Why is a Vacuum Needed?

To move a particle in a (straight) line over a large distance

Why is a Vacuum Needed?

Contamination(usually water)

Clean surface

Atmosphere (High)Vacuum

To provide a clean surface

Pressione Numero molecole/cm3

un pacco di caffè imballato sotto vuoto 104 Pa 2.7 x 1018

un tubo catodico in un televisore 10-4 Pa 2.7 x 1010

un acceleratore di particelle in fisica nucleare 10-8 Pa 2.7 x 106

una camera con il miglior vuoto che attualmente si può produrre in laboratorio 10-12 Pa 2.7 x 102

nostra galassia 10-14 Pa 1-10spazio intergalattico ? 1 al m3

Tabella 1a: La pressione in alcune tipiche applicazioni

Altitudine PressioneAl livello del mare 101000 PaSulla vetta del Monte Bianco 50000 PaAlla quota di crociera di un Jumbo-Jet (20000 m) 5000 PaSu un satellite artificiale alla quota di 35000 km 2 x 10-3 PaSulla superficie della luna 5 x 10-5 Pa

Tabella 1b: Cambiamento della pressione in funzione dell’altitudine

HOW DO WE CREATE A VACUUM?

VACUUM PUMPING METHODS

Sliding VaneRotary Pump

MolecularDrag Pump

TurbomolecularPump

Fluid EntrainmentPump

VACUUM PUMPS(METHODS)

ReciprocatingDisplacement Pump

Gas TransferVacuum Pump

DragPump

EntrapmentVacuum Pump

Positive DisplacementVacuum Pump

KineticVacuum Pump

RotaryPump

DiaphragmPump

PistonPump

Liquid RingPump

RotaryPiston Pump

RotaryPlunger Pump

RootsPump

Multiple VaneRotary Pump

DryPump

AdsorptionPump

Cryopump

GetterPump

Getter IonPump

Sputter IonPump

EvaporationIon Pump

Bulk GetterPump

Cold TrapIon TransferPump

Gaseous Ring Pump

TurbinePump

Axial FlowPump

Radial FlowPump

EjectorPump

Liquid JetPump

Gas JetPump

Vapor JetPump

DiffusionPump

DiffusionEjector Pump

Self PurifyingDiffusion Pump

FractionatingDiffusion Pump

Condenser

SublimationPump

BAROMETER

WATER MERCURY

760mm

Mercury: 13.58 times heavier than water: Column is 13.58 x shorter :10321 mm/13.58=760 mm (= 760 Torr)

10.321mm

29,9in

(Page 12 manual)

PRESSURE OF 1 STANDARD ATMOSPHERE:

760 TORR, 1013 mbar

AT SEA LEVEL, 0O C AND 45O LATITUDE

Pressure Equivalents

Atmospheric Pressure (Standard) =

014.729.9760760760,000101,3251.0131013

gauge pressure (psig)pounds per square inch (psia)inches of mercurymillimeter of mercurytorrmillitorr or micronspascalbarmillibar

THE ATMOSPHERE IS A MIXTURE OF GASES

PARTIAL PRESSURES OF GASES CORRESPOND TO THEIR RELATIVE VOLUMES

GAS SYMBOLPERCENT BY

VOLUMEPARTIAL PRESSURE

TORR PASCAL

NitrogenOxygenArgonCarbon DioxideNeonHeliumKryptonHydrogenXenonWater

N2

O2

ACO2

NeHeKrH2

XH2O

7821

0.930.03

0.00180.00050.0001

0.000050.0000087Variable

5931587.1

0.251.4 x 10-2

4.0 x 10-3

8.7 x 10-4

4.0 x 10-4

6.6 x 10-5

5 to 50

79,00021,000

940331.8

5.3 x 10-1

1.1 x 10-1

5.1 x 10-2

8.7 x 10-3

665 to 6650

(Page 13 manual)

VAPOR PRESSURE OF WATER AT VARIOUS TEMPERATURES

T (O C)

100

25

0

-40

-78.5

-196

P (mbar)

1013

32

6.4

0.13

6.6 x 10 -4

10 -24

(BOILING)

(FREEZING)

(DRY ICE)

(LIQUID NITROGEN)

(Page 14 manual)

(Page 15 manual)

Vapor Pressure of some Solids

(Page 15 manual)

PRESSURE RANGES

RANGE

ROUGH (LOW) VACUUM

HIGH VACUUM

ULTRA HIGH VACUUM

PRESSURE

759 TO 1 x 10 -3 (mbar)

1 x 10 -3 TO 1 x 10 -8 (mbar)

LESS THAN 1 x 10 -8 (mbar)

(Page 17 manual)

Viscous and Molecular Flow

Viscous Flow(momentum transferbetween molecules)

Molecular Flow(molecules moveindependently)

FLOW REGIMESViscous Flow:

Distance between molecules is small; collisions between molecules dominate; flow through momentum transfer;generally P greater than 0.1 mbar

Transition Flow: Region between viscous and molecular flow

Molecular Flow: Distance between molecules is large; collisions betweenmolecules and wall dominate; flow through random motion;generally P smaller than 10 mbar-3

(Page 25 manual)

MEAN FREE PATH

MOLECULAR DENSITY AND MEAN FREE PATH

1013 mbar (atm) 1 x 10-3 mbar 1 x 10-9 mbar

#mol/cm3

MFP

3 x 10 19

(30 million trillion)4 x 10 13

(40 trillion)4 x 10 7

(40 million)

2.5 x 10-6 in6.4 x 10-5 mm

2 inches5.1 cm

31 miles50 km

kTmolecole) diametro(P

2

12

Il libero cammino medio è inversamente proporzionale alla pressione ed alla sezione d’urto della molecola di gas

A A’P1

P2Flusso

P1 > P2

Portata:

dt

dnkT

dt

dVPQ

Q è costante lungo il tubo e pertantodt

dVP

dt

dVP 2

21

1

Conduttanza:

21 PP

QC

Conduttanza in parallelo:

C1

C2

P1 P2

Q1

Q2

)CC(C

)PP()CC(QQQ

21

212121

Conduttanza in serie:

)CC(C

)PP()CC(QQQ

21

212121

C1C2

P1P2

QP3

Flusso totale = somma dei flussi

1

21

2131

232

121

322211

C

1

C

1C

:seriein econduttanz due le per

C

Q

C

1

C

1QPP

C

QPP

C

QPP

quindi

)PP(C)PP(CQ

Flusso costante:

VELOCITA’ DI POMPAGGIO DI UNA POMPA

C

PCamera

Ppompa

Q

pompa

pompacamera

pompa P

PPC

P

QS

Cam

era

Pom

paVELOCITA’ EFFETTIVA DI POMPAGGIO DI UN SISTEMA:

1

camera

pompa

camera

pompacamera

cameraeff C

1

S

1

P

PS

P

PPC

P

QS

L’effetto della conduttanza è quello di ridurre la velocità di pompaggio efficaceRispetto alla velocità di pompaggio all’imbocco della pompa

FLOW REGIMES

Mean Free PathCharacteristic Dimension

Viscous Flow: is less than 0.01

Mean Free PathCharacteristic Dimension

Molecular Flow: is greater than 1

Mean Free PathCharacteristic Dimension

Transition Flow: is between 0.01 and 1

Conductance in Viscous Flow

Under viscous flow conditions doubling the pipe diameter increases the conductance sixteen times. The conductance is INVERSELY related to the pipe length

(Page 28 manual)

d = diameter of tube in cml = length of tube in cmP1 = inlet pressure in torrP2 = exit pressure in torr

)s/l(2

PP

l

d138C 21

4

EXAMPLE:d = 4 cm P1 = 2 torrl = 100 cm P2 = 1 torr

C=530 l/s

Conductance in Molecular Flow

Under molecular flow conditions doubling the pipe diameter increases the conductanceeight times.The conductance is INVERSELY related to the pipe length.

d = diameter of tube in cml = length of tube in cmT = temperature (K)M = A.M.U.

)s/l(M

T

l

d81.3C

3

EXAMPLE:T = 295 K (22 OC) d = 4 cm M = 28 (nitrogen) l = 100 cmC=7.9 l/s

GAS LOAD

Outgassing

Leaks

Virtual

Real

BackstreamingDiffusion

Permeation

GAS LOAD (Q) IS EXPRESSED IN:mbar liters per second

Pumpdown CurveP

ress

ure

(m

bar

)

Time (sec)

10-11

10 1 10 3 10 5 10 7 10 9 10 11 10 13 10 15 10 17

10+1

10-1

10-3

10-5

10-7

10-9

Volume

Surface Desorption

Diffusion

Permeation