Surface World December 2018 Surface World Magazine December 2018 - Page 46

TESTING & MEASUREMENT Surface Profile is the depth from the top of the peaks to the bottom of the valley of a blasted surface, and there are several methods available to measure this value as described in the relevant standards ISO 8503 and ASTM D 4417. Chronologically, the first method is the Surface comparator. A Surface Comparator is a “planar plate of four segments on which are imparted reference surface profiles”. “ISO surface profile comparators shall be manufactured by forming positive replicas from nickel or other corrosion-resistant metals using master coupons prepared from mild steel.” This is Method A in ASTM D 4417 and ISO 8403-1 The next method is the profile depth gauge. This gauge is a dial depth gauge, with a needle as the contact point. The gauge is placed on the surface and the spring mounted needle measures the depth of the valley in which it lands relative to the peaks on to which the gauge is placed. Obviously measuring one valley on a blasted surface will not characterise the surface, so several measurements are taken (10 according to ASTM D 4417). In order to characterise the whole surface under inspection correctly, the average of these 10 readings should be taken to give a more accurate assessment. This aligns with good Statistical Analysis practice. SSPC PA 17 states that “Unless otherwise specified, select a minimum of three 15 x 15 cm (or three 6 x 6 inch) locations in which to take readings for each specific surface preparation apparatus used during each work shift or twelve-hour period, whichever is shorter.” In more recent times these gauges have been digitised and whilst they work on the same principle, data management and statistical analysis within the gauge is now possible. These gauges are now capable of having the measuring or contact point at the end of a cable to enable access in more hard to reach locations. The measurement procedure remains the same. Fig 7. Elcometer 125 ISO Comparator The comparator is placed on the blasted surface which is visible through the hole in the centre of the comparator. The surface is then compared to the 4 reference profile and the “relative” profile is noted. Smoother than segment 1, rougher than 1 but smoother than 2 etc up to rougher than 4. This is obviously, therefore, a subjective test and relies on the eye and interpretation of the inspector. There is no value which can be ascribed to the surface profile. Fig 8. Elcometer 123 Profi le Depth Gauge Fig 9. Elcometer 224 Digital Surface Profi le Gauge The next method to be developed was replica tape. Replica tape contains a compressible foam attached to a flexible, incompressible plastic substrate of uniform thickness. The tape is pressed onto the blasted surface and a burnishing tool is used to press the compressible foam into the profiled surface being measured. The result is that the inspector now has a reversed replica profile of the surface being tested. CONTINUED ON PAGE 48 44 DECEMBER 2018 read online: