Vacuum System Design ConsiderationsMaterialsPlumbingPumpingThroughputUltimate pressureDynamic equilibriumPumping speedLeaks

LeaksRealHoles in the system!VirtualSurface adsorption OutgassingHubers ruleWater desorbs very slowly from all surfacesAlways backfill your vacuum system with dry nitrogenMinimize surface area

Ultimate vacuum/limiting pressureSystem bakeoutP(T) = P0 exp(-Hv/R(1/T 1/T0))Applies to evaporationApplies equally well to desorptionReplace Hv with some desorption energy

Vacuum System BakeoutHeating tapeGlassCopperBrassAluminum

Materials for vacuum systems1. What is its vapor pressure?What is its specific surface area?Typical materials of choiceGlassHardnon-porous and structurally rigidSmoothminimum specific surface areaBakeablePyrex or Kimax (70% SiO2) good to 550 CQuartz or Vycor (96% SiO2) good to 1100 CChemically inert

Materials for Vacuum ApplicationsCeramicsElectrical insulatorsThermal insulatorsBakeable to very high temperaturesCan be machinableLavaMust be fired after machiningExpands 2% on firingMacor

Materials for Vacuum ApplicationsStainless steel304 and 316 are idealThe chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film. [Ditto for bumpers!] The sizes of chromium atoms and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. Non-porous, impervious to infiltrationBakeable to high temperature

Materials for Vacuum ApplicationsAluminumMuch easier to machine than stainlessAlso forms impervious oxideStrength-to-weight ratio is greater than steelOutgassing rate is 5-10X that of stainless

Materials for Vacuum ApplicationsBrass and copperWhats brass?Easily machinedEasily joined with soft or silver solderFittings available from commercial plumbing suppliersVolatile zinc above 200 C

Materials for Vacuum ApplicationsPlasticsEasily formedNot bakeable to very high temperatureNylon and Delrin are most stableOutgas water and airTeflon Bakes over 200 CSoft; poor mechanical strengthPolyimide (Kapton)Very low vapor pressureUsed for tape!

O-ring sealsGroove design criteriahttp://www.oringsusa.com/html/gland_design.html

Vacuum ValvesGlassStopcocksAce valvesNeither is bakeable to very high temperature

Vacuum Valves: Glass StopcockOutletMating ground glass surfacesMust be [heavily] greasedInletThrough hole aligns with inlet (open) or doesnt (closed)

Vacuum Valves: Ace ThredInletThread for stem driveStem seal O-ringsOutletValve-sealingO-ringTapered Glass Seat

Vacuum Valves: Diaphragm valveValve seatDiaphragm!

Vacuum Valves: Bellows ValveValve sealO-ringActuator knobBellowsValve seatStem seal O-ring

Vacuum Valves: Gate ValveManual actuatorSealing plate, fully retractableMinimal reduction in throughput!

Joinery: copper conflatsKnife edgesCopper gaskets (conflats)

TrapsP(T) = P0 exp(-Hv/R(1/T 1/T0))At 275 K, pump oil has very low vapor pressureHence the water baffle

At 77 K, water and many other contaminants have very low vapor pressure.Hence the LN2 trap.Diff pump trapGlass in-line trap and dewar