Molecular Flow Conductance Calculator

Formula Used

For a long circular tube in molecular flow, this calculator uses:

Cair = 12.1 × D³ / L

D and L are in centimeters. Cair is in liters per second for air near 20°C.

For a short circular tube, it applies an entrance correction:

Cair = (12.1 × D³ / L) / (1 + 4D / 3L)

For a thin aperture, it uses:

Cair = 11.6 × A

A is the aperture area in square centimeters.

Gas and temperature correction:

Cgas = Cair × √[(T / 293.15) × (28.97 / M)]

T is absolute temperature in kelvin. M is gas molar mass in g/mol.

For series conductance:

1 / Ctotal = 1 / Ccomponent + 1 / Cseries

For effective pumping speed:

1 / Seff = 1 / Spump + 1 / Ctotal

How to Use This Calculator

1. Select long tube, short tube, or thin aperture.
2. Enter tube dimensions or aperture area.
3. Choose the gas or enter a custom molar mass.
4. Enter temperature and pressure values.
5. Add nominal pump speed if you want effective speed.
6. Add extra series conductance if another restriction exists.
7. Press the calculate button.
8. Download CSV or PDF results if needed.

Example Data Table

Understanding Molecular Flow Conductance

Molecular flow conductance describes how easily gas moves through a high vacuum restriction. In this region, molecules hit walls more often than they hit each other. Tube size, aperture area, gas mass, and temperature control the result. Pressure does not change ideal molecular conductance directly, but pressure still matters for throughput and flow validation.

Why This Calculator Helps

Vacuum systems often lose speed through small pipes, adapters, valves, and ports. A pump may be rated for high speed at its inlet. The chamber sees less speed after conductance limits are included. This calculator estimates that limit before hardware is selected. It also compares the restriction with the chosen pump speed.

Inputs That Matter

Diameter has the strongest effect for round tubes. A small increase can raise conductance sharply. Length has the opposite effect. Longer tubes reduce conductance because molecules meet more wall surfaces. For a thin aperture, area is the main input. Gas choice matters because lighter molecules move faster. Higher temperature also raises molecular speed.

Using Results Carefully

The calculator gives engineering estimates. Real parts may include elbows, screens, rough walls, and valve seats. These details can reduce transmission. Use manufacturer data when exact values are required. The Knudsen advisory is only a guide. Molecular flow is most reliable when the mean free path is larger than the chosen characteristic size.

Design Tips

Place pumps close to the chamber when possible. Use larger and shorter paths for critical vacuum lines. Avoid unnecessary narrow fittings. Compare conductance with pump speed. When conductance is much lower than pump speed, the line becomes the main bottleneck. In that case, a larger pump alone will not fix the problem.

Exporting and Records

Use the export buttons to save the current result. A CSV file helps with spreadsheets. A PDF file is useful for design notes, maintenance logs, and procurement discussions. Keep the selected gas, units, and temperature with each estimate. That makes later review easier and prevents unit mistakes.

For early layouts, test several diameters and lengths. Record the effective speed, not only raw conductance. This shows whether the pump, pipe, or aperture controls performance. Review safety margins before ordering parts. Document assumptions whenever final drawings change during design.

FAQs