Formula Used
For a long cylindrical tube, the base molecular conductance is:
C = 12.1 × D³ / L × √((T / 293.15) × (28.0134 / M))
D and L are in centimeters. C is in liters per second. M is molar mass in g/mol.
For a thin aperture, the base conductance is:
C = 11.6 × A × √((T / 293.15) × (28.0134 / M))
A is aperture area in square centimeters. Temperature is in kelvin.
Repeated components are combined as:
Ceq = (Csingle × fitting factor ÷ series count) × parallel paths
Throughput is calculated as:
Q = Ceq × ΔP
Effective pumping speed is calculated as:
Seff = 1 / ((1 / S) + (1 / Ceq))
How to Use This Calculator
- Select the gas or choose custom gas.
- Choose tube or aperture geometry.
- Enter diameter, length, or aperture area.
- Add temperature, pressure values, and pressure unit.
- Use the bend factor for elbows, screens, traps, or fittings.
- Enter series and parallel counts for repeated paths.
- Add pump speed to estimate chamber-side pumping speed.
- Press calculate, then export CSV or PDF as needed.
Example Data Table
| Gas |
Geometry |
Diameter |
Length |
Temperature |
Expected Use |
| Nitrogen |
Cylindrical tube |
5 cm |
50 cm |
293.15 K |
General vacuum line estimate |
| Helium |
Thin aperture |
2 cm |
Not used |
300 K |
Leak test path comparison |
| Argon |
Cylindrical tube |
8 cm |
120 cm |
295 K |
Sputtering gas conductance review |
Understanding Molecular Flow Conductance
Molecular flow appears when gas molecules strike chamber walls more often than they strike each other. This condition is common in high vacuum lines, analytical instruments, coating systems, and research chambers. Conductance tells how easily gas travels through a tube, opening, or restriction. A high value means the pump can remove gas with fewer losses. A low value means the path limits pumping speed, even when the pump itself is large.
Why Gas Choice Matters
The calculator adjusts conductance for molar mass and temperature. Light gases move faster, so they pass through openings more quickly. Helium and hydrogen often show higher conductance than nitrogen through the same part. Heavy gases, such as argon or carbon dioxide, move more slowly and reduce conductance. Temperature also changes molecular speed. Warmer gas usually gives a higher value.
Design Use
Vacuum conductance helps compare tube diameters, lengths, elbows, valves, and parallel paths before parts are purchased. Diameter has a strong effect because tube conductance scales with the cube of diameter. A small increase in diameter can produce a large gain. Length works in the opposite direction. Long narrow lines can waste pump capacity and slow pump down.
Reading The Outputs
The equivalent conductance combines repeated parts in series and parallel. Series parts reduce conductance because every restriction must be crossed. Parallel paths add capacity because gas can use more than one route. Throughput uses the pressure difference across the part. Effective pumping speed combines the line conductance with pump speed, which shows the speed available at the chamber flange.
Practical Limits
The Knudsen number check estimates whether molecular flow assumptions are suitable. Values well above ten indicate molecular behavior. Lower values warn that transitional or viscous equations may be needed. Always confirm results with manufacturer data for valves, traps, and complex fittings. Real surfaces, contamination, screens, and sharp bends can reduce conductance. Use the bend factor as a conservative allowance during early design.
Better Planning
Try several diameters and lengths. Save the CSV record for comparison. Export the report when documenting assumptions for a lab notebook, maintenance plan, or vendor review. These checks make pump selection clearer and reduce expensive redesigns after hardware arrives during commissioning and routine troubleshooting later.
FAQs
What is molecular flow conductance?
It is the ability of a vacuum path to pass gas when molecule-wall collisions dominate. Higher conductance means less restriction between chamber and pump.
When is this calculator valid?
It is best for molecular flow. Check the Knudsen number output. Values above ten usually support molecular flow assumptions.
Why does tube diameter matter so much?
Tube conductance scales with diameter cubed. A wider tube can greatly improve conductance, even when length stays the same.
Why does gas type change the result?
Light molecules travel faster than heavy molecules. Faster molecular speed increases conductance through the same tube or aperture.
What does the bend factor mean?
It is a practical multiplier for elbows, fittings, screens, traps, and rough path losses. Use values below one for conservative estimates.
How are series parts handled?
Identical series parts reduce total conductance. This calculator divides adjusted single conductance by the number of series components.
How are parallel paths handled?
Parallel paths add conductance. The calculator multiplies the series-adjusted value by the number of parallel routes.
Can this replace manufacturer data?
No. Use it for design estimates. Real valves, traps, gauges, and chambers may need tested or manufacturer-rated conductance values.