Turbomolecular Pump Performance Calculator

Calculate turbomolecular pump effective pumping speed (Seff), inlet conductance loss, and gas-dependent compression ratio per ISO 21360.

Pump & Gas Specifications

Nominal Pump Speed S₀ (L/s): e.g., DN 160 ISO-K or CF flange

Test Gas Type:

Inlet Duct Diameter (mm):

Inlet Duct Length (mm):

Foreline Backing Pressure (mbar): Backing line to scroll or diaphragm dry pump

Vacuum Throughput & Speed

Effective Pumping Speed (Seff):

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Conductance Efficiency (Seff/S₀):

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Inlet Duct Conductance (C):

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Compression Ratio (K₀):

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Theoretical Ultimate Inlet Pressure:

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Frequently Asked Questions

Why is compression ratio much lower for Hydrogen (H₂) and Helium (He)?

Turbomolecular blades impart directional momentum based on blade speed relative to molecular thermal velocity. Light gases have high thermal velocity, making back-diffusion against rotor blades significantly higher.

What causes the difference between nominal and effective pumping speed?

Any vacuum duct, elbow, or valve connected to the pump inlet introduces flow resistance (finite conductance C). Per Knudsen formula, 1/Seff = 1/S₀ + 1/C.

What is the critical backing pressure of a TMP?

Critical backing pressure is the maximum foreline pressure (typically 0.5 to 5 mbar) before the turbo compression mechanism stalls into viscous laminar flow.