Surface World December 2018 Surface World Magazine December 2018 - Page 52

TESTING & MEASUREMENT Comparison to Digital Profi le Gauge As per ASTM D4417-14 a profile measurement was recorded for each sample using: Number of samples per measurement Time per measurement Error compared with Testex 5 1.5 secs 4.3% - Digital Profile Gauge (Method B mean of location maximums) and 10 3 secs 2.4% - Digital Profile Gauge (Method B mean of location means) 25 7.5 secs 0.6% - New Gauge Profile mode where 10 profile samples are taken for each location and the mean profile recorded for each location. (Similar to Method B but using location means rather than location maximums). 50 15 secs 0.5% Measurement speed The table below provides an estimate of the measurement times associated with the various techniques. Technique Test time per location Digital Profi le Gauge (10 samples as per ASTM) 20 secs Replica tape 60-120 secs (2 measurements as per ASTM - depending on number of tapes needed) Stylus Roughness Gauge PS10 27-30 secs (length = 17.55mm) Fig 14. A comparison of the new gauge with a Digital Profi le Gauge Comparison to Stylus Roughness Gauge For each test sample, 9 test locations were selected. For each location: - 4 x Stylus Roughness Gauge PS10 measurements were made, each over a differing path. Note that this deviates from the current method described in ISO 4288:1996, where 10 measurements are made over 5 paths. - New Gauge in Roughness mode where 10 profile samples are taken for each location and the mean roughness recorded for each location. New Gauge Profi le / Roughness (10 samples) 3 secs New Gauge in Profile mode provides good correlation with the established techniques – particularly when the measurement variability is considered. The number of New Gauge samples per measurement has a strong effect on measurement accuracy. 10 readings per measurement provides a reasonable accuracy. With this number of samples per measurement the user will not identify significant differences between the New Gauge in profile mode and Replica tape/Digital Profile Gauge. The New Gauge technique measurement time can provide a major benefit over competing techniques. If we compare the figures above, it will take 30-45 seconds to take 10 readings of 10 measurements using the new gauge and up to 1200 seconds (20 minutes) to obtain the same 10 readings using replica tape for example. Conclusion Fig 15. A comparison of the new gauge with a Surface Roughness Gauge Effect of number of samples per reading on accuracy The results obtained have been based on using 10 samples per location. Decreasing the number of samples will obviously have a deleterious effect on error. The table below demonstrates this effect when the New Gauge Profile mode is compared with Replica tape There is no definitive answer as to whether profile or roughness should be measured, however the inspector must be led by what is asked for on the specification sheet of the coating that has been applied. In the search of paint specifications for protective coatings carried out on the internet from as many manufacturers as are available across every continent, the overwhelming request was merely for a profile with units of microns or mils. No reference was made to Ra, Rz or any other roughness parameter and therefore the inspector must presume that a simple profile depth is required. Ultimately the onus is on the coating manufacturer to specify what is required to ensure the maximum performance of their product and the majority appear to be content with a profile depth. The methods for measuring profile all provide different parameters of profile depth and therefore different values for the same blasted surface. It is for this reason that the method of measurement is specified and the results cannot be compared. CONTINUED ON PAGE 54 50 DECEMBER 2018 read online: www.surfaceworld.com