Sampson flow is defined as fluid flow through an infinitely thin orifice in the viscous flow regime for low Reynolds number. It is derived from an analytical solution to the Navier-Stokes equations. The below equation can be used to calculate the total volumetric flowrate through such an orifice:
formula_1
Here, formula_2 is the volumetric flowrate in formula_3, formula_4 is the pressure difference in Pa, formula_5 is the pore diameter in m, and formula_6 is the fluid's dynamic viscosity in Pa·s. The flow can also be expressed as a molecular flux as:
formula_7
Here, formula_8 is the molecular flux in atoms/m2·sec, formula_9 is the average of the pressures on either side of the orifice, formula_10 is the Boltzmann constant, (formula_11 J/K), and formula_12 is the absolute temperature in K.
Sampson flow is the macroscopic analog of effusion flow, which describes stochastic diffusion of molecules through an orifice much smaller than the mean-free-path of the gas molecules. For pore diameters on the order of the mean-free-path of the fluid, flow will occur with contributions from the molecular regime as well as the viscous regime, obeying the dusty gas model according to the following equation: 
formula_13
Here, formula_14 is the total volumetric flowrate and formula_15 is the volumetric flowrate according to the law of effusion. As it turns out, for many gasses, we notice equal contributions from molecular and viscous regimes when the pore size is significantly larger than the mean-free-path of the fluid, for nitrogen this occurs at a pore diameter of 393 nm, 6.0× larger than the mean-free-path.
References.
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