Calculates viscosity from capillary tube pressure-flow data: η = πr⁴ΔP/(8QL) using the Hagen-Poiseuille law. The parabolic velocity profile assumed by H-P is only valid for Re < 2,100 (laminar flow) — viscometers using this principle are calibrated to correct for kinetic energy effects at tube entry.
Inputs
R Mm
Distance from centre to edge of a circle. Radius = diameter / 2.
L Mm
Linear measurement. Ensure consistent units: 1 m = 1,000 mm = 3.281 ft.
Dp Kpa
Force per unit area (Pa). Atmospheric pressure at sea level: 101,325 Pa. Check whether gauge or absolute pressure is required.
Q Mul S
Volume of fluid per unit time. Used to size pipes, pumps, and ducts.
Results
dynamic viscosity η (Pa·s)
Sample size or count used in the calculation.
wall shear stress τ_w (Pa)
Internal force per unit area (Pa or MPa). Must stay below yield strength in service. Beyond yield: permanent deformation. Beyond tensile strength: fracture.
wall shear rate γ̇_w (1/s)
The value at the specified point or condition.
viscosity (mPa·s = cP)
Reference formula or conversion factor shown for context.
η = πR⁴ΔP/(8QL)
Reference formula or conversion factor shown for context.