A method used to check an expression or a solution to a problem used to describe an observable phenomenon. There must be dimensional consistency between the variables and the phenomenon. The relationship of the variables are considered in terms of their dimensions, such as mass, length, time, heat, and temperature, and are regrouped into dimensionless groups. These groups, by virtue that they have no scale of size, mass, or time, can then be used to study the effects of any terms in any situation independent of its application. Dimensionless groups are therefore important in both scale-up and scale-down of processes in chemical engineering.