(1) A proportional representation of a real object, at a smaller size than its actual dimensions. See also architectural scale and engineering scale. (2) Fouling or lining on interior surfaces of heat transfer and water storage equipment caused when excessive minerals in the water create dense crystal deposits that reduce heat transfer efficiency. It is a deposit of minerals that have precipitated out of a solution and have attached to a metal surface. Water temperature, flow rate, and water hardness affect the rate of scale formation.