Module 00: The Mean Free Path
← AcademyHow far does a molecule travel before hitting another? This number decides everything else about vacuum: the regime, the pump you need, how a chamber breathes.
Mean Free Path Explorer
Drag the pressure slider and watch the physics change. The tracer molecule shows its actual path between collisions. Switch gas species and temperature to see how each affects the mean free path.
1 Torr
10³110⁻³10⁻⁶10⁻⁹10⁻¹²
68 °F
34 mm / 1.34 in
LIVE
50 µm
About the width of a human hair
VISCOUS
TRANSITION
MOLECULAR
Kn
Viscous flow: molecules collide with each other constantly, behaving like a fluid.
Pressure
1 Torr
Temperature
68 °F
Molecular Density
--
Knudsen Number
--
Regime
Viscous
Collision Rate
--
Mean free path: 50 µm -- about the width of a human hair.
Monolayer Formation Time
How long until a clean surface is covered by a single layer of gas molecules? At atmosphere, instantly. At UHV, you have hours or days of working time.
s = 1.0
0.3 s
At this pressure, monolayer formation takes 0.3 seconds.
Race Mode
Compare two gases side by side at the same pressure and temperature. See why H₂ is hard to pump and why He is used for leak detection.
Gas A:
vs Gas B:
Pump-Down Time Estimator
Estimate how long it takes to pump a chamber from atmosphere to UHV. Adjust volume, surface area, outgassing rate, and pump speed.
Your Vacuum Right Now
Connect to a live bake station and see its current pressure in the simulator.
Not connected
Numbers to Carry
- λ·P ≈ 5 cm·mTorr (N₂, 20°C)
- 1 monolayer/second at 10⁻⁶ Torr (s=1)
Sources · 3
- J.F. O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed., Wiley, 2003.
- P.A. Redhead, J.P. Hobson, E.V. Kornelsen, The Physical Basis of Ultrahigh Vacuum, AIP, 1968.
- K. Jousten (ed.), Handbook of Vacuum Technology, 2nd ed., Wiley-VCH, 2016.