Vacuum Conductance & Molecular Flow Calculator

Q: Why does Clausing factor matter for short tubes?

For tubes with L/d < 10, the entrance orifice resistance cannot be ignored. The Clausing probability factor α accurately bridges the transition between an ideal thin aperture and a long tube.

Q: How does an elbow or bend affect conductance?

A standard 90° elbow adds equivalent length approximately equal to 1.33 times its axial centerline length plus extra wall reflection impedance.

Calculate circular tube and aperture conductance (L/s), Clausing transmission probability, and Knudsen flow regime for vacuum piping systems.

Tube Geometry & Conditions

Internal Diameter d (mm):

Tube Length L (mm):

Gas Temperature T (K):

Gas Molecular Weight (g/mol): Air/N2: 28, He: 4, H2: 2, Ar: 40

Conductance & Transmission

Net Tube Conductance C (L/s):

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Orifice / Aperture Conductance C₀:

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Clausing Transmission Probability α:

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L/d Aspect Ratio:

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Frequently Asked Questions

What is molecular flow vs viscous flow?

In molecular flow (Knudsen number Kn > 1, pressure < 10⁻³ mbar), gas molecule mean free path is greater than the pipe diameter. Molecules collide with the walls rather than each other, making conductance independent of pressure.

Why does Clausing factor matter for short tubes?

For tubes with L/d < 10, the entrance orifice resistance cannot be ignored. The Clausing probability factor α accurately bridges the transition between an ideal thin aperture and a long tube.

How does an elbow or bend affect conductance?

A standard 90° elbow adds equivalent length approximately equal to 1.33 times its axial centerline length plus extra wall reflection impedance.