Surface World December 2018 Surface World Magazine December 2018 - Page 50

TESTING & MEASUREMENT Replica tape with Plastic backing Replica tape pressed into blasted surface Fig 12. A High Point is a Peak when it crosses both medians consecutively Surface profi le captured in foam Tape measured using caliper gauge Fig 10. How Replica Tape works Using a calliper gauge, with rounded anvils and a specially damped mechanism to minimise crushing, the overall thickness of the tape, including the “deformed” foam can be measured. This will be the distance from the highest peak to the lowest valley. There are several different tapes to use, depending on the expected depth of the profile, and the inspector must be aware of the “cross- over” between the upper range of one spec of tape and the lower end of the other. The average of the readings from two tape samples constitutes a measurement. Fig 11. Elcometer 122 Replica Tape This is a simple straight forward method which gives the profile measure as the distance from the top of the highest peak to the bottom of the lowest valley. The accuracy of this measured value is dependent on the correct setup and calibration of the caliper gauge and the assumption that the tape has not been crushed further by the user or the caliper gauge after the sample “measurement” has been taken. A recent development in the industry combines the parameters measured in both profile and roughness and consequently the method of using a stylus roughness gauge has been incorporated into the ASTM standard to “authenticate” the use of this new equipment as a profile measuring device. This device takes the replica tape and by shining a light through it is able to detect the high points as impressed into the tape. This high point count produces an approximation to Rz as measured with a stylus gauge which uses peak count. The difference between a high point and a peak, as defined in Surface roughness terms is shown in Fig X. The manufacturers have produced a paper which shows test results corroborating the results and their correlation to the stylus gauge. However the majority of paint specifications that do state a required profile only quote a profile depth, not Rz or Ra etc. In the field of coatings inspection, time is money, and with that in mind Elcometer has developed a gauge that can provide reliable accurate profile information in a more timely manner. The gauge has a scanning probe and by scanning across the surface the gauge is able to measure the profile and report the result in various ways. The samples used to assess the new gauges profile performance consisted of a range of grit blasted ferrous sheets in the nominal range of 0-1mil (ground sample), 1-2mils, 2-3mils and 3-4 mils. These prepared sheets were supplemented with a small, random selection of grit blasted samples of differing ferrous composition. On each test sample a number of test locations were either marked on the surface or could be identified using a guide sheet. Measurements and calibration were performed using a probe that was moved continuously over the test location. A 250um foil was used to simulate a probe cap. Percentage deviations were assessed for the measurements made by the new gauge and the alternative techniques. It should be noted that there is a significant experimental uncertainty with each method comparison. This uncertainty arises from the random nature of the test surfaces and the inherent variability in the technique the gauge is being compared to. An attempt has been made to quantify this variability for each method. The percentage deviations do not include an adjustment for the alternative technique/test plate variability. Comparison to Replica tape As per ASTM D4417-14 a profile measurement was recorded for each location using: - Replica tape (Method C) and - New gauge Profile mode where 10 profile samples are taken for each location and the mean profile recorded. (Similar to Method B but using location means rather than location maximums). Fig 13. Comparison of new gauge with Replica Tape CONTINUED ON PAGE 50 48 DECEMBER 2018 twitter: @surfaceworldmag