Mitutoyo Portable Surface Roughness Tester with External Display: SJ-210, 5 um Probe Tip Radius. Portable surface roughness testers, also known as surface testers, are built with a probe that can extend from the body and digital display to reach large workpieces, hard-to-reach features, and take other measurements in complex positions. They can cycle through standard measurements such as Ra and Rz of a surface.