The surface finish of pipes and vessels. There are various ways it is measured: The parameter Ra is the mean of the absolute values of height difference measured along the surface. Rt is the total roughness and describes the peak-to-valley height in a profile. Rz is the average roughness based on the successive measuring points or an individual peak and valley height. A profilometer is commonly used to measure a surface profile and uses a diamond stylus, which moves vertically in response to the roughness as it is guided over the surface. Other techniques include visual methods and scanning electron microscopy.