Hard water
Water's hardness is determined by the concentration of multivalent cations in the water. Multivalent cations are positively charged metal complexes with a charge greater than 1+. Usually, the cations have the charge of 2+. Common cations found in hard water include Ca and Mg. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they contain few ions.
Surface roughness
Each of the roughness parameters are calculated using a formula for describing the surface. Standard references that describe each in detail are Surfaces and their Measurement.
There are many different roughness parameters in use, but formula_1 is by far the most common, though this is often for historical reasons and not for particular merit, as the early roughness meters could only measure formula_1. Other common parameters include formula_3, formula_4, and formula_5. Some parameters are used only in certain industries or within certain countries. For example, the formula_6 family of parameters is used mainly for cylinder bore linings, and the "Motif parameters" are used primarily in the French automotive industry. The MOTIF method provides a graphical evaluation of a surface profile without filtering waviness from roughness. A "motif" consists of the portion of a profile between two peaks and the final combinations of these motifs eliminate ″insignificant″ peaks and retains ″significant″ ones. Please note that formula_1 is a dimensional unit that can be micrometer or microinch.