by 20%, and the average of the 5 spots, the area measurement, must be within specification.
For the purpose of final acceptance of multi-layer applications, the cumulative thickness of all coating layers shall be no less than the cumulative minimum specified thickness and no greater than the cumulative maximum specified thickness.
Note: If unspecified, Level 3 is the default

31. Measurement Tolerance
EXAMPLE 1:
Target DFT: mils
Coating Thickness Restriction Level 3 (default)
Individual gage readings unrestricted
Spot measurements must be between 3.2 mils and 7.2 mils
Area measurement must be between 4 and 6 mils
If spot or area measurements are out of tolerance, the magnitude of the nonconforming thickness must be determined and demarcated.
In this example the coating thickness is 4 to 6 mils and the default Level 3 restriction level is applied.
Therefore the individual gage readings that are obtained are unrestricted.
The average of the gage readings – the spot measurements – can range between 3.2 and 7.2 mils, which is 80% of 4 mils and 120% of 6 mils. That is, 20% below the minimum and 20% above the maximum specified thicknesses.
The average of the five spot measurements – the area measurement – must be between the specified thicknesses of 4 and 6 mils.
If spot or area measurements are outside of the allowable tolerance, the magnitude of the non-conforming area must be determined and the area demarcated as described in the previous step.

32. Measurement Tolerance
EXAMPLE 2:
Target DFT: mils
Coating Thickness Restriction Level 2
Individual gage readings unrestricted
Spot measurements must be between 4 mils and 7.2 mils
Area measurement must be between 4 and 6 mils
If spot or area measurements are out of tolerance, the magnitude of the nonconforming thickness must be determined and demarcated.
In this 2nd example, the specified coating thickness remains at 4 to 6 mils but the specifier has invoked a more restrictive level 2.
Again, the individual gage readings can be anything. The average of the gage readings – the spot measurements – can range between 4 and 7.2 mils since Level 2 only allows 20% error on the maximum thickness and not on the minimum. The average of the five spot measurements – the area measurement – remains as specified between 4 and 6 mils.

33. SSPC-PA 9 Appendices
Numerical Example of Average Thickness Measurement
Gage Adjustment
That completes a discussion of the 3 major PA 9 steps.
There are now two appendices to the standard. None of the appendices are mandatory unless they are invoked by the contract documents for a project. They include a “Numerical Example of Average Thickness Measurement” and a discussion on when a Gage Adjustment may be required.
The first appendix, which has already been described, also appeared in the original 2008 version of PA 9.

34. Appendix 2 – Gage Adjustment
Principle of Operation - thickness = time of travel
Measurement Accuracy - default sound velocity value produces good measurement accuracy on most polymer coatings
Reference Sample - to verify accuracy or to improve measurement accuracy
Reference Sample Measurement Methods - micrometer, destructive, steel coupon
Tolerance - may increase
Appendix 2 begins by explaining that coating thickness readings are obtained by measuring the time taken for an ultrasonic signal to propagate from the probe to the substrate and back. The travel time is divided by two and multiplied by the velocity of sound in the coating to obtain the thickness of the coating. Measurement accuracy is therefore related to the sound velocity value used by the gage.
Most coatings have a similar sound velocities which gages use as their default value. Therefore good measurement accuracy can be obtained on most polymer coatings.
To improve measurement accuracy, particularly on soft coating materials, it might be necessary to adjust the gage for that particular coating material. Measuring a reference sample of known coating thickness determines if an adjustment is necessary. If so, an adjustment usually takes the form of changing the displayed measurement on the gage to match the known thickness of the reference sample.
The thickness of the reference sample coating can be determined a number of ways including micrometer measurement, destructive measurement, or by applying the coating to a steel coupon and measuring with both a magnetic gage and the ultrasonic gage.
As with any measuring instrument, an adjustment to a thickness value that was determined by another measuring device will increase the tolerance of resultant gage measurements. An example is presented in the appendix.

35. ??????????????? Update to SSPC-PA 9 Paint Application Specification
September 2015
Update to SSPC-PA 9 Paint Application Specification
Measurement of Dry Coating Thickness Using Ultrasonic Gages
Any Questions
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That concludes this webinar. Thank you for your time. I’ll turn the presentation back over to Josiah.