A dimensionless number used in the calibration of rotameters, equal to 1.042 m_f gρ (1-ρ/ρ_f )R³/μ², where ρ and μ are the density and dynamic viscosity of the fluid respectively, m_f and ρ_f are the mass and density of the float respectively, and R is the ratio of the radius of the tube to the radius of the float. Symbol St₂.