One difficulty lies in
the determination of the roughness of the pipe. Nikuradse used artificially roughened pipes produced
by gluing sand grains of known size onto pipe walls to produce pipes with sandpaper-type
surfaces. In commercially available pipes the roughness is not as uniform and well defined as in
the artificially roughened pipes used by Nikuradse. However, it is possible to obtain a measure of
the effective relative roughness of typical pipes and thus to obtain the friction factor.