These letters stands for “root mean square”, a measure of the surface finish or smoothness of a shaft or seal face measured in micro-inches.

The number represents the square root of the mean (middle) distance between the peaks and valleys that make up the surface of the seal face or pump shaft.

The metric system uses C.L.A. or centerline average for the same purpose. The metric measurement is made in microns.


  • On February 16, 2018