Some seal companies use a concave taper to prevent the ingress of solids at start up. This is one of the reasons for the three light band allowable tolerance. Normal lapping produces a slight convex taper because the outer diameter of the seal face is larger than the inner diameter causing more wear as the piece rotates. Some seal companies use a convex lapping surface to compensate for this. If the seal faces stay flat within three helium light bands, and the lapped seal faces stay in contact, a single stationary type mechanical seal can easily pass fugitive emission specifications of less than one hundred parts per million.
Carbon faces that have been pressed into a metal holder have special flatness problems.
Carbon pressed into a metal holder usually stays flatter than a design where the carbon is inserted into a holder that has been compressed with an induction coil. The Mc Nally Institute 1986 S. Belcher Rd.