Surface World December 2018 Surface World Magazine December 2018 - Page 42

TESTING & MEASUREMENT Rc Rz(iso) Ry Wt,W S Sm, RSm D Pc HSC a q ∆a ∆q Lo lr Rsk,Sk Mean Height of Profile Irregularities Roughness Height Maximum Height of the Profile Waviness Height Mean Spacing of Local Peaks of the Profile Mean Spacing of Profile Irregularities Profile Peak Density Peak Count (Peak Density) High Spot Count Average Wavelength of the Profile Root Mean Square (RMS) Wavelength of the Profile Average Absolute Slope Root Mean Square (RMS) Slope Developed Profile Length Profile Length Ratio Skewness These parameters can all be measured using a Surface Roughness Tester, which is actually designed to measure the surface finish of machined parts, or smoothness. The gauge is not particularly robust and most manufacturers would not recommend field use for the types of application often found in the construction or protective coatings industry. Of the 24 parameters listed above, only a few, if any, appear in coating specifications Ra, Rt and Rz. However an important parameter is waviness, as this will have an effect on the Ra, Rt and Rz value. The waviness value of a surface will effectively be added to the more localised surface roughness measurement giving a “compound” value of any measurement. Rz is sometimes used; Rz is the arithmetic mean value of the single roughness depths Rzi of consecutive sampling lengths. Fig 4. Defi nition of Rz Rt, or Pt (total height of P-profile) as it is sometimes called, is the sum of the largest profile peak height and the largest profile valley depth of the P-profile within the evaluation length ln (reference length). The reference length has to be stated. The most common way of measuring these parameters is to use a stylus roughness gauge. This is a slight misnomer as, typically, these gauges are used to measure surface finish (smoothness) of a surface after a machining operation such as turning, grinding or milling. Fig 2. Surfaces may be wavy as well as profi led The recommended way to minimise this effect, if a stylus type gauge must be used is to use a skidless stylus which will be less influenced by the waviness of the surface. The most commonly used parameter is Ra; Ra is the arithmetic average of the absolute values of the roughness profile ordinates. Fig 3. Defi nition of Ra Fig 5 Elcometer 7062 Surface Roughness Gauge It must also be said that they are NOT field instruments as they are delicate and fragile. An internet search of paint specifications from the paint manufacturers showed that most/all of the specs merely ask for a profile depth rather than an Ra, Rz or Rt value. Fig 6. Paint specifi cations typically require profi le depth CONTINUED ON PAGE 44 40 DECEMBER 2018 twitter: @surfaceworldmag