For the discussions that follow, Z(x,y) is the function representing the height of the surface relative to the best fitting plane, cylinder, or sphere. Note that the “a” used in the following integral expressions implies that the integration is performed over the area of measurement and then normalized by the cross-sectional area “A” of the measurement.
Sp (Max Peak Height), Sv Max Valley Depth), Sz (Max Height of Surface)
Sp, Sv, and Sz are parameters evaluated from the absolute highest and lowest points found on the surface. Sp, the Maximum Peak Height, is the height of the highest point, Sv, the Maximum Valley Depth, is the depth of the lowest point (expressed as a negative number) and Sz the Maximum Height of the Surface), is found from Sz = Sp – Sv.
Note: earlier standards referred to Rz as an average of the 10 highest to 10 Lowest Points and other variations. The ISO community agreed for the newer standard, ISO 25178-2 to establish Sz as strictly the peak to valley height over a areal measurement.
A surface used in the printing ndustry characterized by deep valley structures with Sv being ~ -15µm.
A polymer surface prepared with asperities as measured by Sp being ~ 0.90 µm.
Since Sp, Sv, and Sz are found from single points, they tend to result in unrepeatable measurements. Thus when using these three parameters, one must properly set spatial filtering bandwidths to eliminate erroneous peaks/valleys and average multiple measurements at random locations along the sample, to obtain a statistically significant result. Typical applications for Sz may include sealing surfaces and coating applications. Sp may find application when considering surfaces that will be used in a sliding contact application. Sv may find application when valley depths relating to fluid retention may be of concern such as for lubrication and coating systems.