Author: Joe Walker
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Advances in technology have lead to vast changes in Dry Film Thickness (DFT) testing methods. These changes have not only improved the overall user experience and repeatability of measurements, but also have necessitated changes in international standards. Traditionally, industries used both visual and analog devices in conjunction with paper reporting systems to ensure that QA/QC requirements were being met. In the early 1980's, the microprocessor became widely available for application in portable test equipment. Manufacturers, in conjunction with the technology industry, developed the first electronic inspection tools. With the advent of affordable mainframes and the swell of PC availability and usage, instruments were, for the first time, able to be linked in real time to DOS programs. The era of electronic Statistical Process Control (SPC) was born. Due to physical and networking limitations, electronic SPC was primarily used in factories and other large-scale production facilities. Gradually, the market demanded that data capture be made more portable, and instruments with onboard memory and data download capability were introduced which ushered in the age of inspection instruments with features heretofore only found in computers.
To meet the demand for even more portable, powerful, and robust instruments, many instrument manufacturers increased the speed, accuracy, and data processing capabilities for their bench top instruments. As processors and chips became smaller, these features migrated into their lines of hand-held test equipment systems. In conjunction with these advances, firmware and processor upgrades were introduced, which radically enhanced reading rates, measuring ranges, and calibration stability. It is these instrument performance enhancements and their subsequent effect on inspection standards that prompted this paper.
In the spring of 2014 a new SSPC-PA21 standard is scheduled to be published. Despite the fact that this is the second major revision in as many years, new technologies and techniques have been introduced since the 2014 standard was revised and sent to ballot. Some examples include a factory-calibrated coating thickness gauge with pre-defined surface profiles, as well as a scanning probe designed to be slid across a cured coating while taking readings in real time. When these new technologies are put into practice, dramatic improvements in overall inspection times and data accuracy can be seen.
This paper will compare and contrast the results of two inspections; one inspection taking place in the field and one taking place on prepared panels in a laboratory setting. The same instrument types and testing methods were used across both settings; the following table specifies both the instrument type and data collection method used. In each inspection, there are three inspectors using three different instruments (one conventional and two recently introduced) and transferring raw data using three different data retention and analysis methods.
|Instrument Type||"Corresponding Data Collection Method (meeting the requirements of SSPC-PA2)"|
|Conventional Type 2 Gauge||Paper-based system|
|Conventional Type 2 Gauge with Scanning Probe (SSPC-PA2 data collection points pre-programmed in batch and sub batch files>||Real time data download from a Type 2 gauge into a software program with cloud capabilities|
|Fixed Calibration Type 2 Gauge||Post inspection data download from a Type 2 gauge into a software program|
The following paragraphs briefly describe each instrument used in the comparison testing. The first is the Conventional Type 2 Gauge. In the industrial protective coating industry, electronic coating thickness gauges have traditionally been used as point and shoot measurement tools; each gauge reading is a snapshot showing the thickness of the protective coating over the substrate below. To accommodate for variation caused by substrate roughness and inherent variation in the spraying process, SSPC-PA2 specifies that a predetermined number of gauge readings within a defined area be averaged to produce a spot measurement. To ensure measurement accuracy, these instruments should adjust the calibration to the surface profile, metallurgical composition, and shape of the substrate. While these instruments are available in both basic models and memory versions, a basic model was used in these tests.
The second instrument is the Fixed Calibration Type 2 Gauge. Although there are a number of potential causes for erroneous DFT readings, the most common ones have to do with instrument calibration.2 Recently, an electronic coating thickness gauge has been developed that is factory calibrated to a variety of surface profiles. These profile calibrations are fixed and cannot be adjusted
by the user. They are stored in the memory chip of the instrument and are easily accessible via a look-up table. To use this instrument, the inspector selects the appropriate profile calibration, chooses the data collection mode, and begins taking gauge readings.
The third instrument is the Scanning Probe. For years, some electronic coating thickness gauges have had the ability to work in an "auto repeat" mode. This mode allows the probe to take continuous measurements without lifting the probe from the surface. The main drawback to using a probe in "auto repeat" mode is the detrimental effect friction has on the probe tip as it is moved over the surface. Friction causes wear that both reduces accuracy and probe longevity. A new scanning probe technology that combines the "auto repeat" feature with a patented "zero offset" capability has been recently introduced to eliminate the effect friction has on the probe tip. With a scanning probe, a precision-milled, highly durable cap is fitted over the probe tip. The "zero offset" feature allows the probe to, in essence, subtract the thickness of the cap from subsequent readings. This replaceable cap allows the probe to be slid over the measurement area without causing probe tip wear, loss of accuracy, or damage to the coating. When the cap is worn beyond the manufacturers' specification, a warning appears in the instrument display, and the cap is replaced.
Time Trial 1: DFT Inspection on Four Carbon Steel Cylinders
Three coating inspectors were asked to perform a DFT inspection according to SSPC-PA2 on four large cylinders made of carbon steel. The cylinders were encased in three levels of scaffolding which gave the inspectors access to the base and upper levels. Each inspector was instructed to perform the inspection using the three measurement tools listed above and input their readings in the recording formats selected for each instrument. The inspectors were timed from the beginning of the inspection to when the data documentation was completed.
|Method||Inspectors (minutes)||Average (minutes)|
| ||1||2||3|| |
The fixed calibration instrument was, on average, 80% faster than the conventional test method. The scanning probe was, on average, 87% faster than the conventional method. While these new technologies are not explicitly accounted for in the most recent revision of SSPC-PA2, they not only meet the requirements of the standard but also complete the inspection in a more efficient manner. A review of the time study showed that most of the increased time required to perform the conventional gauge inspection was due to the manual input of data onto the customer specified forms. Therefore, another test using coated panels in a laboratory environment was justified. In this test, both the data collection and data input times would be tracked separately so that a better determination as to the efficiency of the instruments themselves could be made.
The inspectors noted that the fixed calibration probe and conventional probe, had to be lifted from the coating surface after each gauge reading. Conversely, the scanning probe was able to slide from gauge reading to gauge reading and only had to be lifted to move to the next spot measurement area - thereby yielding a faster reading rate and, as a result, a faster inspection.
Time Trial 2: DFT Inspection on Six Coated Carbon Steel Panels
In this trial, the same three instruments and data output methods used in the field trials were compared using coated panels with surface profiles varying from 1 to 1.5 mils. The panels were divided into six sections and each instrument collected three gauge readings to obtain one spot measurement per section. Times listed included time to calibrate, verify calibration, and program the data collection mode to the instrument firmware where applicable.
|Method||Inspectors (minutes)||Average (minutes)|
| ||1||2||3|| |
The time to completion results are consistent with the results from Test 1 in that the scanning probe has the largest effect on overall time of inspection, and both the scanning probe and fixed calibration instrument are noticeably faster than the conventional method. The primary reason for this was the inspectors using the conventional method had to take time to write each gauge reading and calculate the spot measurement. The fixed calibration instrument is, on average, 51% faster than the conventional method and the scanning probe is, on average, 73% faster than the conventional method.
Repeatability and Data Output
Despite the fact that substrate profile was approximately 20% of the DFT, there was little discernible difference in the results produced by each instrument when their results on the laboratory panels were compared:
|Method||Panel (average of DFT sections and inspectors in mils)||Overal Avg. (mils)|
| ||1||2||3||4||5||6|| |
The results from the laboratory coated panel tests showed that there was little variation in the
overall thickness results between the three instruments. While there was some variation between
gauge readings and spot measurements, there were no outliers that could not be identified as normal
variations due to the substrate profile and film thickness.
The data also shows that the fixed calibration instrument, when factory calibrated to similar metal and profile as the work piece, is as effective as those calibrated in the field or laboratory. This is of interest because calibration errors have a marked effect on whether a coating application meets the specification requirements.
The time and repeatability comparison tests in both the field and laboratory inspections clearly demonstrate that using the scanning probe significantly reduces the time required to complete a DFT inspection according to SSPC-PA2. Furthermore, the increased data collection has no adverse effect on data accuracy. When any of the aforementioned devices are used in conjunction with digital data collection and downloading, the savings increase exponentially.
In the protective coating application and inspection industry, as in every human endeavor, time is money. The cost savings that can be yielded from a faster inspection are many. However, faster reading rates, data integrity, and real time data analysis are only the beginning of the potential savings. If an inspection can be performed quickly and the data analyzed efficiently, the cost of remediation is reduced as the equipment and personnel needed to perform the work are likely to still be in place or on site. The biggest savings comes from the contractor, inspector, and facility owner having the ability to compile and analyze the inspection data in an efficient and collaborative manner. To achieve this level of collaboration, it is essential that all parties have a thorough understanding not only of the tools necessary to increase productivity in coating process, but also the tools necessary to increase productivity in the inspection process.
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