VACUUM – RULES OF THUMB

Engineering Fundamentals

INTRODUCTION

This sheet will give some basics about a vacuum chamber itself and about mechanical setups that are placed in it.

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ClassificationLevel [mbar]PumpsSensorsExamples
LVLow103 … 100 Dry/wetMechanicalFood seals, domestic vacuum cleaner
MVMedium100 … 10-3Dry/wetMechanicalLight bulb
HVHigh10-3 … 10-6+Turbo ElectronicThermos bottle insulation
VHVVery high10-6 … 10-9+Turbo ElectronicVacuum vessel LHC CERN (particle accelerator)
UHVUltra high10-9 … 10-12+Turbo ElectronicAtomic physics research (e.g. gravity wave)
EHVExtreme high< 10-12(Space)ElectronicSpace
Pressure conversion

1\MPa=1\frac {N}{mm^2}=145psi=10bar
1mbar=10^{-4}MPa=0.0145psi=100\frac {N}{m^2}

Gas law

pV=nRT with R=8.314JK^{-1}mol^{-1}

Estimates for wall thickness (p = 1bar)

\tau _{0.2}{\approx}0.4\ {\cdot}\ \sigma _{0.2} (JPE Estimate for metals), t= thickness, \delta = deformation

Welded flanges:

Vacuum - Welded flanges

O-ring-sealed flanges:

Vacuum - O-ring-sealed flanges

Thin walled tubes:

Vacuum - Thin walled tubes
Types of leakage
  • Real leakage (holes, porous materials)
  • Virtual leakage (pocket sealed with bolt)
  • Desorption / outgassing (solvents)
  • Diffusion (e.g. H-atoms from steel)
  • Permeability (O-rings)
  • Evaporation (finger prints, lubricants)
  • Backflow pump (dry > wet > turbo)
Paschen’s law

This law states that the breakdown potential (necessary to start a discharge between 2 electrodes) is pressure dependent. JPE Rule: vacuum must be below 10-1mbar

In-vacuum-design rules of thumb
  • Prevent virtual leakage: drill holes in bolts etc.
  • Do not use lubricants
  • Check outgassing spec’s from synthetic materials
  • Be aware of high friction (dry environment)
  • Use hybrid bearings
  • Heat must be conducted through material (no convection)

 

Pumps (mbar)
  • Dry:

Diaphragm (10^0), Screw (10^{-2})

  • Wet:

Roots (10^{-2}), Rotary vane (10^{-4})

  • +Turbo:

Molecular pump (10^{-10})

  • Ionization (10^{-11})
Sensors (mbar)
  • Mechanical:
    • Bourdon (10^1)
    • Diaphragm (10^0)
  • Electronic:
    • Piezo based (10^0)
    • Capacitive (10^{-3})
    • Thermal conductivity (Pirani) (10^{-4})
    • Ionization

cold:

Penning (10^{-8})

hot:

Inverted magnetron (10^{-12})

This page uses QuickLaTeX to display formulas.