Empirical values used for the critical properties in multicomponent chemical systems. They are based on mixing rules of pure components and often have very different properties from their true critical points, and have no real physical significance. For example, the pseudocritical viscosity is a viscosity parameter used in an empirical relationship to determine the viscosity of liquids as a function of temperature. Tabulated data are available for hydrocarbons, ethers, ketones, aldehydes, acetates, alcohols, and organic acids.