This brief article will deal with a small part of a much bigger topic: surface finish. We will be looking at commercially available finish options and how they are measured. The surface finish of process systems components plays a very integral role in the cleanability and sterility of the system.
The most commonly used unit for surface finish is Ra (Arithmetic Average Roughness). A device called a “profilometer” is used to measure Ra. Measurements are taken by drawing a stylus along the surface to be measured. The motion of the stylus is perpendicular to the surface and registers the mean of the peaks and valleys along said surface. This reading is typically expressed in microinches or micrometers.
The following table gives examples of commonly specified finishes in both microinches and micrometers.
| Roughness Average (Ra) | |
| micrometer (μm) | microinch (μin) |
| 0.76-0.89 | 30-35 |
| 0.51-0.64 | 20-25 |
| 0.38-0.51 | 15-20 |
| 0.23-0.28 | 9-11 |
Unit Conversions:
1μm = 39.37μin
1μin = 0.0254μm
Another surface designator used in the past, but typically not used today is “Grit”. Grit refers to the grit size of the finishing materials. There is no mathematical conversion between Grit and Ra, but the chart below is commonly agreed upon by most in the industry.
| Surface Finish of Stainless Steel: Ra to Grit | |
| Grit(Ref.) | Ra(μin) |
| 150 | 30 |
| 180 | 25 |
| 240 | 20 |
| 320 | 10 |
Surface finish measurement procedures, general terminology, definitions of most parameters and filtering information can be found in American Standard ASME B46.1 – 2002, Surface Texture, ASME BPE Part SF, and in International Standards, ISO 4287 and ISO 4288.
