Ra Surface-Finish Calculator (ASME BPE SF Designations) — Sample Report

1 · Design inputs

Incoming electropolished tube. The profilometer reads 18 µin; the drawing's SF designation carries a maximum of 20 µin.

2 · Method

1 µin = 0.0254 µm (exact)

ratio = measured Ra / grade Ra max (≤ 1 passes)

Ra is the arithmetic mean of the absolute deviations of the measured profile from its mean line over the sampling length — an averaged quantity, which is what makes it robust for acceptance and blind to individual defects. The calculator's job is comparison, not measurement. Both the reading and the ceiling are normalised to microinches internally, the ratio is formed, and the reading, the ceiling and the ratio are all reported in both µin and µm so a certificate in one unit and a specification in the other can be reconciled on one line without hand arithmetic. The conversion is exact rather than the field shorthand of 40 µin to the micron, which matters at the tight end of the scale: a mechanically-polished grade specified near 20 µin sits at 0.508 µm, and comparing a 0.505 µm reading against a nominal "0.5 µm" grade fails, while comparing it against the exact 0.508 µm conversion of the 20 µin limit that was actually specified passes. A ratio at or below 1 passes; above it fails, with a warning that the surface does not meet the specified grade. The grade → Ra-max mapping is copyrighted table content and is not embedded: the ceiling arrives as a user-supplied input read from your licensed SF-2.4.1-1 or SF-3.4-1, which also means a project contracted to an older edition can be checked against the limits that edition actually carried.

3 · Calculation

1. Reading in metric: 18 × 0.0254 = 0.4572 µm.
2. Ceiling in metric: 20 × 0.0254 = 0.508 µm.
3. Ratio = 18 / 20 = 0.900 → at or below 1: PASS.

4 · Results PASS

5 · Verification statement

The result values above are locked as fixture ra-finish.json, case “18 uin vs 20 uin grade -> pass”, tolerance 0.001, in the calc-core continuous verification gate (see https://pipingtoolset.com/trust/validation.html). Allowable-stress and factor data are user-supplied inputs; citations: ASME BPE — Tables SF-2.4.1-1 (metallic) and SF-3.4-1 (polymeric) surface designations; the grade → Ra-max mapping is user-supplied, not embedded.