Vacuum Conductance Calculator

Advanced Conductance Inputs

Example Data Table

These sample rows show typical inputs. They help users test tube, aperture, and duct cases.

Formula Used

Molecular round tube: C = 12.1 × D³ / L × Fg

Molecular circular aperture: C = 11.6 × A × Fg

Molecular rectangular duct: C = 30.9 × a²b² / [L(a+b)] × Fg

Viscous round tube: C = πr⁴Pavg / (8μL)

Series conductance: 1 / Ce = 1 / C1 + 1 / C2

Effective pumping speed: Se = S × C / (S + C)

Molecular formulas use centimeters and return liters per second. Viscous formulas are converted from SI units to liters per second. The gas factor adjusts molecular speed by molar mass and temperature. The Knudsen number helps choose the likely flow regime.

How to Use This Calculator

1. Select the vacuum path geometry.
2. Choose a gas and pressure unit.
3. Enter the line dimensions and pressures.
4. Select a flow model, or keep automatic mode.
5. Add a second conductance if needed.
6. Enter pump speed for effective speed estimation.
7. Press the calculate button.
8. Download the CSV or PDF report.

Article: Understanding Vacuum Conductance

Why Conductance Matters

Vacuum conductance describes how easily gas moves through a line, port, opening, or duct. It is not the same as pump speed. A strong pump can still perform poorly when a narrow tube restricts flow. This is common in chambers with long fittings. Conductance helps predict that loss before hardware is built.

Flow Regime Selection

Gas behavior changes with pressure. At low pressure, molecules hit walls more often than other molecules. This is molecular flow. At higher pressure, gas behaves more like a fluid. This is viscous flow. The Knudsen number compares mean free path with a chamber dimension. It gives a practical guide for choosing a model.

Geometry Effects

Diameter has a very strong effect. In a molecular round tube, conductance rises with the third power of diameter. In viscous round flow, radius appears to the fourth power. Small increases in bore size can greatly improve performance. Length has the opposite effect. Longer paths reduce conductance and slow pump down.

Gas and Temperature Effects

Lighter gases move faster. Helium and hydrogen often show higher molecular conductance than air. Heavy gases move more slowly. Temperature also changes molecular speed and viscosity. This calculator applies gas and temperature corrections. These estimates are useful for design checks.

System Design Use

Use the result to compare line options. Test a short port against a long tube. Add another conductance in series to simulate a valve. Use parallel mode for two open paths. Then compare effective pumping speed. The best design usually needs high conductance near the chamber. It also needs realistic pump speed. Always confirm critical systems with measured data.

FAQs