Conductance Bottleneck Animator

Module 03 • Compare gas flow through wide vs narrow forelines in viscous & molecular regimes

Speed: 1×

Wide Foreline

25 mm bore • 0.6 m • straight
Wide foreline schematic
Effective Speed: ~8 L/s
Remaining: 0 / 0

Narrow Foreline

15 mm bore • 1.5 m • two 90° bends
Narrow foreline schematic
Effective Speed: ~2 L/s
Remaining: 0 / 0

Current Regime

Key Insight

In molecular flow, conductance depends on the cube of the diameter and is inversely proportional to length. A narrower, longer foreline with bends dramatically reduces effective pumping speed at the chamber.
About this animation: Both chambers start with the same number of molecules so evacuation times are directly comparable. Molecule sizes and chamber dimensions are simplified for clarity — focus on the relative evacuation rates, not absolute counts. In viscous flow, molecules follow a parabolic velocity profile (Poiseuille flow — fastest at the centre, stationary at the walls). In molecular flow, molecules travel in straight lines between wall collisions and re-emit in a random direction weighted by Knudsen’s cosine law. The narrow foreline’s smaller bore, greater length, and two 90° bends cause significantly more backscatter in molecular flow, dramatically reducing effective pumping speed at the chamber. This is a 2D cross-section view — in a real cylindrical pipe, wall collisions occur much more frequently (roughly every pipe diameter), producing shorter hops and even greater backscatter in molecular flow than shown here.