The size of a decanter is established by the time required for separation, which in turn depends on the difference between the densities of the two liquids and on the viscosity of the continuous phase. Provided the liquids are clean and do not form emulsions, the separation time may be estimated from the empirical equation
\begin{equation}
t = \frac{100\mu}{\rho_A - \rho_B}
\end{equation} (2.15)
where
$t$ = separation time, h
$\rho_A$, $\rho_B$ = densities of liquids A and B, kg/m³
$\mu$ = viscosity of the continuous phase, cP
Equation (2.15) is not dimensionless, and the indicated units must be used.