Why is a Vacuum Needed?
To move a particle in a (straight) line over a large distance
Why is a Vacuum Needed?
Atmosphere
Contamination
(usually water)
(High)Vacuum
Clean surface
To provide a clean surface
un pacco di caffè imballato sotto vuoto
un tubo catodico in un televisore
un acceleratore di particelle in fisica nucleare
una camera con il miglior vuoto che attualmente
si può produrre in laboratorio
nostra galassia
spazio intergalattico
Pressione
104 Pa
10-4 Pa
10-8 Pa
Numero molecole/cm3
2.7 x 1018
2.7 x 1010
2.7 x 106
10-12 Pa
10-14 Pa
?
2.7 x 102
1-10
1 al m3
Tabella 1a: La pressione in alcune tipiche applicazioni
Altitudine
Pressione
Al livello del mare
101000 Pa
Sulla vetta del Monte Bianco
50000 Pa
Alla quota di crociera di un Jumbo-Jet (20000 m)
5000 Pa
Su un satellite artificiale alla quota di 35000 km
2 x 10-3 Pa
Sulla 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
VACUUM PUMPS
(METHODS)
Gas Transfer
Vacuum Pump
Entrapment
Vacuum Pump
Kinetic
Vacuum Pump
Positive Displacement
Vacuum Pump
Rotary
Pump
Reciprocating
Displacement Pump
Drag
Pump
Diaphragm
Pump
Liquid Ring
Pump
Gaseous
Ring Pump
Piston
Pump
Rotary
Piston Pump
Turbine
Pump
Multiple Vane
Rotary Pump
Dry
Pump
Fluid Entrainment
Pump
Ion Transfer
Pump
Ejector
Pump
Liquid Jet
Pump
Diffusion
Pump
Diffusion
Ejector Pump
Sliding Vane
Rotary Pump
Axial Flow
Pump
Gas Jet
Pump
Rotary
Plunger Pump
Radial Flow
Pump
Vapor Jet
Pump
Roots
Pump
Adsorption
Pump
Cold Trap
Bulk Getter
Pump
Getter
Pump
Getter Ion
Pump
Sublimation
Pump
Self Purifying
Diffusion Pump
Evaporation
Ion Pump
Fractionating
Diffusion Pump
Sputter Ion
Pump
Molecular
Drag Pump
Cryopump
Turbomolecular
Pump
Condenser
BAROMETER
10.321
mm
Mercury: 13.58 times
heavier than water:
Column is 13.58 x shorter :
10321 mm/13.58=760 mm
(= 760 Torr)
WATER
760
mm
29,9
in
MERCURY
(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) =
0
14.7
29.9
760
760
760,000
101,325
1.013
1013
gauge pressure (psig)
pounds per square inch (psia)
inches of mercury
millimeter of mercury
torr
millitorr or microns
pascal
bar
millibar
THE ATMOSPHERE IS A MIXTURE OF GASES
PARTIAL PRESSURES OF GASES CORRESPOND TO THEIR RELATIVE VOLUMES
GAS
Nitrogen
Oxygen
Argon
Carbon Dioxide
Neon
Helium
Krypton
Hydrogen
Xenon
Water
SYMBOL
N2
O2
A
CO2
Ne
He
Kr
H2
X
H2 O
PERCENT BY
VOLUME
78
21
0.93
0.03
0.0018
0.0005
0.0001
0.00005
0.0000087
Variable
(Page 13 manual)
PARTIAL PRESSURE
PASCAL
TORR
593
158
7.1
0.25
1.4 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
79,000
21,000
940
33
1.8
5.3 x 10-1
1.1 x 10-1
5.1 x 10-2
8.7 x 10-3
665 to 6650
VAPOR PRESSURE OF WATER AT
VARIOUS TEMPERATURES
T (O C)
100
P (mbar)
(BOILING)
32
25
0
1013
(FREEZING)
6.4
0.13
-40
-78.5
(DRY ICE)
6.6 x 10 -4
-196
(LIQUID NITROGEN)
10 -24
(Page 14 manual)
(Page 15 manual)
Vapor Pressure of some Solids
(Page 15 manual)
PRESSURE RANGES
RANGE
PRESSURE
ROUGH (LOW) VACUUM
759 TO 1 x 10 -3 (mbar)
HIGH VACUUM
1 x 10 -3 TO 1 x 10 -8 (mbar)
ULTRA HIGH VACUUM
LESS THAN 1 x 10 -8 (mbar)
(Page 17 manual)
Viscous and Molecular Flow
Viscous Flow
(momentum transfer
between molecules)
Molecular Flow
(molecules move
independently)
FLOW REGIMES
Viscous 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 between
molecules and wall dominate; flow through random motion;
generally P smaller than 10-3 mbar
(Page 25 manual)
MEAN FREE PATH

1
P    (diametro molecole) 2
2
kT
Il libero cammino medio è inversamente proporzionale
alla pressione ed
alla sezione d’urto della molecola di gas
MOLECULAR DENSITY AND MEAN FREE PATH
1013 mbar (atm)
1 x 10-3 mbar
1 x 10-9 mbar
#
mol/cm3
3 x 10 19
(30 million trillion)
4 x 10 13
(40 trillion)
4 x 10 7
(40 million)
MFP
2.5 x 10-6 in
6.4 x 10-5 mm
2 inches
5.1 cm
31 miles
50 km
Portata:
P1
A’
A
Flusso
dV
dn
Q  P
 kT
dt
dt
P2
P1 > P2
Q è costante lungo il tubo e pertanto
Conduttanza:
P1 
dV1
dV
 P2  2
dt
dt
Q
C
P1  P2
Conduttanza in parallelo:
Conduttanza in serie:
Q  Q1  Q 2  (C1  C 2 )  (P1  P2 )
C  (C1  C 2 )
C1
P1
P2
C2
C2
C1
Q1
P1
P2
Q
P3
Q2
Flusso costante:
Q  C1  (P1  P2 )  C 2  (P2  P3 )
Flusso totale = somma dei flussi
Q  Q1  Q 2  (C1  C 2 )  (P1  P2 )
C  (C1  C 2 )
Q

P

P

 1 2 C
 1
1  Q
1
 
quindi 
 P1  P3  Q   
Q
 C1 C 2  C
P2  P3 

C2
per le due conduttanz e in serie :
 1
1 

C   
 C1 C 2 
1
VELOCITA’ DI POMPAGGIO DI UNA POMPA
Ppompa
Camera
PCamera
S
Pompa
C
Q
Q
Ppompa

C  Pcamera  Ppompa


Ppompa
VELOCITA’ EFFETTIVA DI POMPAGGIO DI UN SISTEMA:
Seff 
Q
Pcamera


C  Pcamera  Ppompa
Pcamera
  S P
pompa
Pcamera
1 1 
  
S C
1
L’effetto della conduttanza è quello di ridurre la velocità di pompaggio efficace
Rispetto alla velocità di pompaggio all’imbocco della pompa
FLOW REGIMES
Viscous Flow:
Mean Free Path
is less than 0.01
Characteristic Dimension
Transition Flow:
Mean Free Path
Characteristic Dimension
Molecular Flow:
Mean Free Path
is greater than 1
Characteristic Dimension
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
d P1  P2
C  138  
(l / s )
l
2
4
EXAMPLE:
d = 4 cm
l = 100 cm
d
l
P1
P2
=
=
=
=
diameter of tube in cm
length of tube in cm
inlet pressure in torr
exit pressure in torr
P1 = 2 torr
P2 = 1 torr
(Page 28 manual)
C=530 l/s
Conductance in Molecular Flow
Under molecular flow conditions doubling the pipe diameter increases the conductance
eight times.
The conductance is INVERSELY related to the pipe length.
C  3.81 
3
d
T

(l / s )
l
M
d = diameter of tube in cm
l = length of tube in cm
T = temperature (K)
M = A.M.U.
EXAMPLE:
T = 295 K (22 OC) d = 4 cm M = 28 (nitrogen) l = 100 cm
C=7.9 l/s
GAS LOAD
Outgassing
Permeation
Real
Leaks
Diffusion
GAS LOAD (Q) IS EXPRESSED IN:
mbar liters per second
Virtual
Backstreaming
Pumpdown Curve
10+1
Pressure (mbar)
10-1
Volume
10-3
10-5
Surface Desorption
10-7
Diffusion
10-9
10-11 1
10
Permeation
10 3
10 5
10 7 10 9 10 11 10 13 10 15 10 17
Time (sec)